Unit 2: Genetics and Evolution

Chapter 1: Principles of Inheritance and Variation

Mendel’s Laws of Inheritance

• Law of Dominance: In a heterozygote, one allele (dominant) masks the other (recessive).
• Law of Segregation: Alleles separate during gamete formation.
• Law of Independent Assortment: Alleles for different traits assort independently.

Key Genetic Terms

• Gene: Unit of inheritance.
• Allele: Alternative forms of a gene.
• Homozygous: Identical alleles (TT, tt).
• Heterozygous: Different alleles (Tt).
• Genotype: Genetic makeup (e.g., Tt).
• Phenotype: Observable traits (e.g., Tall).

Deviations from Mendelian Inheritance

• Incomplete Dominance: Blended phenotype (e.g., Pink flowers from Red x White).
• Codominance: Both alleles are fully expressed (e.g., AB blood group).
• Multiple Alleles: More than two alleles for a gene (e.g., ABO blood groups).
• Polygenic Inheritance: Trait controlled by multiple genes (e.g., skin color).
• Pleiotropy: One gene affects multiple traits (e.g., PKU).

Chromosomal Theory & Sex Determination

• Chromosomal Theory: Genes are located on chromosomes.
• Sex Determination:
  • Humans (XY): Male XY, Female XX.
  • Birds (ZW): Male ZZ, Female ZW.
  • Honeybee (Haplodiploidy): Males haploid, Females diploid.

Linkage and Crossing Over

• Linkage: Genes on the same chromosome are inherited together.
• Crossing Over: Exchange of genetic material between homologous chromosomes, leading to recombination.

Mutation

• Definition: A sudden, heritable change in the DNA sequence.
• Types:
  • Point Mutations: Change in a single base pair (e.g., Sickle-cell anemia).
  • Frameshift Mutations: Insertions or deletions that alter the reading frame.

Sex-linked Inheritance

• Inheritance of genes on sex chromosomes.
• Examples in Humans:
  • Haemophilia: X-linked recessive, impaired blood clotting.
  • Colour Blindness: X-linked recessive, inability to distinguish certain colors.

Mendelian Disorders

• Caused by a mutation in a single gene.
• Examples:
  • Thalassemia: Autosomal recessive, reduced hemoglobin.
  • Sickle-cell Anaemia: Autosomal recessive, abnormal red blood cells.
  • Phenylketonuria (PKU): Autosomal recessive, metabolic disorder.

Chromosomal Disorders

• Caused by an abnormal number or arrangement of chromosomes.
• Examples:
  • Down’s Syndrome (Trisomy 21): Extra chromosome 21.
  • Turner’s Syndrome (XO): Missing an X chromosome in females.
  • Klinefelter’s Syndrome (XXY): Extra X chromosome in males.