genetics

GENETICS & INHERITANCE

Paper 2 - 48 Marks

GREGOR MENDEL

FATHER OF GENETICS

  • Explained how genes are passed from parents to offspring.

  • Conducted experiments on the genetics of peas.

Table 2: Traits Compared by Mendel

  • Seed traits:
      - Size: Round vs. Wrinkled
      - Colour: Yellow vs. Green

  • Pod traits:
      - Shape: Full vs. Constricted
      - Colour: Green vs. Yellow

  • Flower traits:
      - Colour: Purple vs. White

  • Height traits:
      - Tall vs. Short

MENDEL'S EXPERIMENTS

  • Mendel observed two heights in garden peas: tall (T) and short (t).

  • Peas are self-pollinating; thus:
      - Tall peas produce tall peas.
      - Short peas produce short peas.

Genetic Crosses

  • First Cross: Tall (T) x Short (t)
      - F1 Generation: All offspring were tall peas.
      - F2 Generation: Interbreeding of F1 peas produced both tall and short peas in a 3:1 ratio (3 tall: 1 short).

MENDEL'S LAWS

  1. Mendel’s First Law: Law of Segregation
       - Each trait is controlled by two factors (alleles).
       - During gamete formation (meiosis), the two factors are separated.
       - A gamete contains one of the two alleles from each parent.

  2. Mendel’s Second Law: Law of Dominance
       - Alleles of a gene can be dominant or recessive.
       - If alleles are different (one dominant, one recessive), the phenotype displays the dominant trait.

  3. Mendel’s Third Law: Law of Independent Assortment
       - Characteristics sort independently during gamete formation due to random assortment at meiosis.
       - Alleles of different genes assort independently into gametes, resulting in various combinations.

ALLELES

  • Alleles occur in pairs:
      - HOMOZYGOUS: identical alleles.
      - HETEROZYGOUS: different alleles.

  • Represented using letters:
      - Capital letters for dominant alleles.
      - Small letters for recessive alleles.

  • Generations:
      - P: Parent generation,
      - F: Filial (offspring) generation;
      - F1: First filial generation,
      - F2: Second filial generation.

GENETIC CROSSES DIAGRAMS

Layout of a Genetic Diagram

  • P₁: Phenotype and Genotype (Parent generation)

  • F1: Resulting Generation (x) Meiosis (Gametes) (x) Fertilization

  • Visible trait: defined as phenotype (e.g., tallness).

  • Genetic makeup: defined as genotype (e.g., TT, Tt, or tt).

Punnett Square

  • Used to determine fertilization outcomes - displays possible genotype and phenotype ratios.

TYPES OF DOMINANCE IN A CROSS

  1. Complete Dominance
       - Dominant allele masks the expression of the recessive allele in heterozygous condition.
       - Example: Round seeds (R) dominate over wrinkled seeds (r).
       - Genetic cross between two heterozygous plants (F1 Generation):
         - Genotypes: Rr x Rr.
         - Phenotypic Ratio: 3 Round: 1 Wrinkled
         - Genotypic Ratio: 1 RR: 2 Rr: 1 rr.

  2. Incomplete Dominance
       - Neither allele is completely dominant; results in a mix of phenotypes.
       - Example: Red (RR) x White (WW) flowers gives all Pink (RW).

  3. Co-dominance
       - Both alleles are dominant and expressed in phenotype.
       - Example: In cattle, mixing red (RR) and white (WW) results in red with white patches (RW).

SEX DETERMINATION

  • Male Genotype: XY

  • Female Genotype: XX

  • F1 Generation: 50% Female (XX) and 50% Male (XY).

SEX-LINKED INHERITANCE

  • Refers to diseases linked to the X-chromosome.

  • Males (XY) at higher risk due to single X; females (XX) have lower risks as they can be carriers.

  • Diseases: Color-blindness and Hemophilia
      - Color-blindness: inability to distinguish colors (e.g., red-green) due to absence of specific photoreceptors.
      - Hemophilia: inability to clot blood due to lack of clotting factors.

Genotypes for Sex-Linkage

  • XHXH: Normal female

  • XBXB: Female (normal vision)

  • XHXh: Carrier female

  • XhXh: Hemophiliac female

  • XHY: Normal male

  • XhY: Hemophiliac male.

MULTIPLE ALLELES - BLOOD GROUPS

  • Four human blood types: A, B, AB, O.

  • Three alleles: IA, IB, i.
      - IA and IB are co-dominant; i is recessive.

  • Possible genotypes:
      - Blood Group A: IAIA, I^Ai
      - Blood Group B: IBIB, I^Bi
      - Blood Group AB: IAIB
      - Blood Group O: ii.

Blood Type Genetic Cross Example

  • Parents both with blood group B (genotype I^Bi):
      - Possible outcomes: 3 Blood group B: 1 Blood group O.

DIHYBRID CROSSES

  • Involves two characteristics.

  • Example with tall (T) vs short (t) and purple (P) vs white (p) flowers in pea plants.

  • Parental Genotype: TtPp x TtPp.
      - Gametes: TP, Tp, tP, tp.

  • Punnett Square outcomes:
      - Resulting phenotypic ratio: 9:3:3:1 (9 tall purple, 3 short purple, 3 tall white, 1 short white).

PEDIGREE DIAGRAMS

  • Used to track inheritance through generations.

  • Indicators: Squares for males and circles for females.

  • Steps for interpretation:
      - Identify dominant and recessive characteristics.
      - Record phenotypes, then work backward to determine genotypes.

MUTATIONS

  • Permanent change in DNA; can be harmful, harmless, or useful.
      - Useful Mutations: Lead to advantageous traits.
      - Harmful Mutations: Cause genetic disorders or physical changes.
      - Harmless Mutations: Occur in non-coding regions, often phenotypically neutral.

Types of Genetic Mutations

  • Examples include:
      - Sickle Cell Anemia: abnormal red blood cells.
      - Albinism: absence of melanin.

CHROMOSOMAL ABERRATIONS

  • Changes during anaphase could lead to conditions like Down Syndrome due to non-disjunction during meiosis.

BIOTECHNOLOGY AND GENETIC ENGINEERING

  • Utilizing organisms for human benefit through methods like:
      - DNA Profiling
      - Genetic Modification (GMO): Includes altering plant traits for better yield or resistance.

RECOMBINANT DNA TECHNOLOGY

  1. Isolate desired gene (e.g., insulin).

  2. Insert into plasmid DNA from bacteria.

  3. Culturing bacteria to produce desired proteins.

ETHICAL CONSIDERATIONS

  • Ethical dilemmas include safety of GMOs, potential for allergies, ecological impacts, and moral decisions regarding genetic manipulation.

STEM CELLS

  • Undifferentiated cells capable of becoming any cell type.

  • Applications include treating heart damage, skin regeneration, and nerve cell therapy.

CLONING

  • Creating genetically identical organisms through asexual reproduction techniques.

  • Notable examples: Dolly the sheep (1996), Futhi the cow (2003).

MITOCHONDRIAL DNA & GENERATIONAL TRACING

  • Contains 37 genes, mutates at a regular rate; important for understanding evolutionary relationships.

  • Inherited maternally; helps trace genetic ancestry.

Evolutionary Evidence in Hominins

  • Graphical presentation shows movements of early hominins and speciation such as Homo sapiens, Homo neanderthalensis, Homo erectus, revealing evolution and migration patterns.
     
    Figure Presentations:

  • Figures 12 & 13: Visual diagrams supporting genetic concepts.

  • Figure 16: Out of Africa hypothesis showing movement of early hominins.