Exam-style Questions: Genetics

1. Tongue-rolling and red-green colour blindness are two genetically controlled conditions which occur in humans. Tongue-rolling is controlled by the dominant allele, T, while non-rolling is controlled by the recessive allele, t.

Red-green colour blindness, is controlled by a sex-linked gene on the X chromosome. Normal colour vision is controlled by the dominant allele, B, while red-green colour blindness is controlled by the recessive allele, b.

a) Complete the genetic diagram to show the possible genotypes and phenotypes which could be produced from the following parents.

 

Female

Colour blind and heterozygous for tongue-rolling.

Male

Normal colour vision and non-roller.

Parental genotypes

 

 

Genotypes of gametes

 

 

Genotypes of children

 

 

Sex and phenotypes of children

 

 

b) Explain why a higher percentage of males than females in a population is red-green colour blind.

c) Sex-linked genes on the y chromosome have been found in humans and other animal species. Suggest and explain one piece of evidence which would support the presence of such a gene.

(Marks available: 6)

Answer

Answer outline and marking scheme for question: 1

Give yourself marks for mentioning any of the points below:

a)

Female

Colour blind and heterozygous for tongue-rolling.

Male

Normal colour vision and non-roller.

Parental genotypes

XbXbTt

XBYtt

Genotypes of gametes

XbT

Xbt

XBt

Yt

Genotypes of children

XBXbTt

XbYTt

XBXbtt

XbYtt

Sex and phenotypes of children

female

normal

roller

male

colour-blind

roller

female

normal

non-roller

male

colour-blind

non-roller

(4 marks)

b) Males have one X chromosome so if allele present they are affected/females have two X chromosomes so need both affected as recessive;

(1 mark)

c) All males posses Y chromosomes so all affected / only males possess Y chromosomes so no females affected.

(1 mark)

(Marks available: 6)

2. a) Doctors in some clinics claim that they can separate individual sperm cells so that the sex of a human child can be pre-determined.
Explain the genetic principles which enable the sex of a child to be pre-determined.

b) The diagram shows the inheritance of a rare hereditary form of rickets in a human family.

Install Flash

The condition is caused by a dominant allele, R. This allele may be present on the X chromosome, XR, but not on the Y chromosome.

Complete the genetic diagram to explain how this type of rickets was inherited by the children of parents A and B.

Parental phenotypes

A

Affected female

B

Unaffected male

Parental genotypes

   

Genotypes of gametes

     

Gentoypes of children

     

Phenotypes of children

affected male unaffected male affected female unaffected female

(Marks available: 5)

Answer

Answer outline and marking scheme for question: 2

Give yourself marks for mentioning any of the points below:

a) two types of sperm; X sperm and Y sperm. Select X sperm for female child / Y sperm for male child.

(2 marks)

b)

Parental phenotypes

A

Affected female

B

unaffacted male

Parental genotypes

XRXr XrY

Genotypes of gametes

XR Xr Xr Y

Gentoypes of children

XRY XrY XRXr XrXr

Phenotypes of children

affected male unaffected male affected female unaffected female

(3 marks)

(Marks available: 5)

3. The inheritance of colour in sweet peas is controlled by two pairs of alleles (A and a, and B and b) at two different gene loci.

The diagram shows how the dominant alleles at these two gene loci determine flower colour by controlling the synthesis of a purple pigment.

Install Flash

a) Plants with the genotypes aaBB, aaBb and aabb all have white flowers. Use infonnation in the diagram to explain why plants homozygous for the mutant allele, a, have white flowers.

b) Use a genetic diagram to explain the genotypes and the expected ratio of phenotypes when plants with the genotypes Aabb and aaBb are crossed.

Parental Phenotypes

White

White

Parental Genotypes

Aabb