10.1 Meiosis

• Fates of Meiosis

  • Meiosis results in the formation of four haploid cells. Haploid cells have half the chromosome number of the parent cell.

  • Crossing over during prophase I leads to the exchange of genetic material. An independent assortment of chromosomes during metaphase I increase genetic diversity.

  • Two homologous chromosomes separate during anaphase I. Each resulting haploid cell possesses a unique combination of genetic material.

  • Random orientation of chromosomes on the equatorial plate during metaphase I. Leads to an independent assortment of alleles and diverse genetic combinations in gametes.

• Replication of Chromosomes and Centromere Joining

  • A process where DNA duplicates itself during the interphase of the cell cycle before meiosis or mitosis.

  • Ensures each daughter cell receives a complete set of genetic information.

  • After replication, two identical copies of a chromosome (sister chromatids) are joined at the centromere.

  • The centromere is a specific region where microtubules attach during cell division, ensuring proper chromosome segregation.

• Formation of Bivalent Chromosome

  • Occurs during prophase I of meiosis.

  • Involves pairing of homologous chromosomes, one from each parent.

  • Two homologous chromosomes align side by side, forming a structure known as a bivalent.

  • Each bivalent consists of four chromatids (two from each homologous chromosome).

  • Random orientation of bivalents on the equatorial plate during metaphase I leads to diverse genetic combinations in gametes.

• Crossing Over and Chiasma Formation

  • Crossing over during prophase I.

  • Enzyme-mediated process with recombinase.

  • Chiasma formation is the point of adherence between non-sister chromatids during meiotic crossing over.

    • It facilitates the efficient exchange of genetic material, enhancing genetic diversity by creating new combinations of alleles.

• Law of Independent Assortment

  • States that the separation of one pair of alleles during the formation of gametes is independent of the separation of another pair of alleles.

  • Introduced by Gregor Mendel based on his studies of genetic relationships, specifically observed during meiosis.

  • Meiosis ensures random orientation and independent assortment.
    

10.2 Inheritance

• Dihybrid Cross

  • Involves the simultaneous consideration of two different traits or gene pairs in a genetic cross.

  • Gregor Mendel conducted the first dihybrid cross using traits such as seed shape and seed colour in pea plants.

  • Each trait is governed by a pair of alleles, and the cross-examines the inheritance patterns of both traits simultaneously.

  • The law of Segregation applies independently to each gene pair in a dihybrid cross.

  • The resulting phenotypic ratio can be determined based on the combination of alleles inherited for both traits.

• Linkage

  • Occurs when two genes are located on the same chromosome and tend to be inherited together.

  • Linked genes defy the law of independent assortment.

  • Autosomal linkage refers to linked genes on non-sex chromosomes, while sex linkage involves genes on the sex chromosomes (X or Y).

  • Drosophila melanogaster Example:

    • It was studied extensively by Thomas Hunt Morgan in the early 20th century.

    • Identified linked genes for body colour (grey or black) and wing length (long or short) in fruit flies.

    • Alleles for body color: 'G' for grey and 'g' for black. Alleles for wing length: 'L' for long and 'l' for short.

    • Combinations like GGLL and GgLl represent grey-bodied and long-winged flies, while ggll represents black-bodied and short-winged flies.

• Types of Variation

  • Continuous and Discrete Variation

    • Continuous: Spectrum of phenotypes.

    • Discontinuous: Few phenotypes, two or more forms.

    • Polygenic inheritance involving multiple genes.

• Chi Squared Tests

  • Purpose and Operation

    • Used to check accuracy of ratios from crosses.

    • Compares observed and expected values.

    • Hypotheses: Ho (traits assort independently) and H1 (traits do not assort independently).

    • Degree of freedom and critical values.
      

10.3 Gene Pools and Speciation

• Gene Pool

  • The total collection of genes and their alleles present in a population at a particular time.

  • Represents the genetic diversity within a population.

  • Changes over time due to factors such as mutation, migration, natural selection, and genetic drift.

  • The gene pool is a dynamic entity that influences the evolution of a population.

• Hardy-Weinberg Equation

  • Equation for calculating allele frequency.

  • (𝑝 + 𝑞)² = 1 for diploid organisms.

• Types of Selection Equilibrium

  • Directional Selection:

    • Favors one extreme phenotype over the other.

    • Leads to a shift in the population towards the favored phenotype.

    • Example: Larger beaks in birds becoming more prevalent during a food scarcity period.

  • Stabilizing Selection:

    • Favors the intermediate phenotype over the extremes.

    • Reduces genetic diversity by favoring the average trait.

    • Example: Intermediate birth weight being advantageous for human survival.

  • Disruptive Selection:

    • Favors both extreme phenotypes over the intermediate.

    • Can lead to the formation of distinct subpopulations.

    • Example: Birds with either small or large beaks having a survival advantage, with intermediate beaks at a disadvantage.

  • Balancing Selection:

    • Maintains genetic diversity by favoring multiple alleles in a population.

    • Can include heterozygote advantage and frequency-dependent selection.

    • Example: Sickle cell anemia providing resistance to malaria in heterozygous individuals, maintaining both alleles in the population.

• Reproductive Isolations

  • Geographical isolations.

    • Geographical isolation is a reproductive barrier that occurs when physical barriers, such as mountains, rivers, oceans, or other landforms, prevent gene flow between populations of a species.

    • This isolation leads to the separation of populations, limiting or preventing the exchange of genetic material, and can contribute to the divergence of species over time.

  • Temporal isolation.

    • Temporal isolation is a reproductive barrier that occurs when two or more species are capable of interbreeding but have differences in their reproductive timing, such as mating seasons, daily activity patterns, or flowering periods.

    • This temporal misalignment prevents the individuals of different populations from encountering each other for mating, reducing the likelihood of successful reproduction between them.

    • As a result, temporal isolation contributes to the development of distinct species by preventing gene flow during specific time intervals.

  • Behavioral isolation.

    • Behavioral isolation is a reproductive barrier that arises due to differences in the behaviors, courtship rituals, or communication signals of individuals from different populations within the same species.

    • Individuals with distinct behavioral patterns may fail to recognize or respond to the mating cues or signals of individuals from other populations, hindering successful mating.

    • This isolation mechanism plays a crucial role in maintaining species integrity by preventing interbreeding between populations with incompatible behaviors, ultimately contributing to the formation of distinct species.

  • Polyploidy as a cause.

  • Speciation is a result of reproductive isolations.

• Rate of Speciation

  • Gradualism: Slow, continuous changes.

  • Punctuated equilibrium: Quick changes, minimal subsequent changes.