Biology Unit 4.3 - Inheritence

Question 1

Explain variability and genetics…

Answer 1

Gregor Mendel suggested that an organism’s characteristics are determined by units (genes), which could be passed from generation to generation (on chromosomes)

Question 2

What are genes?

Answer 2

Basic units of inheritence compaosed of the chemical DNA, which can seperate, pair up and mutate, and is part of a chromosome which codes for a specific polypeptide

Question 3

What is a locus?

Answer 3

Position of a gene on a chromosome, where a diploid have two and each matching pair is called a homologue

Question 4

What is an allele?

Answer 4

Alternative form of the same gene, which is found at the same locus

Question 5

Explain the different allele combinations?

Answer 5

• Heterozygous - different alleles
• Homozygous dominant - two dominant alleles
• Homozygous recessive - two recessive alleles

Question 6

What is a dominant allele?

Answer 6

Allele that is expressed in the phenotype whenever it is present

Question 7

What is a recessive allele?

Answer 7

Allele which is hidden when a dominant allele is present

Question 8

What is a genotype?

Answer 8

The genetic makeup of an individual

Question 9

What is a phenotype?

Answer 9

Description of how the genotype is shown in the organism

Question 10

What is F1 and F2?

Answer 10

Shorthand to indicate the 1st and 2nd generation

Question 11

What is Mendel’s first law?

Answer 11

Characteristics of an organism are determined by internal factors which occur in pairs, where only one of a pair of such factors can be represented in a single gamete

Question 12

What is a monohybrid cross?

Answer 12

Simple cross, describing the inheritence of a single pair of genes, which can be demonstrated using a Punnet square

Ratio of dominant to recessive is 3:1

Question 13

What is Mendel’s second law?

Answer 13

Each member of an allele pair may combine randomly with either of another pair

Question 14

What is a dihybrid cross?

Answer 14

Describes the inheritence of two characteristics; two pairs of genes

Ratio of homozygous dominant:heterozygous:heterozygous:homozygous recessive is 9:3:3:1

Question 15

How do you calculate the expected value is a Chi-squared test?

Answer 15

• Calculate the total number
• Multiply the total by the ration number
• Divide by the total number of possible genotypes

Question 16

What is a null hypothesis?

Answer 16

States that there is no significant difference between the observed and expected results

Rejected if the Chi-squared number is higher than the critical value - significant difference between observed and expected results

Accpected if the Chi-squared number is lower than the critic value - no significant difference between observed and expected results

Question 17

How do you calculate the degrees of freedom?

Answer 17

Number of classes - 1 = Degrees of freedom

Question 18

What is a test cross?

Answer 18

Determines the true genotype which is achieved by cross dominant phenotypes with homozygous recessive plants

50% of offspring will show a recessive phenotype, meaning the unknown genotype must be heterozygous

Question 19

What is co-dominance?

Answer 19

When both alleles are expressed in the phenotype

Question 20

Explain sex chromosomes…

Answer 20

Non-autosomal chromosomes

Females - XX
Males - XY

Question 21

What is sex-linked inheritence?

Answer 21

Alleles are carried on the X chromosome, where any recessive allele carried on the X chromosome wille be expressed in the phenotype in males as there are no pairs, thus no dominant alleles exist

Question 22

What is haemophilia?

Answer 22

Possibly fatal sex-linked condition, which means the person cannot produce a blood clotting hormone, meaning they may bleed to death

Mainly affects males as allele’s locus is on the X chromosome meaning only one recessive allele is needed for the condition to be expressed

Question 23

What is linkage?

Answer 23

When two different genes are found on the same chromosome and are inherited together, and passed into gametes together

Genetic corsses do not follow the typical Mendelian pattern

Question 24

What is complete linkage?

Answer 24

Occurs whn the genes are close together on the same chromosome, where recombination due to crossing over is very unoikely, meaning the genes are inherited together

Question 25

What is incomplete linkage?

Answer 25

* May occur when genes are far or further apart * Crossing over at chiasmata occurs during prophase I of meiosis, which seperates linked genes * Four types of gametes are formed * Large number of parental types and a small number of recombinant types * Equal numbers of parental types * Recombinant types are found in equal numbers * Further apart the genes the more likley they are to be seperated by crossing over

Question 26

What are mutations?

Answer 26

* Any change in the size, order or structure of DNA in an organism, which can affect genes or entire chromosomes * Those which occur during the formation of gametes are the only ones that can be inherited * Most occur during crossing over in prophase I and non-disjuction in anaphase I and anaphase II * Happen at random * Affect natural selection and evolution

Question 27

What is mutation rate?

Answer 27

Very low in organisms such as humans, but is far higher in organisms with a short life cycle and a high rate of meiosis

Question 28

What environmental factors affect mutation rate?

Answer 28

* X-rays * Gamma radiation * Ultraviolet light * Chemical such as polycyclic hydrocarbons in cigarette smoke

Question 29

What are carcinogens?

Answer 29

Mutagens which increase the rate of mutation, where an increase in exposure to mutagens will increase the mutation rate

Question 30

Summarise the effect of carcinogens on genes?

Answer 30

* Carcinogen is an agent that causes cancer and is described as carcinogenic * Some mutagens are carcinogenic and cause mutations in DNA * Tumour suppressor genes halt cell division when enough cell have been produced by regulating mitosis * Mutation may prevent tumour suppressor genes from functioning * Mitosis continues repeatedly and continuously forming a tumour, which may be harmless or benign * If it spreads around the body it becomes cancerous * Proto-oncogenes can be switched on continuously by mutations, meaning excessive proteins are produced causing repeated mitosis * Mutated proto-oncogenes are called oncogenes, which cause cancer * Happens in mutation rearranges the DNA in a chromosome, and places the proto-oncogene next to a DNA sequence that activates it permanently or if there is an extra copy of a proto-oncogene * Tobacco contains 4000 chemicals, where over 40 are carcinogenic and 400 are known to be toxic

Question 31

How can gene and chromosomal mutations occur?

Answer 31

* DNA can be copied incorrectly before cell division * Chromosomes can be damaged, broken or fail to seprate

Question 32

What is a gene mutation?

Answer 32

Caused by any change in the sequence of nucleotides in the DNA molecule, causing the polypeptide to have the wrong sequence of amino acids meaning it won't function as it should

Question 33

What are the different types of gene mutations?

Answer 33

* Duplication * Insertion * Deletion * Inversion * Substitution of bases

Question 33

What is duplication?

Answer 33

Sequence of bases is repeated

Question 34

What is insertion?

Answer 34

Organic base/extra nucleotide is added into a sequence

Question 35

What is deletion?

Answer 35

Removal of an organic base from a sequence

Question 36

What is inversion?

Answer 36

Two organic bases are swapped in the sequence

Question 37

What is substitution of bases?

Answer 37

Organic base is replaced by another base

Question 38

Summarise sickle-cell anaemia...

Answer 38

* Normal haemoglobin HbA is produced by allele Hb*A* * Mutant haemoglobin HbS is produced by allele Hb*S* * Mutation is a substitution in a gene coding for the beta haemoglobin polypeptide * One base is susbtituted with another * Triplet CTC codes for glutamate * T is substituted for A, where CAC codes for valine * This change affects the folding of the polypeptide and its properties * Glutamate is polar, while valine is non-polar * Low partial pressure the haemoglobin aggregates and precipitates * Cell membrane collapses on the haemoglobin and becomes sickle shaped, and may break in the capillaries * Symptoms include joint pain and organ damage * Ability to carry oxygen in the blood is reduced, resulting in anemia and possible death

Question 39

What are chromosomal mutations?

Answer 39

Mutations caused by changes in the structure or number of chromosomes

Question 40

How does the chromosome structure change?

Answer 40

Usually occurs during meiosis, where in prophase I, homologous chromosomes pair up and mistakes occur during crossing over at chiasmata, meaning pairs may end with different gene sequences

Question 41

How does the number of chromosomes change?

Answer 41

Duaghter cells end up with the wrong number of chromosomes as paired chromosomes line up on the crowded equator, where an error occurs during anaphase, meaning chromosomes are not equally shared

Question 42

Summaise Down's syndrome...

Answer 42

Disjunction: * Mutation affects chromosome 21 * Disjunction happens during oogenesis/meiosis at anaphase I and II * May be due to faulty spindle fibres * Chromosomes are not shared equally between daughter cells * Secondary oocyte either has no chromosome 21 or has two copies * First is unviable, while the latter will form a zygote with three copies of chromosome 21 * Individual will have 47 chromosomes Translocation: * Fragment of one chromosome attaches to another * Fragment of chromosome 21 attaches to chromosome 14 * Abnoraml secondary oocyte fuses with a normal sperm, producing a zygote with two normal copies of chromosome 21 and an additional one attached to chromosome 14 * 5% of people with Down's syndrome have 46 chromosomes

Question 43

What is the control of gene expression?

Answer 43

DNA can be modified post-replication, which changes the ability of a gene to be transcribed and can be caused by the environment

Question 44

What are epigenetic modifications by DNA methylation?

Answer 44

Cytosine can have a methyl or hydroxymethyl group added, where heavily methylated regions may not be recognised and are less likely to be transcribed

Question 45

What is histone modification?

Answer 45

Histone proteins can be modified, where unmodified histones allow DNA to be packed more tightly, making DNA less accessible to enzymes, reducing transpiration, but when they are modified DNA is coiled more loosely allowing transcription enzymes to access particular sections of DNA

Question 46

What is DNA damage?

Answer 46

Process which occurs about 60,000 times a day, where most is repaired, but epigenetic changes remain at the site of repair

Question 47

What are the consequences of epigenetic changes?

Answer 47

* Genomic imprinting - gene may be permanently switched off by DNA methylation, which may lead to a medical condition * X inactivation - whole chromosome is switched off, where in male mammals the other chromosome is inactivated and becomes a mass of densely staining chromatin called the Barr body * Implicated in autoimmune decisions, mental illness, diabetes, and many cancers