Mendel and the Gene Idea: Principles of Inheritance

Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of heredity, which are central to modern genetics. His scientific approach led to the identification of two key laws of inheritance: the law of segregation and the law of independent assortment.

Mendel’s Experimental Approach

Mendel used garden peas for his experiments due to their variety, short generation time, and ability to control mating. He focused on characters (heritable features) and traits (variants of characters), such as flower color.

• 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.

• P generation: True-breeding parents.

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Offspring from self- or cross-pollination of F1 hybrids.

The Law of Segregation

Mendel observed that traits do not blend but are inherited as discrete units. When crossing true-breeding purple and white flowers, all F1 hybrids were purple, but the F2 generation showed a 3:1 ratio of purple to white flowers.

• Dominant trait: Trait that appears in the F1 generation (purple).

• Recessive trait: Trait that reappears in the F2 generation (white).

• Gene: The basic unit of heredity, what Mendel called a "heritable factor."

Mendel’s Model of Inheritance

Mendel proposed a model to explain the 3:1 ratio observed in the F2 generation, consisting of four concepts:

1. Alleles: Alternative versions of a gene account for variations in inherited characters. Each gene resides at a specific locus on a chromosome.

2. Inheritance of alleles: An organism inherits two alleles, one from each parent. Alleles may be identical (homozygous) or different (heterozygous).

3. Dominance: If alleles differ, the dominant allele determines appearance; the recessive allele has no noticeable effect.

4. Law of segregation: The two alleles for a character segregate during gamete formation, ending up in different gametes.

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 is used to determine the genotype of an individual with a dominant phenotype by breeding it with a homozygous recessive individual. If any offspring display the recessive phenotype, the mystery parent is heterozygous.

The Law of Independent Assortment

Mendel’s second law states that each pair of alleles segregates independently during gamete formation. This applies to genes on different, nonhomologous chromosomes or those 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: $P(A \text{ and } B) = P(A) \times P(B)$

• Addition rule: $P(A \text{ or } B) = P(A) + P(B)$

Complex Patterns of Inheritance

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

Degrees of Dominance

• Complete dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete dominance: Heterozygote phenotype is intermediate between parental phenotypes.

• Codominance: Both alleles are fully expressed in the heterozygote.

Multiple Alleles

Most genes exist in populations in more than two allelic forms. The ABO blood group in humans is determined by three alleles: IA, IB, and i.

Pleiotropy

Pleiotropy occurs when a single gene affects multiple traits. Examples include Marfan syndrome, cystic fibrosis, and sickle-cell disease.

Epistasis

Epistasis is when the expression of one gene affects the expression of another gene. In Labrador retrievers, coat color is determined by two genes, resulting in modified phenotypic ratios.

Polygenic Inheritance

Polygenic inheritance involves multiple genes independently affecting a single trait, leading to quantitative characters that vary along a continuum. Examples include height and skin pigmentation.

Nature and Nurture: Environmental Impact on Phenotype

Phenotype can be influenced by both genotype and environment. Multifactorial traits depend on multiple genes and environmental factors, resulting in a broad phenotypic range.

A Mendelian View of Heredity and Variation

An organism’s phenotype reflects its genotype and environmental history, encompassing physical appearance, internal anatomy, physiology, and behavior.

Term	Definition	Example
Allele	Alternative version of a gene	Purple vs. white flower color
Homozygous	Two identical alleles	PP or pp
Heterozygous	Two different alleles	Pp
Phenotype	Physical appearance	Purple flowers
Genotype	Genetic makeup	PP, Pp, or pp