Mendel's Principles of Inheritance Part 3

Dihybrid Cross and Mendel's Principles of Inheritance

Introduction

  • The lecture covers:

    • Dihybrid cross

    • Mendel's principle of independent assortment

  • This is part three of the Mendelian principles of inheritance recorded lectures.

Comparison of Mendel's Key Principles

Mendel's First Law: Law of Segregation

  • Each organism has two alleles for a trait.

  • During meiosis, these alleles separate when gametes are formed.

  • Each gamete takes only one allele.

  • This occurs during anaphase I when homologous chromosomes (each containing two sister chromatids) split and move to opposite daughter cells.

Mendel's Second Law: Law of Independent Assortment

  • When considering more than one gene, the chromosomes carrying those genes align randomly at the metaphase plate.

  • The separation of one pair of chromosomes does not affect another pair.

  • Genes located on different loci assort independently.

  • Example:

    • Eye color (brown on one locus of chromosome 22) and hair color (on chromosome 3).

    • Independent assortment means these genes are randomly segregated into gametes.

  • Key Difference:

    • Segregation relates to the splitting of two alleles for the same gene.

    • Independent Assortment involves how different genes on different chromosomes shuffle independently.

Meiosis Overview

Stages of Meiosis

  1. Meiosis I: Homologous chromosomes separate.

    • Metaphase I: Homologous pairs line up in the center with bivalents (paired homologs). Orientation is random.

    • Anaphase I: Homologous chromosomes separate; sister chromatids remain intact.

  2. Meiosis II: Sister chromatids separate.

    • Final product is four gametes.

    • Without crossing over, there is no variation in gametes, which will always reflect parental allele combinations.

    • In scenarios without crossing over, gametes are identical to parental contributions (no new allele combinations).

Crossing Over in Meiosis

  • During Prophase I, homologous chromosomes pair and exchange genetic segments via chiasmata.

  • Example without crossing over:

    • Starting with two pairs of homologous chromosomes (e.g., capital A's, capital B's; lowercase a's, lowercase b's).

    • Resulting in identical gametes after meiosis.

  • Example with crossing over:

    • New combinations emerge (e.g., capital B switches with lowercase b).

    • Resulting in diverse gametes and increased genetic variation.

  • Without crossing over: only parental gametes.

  • With crossing over: results in recombinant gametes—key for genetic diversity.

Monohybrid Cross Recap

  • Monohybrid Cross of true-bred homozygous round seeds and homozygous wrinkled seeds yields:

    • Parental generation (P): Round (capital R) vs. Wrinkled (lowercase r)

    • F1 Generation: All offspring appear round (capital R, lowercase r) due to the dominance of capital R.

    • Self-Fertilization of F1 plants: Results in variation in the F2 generation.

Punnett Square Results for Monohybrid Cross

  • Genotype Ratio: 1 (homozygous round) : 2 (heterozygous round) : 1 (homozygous wrinkled).

  • Phenotype Ratio: 3 (round) : 1 (wrinkled)

  • Mendel's Conclusion: Traits do not blend; recessive traits can reappear in later generations.

Dihybrid Cross Introduction

  • Focus shifts from a single trait to examining two traits simultaneously, considering independent assortment.

  • Example traits:

    • Hair color on chromosome 2 and eye color on chromosome 10.

  • Traits assort independently.

Dihybrid Cross Methodology

  • Start with true breeding plants for both traits (homozygous):

    • Example: Wrinkled green seeds (lowercase r's, lowercase y's) crossed with round yellow seeds (capital R's, capital Y's).

  • All F1 plants are heterozygous (capital R, lowercase r, capital Y, lowercase y).

F2 Generation Outcomes

  • F2 generation results from crossing F1 heterozygotes.

  • Uses a 16-box Punnett square to map potential combinations, leading to a 9:3:3:1 phenotypic ratio in the F2 generation.

  • Breakdown of traits:

    • Round yellow (dominant) vs. wrinkled green (recessive).

    • Verify results through counting appearances of each phenotype.

Probability in Dihybrid Crosses

  • Probability of obtaining specific traits is determined using multiplication rules due to independent assortment.

  • Each trait can be calculated separately and combined.

Example Calculations

  1. Probability of Round (R): 3 out of 4 (from shape trait).

  2. Probability of Yellow (Y): 3 out of 4 (from color trait).

  3. Combined Probability for Round Yellow:
    P(Round ext{ and } Yellow) = P(Round) imes P(Yellow) = (3/4) imes (3/4) = 9/16

  4. Other combinations can be derived similarly, leading to the complete phenotype ratios.

  5. Understanding genotype vs. phenotype probabilities is critical in determining outcomes in the offspring.

Continuation of Calculation Strategies

  • Probability rules (additive and multiplicative) are fundamental in predicting outcomes of genetic crosses.

  • Understanding overlapping categories (similar phenotypes) requires summing the probabilities of individual events where needed.

Conclusion

  • Mendel's principles of inheritance (Law of Segregation and Law of Independent Assortment) provide a foundational understanding of genetic variation in offspring.

  • Dihybrid crosses exemplify these principles in practice, leading to greater genetic diversity in sexually reproducing organisms.