4.3.2 Sex Determination In Humans · 4. Explaining Change | Aqa GCSE Combined Science Synergy 8465: Life And Environmental Sciences

Section 1 of 9Sex chromosomes

Human body cells contain 23 pairs of chromosomes.

Twenty-two pairs are the autosomes, which look the same in males and females.

Sex chromosomes

The 23rd pair are the sex chromosomes, which determine whether a person is male or female.

Females have two X chromosomes (XX).

Males have one X and one smaller Y chromosome (XY).

The Y chromosome carries a gene that triggers the development of testes in the embryo, and the testes then produce testosterone,

which leads to male characteristics.

Without this gene the embryo develops ovaries.

Section 2 of 9Inheritance of sex

All eggs made in the ovaries of a female carry an X chromosome, since she is XX, so each egg has X after meiosis. In males, half of the sperm carry an X and half carry a Y. The sex of a child therefore depends on which sperm fertilises the egg.

	X (egg)	X (egg)
X (sperm)	XX female	XX female
Y (sperm)	XY male	XY male

There is a 50 percent chance of a male and a 50 percent chance of a female, a ratio of 1:1. This is true for each child, no matter what sex the earlier children were, since each fertilisation is an independent event. A family of five girls does not make the next child any more likely to be a boy.

Section 3 of 9Using a genetic diagram

1

State the parents' genotypes: mother XX, father XY.

2

Write the gametes: X and X from the mother, X and Y from the father.

3

Draw a Punnett square or lines linking the gametes.

4

Write the offspring genotypes: XX, XX, XY, XY.

5

State the ratio: 1 female : 1 male, or 50 percent each.

The father determines the sex of the child because only he can produce gametes with different sex chromosomes. This is why some historical views, that blamed mothers for not producing sons, were incorrect.

Section 4 of 9Statistics and real populations

In large populations, about 50 percent of births are male, though the actual ratio is slightly higher, with about 105 boys born for every 100 girls.

Statistics and real populations

In a small number of families the numbers can be far from equal by chance.

If a couple has four children, the probability that all four are girls is (1/2)⁴ = 1/16 or 6.25 percent, which is not unusual.

Sometimes the number of sex chromosomes is abnormal because of an error during meiosis; for example, people with XXY or a single X have conditions such as Klinefelter and Turner syndromes.

Section 5 of 9Links

Sex determination is an example of the principles of monohybrid crosses (6.1.6), with gametes produced by meiosis (6.1.2) and random fertilisation (6.1.1).

Links

Genes on the X chromosome, such as the allele for red-green colour blindness, are sex-linked,

so males with a single X are more likely to show recessive X-linked conditions.

The hormones testosterone and oestrogen, covered in 5.3.1, then control the development of the secondary sexual characteristics.

Section 6 of 9Sex-linked inheritance

Genes located on the X chromosome show a special pattern because males have only one X.

1

Red-green colour blindness is caused by a recessive allele on the X chromosome.

2

Writing it as X^b, a male with X^bY is colour blind, because there is no second X with a dominant allele to mask it.

3

A female must have two copies (X^bX^b) to be colour blind, so the condition is much more common in males.

4

A carrier mother (X^BX^b) and a normal father (X^BY) have the following possible children: X^BX^B, X^BX^b, X^BY and X^bY.

5

Half of their sons have the condition, and no daughters are affected, although half are carriers.

A father cannot pass an X-linked allele to his son, because the son receives the Y chromosome from him. This is a useful point in family tree questions. Haemophilia, in which blood clots poorly, is another example, and it is also more frequent in males.

Knowing which chromosome carries a gene helps doctors predict risks for children and explains why some conditions seem to affect mostly one sex.

Section 7 of 9Exam-style questions

Draw a genetic diagram to show how the sex of a baby is determined. [4]4 marks
A couple have three daughters. Calculate the probability that their fourth child is a son and explain your answer. [2]2 marks
Explain why the sex of a baby is determined by the father. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Sex chromosomes	The pair of chromosomes that determine sex, XX or XY.
Autosome	Any chromosome that is not a sex chromosome.
X chromosome	The larger sex chromosome carried by both sexes.
Y chromosome	The smaller sex chromosome carried only by males.
Gamete	A sex cell with half the usual chromosomes.
Testosterone	Male sex hormone.
Independent event	An event whose outcome is not affected by previous events.

Section 9 of 9Exam technique

1

Saying the egg determines the sex of the child.

2

Writing that sperm all carry Y.

3

Believing that after three girls a boy is more likely.

4

Writing XX for males.

5

Forgetting to give the ratio or percentage.