AHL 10 Genetics

10.1 Meiosis

10.1.1 Describe the behavior of chromosomes during meiosis.

• Terms:– homologous

chromosomes– synapsis– tetrad/ bivalent– sister chromatids

10.1.2 Chiasmata

How many chiasmata?

Proc

ess

of C

ross

ing

Ove

r

A cell replicates its DNA and then starts to divide by meiosis. What is the expected arrangement of chromosomes if crossing over has taken place between the two genes shown?

A . B .

C . D .

May 2008 HL Paper 1 Q29• This question revealed some widespread

misunderstanding with less than 25% of candidates choosing the correct answer, which was C. Far more chose answer D, which shows a chiasma that has formed by the ends of two chromatids swapping partners. Chiasmata are formed by crossing over, which involves the breakage and rejoining of chromatids or DNA molecules. This is one area where teachers need to ensure that they are teaching the topic correctly.

10.1.3 Genetic variety in gametes

Unlimited creativity!

Random Orientation in Metaphase I

• If having 3 pairs of chromosomes gives rise to 8 combinations of chromosome assortment in gametes, how many combinations would human have for having 23 pairs of chromosomes?

10.1.4

• allele pairs separate independently during the formation of gametes

10.1.4 Mendel’s Law of Independent Assortment

10.2 Dihybrid crosses

• 10.2.1, 2 – Linked genes, unlinked genes– Autosomes, sex chromosomes