14.4 Segregation

14.4 Segregation

Mendel's Discoveries

  • F1 Progeny: The first generation offspring from a cross between plants with different traits.

    • Did not breed true, showing mixed traits.

    • F2 generation produced distinct ratios due to self-fertilization.

    • Reappearance of recessive traits in a definite numerical proportion.

  • Key Ratios Observed:

    • Dominant to recessive trait ratio close to 3:1.

    • Overall ratio from Mendel's experiments: 14,949 to 50010 (approximately 2.98:1, close to 3:1).

Experiment Observations

  • Seed Color Trait:

    • Yellow (dominant) vs. Green (recessive): Ratio 6,022 (yellow) to 2,001 (green).

  • Allele Representation:

    • Yellow seeds designated as AA (homozygous), green seeds as aa (homozygous).

  • Each true breeding strain in the P1 generation has identical alleles.

Principles of Segregation

  • Each plant cell contains 2 alleles, one inherited from each parent for each gene.

  • Alleles separate equally into gametes (principle of segregation).

  • Homozygous Plants:

    • AA provides all gametes with A allele.

    • aa gives all gametes with a allele.

Formation of Gametes and Zygotes

  • The zygote is formed by a random union of gametes:

    • F1 progeny: AA x aa = 100% Aa (yellow seed phenotype - dominant).

  • F1 gametes separate into A and a during gamete formation.

Using Punnett Squares

  • Punnett Square: A tool for predicting genotype combinations in offspring.

    • Arrangements of gametes show all possible genotypes.

    • Expected ratio: 1/4 AA, 1/2 Aa, 1/4 aa.

    • Resulting phenotype ratio reflects dominance: 3 (dominant) to 1 (recessive).

Hypothesis Testing

  • Test Cross: Cross between an unknown genotype and homozygous recessive.

    • Helps distinguish between homozygous dominant (AA) and heterozygous (Aa).

    • Expected outcomes depend on the unknown parent's genotype.

Segregation in Meiotic Process

  • Mendel's principles correlate with meiosis:

    • Homologous chromosomes align and separate during anaphase I.

    • Segregation of alleles corresponds to the separation of chromosomes.

Exceptions to Simple Dominance

  • Variations such as incomplete dominance and co-dominance are observed:

    • Incomplete Dominance: Heterozygous phenotype is intermediate.

      • Example: Snapdragon flower colors.

        • Genotype ratios: 1 red, 2 pink, 1 white phenotypes (1:2:1).

    • Co-dominance: Both alleles expressed.

      • Example: ABO blood types.

        • Genotypes: 2A (type A), 2B (type B), 1A1B (type AB).

Probability and Genetic Predictions

  • Probability Theory in genetics:

    • Values range from 0 (impossible) to 1 (certain).

    • Addition Rule: For mutually exclusive outcomes; sum probabilities.

    • Multiplication Rule: For independent events; product of probabilities.

Application of Probability in Traits

  • Estimations for multiple offspring using formulas:

    • Example: Probability of a certain phenotype occurring in a specified number of offspring derived from Mendelian ratios.

Statistical Representation in Genetic Studies

  • Larger sample sizes yield results more aligned with predicted ratios.

  • Overall Mendelian ratios provide foundational concepts for understanding inheritance patterns and trait distributions in various organisms.