3.7.1 - INHERITANCE

Question 1

in fruit flies, males have the sex chromosomes XY and females have XX. in fruit flies, a gene for eye colour is carried on the X chromosome. the allele for red eyes, R, is dominant to the allele for white eyes, r.

male fruit flies are more likely than females to have white eyes.

explain why (2)

Answer 1

1. males have one allele (only one X)
2. females have two alleles so could be heterozygous

Question 2

mutation is one cause of genetic variation.

give two other causes of genetic variation (2)

Answer 2

1. crossing over
2. independent assortment
3. random fertilisation

Question 3

in a species of flowering plant, the T allele for tallness is dominant to the t allele for dwarfness. in the same species two alleles C^R (red) and C^W (white) code for the colour of flowers. when homozygous red-flowered plants were crossed with homozygous white-flowered plants, all the offspring had pink flowers.

name the relationship between the two alleles that code for flower colour (1)

Answer 3

codominance

Question 4

what is meant by the term phenotype? (2)

Answer 4

1. characteristic/expression/appearance due to genetic constitution
2. and environment

Question 5

in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratio.

suggest two reasons why (2)

Answer 5

1. small sample size
2. fertilisation of gametes is random
3. linked genes
4. epistasis
5. lethal genotypes
6. selective advantage

Question 6

in fruit flies, the genes for body colour and wing length are linked. explain what this means (1)

Answer 6

genes/loci on same chromosomes

Question 7

what is meant by codominant alleles? (1)

Answer 7

both alleles are expressed in the phenotype

Question 8

in birds, males are XX and females are XY. use this information to explain why recessive, sex-linked characteristics are more common in female birds than in male birds. (1)

Answer 8

1. females have one allele
2. males could be heterozygous

Question 9

explain what is meant by a recessive allele (1)

Answer 9

only expressed in phenotype when homozygous

Question 10

the fruit fly is a useful organism for studying genetic crosses. female fruit flies are approximately 2.5 mm long. males are smaller and possess a distinct black patch on their bodies. females lay up to 400 eggs which develop into adults in 7 to 14 days. fruit flies will survive and breed in small flasks containing a simple nutrient medium consisting of mainly sugars

use this information to explain two reasons why the fruit fly is a useful organisms for studying genetic crosses (2)

Answer 10

1. large number of eggs/offspring - large sample size
2. small size
3. markings on males so easy to identify
4. short generation time - so results obtained quickly

Question 11

explain what is meant by a dominant allele (1)

Answer 11

is always expressed in phenotype

Question 11

a breeder crossed a black male cat with a black female cat on a number of occasions. the female cat produced 8 black kittens and 4 white kittens.

explain the evidence that the allele for white fur is recessive (1)

Answer 11

1. parents are heterozygous
2. kittens receive white allele from parents

Question 12

hair type in dachshund dogs is controlled by two genes which are on different chromosomes.

dogs with the H allele have wiry hair and dogs with the genotype hh have non-wiry hair.

the length of wiry hair is always the same. dogs with non-wiry hair have either long or short hair. the length of non-wiry hair is controlled by another gene. dogs with the D allele have short hair and those with the genotype dd have long hair.

give all the possible genotypes for dachshunds with non-wiry, short hair (1)

Answer 12

hhDD + hhDd

Question 13

hair type in dachshund dogs is controlled by two genes which are on different chromosomes.

dogs with the H allele have wiry hair and dogs with the genotype hh have non-wiry hair.

the length of wiry hair is always the same. dogs with non-wiry hair have either long or short hair. the length of non-wiry hair is controlled by another gene. dogs with the D allele have short hair and those with the genotype dd have long hair.

what type of interaction is occurring between the two genes? explain your answer (2)

Answer 13

1. epistasis
2. one gene controlling another

Question 14

a woman comes from a family with a history of the sex-linked condition haemophilia. a test was carried out to discover the sex of one of the embryos produced by IVF.

explain how observation of the chromosomes from an embryo cell could enable the sex to be determined (2)

Answer 14

1. female XX, male XY
2. Y shorter than X

Question 15

during meiosis, when the X and Y chromosomes pair up, they do not form a typical bivalent as do other chromosomes. explain why (2)

Answer 15

1. X and Y chromosomes are different sizes
2. chromosomes unable to line up and form bivalent