Mendelian Genetics

Derives from the laws of inheritance proposed by Gregor Mendel.
Mendel discovered the main mechanics of transmission genetics:
- Transmission genetics refers to the passing of genetic material from one generation to the next.

Definition: A monohybrid cross involves mating true-breeding individuals differing in one pair of contrasting traits.
Involves mating true-breeding individuals from two parent strains, leading to homozygosity.
- Homozygous: Individuals that are true-breeding for specific traits.
Reveals how one trait is transmitted from generation to generation.

P1 generation: Original parents (parental generation).
F1 generation: Offspring of the P1 generation (first filial generation).
F2 generation: Offspring resulting from the self-fertilization of F1 generation (selfing), referred to as second filial generation.

Definition: A testcross determines whether an individual displaying a dominant phenotype is homozygous or heterozygous for that trait.
Example: The F1 generation plants could possess either the DD (homozygous dominant) or Dd (heterozygous) genotype; the testcross helps establish the genetic makeup.
A testcross always involves crossing with a homozygous recessive individual.

Scenario: If a mouse exhibits a dominant phenotype (P−), how to determine if it is homozygous (PP) or heterozygous (Pp)?
- Options:
- A. Cross it to a homozygous dominant mouse.
- B. Cross it to a mouse with the dominant trait but similarly unknown genotype.
- C. Cross it to a mouse exhibiting the recessive trait.
- D. Cross it to a heterozygous mouse.
- E. It cannot be determined.
- Correct Answer: C. Cross it to a mouse with the recessive trait.
- Performing a testcross with a homozygous recessive mouse helps confirm the genotype:
  - If all offspring display the dominant phenotype, the mouse is homozygous dominant.
  - If some offspring exhibit the recessive trait, the mouse is heterozygous.

Understanding how to use Punnett squares is essential for visualizing genetic crosses.

HH (homozygous dominant) × hh (homozygous recessive)
- Result: All offspring (F1) will be heterozygotes.
Hh (heterozygote) × Hh (heterozygote)
- Result yields a 1:2:1 phenotypic ratio:
- HH = 1
- Hh = 2
- hh = 1
Hh (heterozygote) × HH (homozygote dominant)
- Result yields half homozygous and half heterozygous:
- HH = 2
- Hh = 2

Definition: A dihybrid cross (two-factor cross) examines the inheritance of two traits simultaneously.
Crosses involve two pairs of contrasting traits, generating unique ratios in the F2 generation.
Typical phenotypic ratio observed: 9:3:3:1.

Scenario: Round-yellow seeds crossed with green-wrinkled seeds.
- F1 generation shows both dominant traits.
- F2 generation shows the phenotypic ratio of 9:3:3:1.

Unit factors in pairs:
- Genetic characters are controlled by unit factors (genes) that exist in pairs within organisms.
Dominance/recessiveness:
- A pair of two unlike unit factors (genes) for a single characteristic contains one dominant and one recessive unit factor.
Segregation:
- Paired unit factors (genes) segregate randomly during gamete formation.

Definition: Traits are inherited independently, meaning the results of a dihybrid cross resemble the result of two monohybrid crosses where traits are inherited independently.

(a) Unit factors in pairs:
- Homologous chromosomes arrange in pairs during the first meiotic prophase.
(b) Segregation of unit factors:
- Homologs segregate during meiosis, occurring in the first meiotic anaphase.
(c) Independent assortment:
- Nonhomologous chromosomes assort independently over various meiotic events, resulting in all possible gametic combinations formed with equal probability.

When examining multiple genes in a genetic cross (e.g., AaBbCcDd x AaBbCcDd), complexity increases significantly and requires additional methods for analysis.

Definition: The product law helps predict the percentage of offspring with specific genotypes by calculating the probabilities of individual genes.
- When all outcomes are equally likely, the probability of a particular outcome is determined by:
  P = \frac{\text{# of ways to obtain that outcome}}{\text{total # of possible outcomes}}
Example probabilities for simple events:
- Probability of heads in a coin toss = \frac{1}{2}.
- Probability of rolling a six on a die = \frac{1}{6}.
- Probability of drawing a Queen of Spades from a deck = \frac{1}{52}.

To find the probability of obtaining a specific genotype (e.g., Aabb) from the cross of AaBb x AaBb:
- Probability of Aa = \frac{1}{2}.
- Probability of bb = \frac{1}{4}.
- Hence, the probability of Aabb = \left(\frac{1}{2}\right) \left(\frac{1}{4}\right) = 0.125 or 12.5%.
- In a sample of 30 offspring, this results in approximately 4 expected offspring (0.125 x 30 = 3.75, round up to 4).

Exercise: Determine the probability of getting the genotype AABbcc from parents with genotypes AaBbCc x AABBcc.
Exercise: Calculate how many offspring out of 500 will have the genotype AaBbCcDd from parents that are AABbCCdd x AaBbCcDd.

Definition: The addition rule applies when calculating probabilities of mutually exclusive events.
- Example scenario:
1. Rolling a die once.
2. The expected outcomes yield that on average one out of six times, you will get a three and another one out of six times, you will get a four.
3. The probability of either getting a three or a four is:
  P(3 \text{ or } 4) = \frac{1}{6} + \frac{1}{6} = \frac{2}{6} = \frac{1}{3}.

To evaluate the probability of obtaining a yellow and round pea from parents that are YyRr:
- Explore all combinations that could yield this phenotype and find their probabilities for each case.
- Cross: YYRR, YyRr, YYRr, and YyRR; each needs its independent probability calculating and then summed together to determine the overall probability of yellow and round phenotype.

Analysis of phenotype probabilities associated with dihybrid crosses can be expanded upon with diagrams describing allele combinations.
For example: Seed shape and color combinations, yielding ratios such as 9/16 for yellow-round, 3/16 for yellow-wrinkled, 3/16 for green-round, and 1/16 for green-wrinkled offspring.