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?
Process of Crossing Over
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