GCSE Biology revision

Sex determination

Reproduction

AQA 4.1.1.1

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AQA student objectives

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ALL 🎯

Identify and describe Human Chromosomes: The Basics, Sex Chromosomes in Males and Females and What Are Sex-Linked Characteristics? in the context of sex determination.

MOST 🎯🎯

Explain the relationships between Human Chromosomes: The Basics, Sex Chromosomes in Males and Females and What Are Sex-Linked Characteristics? and how they contribute to sex determination.

SOME 🎯🎯🎯

Apply and analyse knowledge of Human Chromosomes: The Basics, Sex Chromosomes in Males and Females and What Are Sex-Linked Characteristics? to interpret unfamiliar information about sex determination.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Human Chromosomes: The Basics

Normal human body cells contain 23 pairs of chromosomes, giving a total of 46 chromosomes.
22 of these pairs are called autosomes, which control general body characteristics.
The remaining one pair are the sex chromosomes, which determine biological sex.

Sex Chromosomes in Males and Females

Females have two X chromosomes, giving the genotype XX.
Males have one X and one Y chromosome, giving the genotype XY.
The presence of the Y chromosome is what determines male biological sex.

What Are Sex-Linked Characteristics?

Some characteristics and genetic conditions are described as sex-linked because the alleles responsible are carried on the sex chromosomes (X or Y).
Sex-linked conditions are more common in one sex than the other because of how alleles are distributed across the X and Y chromosomes.
Most sex-linked alleles are carried on the X chromosome, as the Y chromosome carries very few genes.

3. What Are Sex-Linked Characteristics?AQA

Notes

Some characteristics and genetic conditions are described as sex-linked because the alleles responsible are carried on the sex chromosomes (X or Y).
Sex-linked conditions are more common in one sex than the other because of how alleles are distributed across the X and Y chromosomes.
Most sex-linked alleles are carried on the X chromosome, as the Y chromosome carries very few genes.

Homozygous and Heterozygous in Sex-Linked Genetics

A male (XY) can only ever have one copy of an X-linked allele, as he has just one X chromosome, making him hemizygous for that gene.
A female (XX) can be homozygous dominant, homozygous recessive, or heterozygous for an X-linked gene.
A heterozygous female carries one dominant and one recessive allele on her two X chromosomes.

4. Homozygous and Heterozygous in Sex-Linked GeneticsAQA

Notes

A male (XY) can only ever have one copy of an X-linked allele, as he has just one X chromosome, making him hemizygous for that gene.
A female (XX) can be homozygous dominant, homozygous recessive, or heterozygous for an X-linked gene.
A heterozygous female carries one dominant and one recessive allele on her two X chromosomes.

Using Punnett Squares for Sex Determination

Punnett squares can be used to predict the probability of offspring being male or female by crossing X and Y chromosomes.
When a female (XX) is crossed with a male (XY), there is a 50% chance of each offspring being male or female.
The sex of the offspring is determined by which sex chromosome is inherited from the father — X produces a girl, Y produces a boy.

5. Using Punnett Squares for Sex DeterminationAQA

Notes

Punnett squares can be used to predict the probability of offspring being male or female by crossing X and Y chromosomes.
When a female (XX) is crossed with a male (XY), there is a 50% chance of each offspring being male or female.
The sex of the offspring is determined by which sex chromosome is inherited from the father — X produces a girl, Y produces a boy.

Setting Up a Sex-Linked Punnett Square

In a sex-linked Punnett square, the allele is written alongside the sex chromosome on which it is carried, for example X^D or X^d.
The column headings represent the alleles from one parent and the row headings represent the alleles from the other parent.
Each box in the Punnett square shows a possible combination of sex chromosomes and alleles in the offspring.

6. Setting Up a Sex-Linked Punnett SquareAQA

Notes

In a sex-linked Punnett square, the allele is written alongside the sex chromosome on which it is carried, for example X^D or X^d.
The column headings represent the alleles from one parent and the row headings represent the alleles from the other parent.
Each box in the Punnett square shows a possible combination of sex chromosomes and alleles in the offspring.

Worked Example: Homozygous Dominant Male × Heterozygous Female

A homozygous dominant male carries the dominant allele on his only X chromosome, written as X^D Y.
A heterozygous female carries one dominant and one recessive allele across her two X chromosomes, written as X^D X^d.
The four possible offspring are: homozygous dominant female (X^D X^D), heterozygous female (X^D X^d), dominant male (X^D Y), and recessive male (X^d Y).

7. Worked Example: Homozygous Dominant Male × Heterozygous FemaleAQA

Notes

A homozygous dominant male carries the dominant allele on his only X chromosome, written as X^D Y.
A heterozygous female carries one dominant and one recessive allele across her two X chromosomes, written as X^D X^d.
The four possible offspring are: homozygous dominant female (X^D X^D), heterozygous female (X^D X^d), dominant male (X^D Y), and recessive male (X^d Y).

Interpreting Results from Sex-Linked Crosses

Males who inherit the recessive allele on their single X chromosome will always express the recessive condition, as there is no second X chromosome to mask it.
Females need two copies of the recessive allele to express a sex-linked recessive condition, making it less common in females.
Punnett squares allow you to calculate the probability and ratio of offspring with particular genotypes or phenotypes.

8. Interpreting Results from Sex-Linked CrossesAQA

Notes

Males who inherit the recessive allele on their single X chromosome will always express the recessive condition, as there is no second X chromosome to mask it.
Females need two copies of the recessive allele to express a sex-linked recessive condition, making it less common in females.
Punnett squares allow you to calculate the probability and ratio of offspring with particular genotypes or phenotypes.

Key Terminology to Remember

An autosome is any chromosome that is not a sex chromosome.
A sex-linked gene is one located on a sex chromosome, most commonly the X chromosome.
The term hemizygous describes males who have only one allele for an X-linked gene, since they have just one X chromosome.