BIO 184 E3 LECTURE 4

Law of Segregation and Independent Assortment

The law of segregation comes from a monohybrid cross (e.g., Tt cross).
The law of independent assortment comes from a dihybrid cross (two genes on different chromosomes).
Linked genes are an exception; they are found on the same chromosome.

The fork-line method illustrates the law of segregation for each chromosome independently.
It demonstrates how the law of independent assortment handles genes independently.
The final gamete types must have one allele from each gene.

The number of different gamete options is determined by the number of heterozygous loci.
Formula: Number of gamete types = 2^n, where n is the number of heterozygous loci.
Homozygous loci provide no choice, only one type of allele (e.g., TT produces only T).
Example 1: One heterozygous locus (Tt) yields 2^1 = 2 gamete types.
Example 2: Two heterozygous loci (YyTt) yields 2^2 = 4 gamete types.
Example 3: Three heterozygous loci (YyTtCc) yields 2^3 = 8 gamete types.

For a trihybrid cross (e.g., YyTtCc), use the splitting method to find the eight gamete types:
- Y can go with T, which can go with C (e.g., YTC).
- Y can go with T, which can go with c (e.g., YTc).
- Continue methodically to find all combinations.

Each gamete type in a trihybrid cross represents 1/8 of the total possibilities.

A test cross helps determine the genotype of an individual by crossing it with a homozygous recessive individual.
The phenotype of the offspring reveals the genotype of the parent.
- Example: YyTt x yytt allows direct observation of the phenotype since the second parent doesn't contribute.
- The resulting phenotypes (tall, yellow, short, yellow, tall, green, short, green) are directly determined by the alleles from the first parent.

Crossing two dihybrids (e.g., YyTt x YyTt) results in a 16-well Punnett square.
Phenotype ratios in the offspring follow a 9:3:3:1 pattern.
- 9: Dominant-Dominant
- 3: Recessive-Dominant
- 3: Dominant-Recessive
- 1: Recessive-Recessive

Separate the crosses and handle them independently due to the law of independent assortment.
Example: Yy x Yy crossed with Tt x Tt
- Yy x Yy results in a 3:1 ratio (3/4 yellow, 1/4 green).
- Tt x Tt results in a 3:1 ratio (3/4 tall, 1/4 short).
Multiply the ratios to find the combined probabilities:
- 3/4 (tall) x 3/4 (yellow) = 9/16 (tall, yellow).
- The 9:3:3:1 ratio is derived from these multiplications.

Cross: TtYy x TtYY
Goal: Find the fraction of tall, yellow offspring.
No green offspring are possible since one parent is YY.
The cross Tt x Tt yields 3/4 tall.
Since all offspring are yellow, the fraction of tall, yellow offspring is 3/4 x 1 = 3/4.

Instead of calculating all possible outcomes, focus only on the desired phenotype.
Example: Finding the fraction of tall, yellow offspring.
- Fraction of tall from Tt x Tt is 3/4.
- Fraction of yellow from Yy x YY is 1.
- Multiply 3/4 * 1 = 3/4 to get the fraction of tall, yellow offspring.

In a trihybrid cross, there are 2^3 = 8 possible gamete types, resulting in 64 wells in a Punnett square.
The numbers in numerators will add up to 64
Example is TtRrYy x TtRrYy which has results in the ratios of each phenotype showing up.

Cross: TtYy x TtYy.
- Isolate crosses Tt and Yy.
What is the fraction of offspring that are both tall and yellow?
Tall (3/4), Yellow (1/2). Fraction of offspring that are both tall and yellow comes to be 9/32

Recessive alleles often result from a loss of function in a gene.
Example: Yellow vs. Green seed color is determined by pigment production where an enzyme is needed.
One functional copy of the dominant allele is sufficient to produce the dominant phenotype.
YY and Yy will yield a yellow pigment due to one functional enzyme copy.
yy results in no yellow pigment (green) due to lack of a functional enzyme.

Box: Male.
Circle: Female.
Diamond: Unknown sex.
Colored shape: Individual has the disease.
Half-shaded shape or mark in the center: Carrier (normal but carries the allele).
Double line between mating pair: Inbreeding or consanguineous mating.
Number inside shape: Number of children of that sex.
Line through shape: Deceased.
Generation numbers: Used to identify individuals (e.g., I-2).
Single split: Twins.
Line connecting twins: Identical twins.

Dominant traits: Appear in every generation without skipping.
Recessive traits: May skip generations.
Horizontal transmission: Affected individuals appear in the same generation (parents unaffected).

Parents who are carriers, and offspring do not have the disease: Calculate probabilities
Consider the possible genotypes of the offsprings of the parent.
Example: what is the chance that offspring are hetrozygous?
Key: The simple answer is one half, and the answer is often wrong. The box may not be the correct option.
Example: The answer is two thirds because you can rule out out being little c little c.
We have 3 boxes left, it's a two thirds chance out of the remaining boxes.
For 2-6, there's no doubt that they're hetozygous.