Principles of Inheritance and Variation

Key Concepts

Introduction to Genetics

• Genetics: The branch of biology dealing with inheritance and variation.
• Inheritance: The process by which characters are passed from parent to progeny.
• Variation: The degree by which progeny differ from their parents.

Mendel’s Laws of Inheritance

Gregor Mendel conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856-1863).

• True-breeding Line: A line that shows stable trait inheritance and expression for several generations through continuous self-pollination.
• Monohybrid Cross: A cross involving one pair of contrasting traits (e.g., Tall vs. Dwarf).
  • F1 Generation: All offspring show the dominant trait.
  • F2 Generation: Phenotypic ratio is 3:1 (Dominant:Recessive); Genotypic ratio is 1:2:1 (TT:Tt:tt).
• Mendel’s Principles:
  1. Law of Dominance: Characters are controlled by discrete units called factors (genes). In a dissimilar pair, one factor dominates the other.
  2. Law of Segregation: Alleles do not show blending; they segregate during gamete formation so that each gamete receives only one allele.
  3. Law of Independent Assortment: In a dihybrid cross, the segregation of one pair of characters is independent of the other pair. Phenotypic ratio: 9:3:3:1.

Exceptions to Mendelian Principles

• Incomplete Dominance: The F1 phenotype is an intermediate between the two parents (e.g., Pink flowers in Snapdragon - Antirrhinum sp.).
• Co-dominance: Both alleles express themselves equally in the F1 generation (e.g., ABO blood grouping in humans).
• Multiple Alleles: More than two alleles govern the same character (e.g., Three alleles IA, IB, and i for ABO blood groups).

Chromosomal Theory of Inheritance

Proposed by Walter Sutton and Theodore Boveri (1902).

• Argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors (genes) they carried.
• Experimental verification was provided by T.H. Morgan using Drosophila melanogaster (fruit fly).

Linkage and Recombination

• Linkage: Physical association of genes on a chromosome. Tightly linked genes show very low recombination.
• Recombination: Generation of non-parental gene combinations due to crossing over.
• Genetic Mapping: Alfred Sturtevant used recombination frequencies to map the positions of genes on a chromosome.

Polygenic Inheritance and Pleiotropy

• Polygenic Inheritance: Traits controlled by three or more genes (e.g., human skin colour, height). The effect is additive.
• Pleiotropy: A single gene exhibits multiple phenotypic expressions (e.g., Phenylketonuria).

Sex Determination

• Male Heterogamety: Males produce two types of gametes.
  • XO Type: e.g., Grasshopper (Males have one X, Females have XX).
  • XY Type: e.g., Humans, Drosophila (Males have XY, Females have XX).
• Female Heterogamety: Females produce two types of gametes.
  • ZW Type: e.g., Birds (Females have ZW, Males have ZZ).
• Haplodiploid System: e.g., Honey bees. Females are diploid (32 chromosomes), males (drones) are haploid (16 chromosomes) and develop via parthenogenesis.

Mutation

Alteration of DNA sequences leading to changes in genotype and phenotype.

• Point Mutation: Change in a single base pair (e.g., Sickle-cell anaemia).
• Frame-shift Mutation: Deletions or insertions of base pairs.

Genetic Disorders

Categorized into Mendelian and Chromosomal disorders.

• Mendelian Disorders: Determined by alteration in a single gene.
  • Colour Blindness: Sex-linked recessive.
  • Haemophilia: Sex-linked recessive (blood clotting protein affected).
  • Sickle-cell Anaemia: Autosomal recessive. Substitution of Glutamic acid by Valine at the 6th position of the beta-globin chain.
  • Phenylketonuria: Autosomal recessive inborn error of metabolism.
  • Thalassemia: Autosomal recessive blood disease (reduced synthesis of globin chains).
• Chromosomal Disorders: Caused by absence, excess, or abnormal arrangement of chromosomes.
  • Aneuploidy: Gain or loss of a chromosome (e.g., Trisomy, Monosomy).
  • Down’s Syndrome: Trisomy of chromosome 21.
  • Klinefelter’s Syndrome: Extra X chromosome in males (47, XXY).
  • Turner’s Syndrome: Absence of one X chromosome in females (45, XO).