Genetics: Genetics Lecture 10, 9/22: Linkage Mapping in Eukaryotes

• Genes are found on chromosomes. There are times when genes found on a chromosome are transmitted in tandem. When this happens it impacts how traits are inherited.
• Meiosis – during prophase 1 homologous chromosomes pair together and crossing over occurs. There is exchange between the Homologous chromosomes. Frequency with which crossing over occurs can give us a relative distance between genes. The higher the frequency of crossing over, the greater the distance between genes. Put another way the closer two genes are, the more likely they will be transmitted together on a chromosome.

o The benefit of this is that it allows us to map genes on chromosomes. We are looking at the order of genes and the relative distance of genes.

• (10:10)
• ABC Mom and abc Dad
• Prophase I → 4 copies of each chromosome. Two types of chromosomes where crossing over occurs. The other two chromosomes will be the non crossover chromosomes. This will always be the highest % of chromosomes.
• 1 A B C
• 2 A B C
• 3 a b c
• 4 a b c
• 2 and 4 cross
• You produce a variety of crossover chromosomes based on where the crossover occurred. Within the crossover chromosomes the higher the % of two genes being transmitted together the closer those genes are.
• (17:05) Complete linkage altering phenotype. There are times when two genes are so closely linked that they are always transmitted together.
• If you cross heterozygotes with these traits the appearance of the F2 is altered.
• Mutant: Brown eyes + heavy wing vein
• Wildtype: red eyes + thin wing veins
• Parent: Brown eyes and thin wings (homozygous) X with red eyes and heavy wings. These are transmitted together and your F1 generation will have Brown Eye and thin wing as well as red eye and heavy wing. Phenotype red-eyes thin wing veins.
• Cross the F1 brown eye, thin wing + red eye heavy wing X brown eye, thin wing +red eye heavy wing

o They were working with drosophila. Morgan crossed two flies. They were yellow bodied white eyed females X wildtype males
o Both of these traits are on the X chromosome.
o He produced and F1 which he crossed again to produce an F2. He found that 98.7% of his offspring had a parental phenotype. 1.3% had yellow body wtype eyes or WT body and white eyes.
o These were strange findings and they performed another cross. White eyed miniaterue winged females and WT males (both traits on X chromosome)
o This time 62.8% had a parental phenotype and 37.2% were wildtype eyes miniature wings or white eyes and wt wings.
o Morgan had two questions.

 ANS: came from cytological evidence. They were looking at chromosomes during meiosis. They saw that the homologous chromosomes wrapped around each other at the chiasmata. Morgan proposed that this served as the mechanism of seperation.

• Why did the frequency of seperation vary depending on the genes being studied.

 ANS: (41:25) Morgan proposed that the relative distance between genes determined the liklihood a chiasmata would form between two genes.
 Therefore the frequency with which genes crossover that share a chromosome is relative to the distance between them.

o Sturtaveant realized that this technique would allow one to map the position and relative distance of genes on a chromosome. Yellow body had a frequency of crossover to white eys of 0.5%. White eyes showed a frequency of crossover to miniature of 34.5%. Finally, yellow body showed a frequency of crossover of 35.4% to miniature.
o Sturtavant proposed that 1% of crossover would equal 1MU (mapping unit)
o 2 Additional observations by sturtavant

• crossing over occurs on both X chromosomes and autosomal chromosomes. In drosophila, crossing over only occurs in females.

Genetics