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Consider a cross between an individual heterozygous for hitchhiker’s thumb and homozygous recessive for freckles (Jjrr, normal thumb and no freckles) and an individual homozygous dominant for both traits (JJRR, Normal thumb and freckles). Determine the genotype and phenotype ratios of the offspring. Assume Mendelian inheritance is occurring.

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Answer:

Genotypic ratios: 1 JJRr : 1 JjRr

Phenotypic ratios: 1 normal thumb and freckels

Explanation:

  • The genotypes JJ and Jj give normal thumb, therefore the homozygous recessive jj would give hitchhiker's thumb.
  • The homozygous recessive genotype rr gives no freckels, so RR or Rr will produce freckels.

The following cross was made:

Jjrr x JJRR

If there is Mendelian inheritance, the genes assort independently.

The homozygous dominant parent only produces JR gametes, so 100% of the offpsring will have normal thumb and freckels.

The other parent produces Jr and jr gametes. When the gametes from both parents combine, the two possible genotypes of the offspring are either JJRr or JjRr.

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