Genetics (Inheritance & Variation)

3.1 Chromosomes and Mechanism of Inheritance

• Heredity (inheritance) is the transmission of genetic information from parents to offspring.

• Gregor Mendel used hybridisation on pea plants to propose the mechanism of inheritance before genes and chromosomes were discovered.

• He studied 7 traits in pea plants (e.g. seed shape, colour, etc.), observing them individually and in combinations.

• Mendel proposed that "factors" (now known as genes) are responsible for traits and occur in pairs. They segregate during gamete formation without blending.

• His success was due to large sample size, precise planning, and mathematical analysis.

• The concept of dominance and recessiveness emerged from his work.

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3.2 Genetic Terminology

• Character: A specific feature, e.g. plant height.

• Trait: A variant of a character (e.g. tall or dwarf).

• Gene/Factor: Unit of heredity.

• Alleles: Alternative forms of a gene.

• Dominant/Recessive: Alleles that express in F1 generation or remain hidden.

• Phenotype: Observable traits.

• Genotype: Genetic constitution (TT, Tt, tt).

• Homozygous: Identical alleles (TT or tt).

• Heterozygous: Different alleles (Tt).

• Pure line: Homozygous population.

• Monohybrid: Heterozygous for one trait.

• F1/F2 Generation: First and second offspring generations.

• Punnett Square: Used to predict results of crosses.

• Homologous chromosomes: Similar chromosomes paired in meiosis.

• Back cross/Test cross: Crossing F1 with parent to test genotype.

• Phenotypic/Genotypic ratio: Physical vs genetic outcome ratios.

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3.3 Mendel’s Laws of Inheritance

1. Law of Dominance: One allele masks the other in F1 generation.

2. Law of Segregation: Alleles separate during gamete formation; no blending occurs.

3. Law of Independent Assortment: Genes on separate chromosomes are inherited independently (proved by dihybrid cross, 9:3:3:1 ratio).

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3.4 Back Cross and Test Cross

• Back cross: F1 crossed with either parent.

• Test cross: F1 crossed with recessive parent to determine genotype.

• If offspring are mixed, F1 is heterozygous; if all are same, it's homozygous.

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3.5 Deviations from Mendel’s Findings

• Real-life genetics revealed deviations known as Neo-Mendelism:

  • Incomplete Dominance: Both alleles partially express, giving intermediate phenotype (e.g. pink flowers in Mirabilis jalapa).

  • Co-dominance: Both alleles express equally (e.g. red + white = roan coat in cattle).

  • Multiple Alleles: More than 2 forms of a gene exist (e.g. blood groups).

  • Pleiotropy: One gene controls multiple traits (e.g. sickle cell anaemia).

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3.6 Chromosomal Theory of Inheritance

• Mendel’s ideas were confirmed in 1900s after chromosomes were discovered.

• Sutton and Boveri proposed that chromosomes carry genes and obey Mendel’s laws.

• During meiosis, chromosomes segregate and assort independently.

• Chromosomes restore diploid state during fertilisation.

• This theory explained the physical basis of heredity.

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3.7 Chromosomes

• Thread-like bodies in nuclei, made of DNA and proteins.

• Visible during metaphase of cell division.

• Ploidy: Refers to number of chromosome sets (haploid = n, diploid = 2n, etc.).

• Aneuploidy: Addition or loss of specific chromosomes (e.g. trisomy).

• Structure: Two chromatids, centromere, telomeres, sometimes satellite body.

• Types based on centromere: Metacentric, Submetacentric, Acrocentric, Telocentric.

• Sex chromosomes: X and Y; X is larger and gene-rich.

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3.8 Linkage and Crossing Over

• Linkage: Genes on the same chromosome are inherited together.

• Complete linkage: No crossing over (e.g. Drosophila males).

• Incomplete linkage: Some recombination due to crossing over.

• Sex linkage: Genes on sex chromosomes (X or Y).

  • X-linked traits: Colour blindness, haemophilia.

  • Y-linked traits: Hair on ear pinna (holandric traits).

• Crossing over: Exchange of segments during meiosis creates new gene combinations.

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3.9 Autosomal Inheritance

• Traits on autosomes (non-sex chromosomes) inherited equally in both sexes.

• Dominant autosomal traits: Widow’s peak.

• Recessive autosomal traits: PKU, cystic fibrosis, sickle cell anaemia.

• PKU: Absence of enzyme to break down phenylalanine, causes mental retardation.

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3.10 Sex-linked Inheritance

• Traits inherited through sex chromosomes.

• X-linked: Expressed more in males (XY) as they have no second X to mask recessive gene.

• Examples:

  • Colour blindness: Cannot distinguish red-green.

  • Haemophilia: Blood doesn’t clot properly.

• Criss-cross inheritance: Trait passes from father to grandson via daughter (carrier female).

• Y-linked: Traits pass directly from father to son (e.g. ear hair).

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3.11 Sex Determination

• Humans: XX (female) and XY (male); father decides sex of child.

• Birds: ZW (female), ZZ (male); mother decides sex.

• Honeybee: Haplodiploidy; fertilised eggs → females, unfertilised eggs → males.

• Environmental sex determination: e.g. Bonellia viridis develops sex based on surroundings.

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3.12 Genetic Disorders

Two types:

1. Mendelian Disorders: Single gene mutations

   • Sickle-cell anaemia: Abnormal RBC shape due to faulty haemoglobin gene.

   • Thalassemia: Defective synthesis of haemoglobin chains.

   • PKU, Colour blindness, Haemophilia – already explained.

2. Chromosomal Disorders:

   • Down’s Syndrome: Extra 21st chromosome; mental retardation, unique facial features.

   • Turner’s Syndrome (XO): Female with one X chromosome, underdeveloped features.

   • Klinefelter’s Syndrome (XXY): Male with feminine traits and infertility.

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