♀ Females: XX
In females the two sex chromosomes are the same. Both are X chromosomes, so a female is written as XX.
Reproduction » Sex Determination
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.6.1.8
- Human body cells have 23 pairs of chromosomes, and one pair decides sex
- Females are XX and males are XY
- How to carry out a genetic cross to show sex inheritance
- How to use ratios and direct proportion to predict the sex of children
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Unlock This Course23 pairs of chromosomes
Ordinary human body cells contain 23 pairs of chromosomes. That is 46 chromosomes in total. In each pair, one chromosome came from the mother and one came from the father.
Not every pair does the same job. 22 of the pairs control characteristics only. These genes decide things such as your eye colour, your blood group and your height. The 23rd pair is different. This pair carries the genes that determine sex. It is called the pair of sex chromosomes.
Key terms:
- Sex chromosomes: the pair of chromosomes that carries the genes that determine sex.
- X chromosome: the sex chromosome that everyone has at least one of.
- Y chromosome: the sex chromosome that only males have.
XX and XY
♂ Males: XY
In males the two sex chromosomes are different. One is an X and one is a Y, so a male is written as XY.
A quick way to remember it: females have two of the same, males have one of each.
What goes into the gametes
You know that gametes are made by meiosis, and that each gamete gets only one chromosome from each pair. This is the key to sex inheritance.
- A female is XX. Every egg cell gets one X chromosome. So all eggs carry X.
- A male is XY. Each sperm cell gets either an X or a Y. So half the sperm carry X and half carry Y.
When fertilisation happens, one sperm joins one egg. The sperm decides the sex of the baby:
- An X sperm joins the X egg. The baby is XX, a girl.
- A Y sperm joins the X egg. The baby is XY, a boy.
Carrying out the genetic cross
You can show this with a genetic cross, just as you would for any single gene. Here the alleles are replaced by the sex chromosomes. Put the mother's gametes down the side and the father's gametes across the top.
Genetic cross: XX female and XY male
Parents: mother XX, father XY
Gametes: mother X and X; father X and Y
The four boxes of the square:
- Father's X with mother's first X: XX (girl)
- Father's Y with mother's first X: XY (boy)
- Father's X with mother's second X: XX (girl)
- Father's Y with mother's second X: XY (boy)
Offspring: 2 XX (girls) and 2 XY (boys).
The cross has four boxes. Two are XX and two are XY. So there is an equal chance of each sex.
Using ratios and proportion
The result of the cross can be written in different ways. Because 2 of the 4 offspring are girls and 2 are boys:
- The ratio of girls to boys is 2 : 2, which simplifies to 1 : 1.
- The chance of each is 2 out of 4, which is 1/2, or 50%.
This also works as direct proportion. If the number of children doubles, the number of boys expected also doubles. The ratio 1 : 1 stays the same.
Worked example
A hospital expects 300 babies to be born in a month. Use the 1 : 1 ratio to predict how many will be boys.
Step 1: the ratio 1 : 1 has 1 + 1 = 2 parts.
Step 2: one part = 300 ÷ 2 = 150.
Step 3: boys are 1 part, so about 150 boys (and about 150 girls).
Worked example 2
Over several years 480 babies are born to families in one town. About how many girls would you expect?
The ratio of girls to boys is 1 : 1, so girls are 1 out of every 2 children.
480 ÷ 2 = 240 girls.
If the number of babies doubled to 960, the number of girls expected would also double to 480. That is direct proportion.
In real life the numbers are close to 1 : 1 but are rarely exact. If you toss a coin 10 times you may not get exactly 5 heads, but with thousands of tosses the result gets closer to half. The same is true for the sex of babies.
The words "about" and "expected" matter. Each baby is a separate chance, like flipping a coin. The cross gives the probability, not a guarantee.
Everyday example
Imagine a couple who already have two girls. A friend says, "You are bound to have a boy next." This is wrong. The egg always carries X, and the chance that the sperm carries Y is still 1 in 2. Each pregnancy starts again with the same 1 : 1 chance.
Common mistakes
Saying the mother decides the sex of the baby. She cannot, because all her eggs carry an X. It is the sperm, with either an X or a Y, that decides. Another mistake is writing the male as YY or the female as XY. Females are XX and males are XY. A third is thinking that 1 : 1 means every family has exactly equal numbers of boys and girls. It is only the chance for each baby.
Exam-style question
Human body cells contain 23 pairs of chromosomes.
(a) How many of these pairs control characteristics only? [1 mark]
(b) Draw a genetic cross to show how the sex of a child is inherited. Use X and Y to show the chromosomes. [3 marks]
(c) State the ratio of boys to girls expected from the cross. [1 mark]
Model answer
(a) 22 (1)
(b) Parents XX and XY (1). Gametes X, X and X, Y (1). Offspring XX, XX, XY, XY shown in a Punnett square (1).
(c) 1 : 1 (1)
Exam tip
In a cross question, always write the gametes first, then fill in the square. Marks are given for the gametes, so do not skip them.