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In some breeds of mice, yellow coat color is
dominant to gray coat color. Inheriting two
copies of the dominant yellow trait, however, is
lethal during embryonic development.
In a cross between two yellow mice, what
proportion of the offspring is expected to be
yellow? What proportion is expected to be
gray?
What is the phenotypic ratio of the offspring?

Respuesta :

marianaegarciaperedo

In the exposed lethal genes cause the individual's death during embryionic development before being born. H0m0zyg0us dominant individuals do not get born. Yellow: 2/3. Grey: 1/3. Phenotypic ratio: 2:1

What is complete dominance?

When the dominant allele completely masks the expression recessive allele, we talk about complete dominance.

This is the case of individuals that are heter0zyg0us for a particular gene and express the dominant trait.

The dominant allele is hiding the expression of the recessive allele.

What are the lethal genes?

Refer to genes that impede the normal development of the individual.

  • They can cause death during embryo development before the individual gets born,
  • They can cause the individual's death after being born and before reaching sèxual maturity.  

Let us assume that mice color is coded by a single diallelic gene that expresses complete dominance.

Available data:

  • Yellow color is dominant over grey color.
  • Yellow is coded by the dominant allele Y
  • Grey is coded by the recessive allele y

  • Genotype YY → Yellow and lethal
  • Genotype Yy → Yellow
  • Genotype yy → Grey

The cross would be as follows,

Cross:  Yy   x    Yy

Gametes) Y    y     Y    y

Punnett square)    Y      y

                      Y     YY    Yy

                      y      Yy     yy

F1) Considering ONLY the successfully born individuals

Genotype

2/3 = 50% of the progeny is expected to be heter0zyg0us, Yy.

1/3 = 25% of the progeny is expected to be h0m0zyg0us recessive, yy.

Phenotype

2/3 = 50% of the progeny is expected to be yellow, Yy.

1/3 = 25% of the progeny is expected to be grey, yy.

The phenotypic ratio is 2:1

You will learn more about lethal genes at

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