6.1.8 Sex determination - Revision Guide

The sex chromosomes

Definition

Sex chromosomes

The pair of chromosomes that determine biological sex: XX in females and XY in males.

Each ordinary human body cell holds its chromosomes as 23 matching pairs.
Twenty-two of those pairs deal only with the body's ordinary characteristics.
The final pair, the sex chromosomes, is the one that settles whether you develop as male or female.
A female carries two matching sex chromosomes, written XX.
A male carries two different ones, written XY.

Note

The Y chromosome carries the genes that lead to male development.

Definition

Punnett square

A grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

Because the mother is $X X$ , all of her egg cells carry one X chromosome.
Because the father is $X Y$ , half his sperm carry an X and half carry a Y.
To show sex inheritance, write the mother as $X X$ and the father as $X Y$ and combine their gametes in a Punnett square.
The four boxes give $X X$ , $X X$ , $X Y$ and $X Y$ : two female and two male.
This is a 1:1 ratio, so each child has about a $50%50\%$ chance of being either sex.

A Punnett square crossing an XX mother with an XY father, giving XX, XX, XY and XY: a 1:1 ratio of female to male offspring.

Common Mistake

Do not say the mother decides the sex; the sperm carries the X or Y that decides it.

Self review

How many pairs of chromosomes are in a human body cell, and how many carry the sex genes?
Which sex chromosomes do females and males have?
Why can the mother's egg only carry an X chromosome?
Which parent's gamete decides the sex of the child?
What ratio of male to female offspring does a genetic cross predict?

Definition

Sex chromosomes

The pair of chromosomes that determine biological sex: XX in females and XY in males.

Each ordinary human body cell holds its chromosomes as 23 matching pairs.
Twenty-two of those pairs deal only with the body's ordinary characteristics.
The final pair, the sex chromosomes, is the one that settles whether you develop as male or female.
A female carries two matching sex chromosomes, written XX.
A male carries two different ones, written XY.

Note

The Y chromosome carries the genes that lead to male development.

Definition

Punnett square

A grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross.

Because the mother is $X X$ , all of her egg cells carry one X chromosome.
Because the father is $X Y$ , half his sperm carry an X and half carry a Y.
To show sex inheritance, write the mother as $X X$ and the father as $X Y$ and combine their gametes in a Punnett square.
The four boxes give $X X$ , $X X$ , $X Y$ and $X Y$ : two female and two male.
This is a 1:1 ratio, so each child has about a $50%50\%$ chance of being either sex.

A Punnett square crossing an XX mother with an XY father, giving XX, XX, XY and XY: a 1:1 ratio of female to male offspring.

Common Mistake

Do not say the mother decides the sex; the sperm carries the X or Y that decides it.

Self review

How many pairs of chromosomes are in a human body cell, and how many carry the sex genes?
Which sex chromosomes do females and males have?
Why can the mother's egg only carry an X chromosome?
Which parent's gamete decides the sex of the child?
What ratio of male to female offspring does a genetic cross predict?