Download The Chromosomal Basis for Inheritance Thomas Hunt Morgan Early

The Chromosomal Basis for
Inheritance
Thomas Hunt Morgan
Early 1900s
Columbia University (New York)
Studied genetics of Drosophila melangaster (the
common fruit fly)
Why Drosophila?
Only four pairs of
chromosomes
A single mating produces
hundreds of offspring
A new generation can be
produced every 2 weeks
Morgan's studies
revealed...
Genes are located on specific chromosomes at specific
loci (locations).
There are many genes on a single chromosome.
Genes can be linked to various degrees (i.e. they can
be inherited together if they are located on the same
chromosome.)
Sex-linkage
Morgan discovered that Drosophila eye color
was linked to the X chromosome, supporting the
chromosome theory of inheritance.
X linkage and Y linkage exist
Practice
A white-eyed female fruit fly is mated with a wildtype male. What are the expected phenotype
and genotype ratios?
X Inactivation
Barr bodies
Inactivation is varied,
leading to a mosaic of
traits
e.g. Tortoiseshell cats,
sweat glands in human
females
Note that various systems of sex
determination exist.
X-Y
X-O
Z-W
Haplo-diploid
Morgan's research supports
Mendel's Laws
Homologous chromosomes account for Mendel's
Law of Segregation.
Non-homologous chromosomes account for
Mendel's Law of Independent Assortment
Linked Genes
Genes located near each other on the same
chromosome tend to be inherited together in
genetic crosses.
Refers to two or more genes on a single
chromosome. (This is not the same as sexlinkage.)
Normal vs
vestigial wings
Practice
Let b+ = gray body and b = black body. Let vg+ =
normal wings and vg = vestigial wings
What is the expected phenotype ratios if a fly
heterozygous for both traits is crossed with one
that is homozygous recessive for both traits?
Genetic Recombination
Parental types
Non-parental types = recombinant types = recombinants
If chromosomes independently assort, 50% recombination
frequency is expected.
Linkage is suspected when recombination is below 50%.
Due to crossing-over.
Linkage/genetic maps
Based on the idea that the farther apart two
genes lie on a chromosome, the more likely a
cross over event will occur between them.
1 map unit = 1% recombination frequency
For example...
Three genes (b, cn, and vg) are found on a
single chromosome.
Recombination frequencies are b-cn 9%, cn-vg
9.5%, and b-vg 17%.
Practice
Genes A, B, and C are located on the same
chromosome. Testcrosses show that the
recombination frequency between A and B is
28%, B and C is 5%, and A and C is 12%. What
is the linear order of these genes?
Practice
Determine the sequence of genes along a
chromosome based on the following
recombination frequencies: A-C 28%, A-B 8%, AD 25%, B-C 20%, B-D 33%.