Non-Mendelian Genetics Notes

Non-Mendelian Genetics

Alleles can show varying degrees of dominance.
Complete dominance:
- In Mendel’s experiments, he worked with traits that showed complete dominance.
- Homozygous dominant and heterozygous individuals are phenotypically the same.
Incomplete dominance:
- Neither allele is fully dominant.
- The F1 generation has a phenotype that is a mix of those of the parental generation.
- Example: Red flowers crossed with white flowers produce pink offspring.
Codominance:
- Two alleles that affect phenotype are both expressed.
- Example: Human blood group: Type AB blood, where A and B are both expressed.
Multiple Alleles:
- Genes exist in forms with more than two alleles.
- Example: Human blood group alleles: I^A, I^B, i.

A black mouse (BB) is crossed with a white mouse (bb), and the resulting offspring are gray. The easiest explanation for this phenomenon is incomplete dominance.
Cattle can be red (RR = all red hair), white (WW = all white hair), or roan (RW = red and white hair). The best explanation for this phenomenon is codominance.
A red cow is crossed with a roan cow. The phenotypic ratio of the offspring would be 1:1 red and roan (50% red, 50% roan).
- Cross: RR x RW
A woman with type A blood has a child with a man who has type B blood. With this limited information, possible genotypes are:
- Woman: I^AI^A or I^Ai
- Man: I^BI^B or I^Bi

In many cases, two or more genes are responsible for determining phenotypes.
Epistasis:
- The phenotypic expression of a gene at one locus affects a gene at another locus.
- Example: Coat color in labs and some mice.
  - One gene codes for pigment, and a second gene determines whether that pigment will be deposited in the hair.
Polygenic inheritance:
- The effect of two or more genes acting on a single phenotype.
- Example: Height, human skin color.

Thomas Hunt Morgan experimented with fruit flies and determined that specific genes can be carried on sex chromosomes.
Sex-linked gene:
- A gene located on either the X or the Y chromosome.
- Y-linked genes: Genes specifically found on the Y chromosome.
  - Very few Y-linked genes, so very few disorders.
- X-linked genes: Genes found on the X chromosome.

Fathers can pass X-linked alleles to all of their daughters but none of their sons.
Mothers can pass X-linked alleles to both daughters and sons.
If an X-linked trait is due to a recessive allele:
- Females will only express the trait if they are homozygous.
- Because males only have one X chromosome, they will express the trait if they inherit it from their mother.
  - They are called hemizygous (since the term heterozygous does not apply).
  - Due to this, males are much more likely to have an X-linked disorder.

Females inherit two X chromosomes, which is double that of males!
During development, most of the X chromosome in each cell becomes inactive.
- The inactive X in each cell of a female condenses into a Barr body.
- Helps to regulate gene dosage in females.

Genetic recombination:
- Production of offspring with a new combination of genes from parents.
- Parental types: Offspring with the parental phenotype.
- Recombinants: Offspring with phenotypes that are different from the parents.
Mendel also observed recombinants during his crosses.
*Example: green wrinkled plant crossed with a yellow-round plant
*yyrr x YyRr
*YR yr Yr yR
*yr YyRr yyrr Yyrr yyRr
*50% Parental phenotypes
*50% Recombinant phenotypes
*50% recombination, however, indicates that genes are unlinked, or on different chromosomes
Linked genes: Genes located near each other on the same chromosome that tend to be inherited together.
Meiosis and random fertilization generate genetic variation in offspring due to:
- Independent assortment of chromosomes.
- Crossing over in Meiosis I.
- Any sperm can fertilize any egg.

Linked genes show parental phenotypes in offspring at higher than 50%.
During crossing over, chromosomes from one paternal chromatid and one maternal chromatid exchange corresponding segments.
Crossing over helps to explain why some linked genes become separated during meiosis.
The further apart two genes are on the same chromosome, the higher the probability that a crossing over event will occur between them and the higher the recombination frequency.

Experiments performed by Sturtevant allowed scientists to map genes and their locations on chromosomes.
Linkage map:
- Genetic map based on recombination frequencies.
- The distance between genes is measured in map units.
  - One map unit is equivalent to a 1% recombination frequency.
  - Expresses the relative distances along chromosomes.
- 50% recombination means that the genes are far apart on the same chromosome or on two different chromosomes.

Some traits are located on DNA found in the mitochondria or chloroplasts.
Both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells.
- In animals, mitochondria are transmitted by the egg, NOT the sperm.
  - Therefore, ALL mitochondrial DNA is maternally inherited.
- In plants, mitochondria and chloroplasts are transmitted in the ovule, NOT the pollen.
  - Therefore, both mitochondrial and chloroplast-determined traits are maternally inherited.

Some genetic disorders can be linked to affected or mutated alleles or chromosomal changes.

Tay-Sachs disease:
- Autosomal recessive disease.
- Mutated HEXA gene.
  - The body fails to produce an enzyme that breaks down a particular lipid.
  - Affects the central nervous system and results in blindness.
Sickle cell anemia:
- Autosomal recessive disease.
- Mutated HBB gene.
  - Sickled cells contain abnormal hemoglobin molecules.

Nondisjunction:
- Chromosomes fail to separate properly in meiosis I or meiosis II.
- Karyotyping can detect nondisjunction.
Example: Down Syndrome:
- Three copies of chromosome 21.