1. Linked genes

2. Introduction Each chromosome has hundreds or even thousands of genes.
“Linked genes” are genes located on the same chromosome that tend to be inherited together in genetic crosses.
The results of breeding experiments deviate from those expected by Mendel’s law of independent assortment when linked genes are being studied.

3. How linkage affects inheritance
The wild type Drosophila had gray bodies and normal-sized wings.
Mutants had black bodies and vestigial wings (much smaller).
Allele symbols: b+ = grey, b = black, vg+ = normal wings, vg = vestigial wings
Mutant alleles were recessive and not located on a sex chromosome.

4. How linkage affects inheritance
A test cross was done between…
true-breeding double-mutant males(b b vg vg) and
dihybrid females (b+ b vg+ vg)
…To find out if the genes were located on the same chromosome or different chromosomes.
*Because all of the male’s alleles were recessive, the phenotype of the offspring would depend on the female’s alleles. Would the alleles for body color and wing shape stay together during the formation of the female’s gametes?

5. How linkage affects inheritance
Morgan classified the offspring from the testcross matings according to phenotype
He found a much higher proportion of parental phenotypes than would be expected if the genes assorted independently.
Body color and wing size must usually be inherited together, and therefore be located on the same chromosome.
However – nonparental phenotypes were also produced, suggesting that body color and wing size genes are only partially linked genetically. (see p.278,279)

6. Genetic Recombination and linkage
When offspring inherit a phenotype that matches one of the parental phenotypes, they are called “parental types.”
When offspring inherit a phenotype that is a new combination of traits, they are called “recombinant types” or “recombinants.”
When half of the offspring are recombinants, there is a 50% frequency of recombination.
This would result from recombination between unlinked genes
Due to the random orientation of homologous chromosomes which leads to independent assortment of alleles (during metaphase 1 of meiosis).
(see p. 278)

7. Genetic Recombination and linkage
Crossing over accounts for the recombination of linked genes.
Some of the female’s gametes had a recombinant chromosome.
Most of the ova had a b+ vg+ or b vg
But some had b+ vg or b vg+

8. Linkage Mapping
One of Morgan’s students began to construct a “genetic map” – an ordered list of the genetic loci along a particular chromosome.
Since crossing over is a random event, the chance of it happening is approximately equal at all points along a chromosome.
His prediction: the further apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency (% of recombinant offspring)

9. Linkage Mapping
Linkage map = a genetic map based on recombination frequencies
Three genes will be used for illustration:
Body color – b
Wing size – vg
Cinnabar – cn (affects eye color… mutants have brighter red)
Recombination between cn and b is 9%, that between cn an vg is 9.5%, and that between b and vg is 17%.
Crossovers between cn and b and cn and vg are about half as frequent as crossovers between b and vg.
A map would then place cn midway between b an vg.

10. Linkage Mapping Distances between genes are called “map units.”
One map unit is equal to 1% recombination frequency.
Does not correspond to actual physical distances, but does show order of genes along a chromosome
The highest value is 50%... This percentage is true of genes located on different chromosomes as well as genes on the same chromosome that are so far apart that a crossover is virtually certain.
Such genes are genetically unlinked, regardless of whether or not they exist on the same chromosome.
Cytogenetic maps – locate genes with respect to chromosome features such as stain bands (constructed using other methods)

11. Bozeman video!