10.1.1

Describe the behaviour of the chromosomes in the phases of meiosis.

2

10.1.2

Outline the formation of chiasmata in the process of crossing over.

2

10.1.3

Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

3

10.1.4

State Mendel’s law of independent assortment.

1

TOK: There are some interesting aspects of Mendel’s work, including those mentioned in 4.3.11. The law of independent assortment was soon found to have exceptions when pairs of genes are linked on a chromosome, but the law that Mendel discovered in the 19th century does operate for the majority of pairs of genes.

10.1.5

Explain the relationship between Mendel’s law of independent assortment and meiosis.

3

10.2.1

Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

3

10.2.2

Distinguish between autosomes and sex chromosomes.

2

10.2.3

Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

3

10.2.4

Define linkage group.

1

10.2.5

Explain an example of a cross between two linked genes.

3

Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example

This format will be used in examination papers, or students will be given sufficient information to allow them to deduce which alleles are linked.

10.2.6

Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

2

In a test cross of

the recombinants will be

and

10.3.1

Define polygenic inheritance.

1

10.3.2

Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.

3

Aim 8: This is one of the most obvious opportunities to develop the theme of parity of esteem for all humans. The selective advantage of dark skin to protect against ultraviolet light and light skin to allow vitamin D production could be mentioned. The correlation between skin colour and intensity of sunlight is clear, though the selective advantages of particular skin colours can now be overcome by the use of sun-block creams and vitamin D supplements.