Laws of Inheritance: Key Concepts from Biology 2e

Learning Objectives

• Understand Mendel's laws of segregation and independent assortment.
• Use methods to predict genotypes and phenotypes.
• Explain linkage and recombination effects on gamete genotypes.
• Discuss epistatic effects between genes.

Mendel’s Postulates

1. Pairs of Unit Factors (Genes):

   • Hereditary factors are discrete units inherited from each parent.
   • Contradicted the blending inheritance theory of the time.

2. Dominance:

   • In heterozygotes, the dominant allele masks the recessive one.
   • Recessive traits appear only when two copies of the recessive allele are present.
   • Not all traits follow simple dominance; other patterns exist.

3. Law of Segregation:

   • Genes segregate equally into gametes; offspring have an equal chance of inheriting either allele.
   • Basis for the Punnett square predictions.
   • Physically grounded in the first meiotic division.

4. Independent Assortment:

   • Genes sort independently into gametes; each combination is equally likely.
   • Illustrated in a dihybrid cross (e.g., seed color and texture).
   • Independent events can be analyzed using the product rule.

Probability and Forked-Line Methods

• Forked-Line Method:

  • Useful for analyzing crosses involving multiple genes.
  • Segregates alleles for each gene independently.
  • Uses product rule to determine phenotype probabilities.

• Probability Method:

  • Calculates expected phenotypic/genotypic proportions without visual aids.
  • Often more efficient than a Punnett square for complex crosses.

Rules for Multihybrid Fertilization

• General rules for determining gamete and phenotype numbers based on heterozygous gene pairs.
• Table 12.5 outlines specific rules for multihybrid crosses.

Linkage and Recombination

• Linkage:

  • Genes close to each other on a chromosome tend to be inherited together.
  • Crossover (recombination) can separate linked genes, allowing independent assortment.

• Gene Mapping:

  • Recombinant frequency used to map gene positions on chromosomes.

Epistasis

• Gene Interactions:

  • Epistasis occurs when one gene’s expression masks or modifies another’s.
  • Examples include pigment production in mice and fruit color in squash.

• Types of Epistasis:

  • Dominant epistasis: A dominant allele masks another gene’s effect.
  • Reciprocal epistasis: Either gene in dominant form can express the same phenotype.

Scientific Method Connection

• Testing Hypotheses:
  • Independent assortment tested through controlled breeding experiments.
  • Mendel’s experiments involved massive sample sizes to learn about expected ratios.

Conclusion

• Understanding inheritance through Mendel's laws provides a foundation for more complex genetic concepts.
• Continued study of gene interactions (like epistasis) reveals the layered complexity of genetic inheritance.