Genetics Ex

1 (a)With reference to Down syndrome and sickle-cell anaemia, discuss the meaning of the term mutation.[5]

(b) In sheep, the fur colour can be white or black. The sheep live in an area that is constantly covered by snow. Explain what will happen to the proportions of allele for white fur and the allele for black fur when the sheep are continually preyed on by wolves.[5]

Answers
(a) Mutation is the sudden or spontaneous change in gene structure or chromosome, or even the chromosome number, and may be inheritable;
Down syndrome occurs through a change in the chromosome number, due to unequal separation of chromosomes (non-disjunction) during meiosis;
This results in an extra chromosome 21 in either the male or female gamete involved in fertilisation, which results in a zygote that has 3 copies of chromosome 21 instead of the usual 2;
Sickle cell anaemia is caused by a gene mutation where a change in a single base means the mutated gene codes for abnormal haemoglobin (due to one amino acid glutamic acid being replaced by valine), and causes the red blood cells to become sickle shaped;
The mutated gene is recessive so it expresses itself only in the homozygous condition when the person has 2 copies of the mutated allele;

(b) The proportion of the allele for white fur in the sheep population will increase while that for black fur will decrease;
This is because the sheep with white fur will have a selective advantage over those with black fur as they are better camouflaged against the snow;
With predation by wolves (selection pressure), they will have increased survival value and higher reproductive success;
This means that their offspring would inherit the alleles for white fur colour [1/2]
However, the sheep with black fur will not have a selective advantage and would be preyed on by the wolves (lower survival value);
This means lower reproductive succcess and hence after some time, the alleles for black fur would decrease in the sheep population [1/2]

2.In a certain strain of maize plant, the colour of the seed coat can be purple or yellow. This character is controlled by a pair of alleles.

A farmer crossed two purple corn plants. The result of the cross produced 160 purple grains and 55 yellow grains in one of the ears. Fig.below shows a sample of an ear of corn.

(a)(i) State which seed coat colour, purple or yellow, is dominant. [1]

(ii) Using a genetic diagram, show how two purple corn grain plants can produce ears of corn with 160 purple grains and 55 yellow grains. In your genetic diagram, use C to represent the dominant allele and c to represent the recessive allele. [4]

(b) The farmer wishes to produce an ear with approximately 50% purple grains and 50% yellow grains. State the genotypes of the two plants that he must use for the cross. [1]

Answers
(a)(i) Purple;

(a)(ii) Let C represent allele for purple seed coat colour and c represent allele for yellow seed coat colour;
Parental phenotypes: Purple x Purple
Parental genotypes: Cc x Cc;
Gametes: C c C c (in circles!)


F1 genotypes: CC Cc Cc cc (with proper lines connected);

F1 phenotype: Purple Purple Purple Yellow
Ratio: Purple : Yellow
3 : 1

(b) Cc & cc;