Biology 0610 · IGCSE · Chromosomes, genes and proteins
Chromosomes, genes and proteins — practice question
The gene responsible for red-green colour vision is located on the X chromosome.
This gene has two alleles:
• The allele for normal colour vision is shown by the letter B.
• The allele for red-green colour blindness is shown by the letter b.
Fig. 5.1 presents a pedigree chart for a family in which some members are red-green colour-blind.
(a)[2]
Explain why every male child of parent 1 and parent 2 is red-green colour-blind.
(b)[5]
Complete the genetic diagram to work out the probability that the offspring of parent 3 and parent 4 will be red-green colour-blind.
Cross: 3 × 4
Phenotypes of the parents: red-green colour-blind male × female with normal colour vision
Parental genotypes: [BLANK] × [BLANK]
Gametes: [BLANK], [BLANK] × [BLANK], [BLANK]
Offspring genotypes
Offspring phenotypes
Chance of offspring showing red-green colour blindness [BLANK]
Worked solution & mark scheme
This 7-mark question has a full step-by-step worked solution and mark scheme. One marking point: “parent 2 is homozygous recessive / carries two alleles for red-green colour blindness” …