Genes located on the same chromosome are linked and inherited together, forming a linkage group. Mendel's law of independent assortment does not apply to linked genes. During meiosis, linked genes may undergo crossing over, where portions of homologous chromosomes are exchanged. This can result in new combinations of alleles and the production of recombinant offspring that differ from their parents' genotypes. A test cross can be used to identify which offspring from a dihybrid cross involving linked genes are recombinants.

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.