In humans, two abnormal conditions, cataracts in the eyes and excessive fragility in the bones, seem to depend on separate dominant genes located on different chromosomes. A man with cataracts and normal bones, whose father had normal eyes, married a woman free from cataracts but with fragile bones. Her father had normal bones. What s the probability that their first child will a) be free from both abnormalities b) have cataracts but no fragile bones c) have fragile bones but not cataracts; d) have both cataracts and fragile bones

a) 1/4 or 25%

b) 1/4 or 25%

c) 1/4 or 25%

d) 1/4 or 25%

Explanation:

Both cataracts and bone fragility are dominant conditions and are independently inherited. Let cataracts allele be represented by C and fragile bone by B.

A man with cataracts (CC or Cc) and normal bones (bb), whose father had normal eyes would have the genotype Ccbb. The cataract would be heterozygous because he had a normal eyes father. The affected allele must have been inherited from his mother.

A woman free from cataracts (cc) but with fragile bones (BB or Bb) whose father had normal bone would have the genotype ccBb. The fragile bone would be in heterozygous condition because she had a normal bone father. The affected allele must have been from the mother.

    Ccbb    x    ccBb

Offspring

4 CcBb - cataract eyes, fragile bone

4 ccBb - normal eyes, fragile bone

4 Ccbb - cataract eyes, normal bone

4 ccbb - normal eyes, normal bone

a) Probability that their first child would be from both abnormalities = 4/16 or 1/4 or 25%

b) Probability that their first child would have cataracts but no fragile bones = 4/16 or 1/4 or 25%

c) Probability that their first child would have fragile bones but no cataracts = 4/16 or 1/4 or 25%

d) Probability that their first child would have both cataracts and fragile bones = 4/16 or 1/4 or 25%

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