Genetics and Inheritance Patterns

Patterns of Inheritance

Gregor Mendel (1822-1884) conducted groundbreaking work in genetics from a monastery garden in the Czech Republic.
Key Points of Mendel's Research:
- Started studying pea plants in 1856.
- Chose pea plants due to their ability to self-pollinate, short generation time, large offspring numbers, and true breeding characteristics.

Monohybrid Crosses:
- For instance, crossing purple and white flower traits resulted in offspring (F1 Generation) that were all purple.
- When F1 plants were self-pollinated, F2 Generation exhibited a 3:1 ratio of purple to white flowers.
Dominant and Recessive Traits:
- Purple flower color (dominant) masked the expression of white flower color (recessive).
- Publication of his work in 1866 emphasized the inheritance of discrete "heritable factors" (now termed genes).

Continuous Variation: Characteristics show a range (e.g., human height).
Discontinuous Variation: Characteristics appear in distinct forms (e.g., flower colors).

Gene: Mendel's heritable factor governing a character (e.g., flower color).
Alleles: Alternative versions of genes that determine traits (e.g., purple and white flower alleles).
Genotype: The allelic makeup of an organism (e.g., TT, Tt, or tt).
Phenotype: The expressed traits of an organism (e.g., purple or white flowers).
Dominant Allele: Expressed if present; represented by uppercase letters.
Recessive Allele: Masked by dominant allele; represented by lowercase letters.

Monohybrid Cross: Parents differ in one trait (e.g., flower color).
Dihybrid Cross: Parents differ in two traits.
Punnett Square: A tool to predict genotype and phenotype ratios from genetic crosses.

Rule of Multiplication: Used to find the probability of multiple independent events occurring together.
Rule of Addition: Used when considering the chance of an event occurring in various ways.

Three key rules:
1. Each individual possesses two alleles for any character (one from each parent).
2. Dominant alleles mask recessive alleles; homozygous dominance and heterozygosity show identical phenotypes.
3. During gamete formation, alleles segregate so that each gamete carries one allele.

States that alleles for different traits segregate independently during gamete formation, applies to genes on different chromosomes.

Frequency of Dominant Traits: Dominant traits are not necessarily more common in populations (e.g., polydactyly).
Test Cross: Used to determine if an organism with a dominant phenotype is homozygous or heterozygous by crossing it with a homozygous recessive.

A character influenced by multiple genes leads to continuous traits (e.g., skin color).

Located on sex chromosomes, causing males (XY) to express traits with only one copy (hemizygous). Females (XX) require two copies for recessive traits.

Commonly affect males due to the presence of only one X chromosome. Notable examples include hemophilia, color blindness, and Duchenne muscular dystrophy.

Mendel proposed that alleles behave in dominant and recessive patterns, segregating into gametes independently. The existence of codominance and incomplete dominance exemplifies that phenomena can vary beyond simple Mendelian inheritance.
Understanding these principles is crucial in genetics, facilitating predictions about inheritance patterns and traits.