Chromosomal Basis of Inheritance

Linked GenesSex-linked Genes

Some review

Where are genes located in cells?

This has an affect of how genes are passed to offspring

Mendel did not know this. Thomas Hunt Morgan was the first to study

the inheritance of genes taking into account the fact that genes are on chromosomes

Fruit Flies

• Morgan used fruit flies (Drosophilia melanogaster)

• Why did he pick them?– Many traits to track– Reproduce quickly– Have three pairs of autosomes

and one pair of sex chromosomes

The phenotype that is seen most often in flies is called wild type (they have a + sign after them)

The alternative trait is called that mutant phenotype

Linked Genes

• Linked genes are genes located on the same chromosome– These deviate from Mendel's law of

independent assortment

An Example with Fruit Flies

In fruit flies, the gene for body color (b) and wing size (vg) are on the same chromosomes. All wild genes are dominant

• Wild type fruit flies have gray bodies and normal size wings

• Mutant fruit flies have black bodies and small wings

An Example with Fruit Flies

In fruit flies, the gene for body color (b) and wing size (vg) are on the same chromosomes. All wild genes are dominant

• Wild type fruit flies have gray bodies and normal size wings

• Mutant fruit flies have black bodies and small wings

So…

• When genes are linked you see a higher proportion of parental phenotypes

• However, Morgan observed that not all flies from this cross looked like the parents?

HOW IS THIS POSSIBLE?

REVIEW: What is Crossing Over

Discovery About Linked Genes

• The farther apart two genes are, the higher the probability that a crossover will occur. – WHY?

• The greater distance between genes the more places crossing over can occur

The distance between genes was defined as map units where one map unit equals 1% recombinant frequence

A Question• The genes b (body color), vg (wing size) and

cn (cinnabar eye color) are on the same chromosome

cn and b recombination frequency 9%

cn and vg recombination frequency 9.5%

b and vg recombination frequency

Draw a map (linkage map) of these genes on a chromosome

Maxiumum Crossover Percentage

• Genes can be so far away from each other that it virtually certain they will cross over. – The maximum recombination frequency is 50%

These genes are considered genetically unlinked (even though they are on the same chromosome) and follow the rules of independent assortment