Unit 1: Mendelian Genetics and its Extension

1. Principles of Inheritance

Mendelian genetics forms the foundation of modern transmission genetics. Formulated by Gregor Johann Mendel through systematic hybridization experiments on the garden pea plant (Pisum sativum), these principles describe how hereditary factors (genes) pass from parents to offspring across generations.

Why Mendel Chose Garden Peas

• Presence of clear, contrasting, mutually exclusive traits (e.g., tall vs. dwarf, round vs. wrinkled seeds).
• Short life cycle allowing evaluation of multiple generations in a short time.
• Bisexual flowers capable of self-pollination as well as controlled cross-pollination.
• Production of large numbers of fertile offspring per cross, enabling statistical analysis.

Principle 1: Law of Segregation (Mendel's First Law)

The Law of Segregation states that during gamete formation, the two alleles of an allelic pair separate (segregate) from each other cleanly without blending, such that each gamete receives only one allele with equal probability.

Mechanism and Monohybrid Cross: When a homozygous tall plant (TT) is crossed with a homozygous dwarf plant (tt):

• F1 Generation: All plants are heterozygous tall (Tt).
• F2 Generation: Self-pollination of F1 gives a phenotypic ratio of 3 Tall : 1 Dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

Principle 2: Law of Independent Assortment (Mendel's Second Law)

The Law of Independent Assortment states that alleles of two or more different genes assort independently of one another during gamete formation, provided the genes are located on different chromosomes or far apart on the same chromosome.

Mechanism and Dihybrid Cross: Crossing round yellow seeds (RRYY) with wrinkled green seeds (rryy):

• F1 Generation: All round yellow (RrYy).
• F2 Generation: Phenotypic ratio of 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green.

Test Cross vs. Back Cross

A back cross is any cross between an F1 individual and either of its parents. A test cross is a specific type of back cross where an F1 individual showing a dominant phenotype is crossed with a homozygous recessive parent to determine the F1 genotype.

Exam Note: A test cross always yields a phenotypic ratio that directly reveals the proportion of different types of gametes produced by the tested individual.

2. Incomplete Dominance and Co-dominance

Incomplete dominance and co-dominance represent extensions of Mendelian inheritance where single-gene phenotypic patterns deviate from the simple dominant-recessive paradigm.

Incomplete Dominance

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over another allele, resulting in a blended intermediate phenotype in heterozygous individuals.

Classic Example: Flower color in Snapdragons (Antirrhinum majus) and Four o'clock plants (Mirabilis jalapa).

• Crossing homozygous red flowers (RR) with homozygous white flowers (rr) yields pink flowers (Rr) in the F1 generation.
• F2 selfing yields: 1 Red (RR) : 2 Pink (Rr) : 1 White (rr).
• Phenotypic Ratio = Genotypic Ratio = 1:2:1.

Co-dominance

Co-dominance occurs when both alleles of a gene pair in a heterozygote are fully and equally expressed in the phenotype, without blending or masking each other.

Classic Example: Human ABO Blood Grouping (specifically alleles IA and IB) and MN blood group system.

• An individual with genotype IA IB expresses both A and B surface antigens on red blood cells, resulting in Blood Type AB.
• In the MN blood group system, heterozygous individuals (LM LN) display both M and N cell-surface glycoproteins equally.

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

Classical genetics viewed a gene as an indivisible unit of function, mutation, and recombination. Modern genetics defines alleles as alternative forms of a specific gene residing at a defined locus on a chromosome.

Multiple Alleles

Multiple alleles refer to the presence of three or more alternative forms of a single gene locus within a population, although any individual organism carries only two of these alleles.

• Human ABO Blood Groups: Controlled by gene I with three alleles: IA, IB, and i. IA and IB are co-dominant to each other, while both are completely dominant over i.
• Coat Color in Rabbits: Controlled by gene C with four alleles in order of dominance: Full color / Agouti (C) > Chinchilla (c-ch) > Himalayan (c-h) > Albino (c).
• Formula for Number of Genotypes: For 'n' multiple alleles, the maximum number of possible genotypes is given by n(n + 1) / 2.

Pseudoalleles

Pseudoalleles are genes that act as alleles by occupying closely adjacent loci on a chromosome, affecting the same phenotypic trait, and rarely undergoing intragenic recombination between each other.

• Functional behavior mimics single-locus alleles in standard genetic tests.
• They can be separated by rare crossing-over events, proving they occupy distinct physical sub-loci.
• Cis-Trans Test (Complementation Test): Discovered by Seymour Benzer. In cis-configuration (a1 a2 / + +), the wild-type phenotype is expressed. In trans-configuration (a1 + / + a2), a mutant phenotype appears if alleles fail to complement each other.
• Example: Eye color loci in Drosophila melanogaster (e.g., apricot and white eye alleles).

Lethal Alleles

Lethal alleles are mutant alleles that cause death of the organism carrying them, usually during embryonic or early developmental stages.

• Recessive Lethal Allele: Causes death only in the homozygous recessive condition. Heterozygotes are viable and may display a altered phenotype.
• Dominant Lethal Allele: Kills both homozygotes and heterozygotes (e.g., Huntington's disease allele in humans, which expresses lethality later in adult life).
• Classic Example: Coat color in mice studied by Lucien Cuénot. Crossing yellow mice (Yy) x yellow mice (Yy) produces a phenotypic ratio of 2 Yellow : 1 Agouti among surviving offspring because YY homozygotes die in utero.

4. Epistasis, Additivity, and Pleiotropy

Gene interaction occurs when two or more different non-allelic genes influence a single phenotypic character.

Epistasis

Epistasis is a non-allelic gene interaction wherein one gene (epistatic gene) masks, suppresses, or modifies the phenotypic expression of another independent gene locus (hypostatic gene).

Types of Epistasis and F2 Ratios:

1. Dominant Epistasis (12:3:1): A single dominant allele at the epistatic locus masks the alleles at the hypostatic locus. Example: Fruit color in summer squash (Cucurbita pepo).
2. Recessive Epistasis / Supplementary Gene Interaction (9:3:4): Homozygous recessive state of one gene masks the expression of alleles at a second locus. Example: Coat color in Labrador retrievers and mice (Agouti, Black, Albino).
3. Duplicate Recessive Epistasis / Complementary Gene Action (9:7): Dominant alleles at both loci are required simultaneously to produce the functional phenotype. Example: Flower color in sweet pea (Lathyrus odoratus).
4. Duplicate Dominant Epistasis (15:1): Presence of a single dominant allele at either of the two loci is sufficient to produce the dominant phenotype. Example: Seed capsule shape in Shepherd's Purse (Capsella bursa-pastoris).

Additivity (Additive Gene Action / Polygenic Inheritance)

Additivity occurs when two or more independent non-allelic genes contribute cumulatively to a single quantitative phenotypic trait, with each additive allele contributing a small equal unit to the final phenotype.

• Produces continuous phenotypic variation across a population rather than discrete classes.
• Does not exhibit dominance; expression depends strictly on the total number of additive dominant alleles present.
• Classic Example: Grain color in wheat (kernel color) studied by Nilsson-Ehle, and human skin pigmentation studied by Davenport.
• In a two-gene additive system (AABB x aabb), the F2 phenotypic distribution forms a symmetrical bell curve with a ratio of 1 : 4 : 6 : 4 : 1.

Pleiotropy

Pleiotropy is the biological phenomenon where a single gene exerts multiple, seemingly unrelated phenotypic effects across different organs or physiological pathways in an organism.

• Mechanism: The primary product of the gene is utilized in multiple biological pathways or affects a fundamental developmental process.
• Example 1: Phenylketonuria (PKU) in humans: Mutation in the gene encoding phenylalanine hydroxylase leads to mental retardation, decreased skin pigmentation, and hair abnormalities.
• Example 2: Sickle Cell Anemia: Mutation in the beta-globin gene causes abnormal hemoglobin, producing sickled red cells, anemia, heart failure, and spleen damage.
• Example 3: Marfan Syndrome: Mutation in the fibrillin-1 gene affects connective tissue causing skeletal deformities, cardiovascular defects, and lens dislocation in the eye.

Important Observation: Pleiotropy refers to one gene affecting multiple phenotypic traits, whereas Epistasis and Polygenic Additivity involve multiple genes influencing a single phenotypic trait.