Chapter 14: Mendel and the Gene

Overview of Mendelian Genetics

This chapter explores how Mendel's principles can predict patterns of inheritance, starting with his experimental system and expanding to the chromosome theory of inheritance and extensions to Mendel's principles. The study of inheritance is foundational to understanding genetics and heredity in biology.

Historical Context and Competing Hypotheses

Before Mendel, two main hypotheses attempted to explain heredity:

• Blending Inheritance: Parental traits blend so offspring have intermediate traits.

• Inheritance of Acquired Characteristics: Parental traits are modified through use and then passed on to offspring.

Examples of blending inheritance include flower color mixing, while inheritance of acquired characteristics is illustrated by the idea that giraffes' necks lengthen as they stretch for food and this trait is passed to their offspring.

Gregor Mendel: The Father of Genetics

Gregor Mendel was an Austrian monk who used quantitative analysis in plant breeding experiments to develop the concept of the gene. His work laid the foundation for the chromosome theory of inheritance.

• Mendel's experiments were conducted on garden peas (Pisum sativum).

• He wanted to understand heredity—the transmission of traits from parents to offspring.

• Traits are any observable characteristics of an individual.

Mendel's Model Organism: Garden Pea

Mendel chose garden peas for their ease of cultivation, short generation time, and ability to produce large numbers of seeds. Peas have many polymorphic traits, making them ideal for genetic studies.

• Peas can self-fertilize or be cross-fertilized.

• True-breeding lines exhibit the same trait generation after generation.

• Cross-fertilization allows for hybridization between different pure lines.

Traits of Pea Plants

Mendel worked with seven easily recognizable traits, each with two distinct phenotypes:

• Seed shape, seed color, pod shape, pod color, flower color, flower and pod position, stem length

• Phenotype: Observable features of an individual

• True-breeding lines: Produced offspring identical to themselves

• Hybrids: Offspring from crosses between pure lines

Mendel's Experiments with a Single Trait (Monohybrid Cross)

Mendel crossed pure lines differing in one trait (e.g., round vs. wrinkled seeds). The parental generation (P) produced the F1 generation, which all had round seeds. The F2 generation showed a 3:1 ratio of round to wrinkled seeds.

• Dominant trait: Displayed in F1 generation (round seeds)

• Recessive trait: Masked in F1, reappeared in F2 (wrinkled seeds)

Reciprocal Crosses

Mendel performed reciprocal crosses to test if sex influenced inheritance. The results were identical, showing that sex did not affect inheritance for the traits Mendel studied.

Dominant and Recessive Traits

For all seven traits, the F1 generation displayed a trait distinctly like one parent. Dominant traits are displayed in F1, while recessive traits are masked but reappear in F2.

Summary Table: F2 Phenotypes from Monohybrid Crosses

The table below summarizes the dominant and recessive phenotypes and their ratios observed in Mendel's monohybrid crosses.

Particulate Inheritance

Mendel's results were inconsistent with blended inheritance and acquired characteristics. He proposed the hypothesis of particulate inheritance: hereditary determinants act as discrete, unchanging particles.

• Hereditary determinants do not blend or change through use.

• They are inherited as distinct units (genes).

Genes, Alleles, and Genotypes

Genes are hereditary determinants for traits. Each individual has two versions of each gene, called alleles. The combination of alleles is the genotype, which affects the phenotype.

• Alleles: Different versions of a gene

• Genotype: Combination of alleles in an individual

• Phenotype: Observable trait

The Principle of Segregation

Mendel developed the principle of segregation to explain the 3:1 ratio in F2 individuals:

• Two members of each gene pair must segregate into different gamete cells during egg and sperm formation.

• Each gamete contains one copy of each gene (one allele).

Homozygotes and Heterozygotes

Homozygotes have two of the same allele (e.g., RR or rr). Heterozygotes have one dominant and one recessive allele (e.g., Rr). Crossing heterozygotes produces:

• Genotype: ¼ RR, ½ Rr, ¼ rr

• Phenotype: ¾ round, ¼ wrinkled

Summary Table: Mendel's Model for Monohybrid Crosses

Punnett Squares

The Punnett square is a diagram used to predict the results of Mendelian crosses:

1. Write each unique gamete genotype for one parent along the top.

2. Write each unique gamete genotype for the other parent down the side.

3. Fill in each box with the gamete genotypes above and to the left.

4. Calculate proportions or ratios of each offspring genotype and phenotype.

Mendel's Experiments with Two Traits (Dihybrid Cross)

Mendel used dihybrid crosses to determine whether alleles of different genes segregate together or independently. His results supported the principle of independent assortment:

• Alleles of different genes are transmitted independently of one another (usually).

• Dihybrid cross produces four possible phenotypes in a 9:3:3:1 ratio.

Testcrosses

A testcross involves crossing a parent with a dominant phenotype but unknown genotype with a parent that contributes only recessive alleles. Analyzing offspring phenotypes reveals the unknown genotype.

Chromosome Theory of Inheritance

The chromosome theory of inheritance arose from observations of meiosis:

• Genes are located on chromosomes at specific loci.

• Physical separation of alleles during meiosis I explains Mendel's principle of segregation.

• Genes for different traits assort independently if they are on different chromosomes or far apart on the same chromosome.

Extensions and Exceptions to Mendel's Principles

While Mendel's work is foundational, many variations exist:

• Sex-linked traits

• Incomplete dominance

• Multiple allele traits

• Polygenic traits (controlled by many genes)

Key Terms in Mendelian Genetics

Summary

Mendel's principles of inheritance—segregation and independent assortment—form the basis of classical genetics. These principles explain how traits are transmitted from parents to offspring and how genetic variation arises. Extensions to Mendel's work, including sex-linked inheritance, incomplete dominance, and polygenic traits, further enrich our understanding of heredity.