(12 marks) In humans, colour vision depends on genes encoding three pigments. The
R (red pigment) and G (green pigment) genes are close together on the X chromosome
(for the sake of this question, let's say these genes are 25 map units apart, and that the X
chromosome is metacentric, with R being closer to the centromere than G ). The B (blue
pigment) gene is autosomal. For the sake of this question, assume that B is located near
the centromere on chromosome 3, which is a telocentric chromosome. A recessive
mutation in any one of these genes causes colour blindness.
(a) Draw the relevant chromosomes (sex chromosomes and an autosome pair) as they
would appear in metaphase of meiosis I from a man who is colour blind due to a mutation
in the G gene. Be sure to include the correct number of chromatids, and to include the
alleles for all three genes. Also assume that mutations in these genes are rare and that the
Y chromosome is acrocentric. Only draw the chromosomes - don't draw any cell
components. (3 marks)
(b) If the man in (a) were to mate with a woman with normal vision, what proportion of
their sons would be expected to be colour blind? (1 mark)
(c) The man in (a) mates with a woman who is colour blind due to a mutation in the R
gene and they have a daughter. Draw the relevant chromosomes (sex chromosomes and
an autosome pair) as they would appear in metaphase of meiosis I of the daughter. Be
sure to include the correct number of chromatids, and to include the alleles for all three
genes. Also assume that mutations in these genes are rare. Only draw the chromosomes -
Ion't draw any cell components. ( 3 marks)