During sexual reproduction, an individual’s biological sex is determined by the chromosomes they inherit.
In humans, the 23rd pair of chromosomes are sex chromosomes; XX in females and XY in males.
A gene on the Y chromosome triggers the development of testes, which make male sex hormones (androgens).
Interpreting, completing and constructing Punnett squares showing the inheritance of human sex chromosomes.
Some disorders are caused by alleles on the sex chromosomes, so have sex-linked inheritance.

Common misconception

Males only have Y chromosomes, only males inherit sex-linked conditions.

Males have one Y and one X chromosome, males only need one recessive allele to suffer from a sex-linked genetic disorder, whereas females need two and therefore the condition is less common in females.

Keywords

Sexual reproduction - The process of producing offspring where a male and a female provide half the genetic material via gametes.
Biological sex - Male or female as determined by the combination of sex chromosomes inherited (i.e. XX female, XY male).
Sex chromosome - The chromosomes that determine the reproductive and secondary sexual characteristics (i.e. X and Y).
Sex hormone - Chemical messengers, secreted by glands, that help to control the reproductive and secondary sexual characteristics.
Sex-linked disorder - An inherited condition determined by alleles carried by the X or Y chromosome.

Emphasise mothers can only pass X chromosomes, to both males and females, it is the sex chromosome of the sperm that will determine the biological sex of the offspring. Also, that males cannot pass on sex-linked conditions (the common ones) to sons, as they pass a Y chromosome, but can to daughters.

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This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.

Which is the best definition of genotype?

An individual’s genetic make-up.

All of an individual’s genes.

An individual’s physical characteristics.

Correct answer: An individual’s combination of alleles.

An individual’s combination of genes.

Q2.

Horses can breed with donkeys to produce offspring called mules. Horse body cells have 64 chromosomes and those of a donkey have 62. How many chromosomes will the zygote that grows into a mule have?

126

Correct answer: 63

Q3.

Tay-Sachs is a single gene disorder caused by a recessive allele. Here is a family tree. It shows three generations. The shaded male has Tay-Sachs. Which statement is true?

All the grandparents must be carriers of Tay-Sachs.

One grandparent is a carrier of Tay-Sachs.

One parent is a carrier of Tay-Sachs

Correct answer: Both parents are carriers of Tay-Sachs.

All the females are carriers of Tay-Sachs.

Q4.

Albinism is caused by a recessive allele. The Punnett square shows the possible genotypes resulting from two carriers having a child. What is the probability that a child will be a carrier?

0.00

0.25

Correct answer: 0.50

0.75

1.00

Q5.

A student thinks you can tell the genotype of an individual by looking at its phenotype. When is this statement true?

Always.

Only if the individual is heterozygous.

Only if the individual is homozygous dominant.

Correct answer: Only if the individual is homozygous recessive.

Never.

Q6.

Although it wasn’t recognised at the time, Mendel made a significant contribution to our modern understanding of inheritance. What was this contribution?

He worked out that proteins are made from amino acids.

He isolated DNA from a nucleus.

He worked out that three DNA bases code for each amino acid.

He discovered chromosomes contain DNA and proteins.

Correct answer: He worked out rules of inheritance of some traits.

Exit quiz

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6 Questions

Q1.

Match each term to its correct meaning.

Correct Answer:Sexual reproduction ,Production of offspring by the fusion of gametes.

Production of offspring by the fusion of gametes.

Correct Answer:Biological sex ,In humans, features determined by the 23rd chromosomes.

In humans, features determined by the 23rd chromosomes.

Correct Answer:Sex chromosomes ,In humans these can be X or Y.

In humans these can be X or Y.

Correct Answer:Sex hormones ,Chemical messengers that control reproduction and some characteristics

Chemical messengers that control reproduction and some characteristics

Q2.

What percentage of the chromosomes in the nucleus of a zygote comes from the father?

25%

Correct answer: 50%

75%

100%

Q3.

Put the statements in order to describe how sex chromosomes cause the development of male biological sex.

1 - A Y chromosome is inherited.

2 - A gene on this chromosome codes for a protein which triggers the testes' growth.

3 - Male sex hormones are produced and released into the blood.

4 - Testosterone causes the development of male sex characteristics.

Q4.

In humans, biological sex is determined by ...

all 46 chromosomes.

only the Y chromosome.

only the X chromosome.

Correct answer: only the 23rd chromosome pair.

Q5.

What is a sex-linked genetic disorder?

A disorder caused only by alleles on the X chromosome.

A disorder caused only by the alleles on the Y chromosome.

A disorder caused by alleles on the X or Y chromosomes.

Correct answer: A disorder caused by the combination of alleles on the X and Y chromosomes.

Q6.

Duchenne Muscular Dystrophy (DMD) is a sex-linked genetic disorder causing muscle weakness. A recessive gene variant (d) on the X chromosome can trigger the disorder. Which statement is true for DMD?

Only biological males can have DMD.

Only biological females can have DMD.

Only biological males can be carriers of DMD.

Correct answer: Only biological females can be carriers of DMD.

Only biological females can pass on the DMD allele.