Lecture 19 Where are my genes

Question 1

Autosomal genes on different chromosomes

Answer 1

• If there are two genes on two different chromosomes: A and B
• What are the possible combinations in the gametes of a mother heterozygous for both A and B (ie AaBb)?
• AB , ab , Ab , aB

Question 2

Flies: Purple eyes (pr) and vestigial wing (vg)

Answer 2

E.g. Thomas Hunt Morgan crossed a true-breeding fruit fly with normal red eyes and normal wings (pr+ / pr + ; vg + / vg+) with a fly with recessive purple eyes and vestigial wings (pr / pr ; vg / vg)

In the testcross offspring, he found an unusually high number of parental phenotypes and a low number of recombinant phenotypes

Morgan proposed that pr and vg are physically associated on the same chromosome – and the behavior of these linked genes is due to chromosome recombination

Question 3

Recombination

Answer 3

Genetic recombination is a process in which two homologous chromosomes exchange segments with each other by crossing-over during meiosis

The frequency of this recombination is a function of the distance between linked genes – the nearer two genes are, the greater chance they will be inherited together

•To determine the distance between two genes on the chromosome, we calculate the recombination frequency, the percentage of testcross progeny that are recombinants

Question 4

Recombinant frequency

Answer 4

Parental vs recombinant
•1339+1195 vs 151+154
•Recombinant frequency = (151+154)/2839
•305/2839 *100 = 10.7% recombinant frequency
•The two genes are linked on the same chromosome

Question 5

Recombinant frequency - Linkage?

Answer 5

Genes are not linked:
•Recombination frequency is ≥ 50%
•Genes are on different chromosomes or
•Genes are far apart on the same chromosome

Genes are linked (or gene is linked to DNA marker):
•Recombination frequency is < 50%
•Genes are more likely to be on the same chromosome

Question 6

Chromosome Maps

Answer 6

Recombination frequencies used to determine relative locations on a chromosome
•
•Linkage map for genes a, b, and c:
•1 map unit = 1% recombination = 1 centimorgan

Question 7

Recombination Occurs Often

Answer 7

Widely separated linked genes often recombine
•Seem to assort independently
•Detected by testing linkage to genes between them

Question 8

Chromosomal Alterations

Answer 8

Deletion: Broken segment lost from chromosome

Duplication: Broken segment inserted into homologous chromosome

Translocation: Broken segment attached to nonhomologous chromosome

Inversion: Broken segment reattached in reversed orientation

Question 9

Changes in Chromosome Number

Answer 9

Due to:
•Failure of homologous pair separation during Meiosis I
•Failure of chromatid separation during Meiosis II

Euploids
•Normal number of chromosomes

Aneuploids
•Extra or missing chromosomes

Polyploids
•Extra sets of chromosomes (triploids, tetraploids)
•Spindle fails during mitosis