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Multihybrid Crosses and Mendel’s Law of Independent Assortment

Introduction

Mendel’s experiments laid the foundation for modern genetics. While monohybrid and dihybrid crosses examine one or two traits respectively, multihybrid crosses study the inheritance of three or more traits simultaneously. These crosses follow Mendel’s Law of Independent Assortment, which explains how alleles for different genes segregate independently during gamete formation.

What Are Multihybrid Crosses?

A multihybrid cross involves organisms that are heterozygous for multiple traits. For example, a trihybrid cross studies three traits (e.g., seed shape, seed color, and plant height) simultaneously.

Notation Example (Trihybrid Cross)

  • Parent 1: AaBbCc

  • Parent 2: AaBbCc

    Where:

  • A/a = seed shape (round/wrinkled)

  • B/b = seed color (yellow/green)

  • C/c = plant height (tall/dwarf)

Law of Independent Assortment

Mendel’s law states:

“Alleles of different genes assort independently of one another during gamete formation.”

  • Each pair of alleles separates independently during meiosis.

  • This produces new combinations of traits in offspring.

Important Note:

  • This law applies only to genes located on different chromosomes or far apart on the same chromosome.

Gamete Formation in Multihybrid Crosses

  • Number of possible gametes = 2ⁿ (where n = number of heterozygous gene pairs)

  • Example: A trihybrid parent (AaBbCc) produces 2³ = 8 different gametes:

ABC, ABc, AbC, Abc, aBC, aBc, abC, abc

Punnett Square for Multihybrid Crosses

  • For a trihybrid cross, a 8×8 Punnett square can be used to predict all offspring genotypes.

  • The phenotypic ratio for a trihybrid cross (all traits showing complete dominance) is typically:

27:9:9:9:3:3:3:1

  • Each number corresponds to a unique combination of dominant and recessive traits.

Applications of Multihybrid Crosses

  1. Predicting offspring traits in plants and animals

  2. Understanding polygenic inheritance

  3. Studying genetic interactions and epistasis

  4. Breeding programs for multiple desirable traits

Example: Trihybrid Cross in Pea Plants

Traits:

  • Seed shape: Round (R) dominant over wrinkled (r)

  • Seed color: Yellow (Y) dominant over green (y)

  • Flower color: Purple (P) dominant over white (p)

Parent Cross: RrYyPp × RrYyPp

  • Number of gametes per parent: 2³ = 8

  • Offspring phenotypic ratio: 27:9:9:9:3:3:3:1

This demonstrates the law of independent assortment as all traits segregate independently.

Conclusion

Multihybrid crosses are essential tools in genetics for analyzing multiple traits simultaneously. Mendel’s Law of Independent Assortment explains how alleles of different genes combine in offspring, producing genetic diversity. Understanding multihybrid crosses is crucial for plant and animal breeding, genetic prediction, and studying polygenic traits.