Tuesday, 25 March 2025

Unit 7 – Genetics and Evolution

 1. Heredity and Variation

Mendelian Inheritance

  • Gregor Mendel is known as the father of genetics for his work on pea plants.
  • Principles of Inheritance:
    • Law of Segregation: Alleles segregate during gamete formation.
    • Law of Independent Assortment: Genes for different traits assort independently.

Deviations from Mendelism

  1. Incomplete Dominance: The phenotype of heterozygotes is intermediate between the phenotypes of homozygotes (e.g., red and white flowers producing pink flowers).
  2. Co-dominance: Both alleles in a heterozygote are fully expressed (e.g., AB blood type).
  3. Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood groups).
  4. Pleiotropy: A single gene affects multiple traits (e.g., sickle cell anemia affecting blood, pain, and immunity).
  5. Polygenic Inheritance: Multiple genes influence a trait, resulting in a continuous range of phenotypes (e.g., skin color).

Chromosome Theory of Inheritance

  • Genes are located on chromosomes, which segregate and assort independently during meiosis.

Sex Determination

  • In humans, sex is determined by the presence of XY chromosomes (males) or XX chromosomes (females).
  • In birds, the ZW system determines sex (ZW females, ZZ males).
  • In honey bees, sex is determined by the number of chromosome sets (diploid females, haploid males).

Linkage and Crossing Over

  • Genes located close together on the same chromosome tend to be inherited together (linkage).
  • Crossing over during meiosis can result in genetic recombination.

Sex-linked Inheritance

  • Traits linked to sex chromosomes; examples include:
    • Haemophilia: A blood clotting disorder linked to the X chromosome.
    • Color Blindness: A genetic condition affecting color perception, also X-linked.

Mendelian Disorders in Humans

  1. Thalassemia: A blood disorder caused by reduced hemoglobin production.
  2. Chromosomal Disorders:
    • Down’s Syndrome: Caused by trisomy of chromosome 21.
    • Turner’s Syndrome: Affects females with a missing X chromosome (45,X).
    • Klinefelter’s Syndrome: Affects males with an extra X chromosome (47,XXY).

2. Molecular Basis of Inheritance

Search for Genetic Material

  • Early experiments indicated that DNA is the genetic material, confirmed by studies such as those by Avery, MacLeod, and McCarty.

Structure of DNA and RNA

  • DNA is a double helix composed of nucleotides (adenine, thymine, cytosine, guanine), while RNA is single-stranded and contains uracil instead of thymine.

DNA Packaging

  • DNA wraps around histone proteins to form nucleosomes, which further coil to form chromatin.

DNA Replication

  • Semi-conservative process where each strand serves as a template for a new complementary strand.

Central Dogma

  • Describes the flow of genetic information from DNA to RNA to protein:
    • Transcription: DNA → mRNA
    • Translation: mRNA → Protein

Genetic Code

  • The sequence of nucleotides in mRNA is translated into amino acids using codons.

Gene Expression and Regulation

  • The Lac Operon in E. coli serves as a model for understanding gene regulation in prokaryotes.

Genome and Human Genome Project

  • The complete set of genetic material in an organism; the Human Genome Project aimed to map all human genes.

DNA Fingerprinting

  • A technique used to identify individuals based on unique patterns in their DNA.

3. Evolution

Origin of Life

  • Theories about how life began on Earth include abiogenesis and panspermia.

Biological Evolution

  • Change in the genetic composition of populations over generations.

Evidence for Biological Evolution:

  1. Paleontology: Fossils show changes over time.
  2. Comparative Anatomy: Homologous structures indicate common ancestry.
  3. Embryology: Similar embryonic development suggests evolutionary relationships.
  4. Molecular Evidence: Genetic similarities among species indicate evolutionary links.

Darwin’s Contribution

  • Proposed natural selection as the mechanism for evolution; survival of the fittest leads to adaptation over time.

Modern Synthetic Theory of Evolution

  • Integrates Darwin's theory with genetics; emphasizes mutation and gene flow as sources of variation.

Mechanisms of Evolution

  1. Variation (Mutation and Recombination): Genetic diversity arises from mutations and sexual reproduction.
  2. Natural Selection: Favorable traits increase an organism's chances of survival and reproduction.
    • Types:
      • Stabilizing Selection: Favors average traits.
      • Directional Selection: Favors one extreme phenotype.
      • Disruptive Selection: Favors both extreme phenotypes.
  3. Gene Flow: Movement of genes between populations through migration.
  4. Genetic Drift: Random changes in allele frequencies due to chance events.

Hardy-Weinberg Principle

  • Describes the conditions under which allele frequencies remain constant in a population (no evolution occurs).

Adaptive Radiation

  • The diversification of a group of organisms into forms filling different ecological niches.

Human Evolution

  • Study of how humans evolved from ancestral primates through various stages, including bipedalism and increased brain size.

Key Diagrams

  1. Structure of DNA showing nucleotide components.
  2. Diagram illustrating Mendelian inheritance patterns.
  3. Phylogenetic tree showing evolutionary relationships among species.

Summary

This unit covers fundamental concepts in genetics and evolution:

  1. Heredity involves Mendelian principles and deviations such as co-dominance and polygenic inheritance.
  2. Molecular genetics explains DNA structure, replication, transcription, translation, and gene regulation mechanisms.
  3. Evolutionary biology discusses evidence supporting evolution, mechanisms like natural selection, genetic drift, and human evolution processes.

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