Lecture 4

Question 1

What was the conclusive evidence that chromosome segregation was the physical method of inheritance predicted by mendel?

Answer 1

Tracking of chromosome segregation in species where chromosomes differ in size and shape
including the grasshopper where presence of an additional chromosome can be an indication of sex
as well as the phenotypic variance seen in datura through the progressive addition of chromosomes
These models both showed the independent assortment predicted by mendel

Question 2

How was linkage discovered?

Answer 2

When two genes in the sweet pea did not show the standard 9:3:3:1 ratio predicted by mendel
This could then be tested through use of a test cross

Question 3

What was Morgans drosophila test cross, and how did it show linkage?

Answer 3

Drosophila were crossed where one was homozygous recessive and the other homozygous dominant, resulting in an F1 generation which was heterozygous for both traits, this generation was then crossed with a tester, which was homozygous recessive for both traits, the F2 generation then showed a much greater proportion having the parental (either recessive for both traits or dominant for both traits) than the recombinant phenotype of being recessive for one trait but dominant for the other type

Question 4

What mathematical values can be used to determine if linkage is present?

Answer 4

Recombination frequency, which is the number of recombinants of the total population
If the recombinant frequency is less than 50% and the parental frequency is greater than 50% then linkage is present

Question 5

What is the difference between a cis and a trans test cross?

Answer 5

In a cis test cross the organism is dominant for both traits while in the trans cross the organism is dominant for one trait while recessive for the other