Unit 7a - Inheritance

Question 1

What is the meaning of the words “genotype” and “phenotype”?

Answer 1

Genotype - the genetic constitution of an organism
Phenotype - observable characteristics of an organism

Question 2

Why do human phenotypes not always closely match their genotypes?

Answer 2

Some of our genes in our phenotypes are not expressed

Question 3

Define:

Gene
Allele
Locus
Chromosome

Answer 3

Gene - a series of nucleotide bases that codes for a polypeptide
Allele - two or more variations of a gene
Locus - position of a gene on a chromosome
Chromosome - thread-like structures that contains genetic information

Question 4

In meiosis, how is variation achieved within the gametes produced by a parent?

Answer 4

Independent segregation and crossing over

Question 5

Explain the value of variation within gametes

Answer 5

The fertilisation of gametes during meiosis are random

Question 6

A father with genotype Hh and a mother with genotype hh have children. H = Huntington’s Disease and h = healthy. Draw a genetic cross diagram to show the different possible offspring. Label the phenotypes of offspring and the probability of each offspring type being born?

Answer 6

Hh = carrier
hh = healthy

50/50 or 1:1 ratio

Question 7

Why are the results of genetic crosses rarely a match for the statistical ratio?

Answer 7

There may be mutations that could occur during meiosis

Question 8

Draw a 16-box diagram to show the outcome of a genetic cross between two Labrador dogs; both parent dogs have the following genotype:
BbHh
…and where each allele codes as follows:
• B - black fur colour
• b - golden fur colour
• H - short hair
• h - long hair
And identify all phenotypes in the offspring using a key.

Answer 8

B-H- (9x) = black fur colour; short hair
B-h- (3x) = black fur colour; long hair
b-H- (3x) = golden fur colour; short hair
b-h- (1x) = golden fur colour long hair

Question 9

What are the normal ratios expected in genetic crosses between animals with a:
• Monohybrid heterozygous cross
• Dihybrid heterozygous cross

Answer 9

Monohybrid cross = 3:1
Dihybrid cross = 9:3:3:1

Question 10

In the event that offspring phenotypes do not match these ratios, what could be the explanation?

Answer 10

Mutation occurred during meiosis

Question 11

Draw a genetic cross diagram to show the blood type phenotypes within human offspring that were born of a union between two parents with the following genotypes:
• Father - |B |A
• Mother - |B |A

Answer 11

|B |B
|A |B
|A |B
|A |A

Question 12

Which chromosomes code for male and female sex in humans?

Answer 12

Female - XX chromosome
Male - XY chromosome

Question 13

What are the other 22 chromosome pairs called?

Answer 13

Autosomes

Question 14

Draw a genetic cross diagram to predict the likelihood of having a child suffering from Duchenne Muscular Dystrophy with parents carrying the following alleles:
• Mother - Xd XD
• Father - XD Y
where Xd is the allele responsible for causing Muscular Dystrophy.

Answer 14

It is less likely to have a child suffering from Duchenne Muscular Dystrophy (Xd) - 25% chance

Question 15

Explain why sex-linked diseases affect male babies far more frequently than females

Answer 15

Male babies only have one X chromosome whereas female babies have two X chromosomes

Question 16

Explain how a female haemophiliac child could be born

Answer 16

When the mother is a carrier and the father is haemophiliac, there is 25% chance that a female haemophiliac could be born

Question 17

Explain why there are very few females with colour blindness; explain why there are very few female haemophiliacs

Answer 17

Females have two X chromosomes so females with colour blindness and haemophiliacs is less likely to occur

Question 18

What does it mean if two genes have autosomal linkage

Answer 18

Fewer genetic combinations of alleles are possible therefore reducing the variation in offspring

Question 19

In cats, the genes for tooth length and claw colour are autosomally linked, with the following alleles:
• T - long teeth
• t - dwarf teeth
• C - beige claws
• c - brown claws
With two parent cats that both have the allele combinations TtCc, state the gametes that they would produce. Draw a genetic cross diagram to show the possible offspring. Label the phenotypes

Answer 19

Gametes that they would produce - male/female

T-C- (9x) = long teeth; beige claws
T-c- (3x) = long teeth; brown claws
t-C- (3x) = dwarf teeth; beige claws
t-c- (1x) = dwarf teeth; brown claws

Question 20

What is epistasis?

Answer 20

When one gene modifies or masks the expression of a different gene at a different locus

Question 21

In mice, the following genes are responsible for fur colour and colour uniformity:
A - hairs with black bands
a - hairs with a uniform colour
B - melanin (pigment)
b - no melanin
Generate gametes for two mice, one of which has the genotype AABb and the other AaBb, using a 16-box diagram and identify the phenotypes.

Question 22

What is the purpose of the X2 test?

Answer 22

To determine the differences between two datas