Unit 1: Mendelism and its Extension

1. Principles of Inheritance

The study of genetics began with Gregor Mendel, who established the fundamental laws of heredity through his work with pea plants.

• Law of Segregation: During gamete formation, the two alleles for a trait separate, so each gamete carries only one allele for each gene.
• Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes, provided they are on different chromosomes.

2. Incomplete Dominance and Co-dominance

Extension of Mendelian genetics shows that many traits do not follow simple dominant-recessive patterns.

Incomplete Dominance

This occurs when the phenotype of the heterozygote is an intermediate blend of the two homozygotes (e.g., red and white flowers producing pink offspring in 4 o'clock plants).

Co-dominance

In co-dominance, both alleles are fully expressed in the heterozygote (e.g., AB blood group in humans where both A and B antigens are present).

3. Allele Concept: Multiple, Pseudo, and Lethal Alleles

The concept of a gene as a single entity with only two forms has been expanded.

• Multiple Alleles: When more than two alleles exist for a single gene within a population, such as the ABO blood group system.
• Pseudo Alleles: Genes that are functionally related and located so close together on a chromosome that they behave as a single unit but can rarely undergo recombination.
• Lethal Alleles: Alleles that cause the death of the organism that carries them, usually when present in a homozygous state.

4. Epistasis, Hypostasis, and Pleiotropy

Gene interactions can lead to complex phenotypic ratios.

Epistasis and Hypostasis

Epistasis: An interaction where one gene masks or interferes with the expression of another gene.
Hypostasis: The gene whose expression is being masked is termed the hypostatic gene.

Pleiotropy

Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. A classic example is sickle cell anemia, where one mutation affects hemoglobin shape, red blood cell survival, and blood flow.

5. Exam Focus Enhancements