Inheritance of sex

Inheritance · Chromosomes, genes and proteins · note 2 of 4

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Inheritance of sexSpec 17.1.4

Human body cells contain one pair of sex chromosomes, which decide whether a person is male or female. There are two types, called X and Y.

Females have two X chromosomes (XX).
Males have one X chromosome and one Y chromosome (XY).

Gametes (sex cells) carry only one sex chromosome. All eggs carry an X chromosome. Half of the sperm carry an X chromosome and half carry a Y chromosome. The sex of a baby depends on which sperm fertilises the egg.

Genetic diagram: inheritance of sex
	Mother (female)	Father (male)
Parental genotypes	XX	XY
Gametes	X and X (every egg carries X)	X and Y (half the sperm carry X, half carry Y)

Punnett square: eggs across the top, sperm down the side
Sperm ↓ Eggs →	X	X
X	XX (girl)	XX (girl)
Y	XY (boy)	XY (boy)

Offspring: 2 XX (girls) : 2 XY (boys). The chance of a baby being a boy is the same as the chance of it being a girl: 1 : 1, or 50% each.

Genetic diagram for inheritance of sex: a mother (XX) and a father (XY) produce eggs that all carry X and sperm that carry X or Y, giving offspring XX, XX (girls) and XY, XY (boys) in a ratio of 1 : 1. — Every egg carries X; half the sperm carry X and half carry Y, so the chance of a girl or a boy is 1 : 1.

Exam tip:

The father decides the sex of the baby, because only his gametes can carry either X or Y. State this in your answer and show the gametes in a genetic diagram.