Genetics, Cell Cycle & Inheritance (SAC 1) Flashcards
1
Q
define Deoxyribonucleic acid (DNA)
A
a double-stranded nucleic acid chain made up of nucleotides. DNA carries the instructions for proteins which are required for cell and organism survival
2
Q
define Nucleotide
A
the monomer unit of nucleic acids. Made up of a nitrogen-containing base, a sugar molecule (ribose in RNA and deoxyribose in DNA), and a phosphate group
3
Q
define Gene
A
a section of DNA that carries the code to make a protein
4
Q
define Genome
A
the complete set of DNA contained within the haploid set of an organism’s chromosomes
5
Q
define Allele
A
Alleles are different forms of the same gene but with small differences in their base sequence.
6
Q
define Locus
A
the fixed position on a chromosome where a particular gene is located
7
Q
where does DNA sit and what does it form?
A
Deoxyribonucleic acid (DNA) sits inside the nucleus of your cells, forms the major component of chromosomes and contains coded genetic instructions.
8
Q
how is DNA formed?
A
It is formed by the continuous pairing of base pairs into a longer, double-stranded nucleic acid chain.
9
Q
what is DNA made up from?
A
The genetic material DNA is a complex molecule built of many basic building blocks or monomers called nucleotides
10
Q
what are nucleotides made up of?
A
- A deoxyribose sugar
- A phosphate group
- A nitrogen-containing base
11
Q
what are the nitrogenous bases?
A
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
12
Q
what are the complementary base pairs?
A
Adenine (A) and Thymine (T)
Guanine (G) and Cytosine (C)
13
Q
what is chargaff’s rule?
A
Amount of Adenine = amount of Thymine
Amount of Guanine = amount of Cytosine
14
Q
what did watson and crick identify DNA to be?
A
identified the 3D structure of DNA as being two nucleotide chains arranged to form a double helix
15
Q
how do genes make proteins and what determines the protein?
A
- A gene is a particular section of DNA that codes for the creation of an individual polypeptide chain (protein).
- It acts like a set of instructions, which a cell will then read to make the proteins required for a range of cellular functions.
- The order of bases in the double helix determines which protein gets made.
16
Q
how many copies of genes does each person have?
A
Each person has two copies of each gene, one inherited from their mother and the other from their father.
17
Q
how many different genes are inside each of your cells?
A
roughly 25 000
18
Q
what is a genome and what does it house?
A
This collection of genes is called your genome, which houses all of the genetic information needed to build and maintain you: a complex organism.
19
Q
how many alleles are present at the gene locus?
A
Only two alleles are present at the gene locus of any one individual, each of which is inherited from one parent (one from mum and one from dad).
20
Q
what contributes to differences in people?
A
Almost all genes are the same across every human being.
However, a small percentage of genes (<1%) are slightly different between people, which contributes to the immense differences we see from person to person.
These differences are the result of alleles.
21
Q
define Histone protein
A
highly basic proteins that associate with DNA inside the nucleus and help it condense into a chromosome allowing it to fit inside the nucleus
22
Q
define Chromosome
A
the structure made of protein and nucleic acids that is DNA tightly wrapped around histone proteins, carries genetic information of a cell
23
Q
define Somatic cell
A
any cell that is not a reproductive cell (such as sperm and egg cells). Somatic cells are diploid (2n), meaning they contain two sets of chromosomes – one inherited from each parent
24
Q
define Chromatid
A
one half of a replicated chromosome. Prior to cell division, chromosomes are duplicated and two copies join together at their centromeres (joined chromatids are known as sister chromatids)
25
define Homologue
a homologous chromosome
26
where does a genome sit in a cell?
An organism’s genome sits inside the nucleus of each of their somatic cells in the form of DNA.
27
how does DNA become chromosomes?
Each molecule of DNA is coiled tightly around histone proteins and packaged into thread-like structures called chromosomes.
28
how many chromosomes does a human somatic cell have?
Human somatic cells usually contain 46 chromosomes.
29
define telomeres
region of repetitive base sequences that is found at the end of every chromosomes from fusing with other nearby chromosomes in the nucleus
30
define DNA molecule
each chromosome is composed of a long DNA molecule that has been coiled tightly around histone proteins
31
define centromere
a specialised sequence of DNA that holds together the two chromatids. It is very important for the process of meiosis.
32
define sister chromatids
the identical daughter strands of a replicated chromosome
33
define short arm
'p arm', section of chromosome that is shorter in length
34
define long arm
'q arm', section of chromosome that is longer in length
35
what do chromosomes vary in?
Chromosomes vary in size depending on the number of nucleotides they contain.
36
what structures can chromosomes be in?
- 1 chromatid
- 2 duplicated sister chromatids
37
define diploid
full set of chromosomes contained within an organism. Two copies of each chromosome. Human somatic cells have 46 chromosomes (2n = 46), i.e. 23 pairs.
38
define haploid
having one copy of each chromosome. A set of unpaired chromosomes. Human gametes (eggs & sperm) have 23 chromosomes (n = 23).
39
define homologous chromosomes
We call each of the sets of 23 chromosomes within a human nucleus a pair of homologous chromosomes.
- They are the same in size and length
- They have the same centromere position
- They share the same genes at the same gene loci
One of the pair is inherited from your mum (maternal chromosome) and the other from your dad (paternal chromosome).
40
define karyotype
A karyotype is a visual representation of an individual’s entire genome organised into homologous pairs.
41
how are chromosomes arranged in karyotypes?
Chromosomes are arranged by size and centromere position into their homologous pairs.
42
how are karyotypes used to assist in analysis?
Karyotypes are used by to assist in the analysis of chromosomes that are present in cells and check for possible genetic abnormalities.
43
what will biologists check in karyotypes?
When reading karyotypes, biologists will check that the correct number of chromosomes are present and that the size and length of each chromosome are correct.
44
what is an autosome?
any chromosome in humans that is not a sex chromosome (pairs 1 - 22).
45
what is an sex chromosome?
a chromosome responsible for determining the biological sex of an organism.
46
what do the sex chromosomes determine in humans?
The sex of the human. In humans, two X chromosomes result in a female, whereas one X and one Y chromosome will result in a male.
47
how are karyotypes used to separate species?
Diploid number in human somatic cells is 2n = 46
However, diploid number across different species varies widely.
Scientists can use karyotypes to represent these differences in chromosome number and determine genetic differences between species.
48
whats the most important use of a karyotype?
Perhaps the most important use of the karyotype is to detect chromosomal abnormalities within species, such as aneuploidy and polyploidy.
49
define Aneuploidy
Aneuploidy refers to a chromosomal abnormality in which an organism possesses an incorrect number of total chromosomes caused by the addition or loss of an individual chromosome
50
what does Aneuploidy mean in humans in relation to number of chromosomes?
In humans, this means having more or less than the usual number of 46.
51
what are the different form of Aneuploidy?
- Monosomy
- Trisomy
- Tetrasomy
52
define Monosomy
if an organism has one missing chromosome (2n - 1)
53
define Trisomy
if an organism has one extra chromosome (2n + 1)
54
define Tetrasomy
if an organism has two extra chromosomes (2n + 2)
55
case study on monosomy?
Name: Turner syndrome (45, X0)
Mutation: single X chromosome
Incidence rate: 1 : 2000
Common symptoms:
- Infertility
- Short stature
- Fused neck and head
- Poor breast development
- No menstruation
56
case study on trisomy 21?
Name: Down syndrome (47, XY, +21)
Mutation: extra copy of chromosome 21
Incidence rate: 1 : 1000
Common symptoms:
- Delayed physical growth
- Possible heart defects
- Flattened facial profile
- Mild to moderate intellectual disability
- Poor muscle tone
57
case study on trisomy?
Name: Klinefelter syndrome (47, XXY)
Mutation: extra X chromosome in males
Incidence rate: 1 : 650 males born
Common symptoms:
- Accelerated growth & taller than average stature
- Small testes and reduced testosterone levels
- Absent, delayed or incomplete puberty
- Decreased muscle mass
- Infertility
- Learning & intellectual disabilities
58
define Polyploidy
Polyploidy refers to a chromosomal abnormality in which an organism has more than two sets of each chromosome.
59
what does polyploidy mean in humans?
In humans, this would mean that rather than being diploid (2n = 46), the individual would be 3n = 72 or more.
60
what happens when humans have polyploidy?
Polyploidy is typically lethal in humans (extremely rare for a foetus to survive to term). However, it is quite common for other organisms, especially plants, to thrive with additional sets of chromosomes.
61
advantages of polyploidy?
Advantages of polyploidy include increased size and hardiness in certain types of fruit and faster growth rate in farmed Atlantic Salmon that are triploid.
62
features of prokaryotes?
- Lack a nucleus
- Single celled organism (unicellular)
- Have a single chromosome - a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid.
- Reproduce by binary fission
- Include bacteria and archaea
63
why do cells undergo cell replication/division?
Cell replication is a critical process for organisms to undergo in order to survive.
64
purposes of cell division?
Its purposes include growth and development, maintenance and repair and reproduction.
65
purpose of cell division - growth and development explanation?
for a multicellular organism to grow and develop, it is important that cells replicate as to grow we simply are made of new cells
66
purpose of cell division - maintenance and repair explanation?
cells are constantly dying as they age or become damaged, cell replication to ensure the proper functioning of an organism
67
purpose of cell division - reproduction explanation?
prokaryotic and eukaryotic cells replicate to reproduce, when replicating they are enlarging population
68
features of binary fission?
- Type of asexual reproduction
- Prokaryotes (e.g.bacteria) reproduce rapidly via binary fission
- Less complex process and faster than the cell cycle in eukaryotes
- One division can be completed can be completed in 20 minutes in some bacteria
69
what does binary fission result in?
Results in two genetically identical copies of a cell
70
what is binary fission composed of?
Composed of the following steps: DNA replication, elongation, septum formation and cell division (D.E.S.C)
71
what is the process of binary fission?
1. there is a prokaryotic cell before cell replication
2. the circular chromosomes is un coiled and the DNA is replicated plasmids also replicated
3. the cell elongates as its prepares to separate into two cells and the duplicated circular chromosomes migrate to opposite ends
4. cell begins cytokinesis (process of separating into two cells), by pinching inwards and creating a septum. Because plasmids replicate independently of the circular chromosome, these will not always be evenly distributed between the two new cells
5. a new cell and membrane are formed down the centre of the cell
6. two genetically
72
what is a plasmid?
small circular loop of DNA that is separate from a chromosome, typically found in bacteria.
73
define gametes
reproductive cells that arise from germline cells and contain half the genetic material (n) of a somatic cell.
74
define zygote
the diploid cell formed by the combination of two haploid gamete cells
75
define Germline cell
cells that are involved in the generation of gametes in eukaryotes
76
define Gonads
the organs that produce gametes from germline cells
77
define Crossing over
the exchange of genetic material between non-sister chromatids during prophase I of meiosis, resulting in new combinations of alleles in daughter cells
78
define Chiasma
the point/location of overlap between two non-sister chromatids
79
define Independent assortment
the random orientation of homologous chromosomes along the metaphase plate during metaphase I
80
what is meiosis
Meiosis is a specialised form of cell division that occurs in sexually reproducing organisms and is used to produce the gametes necessary for sexual reproduction.
81
what are gametes from meiosis used for after?
In humans, these gametes are sperm and egg (also known as ova) cells, which contain only one copy of each chromosome (n) and will fuse together during fertilisation to create two copies of each chromosome (2n) inside a developing zygote.
82
what does meiosis result in?
Meiosis involves a single cell dividing into four haploid cells, each of which is genetically different from one another.
83
what is the dividing cell in meiosis?
In humans, the dividing cell is known as a germline cell and is found in the gonads (testes for males and ovaries in females).
84
in nucleotides what changes?
- sugar and phosphate parts are the same in all four nucleotides
- the difference is between the nitrogenous bases they contain
85
is DNA symmetrical?
yes
86
what are adenine and guanine types?
purines
87
what are thymine and cytosine types?
pyrimidines
88
what is exponential growth and how does binary fission allow this?
- Cells replicate exponentially - after each round of division the number of cells doubles
- Binary fission in bacteria allows a single bacterium to replicate into a few million bacteria in a very short span of time
89
features of eukaryotes?
- Contain a nucleus and membrane-bound organelles
- Nucleus houses the cell's DNA and directs the synthesis of proteins and ribosomes
- Replication of cells occurs through the cell cycle (mitosis)
90
what is the eukaryotic cell cycle?
Process that results in the production of two new cells, each identical to the parent cell that gave rise to them
91
how long does it take for typical humans to complete the the eukaryotic cell cycle?
For a typical human, 24 hours are required to complete one cell cycle
92
Three stages of the eukaryotic cell cycle include?
Interphase - cellular growth and duplication of chromosomes
Mitosis - separation of sister chromatids and the formation of two new nuclei
Cytokinesis - division of the cytoplasm and formation of two daughter cells
93
what are the stages of growth and division in the eukaryotic cell cycle and their times?
- G1 stage - 11 hours
- S stage - 8 hours
- G2 stage - 4 hours
- M stage (Mitosis) - 1 hour
- Cytokinesis
94
what is interphase?
period of DNA replication, which is the first and longest stage of the cell cycle
95
what happens during interphase?
During interphase, the cell synthesises the necessary DNA, proteins and organelles required for growth and replication.
96
what does DNA exist as in interphase?
DNA in the nucleus exists as long chromatin threads instead of chromosomes.
97
Interphase is divided into three sub-stages?
- Gap 1 (G1) phase
- Synthesis (S) phase
- Gap 2 (G2) phase
98
what is the G1 phase?
In the G1 phase (first growth stage), the cell prepares to copy its DNA and grows
99
how does the cell prepare to copy DNA and grow in G1?
- Increasing the volume of its cytosol
- Synthesising proteins required for DNA replication
- Replicating its organelles
100
what happens at the end of the G1 phase?
At the end of this phase, the cell either proceeds to the S phase or exits the cell cycle and enters the G0 phase (non-dividing quiescent phase)
101
what is the G0 phase?
Cells that are not required to replicate rest in the G0 phase.
102
what are cells in G0 phase?
Cells in G0 are either quiescent or terminally differentiated.
103
what is a quiescent cell?
Quiescent cells are dormant and have the ability to re-enter the cell cycle. Example: liver cells
104
what is a terminally differentiated cell?
Terminally differentiated cells are fully specialised and no longer replicate (remain in G0 indefinitely). Example: nerve cells
105
what happens in the synthesis phase?
Cell replicates its DNA, turning one chromosome into two genetically identical sister chromatids.
106
what happens at the end of the synthesis phase?
At the end of this stage, the parent cell contains 2 identical copies of its original DNA.
107
what are chromosomes regard to as when sister chromatids are held together by the centromere?
a single chromosome
108
what are sister chromosomes regarded to as when indivivdual?
a single chromosome
109
what is the G2 phase?
Final stage of interphase where cell continues to grow and prepare itself for mitosis. Similar to G1 phase/
110
how is G2 phase similar to G1 phase?
- Cell continues increasing the volume of the cytosol.
- Cell synthesises proteins in preparation for mitosis.
111
what is mitosis (M phase)?
- Important part of the cell cycle
- Involves the organisation and separation of the newly replicated chromosomes into two new nuclei
112
four sub-stages of mitosis?
Prophase
Metaphase
Anaphase
Telophase
113
define Chromosome condensation
the shortening and thickening of chromosomes, as DNA is tightly coiled around histone proteins
114
define Centrioles
cylindrical structures composed of protein which form the spindle fibres during mitosis
115
define Spindle fibres
structures which aid in the movement of chromosomes to either pole of the cell during mitosis and meiosis
116
define equator
the centre line between opposite ends of the cell that the chromosomes line up on during metaphase
117
define Cleavage furrow
an indentation of the plasma membrane during cytokinesis
118
define cell plate
a component involved in the formation of a cell wall
119
what happens in prophase?
- DNA coils tightly and chromosomes condense becoming visible as distinct chromosomes (double-stranded structures)
- Nuclear membrane breaks down and nucleolus disappears
- Centrioles (organelles made up of microtubules) migrate to opposite ends (or poles) of the cell
- Spindle fibres begin to form
120
what happens in metaphase?
- Mitotic spindle fibres fully form and attach to the centromere of each chromosome
- Spindle fibres guide the double-stranded chromosomes towards the equator (middle) of the cell where they line up
121
what happens in anaphase?
- Spindle fibres contract (shorten), splitting the centromere of each chromosome
- Sister chromatids are then pulled by the spindle fibres to opposite poles of the cell
122
what happens in telophase?
- Separation of chromosomes is completed
- Chromosomes densely pack together at either end of the cell
- New nuclear membranes form around each separate group of chromosomes producing two genetically identical nuclei
- Spindle fibres disintegrate and chromosomes gradually decondense (no longer visible as distinct chromosomes)
123
when does cytokinesis occur?
Occurs after mitosis
124
what happens in cytokinesis?
- the cytoplasm divides
- the organelles evenly distribute themselves before separating into two genetically identical daughter cells
125
how does cytokinesis occur in animals?
In animals, this occurs when a cleavage furrow develops and pinches the plasma membrane into two new cells
126
how does cytokinesis occur in plants?
In plants, because they have a cell wall, a cell plate first forms at the equator before separating into two cells
127
steps of cytokinesis in animals?
1. formation of cleavage furrow
2. the cleavage furrow reaches the centre
3. the cells are pinched apart into two cells
128
steps of cytokinesis in plants?
1. vesicles start to accumulate in the centre of the cell
2. a cell plate begins to form along the equator of the cell
3. new cell walls are formed producing two new cells
129
how does the eukaryotic cell cycle regulate itself?
The cell cycle has three checkpoints where the cell inspects itself for errors before proceeding to the next stage
130
when are the checkpoints in the eukaryotic cell cycle?
These checkpoints occur at the end of the G1 and G2 phases and during metaphase
131
what happens if errors are detected at one of the checkpoints in the eukaryotic cell cycle?
- If any errors are detected, the cell can pause for repairs
- If the damage is irreparable, then the cell undergoes programmed cell death
132
what does the G1 checkpoint check?
- If the cell has grown to the correct size
- If the cell has synthesised enough protein for DNA replication
- If the original DNA has been damaged during mitosis and cell growth
- If there are enough nutrients and oxygen (favourable conditions for mitosis)
133
what happens if the cell passed the G1 checkpoint?
If a cell passes this checkpoint, it can advance to the S stage of the cell cycle.
134
what does the G2 checkpoint check?
- DNA has replicated properly in the S phase and there is no damage
- The cell has enough resources for mitosis
135
what happens if the cell passed the G2 checkpoint?
If a cell passes this checkpoint, it can advance to the mitosis stage of the cell cycle.
136
what does the M checkpoint check?
- The formation of the spindle fibres
- That the spindle fibres are attached to the chromosomes correctly
137
what happens if the cell passed the Metaphase (M) checkpoint?
If the chromosomes are lined up in the correct location, the cell proceeds to anaphase.
138
how many processes of meiosis are there?
Meiosis I and Meiosis II
139
what happens in meiosis I?
which separates each homologous chromosome into two different cells, begins with a diploid germline cell and results in two genetically distinct daughter cells
140
what happens in meiosis II?
which separates each sister chromatid into four different cells, haploid cells that were created in Meiosis I and, by splitting each of the chromosomes into two sister chromatids, creates four separate haploid cell
141
what happens in meiosis interphase?
cell grows and duplicates all of its chromosomes in preparation for division
142
what happens in prophase I?
this is where crossing over occurs, nuclear membrane breaks down as chromosomes condense and line up in homologous pairs
143
what happens in metaphase I?
this is where independent assortment occurs, homologous chromosomes will then line up randomly on opposite sides of the metaphase plate, with once copy (paternal) or (maternal) on either side, each chromosome is then attached to the microtubules of whichever pole it is closest to in preparation to be separated during the next stage
144
what happens in anaphase I?
homologous chromosomes are moved apart towards opposite poles of the cell, however for now, sister chromatids remain attached to one another at the centromere
145
what happens in telophase I?
the chromosomes arrive at opposite ends of the cell as the nuclear membrane is cleaved, a cleavage furrow forms in preparation for the cell to undergo cytokinesis
146
what happens in cytokinesis I?
splits each of the chromosome sets at opposite ends of the cell into two haploid cells, the cytoplasm divides
147
what happens in prophase II?
two cells prepare for another division, nuclear envelope begins to break down, the chromosomes condense, and the spindle fibres from in preparation to pull apart the sister chromatids of each chromosome
148
what happens in metaphase II?
each chromosome lines up along the metaphase plate of the cell, as microtubules from opposite poles of he cell prepare to pull the chromosomes apart
149
what happens in anaphase II?
sister chromatids are now separated and pulled towards opposite poles of the cell by microtubules, which are now attached at each centromere
150
what happens in telophase II?
individual chromatids now at each pole of the cell, separate nuclear membranes begin to form around each set as the chromosomes begin to decondense and unravel
151
what happens in cytokinesis II?
splits each of the chromosomes seats at opposite ends of the cell into fur haploid cells, the cytoplasm divides
152
what is the aim of meiosis?
The aim of meiosis is to produce gametes that are genetically distinct from each other, and from the parent cell from which they originated.
153
what does meiosis do?
Meiosis increases the genetic diversity present in resulting gamete cells through the processes of crossing over and independent assortment
154
when does crossing over occur?
prophase I
155
when does independent assortment occur?
metaphase I
156
what is crossing over?
which exchanges genetic material between homologous chromosomes at the charisma and results in recombinant chromatids with their own unique combination of alleles
157
how does crossing over increase genetic diversity?
adds to the genetic diversity of the daughter cells as the sister chromatids they will inherit are no longer identical
158
what is independent assortment?
which allows homologous chromosomes to arrange randomly along the equator of the cell, irrespective of the orientation of the other homologous pairs and results in the random splitting of chromosomes into different daughter cells
159
how does independent assortment increase genetic diversity?
adds to genetic diversity as the resulting combination of alleles in each daughter cell is randomised, give rise to different chromosome combinations in gametes
160
differences between meiosis and mitosis?
- mitosis is used by almost every cell in your body, whereas meiosis is used by germ cells
- mitosis is important in development and growth and is also used to replace old or malfunctioning cells within our body with identical copies, whereas meiosis is important for sexual reproduction so that when two gametes fuse during fertilisation, the resulting offspring does not inherit double the amount of necessary genetic material
- mitosis results in the production of two identical copies of the original cell, whereas meiosis serves one specific purpose – to produce gametes which have exactly half the genetic material of the original cell
- mitosis starts with 1 somatic cell (2n) whereas meiosis starts with 1 germline cell (2n)
- mitosis produced two identical somatic cells (2n), whereas meiosis produces four genetically unique gamete cells (n)
- mitosis has one PMAT phase, whereas meiosis has two PMAT phases
161
define diploid?
having two sets (2n) of each chromosome, one from each parent
162
define homozygous
having identical alleles for the same gene on homologous chromosomes
163
define heterozygous
having different alleles for the same gene on homologous chromosomes
164
define complete dominance
a pattern of dominance where only the dominant allele from the genotype of a heterozygous individual is expressed in the phenotype of that organism
165
define carrier
an organism that has inherited a copy of a recessive allele for a genetic trait but does not display the trait due to it being masked by the presence of a dominant allele
166
define Sex-linked genes
genes that are located on a sex chromosome
167
how many alleles do diploid organisms inherit?
Every diploid organism inherits a particular combination of two alleles from their parents
168
what happens if an individual inherits two identical alleles for the same gene from both their mother and father?
they are said to be homozygous for that gene
169
what happens if an individual inherits two different alleles for the same gene from their parents?
they are said to be heterozygous for that gene
170
what is a dominant allele?
the variant of a gene that is always expressed in the heterozygote and masks the effect of a recessive allele of the same gene on a homologous chromosome
171
how are dominant alleles represented?
Dominant alleles are represented by CAPITAL letters
172
what is a recessive allele?
the variant of a gene that is masked by a dominant allele on a homologous chromosome. It can only be expressed if the person has both recessive alleles
173
how are recessive alleles represented?
Recessive alleles are represented by LOWERCASE letters.
174
what is a genotype?
the genetic composition of an organism at one particular gene locus (represented using letter symbols)
175
what do genotypes show us?
Genotypes show us if an individual is homozygous dominant, homozygous recessive or heterozygous for a certain trait.
176
how do we represent genotypes?
Any letter can be used, although we typically use letters that relate to the trait in question
In diploid organisms, the genotype is typically written as a pair of alleles.
177
what is a phenotype?
the observable physical, behavioural or biochemical characteristics of an organism that are the result of gene expression and the environment
178
what are phenotypes results of?
A phenotype may sometimes result from interactions between genes at two or more gene loci.
179
what are phenotypes influenced by?
An organism’s phenotype is influenced both by its genotype and by the environment in which it lives, and is therefore susceptible to change over time.
180
what is complete dominance?
Complete dominance is when a dominant allele is fully expressed in a phenotype and masks the expression of a recessive allele.
181
are dominant alleles for common?
dominant alleles are not always more common than recessive alleles
182
who is a carrier?
Although a dominant allele may mask the expression of a recessive allele, a person who is heterozygous at a specific gene locus is still a carrier.
183
what does being a carrier mean?
This means they are still able to pass on the recessive allele to their offspring despite not displaying the trait.
184
Examples of recessive disorders?
- Cystic fibrosis
- Albinism
- Thalassaemia
185
examples of dominant traits?
- Peaked hairline (widow’s peak) (W)
- Free earlobes (F)
- Shortened fingers (brachydactyly) (S)
- Normal pigmentation (A)
- Non-red hair (R)
- Dwarf stature (achondroplasia) (N)
- Rhesus positive (Rh +ve) blood (D)
186
examples of recessive traits?
- Straight hairline (w)
- Attached earlobes (f)
- Normal length fingers (s)
- Pigmentation lacking (albinism) (a)
- Red hair (r)
- Average stature (n)
- Rhesus negative (Rh −ve) blood (d)
187
what is co dominance?
Codominance occurs when both alleles from the genotype are fully expressed in the phenotype of a heterozygote.
In this case, both alleles can be thought of as dominant and neither allele can mask the expression of the other allele.
188
what is incomplete dominance?
Incomplete dominance occurs when neither allele from the genotype is fully expressed in the phenotype of a heterozygote.
The resulting observable trait is a blending of both alleles.
189
how are alleles for codominance & incomplete dominnance represented?
We use a standard capital letter that stays the same and two different superscript letters to represent the two different alleles.
Any letter of the alphabet can be used for the base letter and the superscripts, as long as they are different from each other.
190
what are the alleles if the ABO blood group?
IA - domiant
IB - dominant
i - recessive
191
what are the blood groups?
- Blood type A
- Blood type B
- Blood type O
- Blood type AB
192
what are autosomal genes?
Genes found on the autosomes (chromosomes 1 - 22)
193
how many autosomes do humans have?
Males and females have 2 copies of each gene, one on each chromosome in a homologous pair.
194
what are sex-linked genes?
Sex-linked genes are genes that are located on either the X or Y chromosomes (sex chromosomes).
195
what does sex linked inheritance refer to in humans?
In humans, sex-linked inheritance usually refers more specifically to X-linked traits, as they are far more common.
Y-linked traits also exist, but are rare and only show up in males.
This is because the X chromosome is much longer than the Y chromosome, containing as many as 4000 more genes.
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what are x linked traits?
X-linked traits are traits controlled by a gene that is located on the X chromosome.
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what are y linked traits?
Y-linked traits are traits controlled by a gene that is located on the Y chromosome.
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why are x linked recessive traits more likely in males?
- X-linked recessive traits are more likely to be expressed in males than they are in females. This is because males only have a single copy of the X chromosome, which comes from their mother.
- The allele present on their X chromosome will be expressed in their phenotype, regardless of whether that allele is dominant or recessive.
- There is no corresponding allele on the Y chromosome to mask the effects of the allele on the X chromosome. This means that if the allele is faulty and causes a condition, then the male has a 100% chance of showing that condition.
- In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.
- Therefore, males only need to inherit ONE recessive allele in order to get a sex-linked trait and a female needs to inherit TWO recessive alleles in order to acquire a sex-linked trait.
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why are males with x linked traits unable to pass them onto their sons?
Males with X-linked traits or conditions are unable to pass the trait on to their sons.
This is because if a male has a son, that son must received their Y chromosome from their father.
Affected males can only pass on the abnormal X-linked gene to their daughters.
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how are sex linked genotypes expressed?
Sex-linked genotypes are written differently, following a similar superscript notation as codominance and incomplete dominance.
In sex-linked genotypes the standard base letter is always X or Y.
This is because as well as communicating which allele is dominant or recessive, they also need to show which sex chromosome carries the allele.
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define environment
the conditions and resources external to an organism with which that organism typically interacts
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define proportionate heritability
the amount of phenotypic variance that can be explained by genes in a given population
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define epigenetics
changes to an organism’s phenotype resulting from modifications to gene expression
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define gene expression
the process of reading the information stored within a gene to create a functional product, typically a protein
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define transcription
the process whereby a sequence of DNA is used to produce a complementary sequence of mRNA
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define translation
the process whereby an mRNA sequence is used to produce a protein
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define DNA methylation
the process by which methyl (-CH3) groups are added to particular nucleotides in a DNA segment so as to modify the expression of a gene
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what are phenotypes influenced by?
Phenotypes are influenced not only by an organism’s genes, but also by the environment in which they live.
An organism’s phenotype can be influenced by environmental factors such as temperature, light, nutrition or predation.
This idea is called proportionate heritability, where an organism’s phenotype is explained partly by genes and partly by environmental factors.
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what does proportionate heritability tell us?
- Proportionate heritability tells us that an organism’s phenotype can be entirely due to the environment
- Other times, genes play the main role in determining phenotype
- However, most of the time, your phenotype is the result of a mixture of genetic influences and environmental influences.
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what are epigenetic modifications?
In some cases, environmental factors may cause changes to a gene that activate or deactivate the expression of that gene.
This affects the amount of protein produced, which subsequently alters an individual’s phenotype.
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how do epigenetics factors influence gene expression?
Epigenetics factors influence gene expression by determining which genes are ‘turned on or off’, but they do not alter the actual DNA sequence.
This is often in response to environmental changes, such as exposure to certain chemicals.
Epigenetic factors may underlie some of the differences seen in identical twins, since these differences cannot be explained by differences in their genotypes.
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How does epigenetics work?
- A gene is expressed when the protein it carries the instructions for is built by a cell.
- This occurs through the processes of transcription and translation.
- Epigenetic changes alter the process of transcription.
- They are caused by molecules that increase or decrease the amount of transcription of a particular gene and therefore alter the amount of protein that is produced.
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how does making proteins work?
DNA --- transcription ---> copy of gene (mRNA) --- translation ---> protein
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what are the two types of epigenetic modifications?
- DNA methylation
- Histone modification
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process of DNA methylation?
occurs when methyl groups (small hydrocarbon molecules) attach to certain nucleotides within the DNA sequence of a particular gene and alter levels of gene expression, typically causing that gene to be silenced (turned off)
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process of histone modification?
modify how tightly a DNA molecule is wrapped around a histone protein . When DNA is wrapped tighter around the histone, genes are less likely to be expressed and when DNA is less tightly wrapped, genes are more likely to be expressed
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how does epigenetics plays a number of important roles in the cells of our body?
Helping to control cell differentiation - development of different cell types is regulated by epigenetic mechanisms that turn off unneeded genes and promote the expression of required genes.
Providing a mechanism for a developing organism to respond to its environment - epigenetic modifications act as a rapid feedback mechanism by which an organism can respond to changes in their environment.
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can epigenetics be passed on and what type of heritability?
- Epigenetic changes can be passed onto daughter cells during mitosis. This means that they could affect an individual organism throughout the entire course of its life.
- For this reason, epigenetic features are described as being somatically heritable (can be passed from somatic cell to somatic cell).
- Most epigenetic changes in an organism are erased when gametes (sperm & egg cells) are formed.
- However, current research suggests that a small proportion of the epigenetic changes that accumulate over a lifespan may in fact remain during the production of gametes.
- This means that those epigenetic traits would pass onto the next generation and suggests that both upbringing and decisions made as young adults could potentially impact future progeny.
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what is a monohybrid cross?
genetic cross performed to observe the inheritance of alleles and phenotypes for a single gene
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what is a punnet square?
a square diagram used to predict the genotypes of offspring and therefore phenotypes
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what is a test cross?
when an individual expressing the dominant phenotype but with an unknown genotype is crossed with a homozygous recessive individual. The results indicate whether the individual with the dominant trait is homozygous dominant or heterozygous
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what is the Parental (P) generation?
the parental generation in a breeding experiment
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what is the F1 generation?
the first generation offspring in a breeding experiment (1st filial generation), from breeding individuals of the P generation
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what is the F2 generation?
the second generation offspring in a breeding experiment (2nd filial generation), from breeding individuals of the F1 generation
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monohybrid cross results?
AA x AA - 100% AA - dominant
AA x Aa - 50% AA, 50% Aa - 100% dominant
AA x aa - 100% Aa - 100% dominant
Aa x Aa - 25% AA, 50% Aa, 25% aa - 75% dominant, 25% recessive
Aa x aa - 50% Aa, 50% aa - 50% dominant, 50% recessive
aa x aa - 100% aa - 100% dominant
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how to complete sex links punnet squares?
To complete Punnett squares for these traits, the method is the same, however, the way alleles are written and the genotypic and phenotypic proportions differ from that of an autosomal Punnett square.
With sex-linked traits, it is important to calculate proportions with respect to sex.
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what is a dihybrid cross?
a genetic cross used to observe the inheritance of alleles and phenotypes for two genes
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what is unlinked genes?
genes located on different chromosomes, or far apart of the same chromosome. Unlinked genes have less chance of being inherited together
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what are linked genes?
genes that are found close together on the same chromosome and are likely to be inherited together
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what are map units?
a measure of the distance between two genes on the same chromatid. Genes that are closer together are more likely to be linked genes
One map unit equates to a one percent chance of crossing over and the offspring containing a recombinant chromosome.
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what is a recombinant chromosome?
a chromosome which is not identical to one of the homologous chromosomes in a diploid cell
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what is a parental chromosome?
a chromosome which contains the same combination of alleles as one of the parents’ chromosomes
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what are the results of unlinked genes?
In crosses involving unlinked genes, the offspring occur in predictable ratios.
The outcome of a dihybrid cross of two heterozygotes is four phenotypes in the ratio of 9:3:3:1.
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how do you determine gametes of individuals?
FOIL
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can linked genes be separated?
Linked genes, however, can occasionally be separated through crossing over.
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what is recombination?
The further apart a pair of alleles are on a chromosome, the more likely it is that crossing over may occur between them, leading to recombination.
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what do the results of a linked gene cross?
two high numbers of parental gamete, two low numbers of recombinant gamete
- need real numbers to calculate
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what is a pedigree chart?
a diagram showing the expression of a trait over multiple generations
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what is consanguineous breeding?
breeding of two individuals that are closely related. Also known as inbreeding
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Important conventions of pedigree charts?
- Horizontal lines represent mating between two individuals
- Vertical lines represent the link between two generations
- Circles are females and squares are males
- Coloured shapes are affected individuals
- Uncoloured shapes are unaffected individuals
- double horizontal lines represent consanguineous
- diamond shape means gender isn't specified
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Reading and naming pedigree charts?
In a pedigree chart, each individual has a unique reference based on their generation and where they are located from left to right.
The generation of an individual is noted with Roman numerals, ascending from oldest to youngest.
The location of an individual in a generation is noted using numbers, ascending from left to right.
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To provide evidence in support of a given mode of inheritance, a pedigree must show:
- An adequate number of individuals, both affected and unaffected
- Individuals of each sex
- Preferably three generations
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Autosomal dominant inheritance on pedigrees?
- If both parents are affected, the offspring may be unaffected.
- If neither parent is affected, the offspring must be unaffected.
- If an offspring is affected, there must be an affected parent.
- The trait cannot skip a generation.
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Autosomal recessive inheritance on pedigrees?
- If both parents are affected, all offspring must be affected.
- If neither parent is affected, the offspring may be affected.
- If an offspring is affected, there may be an affected parent.
- The trait can skip a generation.
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X-linked dominant inheritance on pedigrees?
- If a male is affected, his mother must be affected.
- If a male is affected, his daughters must be affected.
- If a female is unaffected, her father must be unaffected.
- If a female is unaffected, her sons must be unaffected.
- The trait cannot skip a generation.
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X-linked recessive inheritance on pedigrees?
- If a female is affected, her father must be affected.
- If a female is affected, her sons must be affected.
- If a male is affected, his mother may be affected.
- The trait can skip a generation.
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Y-linked inheritance on pedigrees?
- Only males can show the trait.
- All males in a lineage will show the same phenotype.
- The trait cannot skip a generation.
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Autosomal dominant inheritance examples?
- Huntington’s disease
- Achondroplasia (dwarfism)
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Autosomal recessive inheritance examples?
- Cystic fibrosis
- Albinism
- Thalassaemia
- Phenylketonuria (PKU)
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X-linked dominant inheritance examples?
- Rett syndrome
- Fragile X syndrome
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X-linked recessive inheritance examples?
- Red-green colour blindness
- Haemophilia
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Y-linked inheritance examples?
- Hypertrichosis pinnae auris
- Y chromosome infertility
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flowchart for pedigrees?
is trait present in every generation?
yes - are all affected individuals male? if so, does every affected male have an affected father and are all sons affected?
yes - y-linked trait
no - does all affected males and healthy females produce affected daughters? AND do all affected females and healthy males produce affected sons?
yes - x-linked dominant
no - autosomal dominant
is trait present in every generation?
no - for every affected female, is their father affected?
yes - x-linked recessive
no - autosomal recessive
