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In humans, widow's peak (A) is dominant over a continuous hairline (a), and short fingers (B) are dominant over long fingers (b). Two individuals with widow's peak and short fingers have a child with continuous hairline and long fingers. Determine the genotypes of the parents.

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Answer:

Both parents are heterozygous for the traits and exhibit the dominant phenotype → AaBb

Explanation:

Available data:

  • widow's peak (A) is dominant over a continuous hairline (a)
  • short fingers (B) are dominant over long fingers (b)
  • Cross:  Two individuals with widow's peak and short fingers
  • F1) a child with continuous hairline and long fingers

Genotypes                        Phenotypes

  • AA and Aa             individuals with widow's peak
  • aa                           individuals with no widow's peak
  • BB and Bb             shorted-fingered individuals
  • bb                           longed-fingered individuals

Cross:

Parentals) A-B-   x   A-B-

Note: the - symbol represents either a dominant or recessive allele.

F1)  child with continuous hairline and long fingers → aabb

Each parent provided one allele for each trait to the child. So the child inherited one recessive allele a and one recessive allele b from EACH parent. The fact that both parents could provide one recessive allele to the child for each character, indicates that both parents are heterozygous for the trait. It is the only way of having a child who expresses the double recessive trait.

  • Parentals genotypes → AaBb    x    AaBb
  • Child →   aabb

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