Transcript Chromosomal Basis of Inheritance Linked

Chromosomal Basis
of Inheritance
Linked Genes
Sex-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
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