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WAEC Biology 2016 Papers

Question : 42

Total: 50

Solution:

Concept:

Colour blindness is a sex-linked recessive trait carried on the X chromosome.

Explanation:

Let

X^{C}

represent the normal allele and

X^{c}

represent the colour-blind allele.

The colour-blind male has the genotype

X^{c}Y

The normal female is homozygous normal with the genotype

X^{C}X^{C}

The father contributes either

X^{c}

Y

to offspring.

The mother contributes only

X^{C}

Daughters receive

X^{c}

from the father and

X^{C}

from the mother, making them carriers with genotype

X^{C}X^{c}

Sons receive

Y

from the father and

X^{C}

from the mother, making them normal males with genotype

X^{C}Y

Thus, one carrier female is produced for each normal male.

Answer:

The genotype ratio of carrier females to normal males is

1:1

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