The process of producing offspring where a male and a female provide half the genetic material via gametes.

Male or female as determined by the combination of sex chromosomes inherited (i.e. XX female, XY male).

The chromosomes that determine the reproductive and secondary sexual characteristics (i.e. X and Y).

Chemical messengers, secreted by glands, that help to control the reproductive and secondary sexual characteristics.

The inheritance of biological sex in humans

Combined science

Pupils will learn about growth through cell division, mitosis as part of the cell cycle, cancer as the result of changes in cells that lead to uncontrolled growth and division, and the formation of gametes by meiosis.

Pupils learned about what the genome is, what it’s made of and where it’s stored, more detail about the chemical structure of DNA, and the relationship between an organism’s genome and its phenotype.

This unit covers key genetic terms, single-gene inheritance, and genetic crosses, including probability and ratios. It explores how most traits result from multiple genes and sex determination in humans. The unit also includes interpreting data and using models to solve problems.

Inheritance, genotype and phenotype

Production of offspring by the fusion of gametes.

In humans, features determined by the 23rd chromosome pair.

Chemical messengers that control reproduction and physical characters.

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

In humans, biological sex is determined by ...

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

Male sex hormones are produced and released into the blood.

Testosterone causes the development of male sex characteristics.

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

The incomplete Punnett square show the inheritance of biological sex in humans. What combination of sex chromosomes **cannot** be produced in a fertilised egg cell?

Only one of their children, later Queen Elizabeth I, survived childhood.

If more children had been conceived, eventually one would have been a boy.

Only half her eggs would contain a Y chromosome, so it was just chance.

All of her eggs contained only an X sex chromosome.

Henry the VIII had his second wife Anne Boleyn executed partly because she failed to give him a male child. Why wasn’t the absence of a male child her fault?

An individual’s physical characteristics.

An individual’s combination of alleles.

Which is the best definition of genotype?

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?

All the grandparents must be carriers of Tay-Sachs.

One grandparent is a carrier of Tay-Sachs.

All the females are carriers of Tay-Sachs.

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?

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?

Only if the individual is homozygous dominant.

Only if the individual is homozygous recessive.

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

He worked out that proteins are made from amino acids.

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

He discovered the double helix structure of DNA.

He worked out rules of inheritance of some traits.

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

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