Chapter 22: Variation and Genetics

Gene: Basic unit of biological information. Smallest part of DNA. Located at specific position on chromosome called Locus.

Alleles: Partners of a gene pair. Alternate forms of a gene. Homozygous (identical alleles, e.g., RR, rr) and Heterozygous (different alleles, e.g., Rr).

Gene Pool: All the genes/alleles found in a breeding population at a given time.

Law of Segregation

Co-existing alleles for each trait isolate/segregate from each other during gamete formation so that each gamete receives only one of the two alleles. Monohybrid cross (3:1 ratio in F2).

Law of Independent Assortment

When two contrasting pairs of traits are followed in the same cross, their alleles assort independently into gametes. Dihybrid cross (9:3:3:1 ratio in F2).

• Complete Dominance: One allele completely masks the other (e.g., Red vs White in pea flowers, Rr is Red).
• Incomplete Dominance: Heterozygote has intermediate phenotype (e.g., 4 O'Clock plant: Red x White = Pink). Ratio 1:2:1.
• Codominance: Both alleles express independently (e.g., MN Blood group).
• Over Dominance: Heterozygote exceeds phenotypic expression of both homozygotes (e.g., Drosophila eye pigments).

ABO System: Controlled by gene 'I' with alleles IA, IB, and i. IA and IB are codominant, i is recessive.

• Type A: IAIA or IAi (Antigen A, Anti-B antibodies)
• Type B: IBIB or IBi (Antigen B, Anti-A antibodies)
• Type AB: IAIB (Antigens A & B, No antibodies) - Universal Recipient
• Type O: ii (No antigens, Anti-A & Anti-B antibodies) - Universal Donor

Epistasis (Bombay Phenotype): Gene H regulates attachment of antigens. hh genotype prevents antigen attachment, making phenotype O even if IA/IB alleles are present.

Rh Factor: Antigen discovered in Rhesus monkey. Controlled by genes C, D, E. D is dominant (Rh+). dd is Rh-.

Erythroblastosis Foetalis

Maternal-foetal Rh incompatibility. Rh- mother carries Rh+ foetus. Mother produces Anti-Rh antibodies which cross placenta and destroy foetal RBCs causing severe anemia and jaundice.

Traits controlled by multiple genes (Polygenes) showing continuous variation.

• Wheat Grain Colour: Controlled by 3 gene pairs (A, B, C). Range from White (aabbcc) to Dark Red (AABBCC).
• Human Height & Skin Colour: Show continuous variation, influenced by environment.

Linkage: Genes on the same chromosome tend to stay together. Do not assort independently.

Crossing Over: Exchange of segments between non-sister chromatids of homologous chromosomes during Prophase I of Meiosis. Produces recombinants.

• XY-XX System: Humans, Drosophila. Male XY (Heterogametic), Female XX (Homogametic). SRY gene on Y determines maleness in humans.
• XO-XX System: Grasshopper. Male XO, Female XX.
• ZW-ZZ System: Birds, Butterflies. Male ZZ (Homogametic), Female ZW (Heterogametic).

Traits determined by genes on sex chromosomes.

X-Linked Recessive Traits

• Drosophila Eye Colour: White eye mutant discovered by T.H. Morgan. Zig-zag inheritance (Mother to Son).
• Hemophilia: Blood clotting failure. Types A, B (X-linked) and C (Autosomal). Queen Victoria was a carrier.
• Color Blindness: Inability to distinguish Red/Green. Dichromacy, Monochromacy.

Other Patterns

• Sex Limited Traits: Expressed in only one sex (e.g., Milk production, Beard growth).
• Sex Influenced Traits: Dominant in one sex, recessive in other (e.g., Pattern Baldness).

Hereditary disorder of carbohydrate metabolism.

• Type I (IDDM): Juvenile, Autoimmune destruction of Beta cells. Insulin dependent.
• Type II (NIDDM): Adult onset, Insulin resistance. Multifactorial (Genes + Environment/Obesity).
• MODY: Maturity Onset Diabetes of the Young (Autosomal dominant).