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At one gene locus in humans there are 2 alleles: one for curly hair and another for straight hair. Neither allele is dominant, and heterozygous individuals have wavy hair. At a second gene locus, 3 alleles control ABO blood type. The alleles at these 2 gene loci assort independently. A man with straight hair and A blood has children with a woman with wavy hair and B blood. Their first child has straight hair and O blood. If their second child is a boy with straight hair, what is the probability that he has A blood? ( Express your answer as a percentage. Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5 )

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Answer:

25%

Explanation:

Genotypes and phenotypes:

  • SS = straight hair
  • CC= curly hair
  • SC = wavy hair
  • AA or A0= type A blood
  • BB or B0= type B blood
  • AB = type AB blood
  • 00 = type 0 blood

A man with straight hair and A blood has the genotype SS AA or SS A0.

A woman with wavy hair and B blood has the genotype SC BB or SC B0.

Their first child has straight hair and O blood: SS 00

Because the child has type 0 blood, both parents must have a 0 allele, so their genotypes are SS A0 and SC B0.

If their second child is a boy with straight hair, what is the probability that he has A blood?

The hair and blood type genes are independent, so the hair phenotype of the child is irrelevant to determine the probability of him having A blood.

The parental cross for blood type is A0 x B0.

If you do a Punnett Square, you'll get the following offspring:

25% AB, 25% B0, 25% A0 and 25% 00.

The answer is the child has a probability of 25% of having A blood.

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