Lesson 6: Non-Mendelian Genetics: Linked Genes

Question 1

Thomas Hunt Morgan

Answer 1

-In the early 1900’s T.H Morgan was working at Columbia University. He was some reservations about the validity of Mendel’s work in light of the emergence of the chromosomal theory of inheritance
-he believed that variation was better explained by environmental influences
-working with fruit flies, drosophilia melanogaster, he made discoveries that identified exceptions to Mendel’s principle of independent assortment

Question 2

Drasophilia melanogaster

Answer 2

-one of the most popular research organisms for genetics.
-drasophilia are small, easy to care for, produce hundreds of offspring in e/ generation and have a very short (10 day) generation time. This makes them ideal as laboratory specimen.
-in terms of genetics, drasophilia offer both complexity and simplicity

Question 3

Morgan’s Experiments p1

Answer 3

-after breeding thousands of flies, Morgan notices a single fly with white eyes instead of the normal red.
-he bred this white eyed fly/ with a normal red-eyed fly. Out of 1200 flies in the first generation only three had the white-eyes trait. In the second generation he saw that there were a lot more white-eyed flies, about one white-eyed fly to every three red-eyes flies. This is what was expected by Mendel’s principle of dominance

Question 4

Morgan’s Experiments p2

Answer 4

-however, all the white-eyed flies were male. This didn’t follow Mendel’s principle of dominance
-this observation led Morgan to determine that what there was an association between the sex chromosome and the colour of the eyes. Sex and eye colour didn’t sort independently

Question 5

Linked genes

Answer 5

-Morgan’s work led to the discovery of about two dozen mutants traits in flies, many of which show this linked inheritance.
-through their experiments geneticists were able to identify a group of genes that were on the X chromosome of Drosophila. Further careful breeding experiments allowed these genes to be arranged in a linear sequence on the chromosome

Question 6

Gene Loci p1

Answer 6

locus: the specific position of a gene on a chromosome
—genes are linked when they are found on the same chromosome (crossing over will allow for recombination). The closer the genes are on the chromosome the more likely they are to be inherited together

Question 7

Gene Loci- linked genes diagram- p2

Answer 7

pr vg

pr^+ vg^+

Question 8

Gene loci-sex linkage via linked genes- p3

Answer 8

-Sex-linkage occurs with genes in the X chromosome; autosomal linkage occurs with genes on autosomal chromosomes.
-e/ line represents a chromosome
-the alleles above are on one chromosome and the alleles are on another.
-the ‘+’ represents the wild type