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 wild-type 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 sex-linkage.)

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%, A-D 25%, B-C 20%, B-D 33%.