A male mouse that is homozygous recessive for the BIG gene ( bb ) is crossed to

Question

A male mouse that is homozygous recessive for the BIG gene (bb) is crossed to a homozygous dominant female mouse (BB). The offspring are all small.
In a reciprocal cross, a homozygous recessive female mouse (bb) is crossed to a homozygous dominant male mouse (BB). This time the offspring are all big.

(a) What are the genotypes of the offspring in each cross?

(b) These crosses demonstrate which inheritance pattern: x-linked inheritance, imprinting, dosage compensation, or cytoplasmic inheritance?

(c) Explain why the phenotypes of the offspring are different in the two crosses.

Explanation / Answer

Answer a) Genotypes of offsprings will be Bb, Bb, Bb, Bb.

Answe b) These crosses demonstrate dosage compensation.

Answer c)

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X-linked dominant inheritance, sometimes referred to as X-linked dominance, is a mode of genetic inheritance by which a dominant gene is carried on the X chromosome. As an inheritance pattern, it is less common than the X-linked recessive type. In medicine, X-linked dominant inheritance indicates that a gene responsible for a genetic disorder is located on the X chromosome, and only one copy of the allele is sufficient to cause the disorder when inherited from a parent who has the disorder. In this case, someone who expresses an X-linked dominant allele will exhibit the disorder and be considered affected.

X-linked dominant traits do not necessarily affect males more than females (unlike X-linked recessive traits). The exact pattern of inheritance varies, depending on whether the father or the mother has the trait of interest. All fathers that are affected by an X-linked dominant disorder will have affected daughters but not affected sons. However, if the mother is also affected then sons will have a chance of being affected, depending on whether a dominant or recessive X chromosome is passed on. When the son is affected, the mother will always be affected.

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