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3.4 explain how cross-breeding experiments can identify the relative position of linked genes
explain how cross-breeding experiments can identify the relative position of linked genes
- Cross-breeding experiments are used to study the relative positions of linked genes on homologous chromosomes.
- In 1911, Sturtevant, a student of Morgan, studied recombination of genes in fruit flies and concluded that when the distance between genes on chromosomes was large, there was a greater chance of crossing over occurring between these genes. From this, chromosome mapping was developed.
- A chromosome map shows the relative positions of genes on a chromosome. Alleles are genes situated on corresponding loci (positions) on homologous chromosomes. Crossing over results in the exact swapping of alleles from one of the chromosome pairs to another.
- If crossing over or recombination occurs 5% of the time, then the two genes are said to be 5 units apart on a chromosome. The greater the percentage of recombination, the larger the distance that the genes are apart.
Example:
Genes A, B and C are linked. In a cross breeding experiment, ABC was crossed with abc and the percentages of recombinations were observed.
AB x ab
|
Results: AB and ab 85%
Ab and aB 15% (recombined through crossing over) |
BC x bc
|
Results: BC and bc 95%
Bc and bC 5% (recombinations)
|
AC x ac
|
Results: AC and ac 90%
Ac and aC 10% (recombinations)
|
A and B are 15 units apart
|
A ________________15 _______________ B |
B and C are 5 units apart
|
B ____5___C |
A and C are 10 units apart
|
A ________10 _________C |
The resulting map is:
|
A __________10__________C______5_____B |