Genetics and Inheritance

The Science of Genetics

Early explanations of heredity were eventually rejected by science.
Hippocrates' Theory of Pangenesis: Particles from all parts of the body travel to the eggs or sperm to be passed on, which was later proven incorrect.
Early 19th-Century Biologists' Blending Hypothesis: Traits from both parents mix in offspring, also later disproven.

Gregor Mendel:
- An Austrian Monk.
- Grew up in a rural area.
- The father of genetics.
- Began breeding pea plants in 1857.
- Published his results in 1866.
Mendel's Conclusion: Parents pass discrete "heritable factors" (genes) responsible for traits in offspring.

Used pea plants because they:

Mendel succeeded where others failed because:

Pure breeding parents: Purple and white.
First generation (F1): Offspring all purple.
Inbred F1 to produce 2nd generation (F2): Purple and white offspring in a 3:1 ratio.
Conclusion: F1 had both purple and white inheritance factors, but only the dominant factor was visible.

Gene Variation: There are alternative forms of genes (alleles) that account for variations in inherited characteristics.
Diploid Genes: For each characteristic, an organism inherits two alleles, one from each parent. These alleles may be the same or different.
Law of Dominance: If the two alleles of an inherited pair differ, then one determines the organism’s appearance (dominant allele), and the other has no noticeable effect (recessive allele).
Law of Segregation: A sperm or egg carries only one allele for each inherited trait because allele pairs separate (segregate) from each other during the production of gametes.
Independent Assortment: Each pair of alleles segregates independently of the other pair of alleles during gamete formation.
- The inheritance of one trait has no effect on the inheritance of another.

Mendel's observation support maternal and paternal alleles line up at metaphase I independently if genes for seed color and shape not linked

Rule of Multiplication:
- Example: Bb male x Bb female. What is the chance offspring will be bb?
  - P(B) = \frac{1}{2}
  - P(b) = \frac{1}{2}
  - P(bb) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}
Rule of Addition:
- Example: Bb male x Bb female. What is the chance for offspring with genotype Bb?
  - Option 1: B from sperm ($\frac{1}{2}$) and b from egg ($\frac{1}{2}$): \frac{1}{2} \times \frac{1}{2} = \frac{1}{4} chance
  - Option 2: b from sperm ($\frac{1}{2}$) and B from egg ($\frac{1}{2}$): \frac{1}{2} \times \frac{1}{2} = \frac{1}{4} chance
  - Add separate probabilities: \frac{1}{4} + \frac{1}{4} = \frac{1}{2} chance Bb offspring

Monohybrid cross (Bb x Bb)
- Principle of segregation
- Each gamete carries 1 allele of a gene
- Alleles separate from each other during Anaphase I of meiosis.
Dihybrid cross (RrYy x RrYy)
- Principle of independent assortment
- When genes are on different chromosomes, each allele of that gene is assorted into the gamete independently of the other
- Different chromosomes line up during Metaphase I of Meiosis I

Mendel got lucky because all characteristics he studied were determined by 2 alleles, 1 completely dominant over the other.
Many characteristics have >2 alleles and/or not always complete dominance of any 1 allele.

Incomplete dominance:
- Snapdragon color
- Hypercholesterolemia
Codominance:
- ABO blood groups
- Roan cattle (mixture of white and colored hair)
Pleiotropy:
- Sickle-cell trait/disease
Polygenic inheritance:
- Skin color

Gene on sex chromosome
X-linked recessive trait:
- Red-green colorblindness
Y-linked trait:
- Hairy auricle (XYh) - not in text
X-linked recessive trait:
- Hemophilia
- Genotypes: XHXH, XHXh, XhXh, XHY, XhY
- Phenotypes depend on the specific genotypes