IB Biology Genetics

1. 10.2 Gene Linkage (autosomal)
Topic 10 Genetics

2. Gene Linkage
 10.2.4 Define linkage group.
 10.2.5 Explain an example of a cross between two linked
genes.
 10.2.6 Identify which of the offspring are recombinants in
a dihybrid cross involving linked genes.

3. Chromosomes
 Human body cells consist of 46 chromosomes in their nucleus.
 Another way of putting that is 23 pairs of chromosomes
 23 from the mother
 23 from the father
 Two of the 46 chromosomes are called the sex chromosomes.
They determine the sex of an individual.
 Females XX
 Males XY
 The other 44 chromosomes are referred to as autosomes.
 So from each parent you get:
 22 autosomes and 1 sex chromosome

4. Gene Linkage
 When Gregor Mendel studies his pea plants, he chose
factors that occur on separate chromosomes.
 Thus he was able to come up with his law of Independent
Assortment of factors.
 He claimed that all genes assort independently of each
other.
 Today we know that this is not always the case.
 Alleles on the same chromosome are inherited together.
 Genes (alleles) which lie on the same chromosome are
said to be linked. They form a linkage group.

5. Recombination in Unlinked Genes
 Unlinked genes are found on different chromosomes.
 During meiosis (metaphase I), when homologous pairs
line on the equator of the cell, the pairs line up
independently of each other (independent assortment)
 Independent Assortment allows for recombination of
unlinked genes because either of a pair of alleles can
combine randomly with either of another pair.
 Linked genes do not obey this law because they do not
assort independently due to being linked on the same
chromosome.
 Instead they pass together into the same gamete.

6. Gene Linkage Representation
 Alleles are normally shown side-by-side in dihybrid
crosses.
 eg: AaBb
 In representing crosses involving linkages groups, it is
more common to show them as vertical pairs.
 A B
a b
It allows us to see that the above genotype could also be:
 A b
a B

7. An example:
 Large head and red body are dominant
characteristics found on chromosome 5.
The recessive characteristics are small head
and black body. A beetle homozygous for
large head and red body is crossed with
one that is homozygous for small head and
black body. Deduce the genotypes and
phenotypes of the offspring of a test cross
on the F1 beetles. Let L = large, l = small;
 R = red, r = black.

8. Recombination in Linked Genes
 Pairs of linked genes are on the same chromosome.
 Recombination of linked genes can only occur when
crossing over occurs.
 Crossing over occurs during prophase I of meiosis.

9. Crossing Over
Ref: Biology Key Ideas

10. A test cross uses a
homozygous recessive
parent for both genes.
Ref: Biology for the IB Diploma, Allott
Notice that recombinants
are formed as a result of
crossing over.

11. IBO guide:
 10.2.4 Define linkage group.
 10.2.5 Explain an example of a cross between two linked
genes.
 10.2.6 Identify which of the offspring are recombinants in
a dihybrid cross involving linked genes.