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

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.