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 has 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, Drosophila melanogaster, he
made some discoveries that identified exceptions to
Mendel’s principle of independent assortment.

Question 2

Drosophila
melanogaster

Answer 2

One of the most popular
research organisms for
genetics.

Drosophila are small, easy to
care for, produce hundreds
of offspring in each
generation and have a very
short (10 day) generation
time. This makes them ideal
as laboratory specimen.

In terms of genetics,
Drosophila offer both
complexity and simplicity.

Question 3

Morgan’s Experiments

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-eyed trait. In the second generation he saw a that
there were a lot more white-eyed flies, about one
white-eyed fly to every three red-eyed flies. This is what
was expected by Mendel’s principle of dominance.

However, all the white-eyed flies were male. This did not
follow Mendel’s principle of dominance.

This observation led Morgan to determine that there was
an association between the sex chromosome and the
colour of the eyes. Sex and eye colour did not sort
independently.

Question 4

Linked Genes

Answer 4

Morgan’s work led to the discovery of about two dozen
mutant 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 5

Gene Loci

Answer 5

The specific position of a gene on a chromosome is
called the locus of the gene.

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.

Sex-linkage occurs with genes on the X chromosome
and autosomal linkage occurs with genes on autosomal
chromosomes.

Linked genes are represented like this:

Each line represents a chromosome

The alleles above are on one chromosome and the
alleles below are on another.

The + represents the wild type.