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WAEC Biology 2016 Papers

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Question : 43

Total: 50

The illustration above represents a cross between a colour blind male and normal female.

Study it carefully and answer the questions.

Which of the genotypes are carriers of colour blindness?

1 and 11 only
1 and 111 only
11 and 111 only
1, 11 and 111 only

Solution:

Concept:

Colour blindness is an X-linked recessive trait.

Carriers are heterozygous females who possess one normal allele and one recessive allele.

Explanation:

Let

X^C

represent the normal allele and

X^c

represent the colour-blind allele.

The colour-blind male has the genotype

X^cY

.

The normal female in the cross has the genotype

X^C X^C

.

Their daughters receive

X^c

from the father and

X^C

from the mother.

Therefore, all daughters have the genotype

X^C X^c

, making them carriers.

Sons receive

Y

from the father and

X^C

from the mother, so they are normal and not carriers.

From the illustration, the genotypes labelled I and II represent these carrier females.

Answer:

A. I and II only

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