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Transcript 
TOPIC 10
GENETICS
10.1 Meiosis (overview)
10.1.1: Describe the behaviour of the chromosomes in the phases of meiosis.
Prophase I of Meiosis I
Metaphase I of Meiosis I
Anaphase I of Meiosis I
Telophase I of Meiosis I
Prophase II of Meiosis II
Metaphase II of Meiosis II
Anaphase II of Meiosis II
Telophase II of Meiosis II
10.1.2: Outline the formation of chiasmata in the process of crossing over.
10.1.2: Crossing over
10.1.2: Crossing over
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.
10.1.3 and 10.1.4.: State Mendel’s law of independent assortment.
10.1.3 and 10.1.4
10.2: Dihybrid Crosses and gene
linkage
Unlinked
genes
10.2.1 ; Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid
crosses involving unlinked autosomal genes.
10.2.2: Distinguish between autosomes and sex chromosomes.
10.2.2: autosomes / sex
chromosomes
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.
10.2.3: Crossing over
10.2.4: Define linkage group.
A pair or set of genes on a
chromosome that tend to be
transmitted together.
10.2.5 and 10.2.6: Explain an example of a cross between two linked genes.
10.2.5 and 10.2.6
Identify which of the offspring are recombinants in a dihybrid cross involving linked
genes.
10.2.5 and 10.2.6
10.3.1: Define polygenic inheritance.
Polygenic Inheritance
10.3.2: Explain that polygenic inheritance can contribute to continuous variation
using two examples, one of which must be human skin colour.
BIG PICTURE of MEIOSIS
LINKED GENES
X
AaBb
A
a
A
B
b
a
?
OR
B
b
Start by assuming that the genes are unlinked because you know
what would be the expected ratios of offspring
AaBb
A
AB
Ab
a
B
aB
b
ab
AaBb x AaBb
A
AB
Ab
a
B
aB
b
ab
AaBb x AaBb
A
AB
Ab
a
B
aB
b
ab
AaBb
A
AB
Ab
a
B
aB
b
ab
AaBb
A
AB
Ab
a
B
aB
X
b
AaBb
A
ab
AB
Ab
a
B
aB
b
ab
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
Aabb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
A______ B_____
A______ bb
aa
B_____
aa
bb
= 9/16
= 3/16
= 3/16
= 1/16
1125
375
375
125
620
702
503
400
What are the expected phenotype ratios if the genes are unlinked
in the following crosses?
Expected
Then determine if the genes appear to be
linked or unlinked
A__B__
AABb
X
AAbb
X
aaBb
500
A__bb
500
aabb
500
X
Aabb
Observed
A__B__
980
aaB__
1020
A__bb
0
aabb
0
Expected
aaBb
500
aaB__
Expected
AaBB
Observed
Observed
A__B__
324
aaB__
562
A__bb
417
aabb
697
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