Question

*why do sex-linked traits (X- linked traits) such as red-green colorblindness (which is recessive) display a different pattern of genetic inheritance from autosomal recessive traits? and does it make a difference whether the sex-linked trait is dominant rather than recessive? how?

*what are the chances a boy will be colorblind if his mothers father was color blind?

Answer 1

1. Males have one X chromosome while females have two X chromosome. Autosomes conversely will have two copies of the gene (Autosomes).

In X linked recessive traits, presence of a single mutated X chromosome is enough to cause disease phenotype in the male. Females on the other hand will be carriers as they have a normal X chromosome. Contribution from both X chromosomes is equal. If there are two copies of mutant X chromosome in females, only then will they be affected by the disease. Thus, females require an X linked recessive genotype (two X) to show symptoms of the disease. Males will never have a homologous recessive pattern of inheritance, as they have only one X chromosome.

Autosomal recessive traits require both copies of the allele to exhibit symptoms of the disease. There is no variation in inheritance pattern based on sex of the disease. Heterozygous individuals are carriers. Thus, there is a difference in exhibition of symptoms in X linked recessive and autosomal recessive diseases.

In sex linked traits, if the mutation is inherited in dominant fashion, presence of a single copy of mutant X chromosome will cause the disease, irrespective of sex of the individual. Thus, heterozygous females will exhibit symptoms as will homozygous dominant. Homozygous recessive will not exhibit symptoms of disease in females. Males can never be homozygous recessive or dominant. They will always be heterozygotes and will always exhibit symptoms of the disease if the mutant copy is inherited. Thus, pattern of inheritance differs if the gene is dominant or recessive in sex linked traits.

2. The father is normal. Hence, his X chromosome is normal. Color blindness is a due to presence of mutated X chromosome.

The mother will have inherited a mutant X chromosome from her father, who was color blind. When a heterozygous female, breeds with a normal male, 50% of the male offspring will be color blind, depending on whether the mutant or normal copy of X chromosome of the mother is passed to the son. The sons will always inherit Y chromosome from the father (the fathers X chromosome is irrelevant).

The daughters will inherit either mutant or normal X chromosome from the mother and normal X chromosome from the father. If the mutant X chromosome if inherited from the mother, the daughters will be carriers. If they inherit the normal copy from mother, they will be normal. Thus, 50% of the daughters will be carriers. There is no chance of colorblind daughters in this case.

Thus, chances of the boy being colorblind is 50%.