13.6 Genetics and Inheritance

Question 1

Define gene

Answer 1

A section of DNA that codes for a specific protein.

Question 2

Define genotype

Answer 2

Full genetic constitution/make-up of an organism and all the alleles it possesses.

Question 3

Define gene pool

Answer 3

All the alleles within an interbreeding population at a specific time.

Question 4

Define population

Answer 4

All individuals of the same species occupying the same area at the same time.

Question 5

Define phenotype

Answer 5

Expression of its genetic constitution/make-up and its interaction with the environment (physical appearance).

Question 6

Define allele

Answer 6

Different versions of the same gene.

Question 7

Define locus

Answer 7

Different alleles for the same gene are found at the same position on the homologous chromosome.

Question 8

Define homozygous

Answer 8

Two copies of a gene are the same allele.

Question 9

Define heterozygous

Answer 9

Two copies of a gene are different alleles.

Question 10

Define dominant allele

Answer 10

Always expressed in phenotype.

Question 11

Define recessive allele

Answer 11

Only expressed in phenotype if there are two copies// if genotype is homozygous.

Question 12

Define codominant alleles

Answer 12

Equally expressed within phenotype

Question 13

Define diploid

Answer 13

Containing two copies of a gene

Question 14

Define haploid

Answer 14

Containing one copy of a gene

Question 15

Define allelic frequency

Answer 15

Number of times an allele occurs within a gene pool.

Question 16

Types of inheritance

Answer 16

• Monohybrid
• Codominance
• Dihybrid
• Epistasis
• Autosomal linkage

Question 17

What does the Hardy-Weinberg principle predict?

Answer 17

Predicts the ‘allelic frequencies (of a particular gene) from one generation to the next will remain constant’.

Question 18

What does the Hardy-Weinberg principle rely on for it to be correct?

Answer 18

It relies on the fact there will be:
- NO migration
- NO gene mutation
- NO selection for or against a particular allele
- a large population
- random mating in population so all individuals have an equal chance of mating

Question 19

Hardy-Weinberg equations

Answer 19

p + q = 1
P^2 +2Pq + q^2 = 1

Question 20

Define monohybrid inheritance

Answer 20

The inheritance of a single gene, which determines a single characteristic.
An example would be where the phenotype is considered by two alleles, one of which is dominant and the other is recessive.

Question 21

Explain codominance.

Answer 21

Where alleles do not show a standard dominant/recessive relationship.
Small superscript letters used for alleles paired with letter ‘C’

Question 22

Explain multiple alleles- monohybrid inheritance.

Answer 22

For many genes, there are several alleles in the population (hair colour)
Show a dominance hierarchy

Question 23

There was 850 fruit flies in a population.
510 fruit flies had the genotype W(H)W(N)
255 fruit flies had the genotype W(N)W(V)
85 fruit flies had the genotype W(V)W(V)
CALCULATE ALLELE FREQUENCY OF ALLELE W(V)

Answer 23

1700 alleles in frequency
2x85=170
1x255=255
255+170=425
424/1700
0.25

Question 24

950 flies in population
frequency of W(V) allele. was 0.2
calculate the number of expected individuals with the genotype W(N)W(V)

Answer 24

2X0.2X0.8=0.32
0.32X950=
304

Question 25

Rules for dominant characteristics

Answer 25

- Affected offspring must have at least one affected parent. - Unaffected parents must only have unaffected offspring. - If both parents are affected and have an unaffected offspring, both parents must be heterozygous.

Question 26

Rules for recessive characteristics

Answer 26

Unaffected parents can have an affected offspring if they are heterozygous

Question 27

Explain sex-linked characteristics- monohybrid inheritance.

Answer 27

One that is controlled by a gene located on one of the sex chromosomes. Gene is on X chromosome.

Question 28

Male chromosomes

Answer 28

XY

Question 29

Female chromosomes

Answer 29

XX

Question 30

Why is it easier for a genetic disease to occur in men?

Answer 30

For sex-linked genes, a female would have to have 2 copies of a recessive allele for it to appear in the phenotype, whereas males only need one.

Question 31

Sex link rules to prove a characteristic is not sex-linked.

Answer 31

DADS AND DAUGHTERS (R) - If a daughter is affected (homo R) and her father is not. - And it is a recessive disease It is not sex-linked. MUMS AND SONS (D) - If a son is affected and his mum is not (homo R). - And it is a dominant disease It is not sex-linked.

Question 32

Define dihybrid inheritance

Answer 32

Involves a phenotype that is inherited as the result of two different genes, thus two characteristics. e.g round yellow The two genes either can be on different chromosomes or linked on the same chromosome.

Question 33

The ratio for dihybrid inheritance with homozygous parents

Answer 33

9:3:3:1

Question 34

How to find possible gametes for dihybrid inheritance

Answer 34

AaBb 1st and 3rd, 1st and 4th 2nd and 3rd, 2nd and 4th = AB, Ab, aB, ab

Question 35

Define autosomal chromosome

Answer 35

Chromosomes that are not directly involved in determining the sex of an organism.

Question 36

Explain autosomal linkage- dihybrid inheritance.

Answer 36

Chromosomes not directly involved in determining sex of organism inherited together.

Question 37

Explain the crossing-over process

Answer 37

Non sister chromatids on homologous pairs cross over to form a bivalent Then forms a chiasma Exchange of alleles to produce random recombinant offspring which display new combination of alleles

Question 38

What happens to the expected ratio when two genes are autosomal linked

Answer 38

Does not follow the expected phenotypic ratio

Question 39

Define epistasis

Answer 39

The allele of one gene affects or masks the expression of another in the phenotype;

Question 40

KP: In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios. Suggest two reasons why.

Answer 40

* Small sample size; * Fusion/fertilisation of gametes is random; * Linked Genes; Sex-linkage / crossing over; * Epistasis; * Lethal genotypes;

Question 41

KP: What does Hardy Weinberg’s equation predict

Answer 41

* The frequency/proportion of alleles (of a particular gene); * Will stay constant from one generation to the next/over generations / no genetic change over time; * Providing no mutation/no selection/population large/population genetically isolated/mating at random/no migration;

Question 42

KP: Expected offspring phenotype ratios from heterozygous parents: 1. Monohybrid 2. Dihybrid 3. Epistasis 4. Autosomal linkage

Answer 42

Dominant : recessive 1. 3:1 2. 9:3:3:1 3. 9:4:3 or 15:1 or 9:7 4. 3:1 (no x over) (no other pattern other than 4 phenotypes with recombination of alleles)

Question 43

KP: How do multiple alleles of a gene arise?

Answer 43

* mutations; * which are different / at different positions in the gene;