Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of heredity. His quantitative approach and careful selection of plant varieties allowed him to deduce laws that explain how traits are inherited from one generation to the next.

Mendel’s Experimental Approach

Mendel used pea plants for their distinct traits, short generation time, and ability to control mating. He tracked characters with two alternative forms and started with true-breeding varieties.

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• True-breeding: Plants that produce offspring of the same variety when self-pollinated.

• Hybridization: Mating two contrasting, true-breeding varieties.

The Law of Segregation

Mendel’s first law states that two alleles for a heritable character segregate during gamete formation and end up in different gametes. This explains the 3:1 ratio observed in the F2 generation of his crosses.

• Dominant trait: The trait that appears in the F1 hybrids (e.g., purple flowers).

• Recessive trait: The trait that reappears in the F2 generation (e.g., white flowers).

• Gene: Mendel’s “heritable factor,” now known as a gene.

Mendel’s Model of Inheritance

Mendel proposed four concepts to explain inheritance patterns:

1. Alleles: Alternative versions of genes account for variations in inherited characters.

2. Inheritance: Each organism inherits two alleles, one from each parent.

3. Dominance: If alleles differ, the dominant allele determines appearance; the recessive allele is masked.

4. Segregation: The two alleles segregate during gamete formation.

Genetic Vocabulary

• Homozygote: An organism with two identical alleles for a gene.

• Heterozygote: An organism with two different alleles for a gene.

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

The Testcross

A testcross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the parent is heterozygous.

The Law of Independent Assortment

Mendel’s second law states that each pair of alleles segregates independently during gamete formation. This law applies to genes on different chromosomes or far apart on the same chromosome.

• Monohybrid cross: Cross between heterozygotes for one character.

• Dihybrid cross: Cross between heterozygotes for two characters.

Probability Laws in Genetics

Mendelian inheritance follows the rules of probability. The multiplication rule calculates the probability of independent events occurring together, while the addition rule calculates the probability of mutually exclusive events.

• Multiplication rule:

• Addition rule:

Extensions of Mendelian Genetics

Inheritance patterns can be more complex than simple Mendelian genetics. These include incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance.

• Incomplete dominance: Heterozygotes have intermediate phenotypes.

• Codominance: Both alleles are expressed in the phenotype.

• Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood groups).

• Pleiotropy: One gene affects multiple phenotypic characters.

Epistasis and Polygenic Inheritance

Epistasis occurs when one gene affects the expression of another gene. Polygenic inheritance involves multiple genes contributing to a single trait, resulting in quantitative variation.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors as well as genotype. Phenotypic plasticity describes the ability to produce different phenotypes under varying conditions. Multifactorial traits depend on both genetic and environmental influences.

Human Mendelian Genetics

Many human traits follow Mendelian patterns. Pedigree analysis is used to study inheritance in families. Recessive and dominant disorders, such as albinism, cystic fibrosis, sickle-cell disease, and achondroplasia, illustrate Mendelian inheritance in humans.

Genetic Testing and Counseling

Genetic counselors provide information to prospective parents about genetic risks. Testing can reveal genetic disorders in fetuses and newborns, aiding in early intervention and informed decision-making.

Summary Tables

The following tables summarize relationships among alleles and genes: