1. Gene Linkage and Crossing Over
Mapping Chromosomes

2. Chromosomes and Genetics
Traits located on different chromosomes:
can be completely independent OR
can influence expression of one another
Traits located on the SAME chromosomes:
can be linked
can cause variability due to crossing over

3. Morgan – Drosophila and Linked genes
Through more experimentation with Drosophila, Morgan observed some unexpected gene combinations when doing Dihybrid crosses
Noted that some traits showed up more often in pairs than in the usual predicted ratio
This lead him to believe that the traits he was observing were located on the same chromosome = they were LINKED
Thomas Hunt Morgan

4. Morgan – linked genes

5. Gene Linkage Genes (traits) located on the same chromosome
Dihybrid cross only results in only two types of gametes (just like monohybrid)
Ex. AaBb
A and B are linked = therefore offspring must also have AB

6. Crossing Over Occurs during meiosis (synapsis)
Through crossing over, the gene combinations on a single chromosome can be altered as it is passed from generation to generation.
In this cross, gametes with the gene combination Ab and aB would not occur without crossing over.
Occurs during meiosis (synapsis)
Exchange of alleles between homologous chromosomes

7. Mapping Chromosomes: Placing genes on chromosomes in order of distance from one another
linkage group
a group of linked genes on a chromosome
locus (plural, loci)
a specific location along a chromosome where a particular gene is found

8. Distances are additive = use this to figure out location on xsome
Mapping Chromosomes
Crossover percentage is related to the physical distance of genes on a chromosome
** % = map distance **
Higher crossover percentage indicates genes located further apart on the chromosome
Order of genes is related to map distance
** map distance is additive **
Can place genes in ORDER on chromosome = MAP OF XSOME
Distances are additive = use this to figure out location on xsome

9. Mapping Chromosomes We know: A and B are linked a and b are linked
We want to know:
What is the crossover frequency??
Number of recombinations = ____ ?
Total number of offspring = _____ ?
Crossover percentage = _______ ?
Mapping Chromosomes

10. Gene Mapping - Practice

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