Mendelian Genetics

Moving from DNA basics to heritability.
Gregor Mendel: The father of genetics.
- Born Johann Mendel in Austria to an agricultural family.
- Joined the priesthood and became interested in teaching.
- Attended the University of Vienna, studying teaching, agriculture, mathematics, and experimental science.
- Returned to the monastery and was put in charge of the garden.

Observed variations in pea plants.
- Red plants having white offspring.
- Characteristics disappearing and reappearing across generations.
Started exploring these observations using experimental design.

Chose pea plants for study.
Planted a pea garden and waited for several years.
Developed true-breeding plants (pure-breeding).
- Crossing two plants with a specific trait always resulted in offspring with the same trait (e.g., full peapods).
- These true-breeding plants served as controls for his experiments.

Selected seven traits to study, including:
- Round peas
- Wrinkled peas
Crossed round peas with round peas and always got round peas; same with wrinkled peas.
True-breeding plants always pass down the same phenotypic trait to offspring.
These true-breeding plants were termed the parental generation (P).

Mendel performed reciprocal crosses.
- He observed that results were the same regardless of which plant donated pollen or received it.

Mendel initially believed in blended inheritance, but the F1 results contradicted this idea.
Instead of getting intermediate peas, all offspring were round.

Mendel observed the same pattern with other traits.
- Crossing true-breeding plants with contrasting traits resulted in the F1 generation expressing only one of the original traits.
- Example: Crossing green and white seed coats always resulted in green seed coats in the F1 generation.

Mendel counted the number of round and wrinkled peas in the F2 generation.
- 5,474 round peas
- 1,850 wrinkled peas
Calculated the ratio of round to wrinkled peas: 2.96:1.

Mendel observed similar ratios for other traits.
- Pod shape: 2.95:1
- Flower color: 3.15:1
- Seed color: 3.01:1
These ratios approximated a 3:1 ratio.

Mendel hypothesized about particles of inheritance (genes).
He considered different scenarios, such as: One particle of inheritance versus two.
He deduced that each organism must have two particles of inheritance for each trait.
- $R$ represents round, and $r$ represents wrinkled.
- $RR$ is round.
- $rr$ is wrinkled.

Mendel's experiments led to the idea of diploidy.
- Each organism inherits genetic information from both parents.
He also proposed the idea of dominance.
- One particle of inheritance (allele) can be dominant over the other.
- Round ( $R$ ) is dominant to wrinkled ( $r$ ).

Punnett squares are a common tool for calculating genetic outcomes, but Mendel did not use them.
Mendel used probability to analyze his results.

The probability method relies on the multiplication and addition rules. Probability of passing on either allele is 50%.
Multiplication Rule:
- Multiply numerators and denominators.
- Example: Probability of getting $R$ from one parent (1/2) and $R$ from the other parent (1/2) = 1/4.
Addition Rule:
- Ensure denominators are the same, then add numerators.
Probability allows to calculate odds that cannot be calculated using Punnett squares.

Mendel wrote a manuscript about his work but faced a problem.
He performed the experiment (counting peas) before formulating his hypothesis, which is not standard scientific practice.
The correct scientific method involves:
- Observation
- Hypothesis
- Experiment
- Analysis.

Mendel performed test crosses to validate his hypothesis.
A test cross involves crossing an unknown genotype with a homozygous recessive individual ( $rr$ ).
Possible outcomes:
- $RR$ x $rr$ : All offspring are round.
- $Rr$ x $rr$ : 1:1 ratio of round to wrinkled offspring.

Genes: Particles of inheritance (DNA sequences).
Alleles: Alternate forms of a gene (e.g., $R$ and $r$ ).
Locus: Location of a gene on a chromosome (Loci is plural).
Genotype: The alleles an organism has for a particular trait (e.g., $RR$ , $Rr$ , $rr$ ).
Phenotype: Physical appearance or trait expressed by a gene (e.g., round or wrinkled).
Homozygous: Both alleles are the same (e.g., $RR$ or $rr$ ).
Heterozygous: Alleles are different (e.g., $Rr$ ).

Mendel's Third Law (Law of Dominance): One trait (allele) will mask the trait of the other allele.
Mendel's First Law (Law of Segregation): During gamete formation, alleles segregate randomly, with each allele having a 50% chance of being passed down.