Mendelian (Classical) Genetics 1.1 Genetics

Genetics

Genetics is the study of genes and how different characteristics are inherited through genes from one generation to the next.
A gene is a portion of a DNA sequence that carries the hereditary information to determine a characteristic and is located on a chromosome.
Chromosomes get passed onto new organisms, thus passing genes.
Different genes are responsible for different characteristics.
Each double-stranded chromosome carries two copies of the same gene due to DNA replication during interphase.

The study of genetics was pioneered by Gregor Mendel, an Augustinian monk.
Mendel studied inheritance patterns in pea plants, leading to the laws of inheritance.
He cross-pollinated different traits in pea plants and noted which traits were expressed in the offspring.
Mendel worked with seven different traits in pea plants:
- Flower color
- Flower position
- Seed color
- Seed shape
- Pod shape
- Pod color
- Stem length
Mendel predicted the outcome of different traits in offspring by determining which traits were dominant and recessive.
He discovered the "unit of inheritance" (genes) through mathematical analysis despite not knowing about DNA.
Mendel described how the traits of pea plants were inherited.

Traits can be described in terms of alleles, which are alternative traits to the same gene.
- Example: purple flowers and white flowers are alleles for flower color.
- Example: Blood type in humans (A, B, AB, O) which has more than two alleles determining phenotype.
The dominant allele is expressed when present, while the recessive allele is not expressed unless no dominant allele is present.
- Purple flower color is dominant, and white flower color is recessive in pea plants.
- Breeding a purple flowered pea plant with a white flowered pea plant gives all purple flowered pea plants in the F1 generation.
- Crossing a purple flowered pea plant from the F1 generation with a white flowered pea plant gives a 3:1 ratio of purple to white flowered pea plants in the F2 generation.

Mendel carefully cross-pollinated pea plants by cutting the anthers off one plant and transferring pollen from another to avoid self-pollination.
He worked with one trait at a time to understand inheritance.
In one experiment, he cross-pollinated a true-breeding purple flowered plant (anthers removed) with pollen from a true-breeding white flowered plant.
True breeding implies the organism is homozygous for a particular gene.
Homozygous: both alleles for the same trait are the same (e.g., AA for two dominant, aa for two recessive).
Mendel ensured true breeding by self-pollinating plants for several generations to ensure homozygosity.
Heterozygous: two different alleles for a trait (e.g., Aa, one dominant and one recessive).
After cross-pollinating true-breeding purple and white flowers, all F1 generation flowers were purple.
He allowed F1 generation flowers to self-pollinate and noted the F2 generation outcome.
In the F2 generation, both purple and white flowers were present in a 3:1 ratio.
Phenotype: the expression of a trait.
Genotype: the actual set of alleles.
Example: F2 generation phenotypic ratio is 3:1, but the genotype ratio is 1:2:1 (1 BB: 2 Bb: 1 bb).
After many experiments with all seven traits, Mendel formulated three laws of inheritance.