Question

If a trait is only passed along from a mother to her son, the mode of inheritance is most likely
A. Autosomal dominant
B. Autosomal recessive
C. Y-linked
D. X-linked
E. Codominant

Answer 1

Hint: Most X-linked patterns are recessive. Here, the recessive X linked character always passes through the maternal line having the condition appearing only in males. It is being carried in females, but not usually expressed.

Complete answer:
X-linked inheritance exactly means that the gene causing the disorder or trait is located on the X chromosome. As we know females have two X chromosomes and males have one X and one Y. Genes present on the X chromosome can be dominant or recessive. They do not express the same in females and males. X-linked recessive genes are expressed in females only in that condition when there are two copies of the affected gene (one on each X chromosome). However, in males, it needs to be only one copy of an X-linked recessive gene in order to express the trait or disorder.
For example, a woman can carry a recessive gene on one of the X chromosomes and pass it on to a son, who will only express the trait. The chance that daughters carry the particular gene and can transmit it to the next generation is 50 percent. There is also a 50 percent chance that a daughter will not have the given gene and, thus, cannot pass it on to the next generation. For the sons, there is a 50 percent chance to not have the gene and be healthy. However, there is a 50 percent chance that a son will have received the gene for trait or disorder and will express it. Red-green color blindness and hemophilia are X linked disorders which are caused by recessive genes located on the X chromosome.

Among the given options:
> Autosomal dominant traits are transmitted in every generation of family.
> Autosomal recessive character is transmitted when two copies of the autosomes carrying the allele for that particular trait are present in the same individual.
> Y-linked character is inherited only in males because the allele for that particular character is present on the Y chromosome.
> Codominance is considered as a form of inheritance wherein both the alleles of a gene express themselves simultaneously in a heterozygote.

So, the correct answer is option D.

Note:
The X-linked trait is inherited by X chromosome. If the trait is X-linked recessive then it expresses only in males because they have only one X chromosome. So, if a mother is affected and she is transmitting the trait to her son, it means the X-linked recessive as the mother has two chromosomes, one of which will be passed to her son and will express the trait in him.