inheritance

Question 1

what is a diploid cell?

Answer 1

A cell that contains 2 complete sets of chromosomes (2n)

These chromosomes contain the DNA necessary for protein synthesis and cell function
Nearly all cells in the human body are diploid with 23 pairs (46) of chromosomes in their nucleus

Question 2

what is a haploid cell?

Answer 2

a cell that contains one complete set of chromosomes (n)

In other words they have half the number of chromosomes compared to diploid cells
Humans have haploid cells that contain 23 chromosomes in their nucleus
These haploid cells are called gametes and they are involved in sexual reproduction
For humans they are the female egg and the male sperm

Question 3

Haploidy and diploidy are terms that can be applied to cells across different species

Answer 3

They describe the number of sets of chromosomes, not the total number of chromosomes

Question 4

whate is a gamete?

Answer 4

reproductive (sex) cells that fuse with another during fertilisation

Question 5

zygote

Answer 5

the diploid cell from which a cell develops
- forms from the fusion of gametes.

Question 6

What happens during fertilization ?

Answer 6

• the nuclei of gametes fuse together to form the nucleus of the zygote

Both gametes must contain the correct number of chromosomes in order for the zygote to be viable. If a zygote has too many or too few chromosomes it may not survive

For a diploid zygote this means that the gametes must be haploid
n + n = 2n

Question 7

what does meiosis produce?

Answer 7

haploid gametes during sexual reproduction

Question 8

What is the first cell division of meiosis called?

Answer 8

a reduction division
This is a nuclear division that reduces the chromosome number of a cell

In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)

The reduction in chromosome number during meiosis ensures the gametes formed are haploid

Question 9

chromosomes have a characteristic shape

Answer 9

They have a fixed length
The position of the centromere is in a particular location

These characteristic features allow for each chromosome to be identified in a photomicrograph

In photomicrographs chromosomes are often grouped into their homologous pairs

Question 10

homologous chromosomes

Answer 10

Carry the same genes in the same positions
Are the same shape

Question 11

During fertilization what type of zygote forms?

Answer 11

a diploid zygote is formed

In a zygote one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete

Having the same genes in the same order helps homologous chromosomes line up alongside each other during meiosis

Question 12

Although homologous pairs of chromosomes contain the same genes in the same order they don’t ….

Answer 12

necessarily carry the same alleles (form) of each gene

Question 13

meiosis

Answer 13

is a form of nuclear division that results in the production of haploid cells from diploid cells

It produces gametes in plants and animals that are used in sexual reproduction

Question 14

what are the 2 divisions of meiosis called:

Answer 14

meiosis I and meiosis II
Within each division there are the following stages: prophase, metaphase, anaphase and telophase

Question 15

prophase I

Answer 15

DNA condenses and becomes visible as chromosomes

DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere

The chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent

As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)

In this stage centrioles migrate to opposite poles and the spindle is formed

The nuclear envelope breaks down and the nucleolus disintegrates

Question 16

Metaphase I

Answer 16

The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres

Question 17

Anaphase I

Answer 17

The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle

The centromeres do not divide

Question 18

Telophase I

Answer 18

The chromosomes arrive at opposite poles

Spindle fibres start to break down

Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Question 19

Cytokinesis

Answer 19

This is when the division of the cytoplasm occurs

Cell organelles also get distributed between the two developing cells

In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half

In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell

The end product of cytokinesis in meiosis I: two haploid cells

These cells are haploid as they contain half the number of centromeres

Question 20

Meiosis II

Answer 20

There is no interphase between meiosis I and meiosis II so the DNA is not replicated

The second division of meiosis is almost identical to the stages of mitosis

Question 21

Prophase II

Answer 21

The nuclear envelope breaks down and chromosomes condense

A spindle forms at a right angle to the old one

Question 22

Metaphase II

Answer 22

Chromosomes line up in a single file along the equator of the spindle

Question 23

Anaphase II

Answer 23

Centromeres divide and individual chromatids are pulled to opposite poles

This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

Question 24

Telophase II

Answer 24

Nuclear membranes form around each group of chromosomes

Question 25

Cytokinesis in Meiosis II

Answer 25

Cytoplasm divides as new cell surface membranes are formed creating four haploid cells The cells still contain the same number of centromeres as they did at the start of meiosis I but they now only have half the number of chromosomes (previously chromatids)

Question 26

Meiosis I or Meiosis II

Answer 26

Homologous chromosomes pair up side by side in meiosis I only This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be occurring The number of cells forming can help distinguish between meiosis I and II If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II

Question 27

The distinguishing features at each stage of Meiosis I

Answer 27

Prophase I: Homologous pairs of chromosomes are visible Metaphase I: Homologous pairs are lined up side by side along the equator of spindle Anaphase I: Whole chromosomes are being pulled to opposite poles with centromeres intact Telophase I: There are 2 groups of condensed chromosomes around which nuclei membranes are forming Cytokinesis: Cytoplasm is dividing and cell membrane is pinching inwards to form two cells

Question 28

The distinguishing features at each stage of Meiosis II

Answer 28

Prophase II: Single whole chromosomes are visible Metaphase II: Single whole chromosomes are lined up along the equator of the spindle in single file (at 90 degree angle to the old spindle) Anaphase II: Centromeres divide and chromatids are being pulled to opposite poles Telophase II: Nuclei are forming around the 4 groups of condensed chromosomes Cytokinesis: Cytoplasm is dividing and four haploid cells are forming

Question 29

# Meiosis - sources of Genetic variation 2 ways that increase the genetic diversity of gametes produced

Answer 29

crossing over and independent assortment (random orientation) result in different combinations of alleles in gametes

Question 30

# Meiosis - sources of Genetic variation Crossing over

Answer 30

is the process by which non-sister chromatids exchange alleles Process: During meiosis I homologous chromosomes pair up and are in very close proximity to each other The non-sister chromatids can cross over and get entangled These crossing points are called chiasmata The entanglement places stress on the DNA molecules As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes There is usually at least one, if not more, chiasmata present in each bivalent during meiosis Crossing over is more likely to occur further down the chromosome away from the centromere

Question 31

# Meiosis - sources of Genetic variation Independent assortment

Answer 31

is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I The different combinations of chromosomes in daughter cells increases genetic variation between gametes In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle Each pair can be arranged with either chromosome on top, this is completely random The orientation of one homologous pair is independent / unaffected by the orientation of any other pair The homologous chromosomes are then separated and pulled apart to different poles The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell For humans this is 223 which calculates as 8 324 608 different combinations

Question 32

How does Meiosis create genetic variation between the gametes produced ?

Answer 32

by an individual through crossing over and independent assortment This means each gamete carries substantially different alleles During fertilization any male gamete can fuse with any female gamete to form a zygote This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

Question 33

what does each chromosome consist of?

Answer 33

a long DNA molecule which codes for several different proteins

Question 34

what is a gene?

Answer 34

A length of DNA that codes for a single polypeptide or protein

Question 35

define locus (plural: loci)

Answer 35

The position of a gene on a chromosome

Question 36

define alleles

Answer 36

**is a different form of the same gene** Each gene can exist in two or more different forms Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome ## Footnote Example of alleles One of the genes for coat colour in horses is Agouti This gene for coat colour is found on the same position on the same chromosome for all horses Hypothetically there are two different forms (alleles) of that gene found in horses: A and a Each allele can produce a different coat colour: Allele A → black coat Allele a → chestnut coat

Question 37

define **genotype** of an organism

Answer 37

the alleles possessed by an organism The genotype of an individual affects their phenotype

Question 38

define **homo**zygous

Answer 38

When the two allele copies are **identical** in an individual

Question 39

define **heter**ozygous

Answer 39

When the two allele copies are different in an individual

Question 40

define phenotype

Answer 40

is the observable characteristics of an organism ## Footnote A horse that has two black coat alleles A has the genotype AA and is homozygous. The phenotype of this horse would be a black coat In contrast a horse that has one black coat allele A and one chestnut coat allele a would have the genotype Aa and is heterozygous

Question 41

define dominant

Answer 41

Some alleles are dominant: they are always expressed in the phenotype This means they are **expressed** in both heterozygous and homozygous individuals

Question 42

define recessive

Answer 42

they are only **expressed** in the phenotype if **no dominant allele** is present This means that it is only expressed when present in a homozygous individual ## Footnote If for horses the allele A for a black coat is dominant and the allele a for a chestnut coat is recessive the following genotypes and phenotypes occur: Genotype AA → black coat Genotype Aa → black coat Genotype aa → chestnut coat

Question 43

define codominance

Answer 43

Sometimes both alleles can be expressed in the phenotype at the same time When an individual is heterozygous they will express both alleles in their phenotype When writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example I^A | ^ : to the power of ## Footnote A good example of codominance can be seen in human blood types The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O Allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO) If both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB) Blood type O (IOIO) is recessive to both group A and group B alleles

Question 44

When a homozygous dominant individual is crossed with a homozygous recessive individual what are the offspring called ?

Answer 44

the F1 generation All of the F1 generation are heterozygous

Question 45

If two individuals from the F1 generation are then crossed, the offspring they produce would be called?

Answer 45

the F2 generation

Question 46

Why is a test cross used and what does it deduce?

Answer 46

the genotype of an unknown individual that is expressing a dominant phenotype The individual in question is crossed with an individual that is expressing the recessive phenotype The resulting phenotypes of the offspring provide sufficient information to suggest the genotype of the unknown individual If there are any offspring expressing the recessive phenotype then the unknown individual must have a heterozygous genotype

Question 47

there are 2 types of linkages in genetics. What are they?

Answer 47

sex linkage and autosomal linkage

Question 48

# linkage Sex linkage

Answer 48

There are two sex chromosomes: X and Y Women have two copies of the X chromosome (XX) whereas men have one X chromosome and one shorter Y chromosome (XY) Some genes are found on a region of a sex chromosome that is not present on the other sex chromosome As the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes Most often sex-linked genes are found on the longer X chromsome Haemophilia is well known example of a sex-linked disease Sex-linked genes are represented in the genotype by writing the alleles as superscript next to the sex chromosome. For example a particular gene that is found only on the X chromosome has two alleles G and g. The genotype of a heterozygous female would be written as XGXg. A males genotype would be written as XGY

Question 49

# linkage Autosomal linkage

Answer 49

This occurs on the autosomes (any chromosome that isn’t a sex chromosome) Two or more genes on the same chromosome do not assort independently during meiosis These genes are linked and they stay together in the original parental combination

Question 50

What does monohybrid inheritance look at?

Answer 50

how the alleles for a single gene are passed on from one generation to the next ## Footnote Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs When two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote This means there is an equal chance of the zygote inheriting either allele from their parent

Question 51

Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made These diagrams are called?

Answer 51

Punnett squares The predicted genotypes that genetic diagrams produce are all based on chance There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions | look at worked examples save my exams 16.2.2 and 16.2.3

Question 52

What do dihybrid crosses look at?

Answer 52

how the alleles of two genes transfer across generations

Question 53

When caan a test cross be used?

Answer 53

to deduce the genotype of an unknown individual that is expressing a dominant phenotype The individual in question is crossed with an individual that is expressing the recessive phenotype. This is because an individual with a recessive phenotype has a known genotype The resulting phenotypes of the offspring provides sufficient information to suggest the genotype of the unknown individual

Question 54

Results for test cross for monohybrid test

Answer 54

If no offspring exhibit the recessive phenotype then the unknown genotype is homozygous dominant If at least one of the offspring exhibit the recessive phenotype then the unknown genotype is heterozygous

Question 55

Results for test cross - dihybrid

Answer 55

If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is homozygous dominant for both genes If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is heterozygous for one gene and homozygous dominant for the other If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is heterozygous for both genes | look at worked examples 16.2.4 save my exams

Question 56

chi squared

Answer 56

16.2.5 save my exams

Question 57

what can a genotype effect?

Answer 57

the phenotype of an organism A gene codes for a single protein The protein affects the phenotype through a particular mechanism

Question 58

what can the phenotype of an individual also be affected by?

Answer 58

the environment

Question 59

TYR gene & albinism

Answer 59

Humans with albinism lack the pigment melanin in their skin, hair and eyes - causes them to have very pale skin, very pale hair and pale blue or pink irises in the eyes There is a metabolic pathway for producing melanin: 1.The amino acid tyrosine is converted to DOPA by the enzyme tyrosinase 2.DOPA is converted to dopaquinone again by the enzyme tyrosinase 3.Dopaquinone is converted to melanin tyrosine → DOPA → dopaquinone → melanin A gene called TYR located on chromosome 11 codes for the enzyme tyrosinase There is a recessive allele for the gene TYR that causes a lack of enzyme tyrosinase or the presence of inactive tyrosinase Without the tyrosinase enzyme tyrosine can not be converted into melanin

Question 60

HBB gene & sickle cell anaemia

Answer 60

Sickle cell anaemia is a condition that causes individuals to have frequent infections, episodes of pain and anaemia Humans with sickle cell anaemia have abnormal haemoglobin in their red blood cells β-globin is a polypeptide found in haemoglobin that is coded for by the gene HBB which is found on chromosome 11 There is an abnormal allele for the gene HBB which produces a slightly different amino acid sequence to the normal allele The change of a single base in the DNA of the abnormal allele results in an amino acid substitution (the base sequence CTC is replaced by CAC) This change in amino acid sequence (the amino acid Glu is replaced with Val) results in an abnormal β-globin polypeptide The abnormal β-globin in haemoglobin affects the structure and shape of the red blood cells They are pulled into a half moon shape They are unable to transport oxygen around the body They stick to each other and clump together blocking capillaries A homozygous individual that has two abnormal alleles for the HBB gene produces only sickle cell haemoglobin They have sickle cell anaemia and suffer from the associated symptoms A heterozygous individual that has one normal allele and one abnormal allele for the HBB gene will produce some normal haemoglobin and some sickle cell haemoglobin They are a carrier of the allele They may have no symptoms

Question 61

F8 gene & haemophilia

Answer 61

Factor VIII is a coagulating agent that plays an essential role in blood clotting The gene F8 codes for the Factor VIII protein There are abnormal alleles of the F8 gene that result in: Production of abnormal forms of factor VIII Less production of normal factor VIII No production of factor VIII A lack of normal factor VIII prevents normal blood clotting and causes the condition haemophilia The F8 gene is located on the X chromosome This means F8 is a sex-linked gene Haemophilia is a sex-linked condition If males have an abnormal allele they will have the condition as they have only one copy of the gene Females can be heterozygous for the F8 gene and not suffer from the condition but act as a carrier

Question 62

F8 gene & haemophilia

Answer 62

Factor VIII is a coagulating agent that plays an essential role in blood clotting The gene F8 codes for the Factor VIII protein There are abnormal alleles of the F8 gene that result in: Production of abnormal forms of factor VIII Less production of normal factor VIII No production of factor VIII A lack of normal factor VIII prevents normal blood clotting and causes the condition haemophilia The F8 gene is located on the X chromosome This means F8 is a sex-linked gene Haemophilia is a sex-linked condition If males have an abnormal allele they will have the condition as they have only one copy of the gene Females can be heterozygous for the F8 gene and not suffer from the condition but act as a carrier

Question 63

HTT gene & Huntington's disease

Answer 63

Huntington’s disease is a genetic condition that develops as a person ages Usually a person with the disease will not show symptoms until they are 30 years old + An individual with the condition experiences neurological degeneration; they lose their ability to walk, talk and think The disease is ultimately fatal It has been found that individuals with Huntington's disease have abnormal alleles of the HTT gene The HTT gene codes for the protein huntingtin which is involved in neuronal development People that have a large number (>40) of repeated CAG triplets present in the nucleotide sequence of their HTT gene suffer from the disease The abnormal allele is dominant over the normal allele If an individual has one abnormal allele present they will suffer from the disease

Question 64

The Role of Gibberellin in Stem Elongation

Answer 64

In some plants species their height is partially controlled by their genes The Le gene dictates the height of some plants It has two alleles: Le and le The dominant allele Le produces tall plants when present The recessive allele le produces shorter plants when present (in a homozygous individual) The gene regulates the production of an enzyme that is involved in a pathway that forms active gibberellin GA1 Active gibberellin is a hormone that helps plants grow by stimulating cell division and elongation in the stem The recessive allele le results in non-functional enzyme It is only one nucleotide different to the dominant allele This causes a single amino acid substitution (threonine -> alanine) in the primary structure of the enzyme This change in primary structure occurs at the active site of the enzyme, making it non-functional Without this enzyme no active gibberellin is formed and plants are unable to grow tall Plants that are homozygous for the recessive allele le are dwarves Some farmers apply active gibberellin to shorter plants to stimulate growth

Question 64

The Role of Gibberellin in Stem Elongation

Answer 64

In some plants species their height is partially controlled by their genes The Le gene dictates the height of some plants It has two alleles: Le and le The dominant allele Le produces tall plants when present The recessive allele le produces shorter plants when present (in a homozygous individual) The gene regulates the production of an enzyme that is involved in a pathway that forms active gibberellin GA1 Active gibberellin is a hormone that helps plants grow by stimulating cell division and elongation in the stem The recessive allele le results in non-functional enzyme It is only one nucleotide different to the dominant allele This causes a single amino acid substitution (threonine -> alanine) in the primary structure of the enzyme This change in primary structure occurs at the active site of the enzyme, making it non-functional Without this enzyme no active gibberellin is formed and plants are unable to grow tall Plants that are homozygous for the recessive allele le are dwarves Some farmers apply active gibberellin to shorter plants to stimulate growth

Question 65

When writing gene by hand

Answer 65

Underline gene to represent it in italics TYR (underline the word)