Interpret a pedigree to follow a trait through generations, and explain sex-linked inheritance using the X and Y chromosomes (Ohio's Learning Standards for Science, Biology, B.H.2).

What this topic is asking

Ohio standard B.H.2 covers how offspring inherit traits from parents. Two tools the EOC uses to test this are the pedigree (a family tree of a trait) and sex linkage (genes on the X and Y chromosomes). The crosscutting idea is patterns: a pedigree reveals whether a trait is dominant or recessive, and sex linkage explains a striking pattern, why some traits affect males more than females. Both build on the Mendelian rules from Mendelian genetics and Punnett squares.

Reading a pedigree

A pedigree is a diagram of a family that shows who has a trait across generations. The standard symbols are:

• Circle = female; square = male.
• Shaded (filled) shape = the individual shows the trait; unshaded = does not show it.
• A horizontal line between two shapes = a mating pair; vertical lines lead down to their children.

The power of a pedigree is that it lets you work out whether a trait is dominant or recessive without a Punnett square.

• If two unaffected parents have an affected child, the trait is recessive. The parents must be carriers (heterozygous): they each carry one recessive allele but do not show the trait, and the child inherited a recessive allele from each.
• If an affected child always has an affected parent, the trait is more likely dominant.

The single most tested rule is the first one: two unaffected parents plus an affected child equals a recessive trait.

How sex is determined

Humans have 23 pairs of chromosomes. One pair is the sex chromosomes, which set biological sex.

• Females are XX (two X chromosomes).
• Males are XY (one X and one Y).

A child gets one sex chromosome from each parent. The mother (XX) always passes an X; the father (XY) passes either an X (giving a daughter) or a Y (giving a son). So the father's gamete decides the sex of the child.

Sex-linked inheritance

A sex-linked (more precisely, X-linked) gene sits on the X chromosome. Because males and females have different numbers of X chromosomes, X-linked traits show an uneven pattern between the sexes.

• A female has two X chromosomes, so she has two copies of every X-linked gene.
• A male has only one X chromosome (the Y is much smaller and lacks most of these genes), so he has only one copy of each X-linked gene.

This is why an X-linked recessive trait, such as red-green color blindness or haemophilia, appears more often in males: a male needs only one recessive allele on his single X to be affected, while a female needs the recessive allele on both of her X chromosomes. A female with one recessive allele is an unaffected carrier who can still pass it on.

Try this

Q1. In a pedigree, two unaffected parents have a daughter who shows the trait. State whether the trait is dominant or recessive and explain why. [2]

• Cue. Recessive: an affected child of two unaffected parents means each parent is a carrier who passed a recessive allele, which would be impossible for a dominant trait.

Q2. Explain why X-linked recessive traits are more common in males than in females. [2]

• Cue. Males have one X chromosome, so one recessive allele is enough to show the trait; females have two X chromosomes and need the recessive allele on both.