Question

How could we know whether a trait is autosomal or sex linked by studying a pedigree?

Answer 1

Hint: Autosomes are all chromosomes except the X and Y chromosomes, and they do not differ between males and females, hence autosomal features are inherited the same way regardless of the parent's or offspring's sex. Sex-linked features are traits that are controlled by genes on the sex chromosomes.

Complete solution:
When it comes to pedigrees, there are five factors to keep in mind.
(1) A person who is unaffected cannot have any alleles of a dominant trait.
(Because an individual is affected by a single allele of a dominant characteristic).
(2) Individuals marrying into the family are believed to have no disease alleles, implying that they will never be affected and cannot be carriers of a recessive trait. (because of the rarity of the characteristic in the population).
(3) A healthy person can be a carrier (carry one allele) of a recessive trait. (Because a person must have two alleles of a recessive trait to be affected).
(4) When a trait is X-linked, a male can be influenced by a single recessive allele. (due to the male being hemizygous — he only carries one allele of an X-linked characteristic).
(5) A father passes on his X-linked gene allele to his daughters but not to his sons. A mother passes on an X-linked gene allele to both her daughters and sons.

Note:
The Y chromosome is present in males, female offspring of an affected father cannot express the characteristic. Each son will have the condition inherited.