Genetics Syllabus - Topic Breakdown

1. Mendelian Genetics; Monohybrid, Dihybrid, and Trihybrid Cross | Complete, Incomplete, and Codominance

• Monohybrid cross: A cross between two individuals differing in ONE trait (e.g., Tt x Tt). Produces a 3:1 phenotypic ratio in F2.
• Dihybrid cross: A cross involving TWO traits (e.g., TtRr x TtRr). Produces a 9:3:3:1 ratio in F2.
• Trihybrid cross: THREE traits considered simultaneously.
• Complete dominance: One allele fully masks the other (e.g., T over t - tall over short).
• Incomplete dominance: The heterozygote shows an intermediate phenotype (e.g., red x white = pink).
• Codominance: Both alleles are expressed simultaneously (e.g., AB blood type - both A and B antigens present).

2. Nonallelic Gene Interaction and Its Inheritance Characteristics

• Epistasis: One gene masks or modifies the expression of another gene at a different locus.
• Types: Dominant epistasis, recessive epistasis, duplicate dominant, complementary, etc.
• Example: Coat color in Labrador dogs involves two genes - the E gene can suppress B gene expression entirely.
• These interactions produce modified Mendelian ratios (e.g., 9:7, 12:3:1, 15:1 instead of 9:3:3:1).

3. Linkage and Crossing Over. Main Genetical Basis of Recombination

• Linkage: Genes located on the same chromosome tend to be inherited together (violating Mendel's law of independent assortment). Named "linked genes."
• Crossing over (recombination): During meiosis I, homologous chromosomes exchange segments at points called chiasmata. This shuffles alleles and produces new combinations (recombinants).
• Genetic basis of recombination: The physical exchange of DNA segments between non-sister chromatids of homologous chromosomes.
• Recombination frequency is used to map genes - genes far apart recombine more frequently than genes close together.

4. Sex Genetics. Sex Linkage. X and Y Chromosome Linkage

• Sex determination: In humans, females are XX and males are XY. The Y chromosome carries the SRY gene which triggers male development.
• Sex-linked inheritance: Genes located on sex chromosomes follow different inheritance patterns.
• X-linked traits: Genes on the X chromosome. Since males have only one X, they express even recessive X-linked traits (e.g., hemophilia, color blindness).
• Y-linked (holandric) traits: Genes on the Y chromosome pass exclusively from father to all sons (e.g., some male fertility genes).
• X-linked dominant vs recessive patterns differ in carrier females vs affected males.

5. Changeability (Variability). Main Types of Variability. Inherited Variability - Mutation

• Non-hereditary (phenotypic/modification) variability: Changes due to environment; not passed to offspring.
• Hereditary (genotypic) variability: Changes in genetic material that ARE passed to offspring. Two types:
  • Combinative variability: New combinations of existing alleles through sexual reproduction (crossing over, independent assortment, fertilization).
  • Mutation: A sudden, heritable change in the DNA sequence.
    • Gene (point) mutations: Change in single nucleotide (substitution, insertion, deletion).
    • Chromosomal mutations: Structural changes - deletion, duplication, inversion, translocation.
    • Genomic mutations: Change in chromosome number - aneuploidy (e.g., trisomy 21 = Down syndrome), polyploidy.