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Identical twins are produced from the same sperm and egg (which splits after the first mitotic division), whereas fraternal twins are produced from separate sperm and separate egg cells. If two parents, one with brown eyes (a dominant trait) and the other with blue eyes (the recessive trait) produce one twin boy with blue eyes, what is the probability that a fraternal twin will transmit the blue eye allele to his or her offspring

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eyitbay

Answer:

This is 25%

Explanation:

Analysis, Since the dominant brown eyed  parents  produce blue eye child, this shows they are heterozygote dominant

However,this fraternal twin is not genetically identical, due to the blue eye color,(recessive) but the fact remain that they  are the parents.(because he inherited the recessive allele from each of the parent)Therefore the probability is 25%.

This is because he is the only one out of the 4 kids( based on Mendelian fashion) this is  1/4,25%

marianaegarciaperedo

Fraternal tweens are produced from different maternal and paternal gametes. The blue-eyed tween has 100% probability of transmitting the blue-eyes allele to the offspring, while the brown-eyed tween has 50% probability.

Available data:

  • Fraternal twins are produced from separate sperm and separate egg cells.
  • Brown eyes ⇒ Dominant trait
  • Blue eyes ⇒ Recessive trait

We will name B to the dominant allele that codes for brown, and b to the recessive allele that codes for blue eyes.

So, the possible genotypes and phenotypes are

Gentoype     Phenotype

BB                  Brown eyes

Bb                  Brown eyes

bb                  Blue eyes    

Solving the problem

  • The parent with blue eyes must be only bb
  • The parent with brown eyes must be either BB or Bb

For the twin boy to have blue eyes, he must have received two recessive alleles, one from the mother and one from the father.

So the father is heter0zyg0us for brown eyes, Bb.

Cross:  Bb  x  bb

F1) 1/2 of the progeny is expected to be brown-eyed, Bb

    1/2 of the progeny is expected to be blue-eyed, bb.

So we have two genotypes for the tweens, Bb and bb.

bb

  • The bb tween has blue eyes and is h0m0zyg0us recessive for the trait.
  • So the probability that this fraternal twin will transmit the blue eye allele to his or her offspring is 100%
  • This tween can only provide recessive alleles to his or her progeny.

Bb

  • The Bb tween has brown eyes and is heter0zyg0us for the trait.
  • So the probability that this fraternal twin will transmit the blue eye allele to his or her offspring is 50%
  • This tween can provide one dominant or one recessive allele to his or her progeny.

So, the blue-eyed tween has 100% probability of transmitting the blue-eyes allele to the offspring. The brown-eyed tween has 50% probability of transmitting the same allele to the descendants.

You will learn more about tweens at

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