Question from last time:
What assumptions does Mendelian genetics make?
Major Points
1. Genes can interact--Epistasis
2.The concept of genetic linkage
3. Sex-linkage versus autosomal linkage
Mendel Assumed
One gene per phenotype
Genes segregate entirely randomly
Dominant vs. recessive
First Problem: Epistasis
Epistasis: "a genetic interaction between different genes which affects
phenotype"
The interaction can be complete or incomplete
Labrador Coat Color
B controls intensity of melanin deposition
B is the dominant allele--it specifies black coat color
The recessive form b causes less deposition and brown coat color
E controls whether melanin will be deposited
Regardless of the B allele present, a homozygous ee dog has
yellow fur (no melanin)
Continuous Variation
Some traits don't show discrete phenotypes
i.e., black/brown/yellow coat color
There are a wide range of intermediate phenotypes
Examples:
Eye color
Height
Second Problem: Linkage
Mendel assumed random segregation of alleles and genes
* For each gene pair
- 50% chance of inheritance of heterozygous alleles
* Among genes
- all segregate randomly and independently
Genes are on Chromosomes
We know, but Mendel didn't, that genes are on chromosomes
* They are physically linked
* Linkage makes them tend to segregate to the same cells in meiosis
Cosegregation of Linked Genes
Chromosomes Recombine
During Prophase I of meiosis chromosomes recombine
* Recombination occurs randomly across chromosomes
* It's equal likely across the genome
Recombination Changes Linkage
Sex Linkage
* Because the sex chromosomes are special, so are genes linked to them
X-linkage
Y-linkage
* It was discovered by Thomas Morgan using mutants of the white locus
Sex-Linked Human Genes
Hemophilia
* Defect in blood clotting
Red-Green Color Blindness
* The genes encoding the blue and green photoreceptor genes
are tandemly repeated on the X chromosome
Patterned Baldness
Autosomal Inheritance
Most genes are on the autosomes
* They are either dominant or recessive
* Autosomal recessive diseases (e.g. Sickle-Cell Anemia)
- Must be inherited from two carriers (Aa types)
* Autosomal dominant diseases (e.g. Huntington's Disease)
- Must be inherited from an affected parent