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All the offspring of a cross between a black-eyed mendelian and an orange-eyed mendelian have black eyes. what is the expected phenotypic ratio of a cross between two orange-eyed mendelian?
a.3 black-eyed:1 orange-eyed
b.0 black-eyed:1 orange-eyed
c.1 black-eyed:3 orange-eyed
d.1 black-eyed:0 orange-eyed

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X3n0Captain
The answer is B. The first cross between the black-eyed mendelian and an orange-eyed mendelian resulted in all of them having black eyes. This means that black-eyed is the dominant trait in the species. That means if a phenotype is orange-eyed then it is homozygous recessive (bb). There will be no black-eyed since all of them are recessive.
Histrionicus

The expected phenotypic ratio of a cross between two orange-eyed mendelian - b.0 black-eyed:1 orange-eyed.

  • When black eyed-Mendelian is crossed with orange-eyed Mendelian, the entire progeny has black eyes.
  • This means that the allele for Black color is dominant to the color for orange color and it masks its effect completely in the next progeny.

F1:

BB X bb = Bb ( All black )

So orange Mendelian is a recessive trait and will have genotype bb. When two orange-eyed Mendelians are crossed:

F2:

bb X bb = bb ( All orange since there is no B allele )

Thus, the phenotypic ratio will be = 0 : 1 ( black eyed : orange eyed )

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