Fix the mistake: Reproduction and Heredity

Mendel's Laws: Dominance, Segregation, and Independent Assortment

Gregor Mendel discovered the basic laws of inheritance through crossbreeding experiments with pea plants. The law of dominance, law of segregation, and law of independent assortment are the starting points of all genetics. Together with modern molecular biological knowledge, let us understand the meaning and limits of application of these laws.

Basic Knowledge

Reasons Mendel succeeded: ① He began experiments with pure lines (homozygotes) ② He selected seven traits with clear contrasting characteristics ③ He statistically analyzed a large number of individuals ④ He used systematic experimental designs, including dihybrid crosses.

Law of dominance: When individuals from pure lines with different traits are crossed, all F₁ offspring show the dominant trait. The phenomenon in which offspring showing a recessive trait appear even when both parents show the dominant trait is called recessiveness. Dominant and recessive do not mean “strong” and “weak”; they describe whether a trait appears in the phenotype.

Law of segregation: The two alleles at one gene locus separate during gamete formation and enter different gametes (based on the separation of homologous chromosomes during meiosis). F₁ × F₁ (self-pollination) → in F₂, dominant:recessive = 3:1.

Law of independent assortment: Genes on different chromosomes are distributed independently during gamete formation. In a dihybrid cross F₁ × F₁ → F₂, the ratio is 9:3:3:1. Note that the law of independent assortment holds only between genes that are not linked.

Example Problem
When yellow (Y) is dominant over green (y) and round (R) is dominant over wrinkled (r) in peas, determine the phenotypic ratio of F₂ produced by self-pollinating F₁ (YyRr).
Answer: Based on the law of independent assortment, consider each gene independently. Yellow:green = 3:1, and round:wrinkled = 3:1. Combining them gives yellow round:yellow wrinkled:green round:green wrinkled = 9:3:3:1. Check: 9/16 are yellow round, 3/16 are yellow wrinkled, 3/16 are green round, and 1/16 are green wrinkled.
Key Points
  • Dominance = a trait expressed in a heterozygote; it does not indicate strength
  • Law of segregation = alleles separate into different gametes during meiosis
  • F₂ ratios = one factor: 3:1 / two factors: 9:3:3:1
  • Independent assortment = genes on different chromosomes are distributed independently
  • Homozygote (AA・aa) / heterozygote (Aa)
  • Mendel's pea plants = pure lines, clear contrasting traits, and statistical analysis
  • The law of independent assortment holds only for unlinked genes

Points to Note

① Dominant and recessive (dominant and recessive traits) do not indicate the “value” or “strength” of a trait. ② The 3:1 ratio results from a cross between F₁ individuals (F₁ × F₁), not from a cross between pure lines. ③ The 9:3:3:1 ratio holds only when two pairs of genes assort independently (are unlinked).

Practice

  1. Explain Mendel's law of segregation in connection with the process of meiosis.
  2. When an AaBb individual is self-pollinated, determine the proportion that is aabb (recessive homozygote).
  3. What conditions are required for the law of independent assortment to hold?

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