Analyze sex-linked inheritance and interpret pedigrees to trace traits through generations (North Carolina Standard Course of Study, Biology, LS.Bio.7).

What this topic is asking

North Carolina LS.Bio.7 asks you to analyze sex-linked inheritance and interpret pedigrees. For the Biology EOC you need to know that sex is determined by the X and Y chromosomes, why X-linked recessive traits appear more often in males, what a carrier is, and how to read a pedigree to decide whether a trait is dominant or recessive. Items often show a pedigree and ask you to reason about a trait.

Sex chromosomes and sex-linked traits

Because the X and Y chromosomes are different, genes on the X are inherited differently in males and females. The Y chromosome is small and carries few genes, so most sex-linked genes are X-linked. A father passes his X to his daughters (and his Y to his sons), while a mother passes one of her two X chromosomes to every child. This pattern is what makes sex-linked inheritance distinctive.

Why X-linked recessive traits favor males

A carrier is an individual who has one copy of a recessive allele but does not show the trait, yet can pass it on. Carriers are central to sex-linked inheritance: a carrier mother (one affected X) can pass the allele to her sons, who would then show the trait, which is how X-linked conditions move through families.

Reading a pedigree

A pedigree is a chart of a family across generations, used to trace how a trait is inherited. By convention, squares are males, circles are females, shaded shapes show individuals with the trait, and horizontal lines join parents, with their children below. The main skill the EOC tests is using a pedigree to decide whether a trait is dominant or recessive:

• Two unaffected parents have an affected child. This points to a recessive trait: the parents must each be carriers ($Aa$), and the child inherited two recessive alleles ($aa$). A dominant trait could not appear in a child whose parents both lack it.
• An affected child has at least one affected parent in every generation, and the trait does not skip. This is more consistent with a dominant trait.

For sex-linked traits, also look at whether the trait appears mostly in males, which hints at X-linked recessive inheritance.

Try this

Q1. Explain why a female can be a carrier of an X-linked recessive trait but a male usually cannot. [2]

• Cue. A female has two X chromosomes, so she can carry one recessive allele masked by a dominant one; a male has only one X, so a single recessive allele is expressed rather than carried.

Q2. In a pedigree, two unaffected parents have an affected child. State whether the trait is dominant or recessive and why. [2]

• Cue. Recessive: the parents must be carriers, and the child inherited two recessive alleles; a dominant trait could not skip both parents.