Mendel and the Gene Idea: Foundations of Inheritance

Chapter 11: Mendel and the Gene Idea

11.1 Mendel's Scientific Approach to Inheritance

Gregor Mendel, an Austrian monk, established the basic principles of heredity by conducting controlled breeding experiments with garden peas. His work led to the identification of two fundamental laws of inheritance.

• Model Organism: Mendel chose peas due to their many varieties with distinct features (characters) and easily observable traits (such as flower color).

• Experimental Control: Peas allowed strict control over mating, enabling Mendel to track inheritance patterns.

Crossing Pea Plants

Mendel's experiments involved crossing true-breeding pea plants and observing the resulting generations.

• Hybridization: Removal of stamens from one flower and transfer of pollen from another.

• P Generation: True-breeding parents.

• F1 Generation: Offspring from the cross, all showing the dominant trait.

• F2 Generation: Produced by self-pollination or cross-pollination of F1 hybrids, revealing both dominant and recessive traits.

Mendel's Experimental, Quantitative Approach

Mendel focused on characters with two distinct forms and used true-breeding varieties to ensure consistency in results.

• True-breeding: Plants that produce offspring identical to themselves when self-pollinated.

The Law of Segregation

This law states that two alleles for a heritable character separate during gamete formation and end up in different gametes.

• Dominant and Recessive Traits: Dominant traits mask recessive traits in the F1 generation; recessive traits reappear in the F2 generation.

• 3:1 Ratio: Mendel observed a 3:1 ratio of dominant to recessive phenotypes in the F2 generation.

• Gene: Mendel's "heritable factor" is now known as a gene.

Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

Mendel tracked seven characters, each with two forms, and consistently observed dominant and recessive traits.

Character	Dominant Trait	Recessive Trait	Ratio (F2)
Flower color	Purple	White	3:1
Seed color	Yellow	Green	3:1
Seed shape	Round	Wrinkled	3:1
Pod color	Green	Yellow	3:1
Pod shape	Inflated	Constricted	3:1
Flower position	Axial	Terminal	3:1
Stem length	Tall	Dwarf	3:1

Mendel's Model of Inheritance

Mendel proposed a model to explain the observed inheritance patterns, introducing the concept of alleles and their behavior during reproduction.

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

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

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

4. Law of Segregation: Alleles separate during gamete formation, so each gamete carries only one allele for each gene.

Mendel's Segregation Model

The segregation model explains the 3:1 ratio in the F2 generation using Punnett squares to predict genetic outcomes.

• Notation: Capital letters represent dominant alleles; lowercase letters represent recessive alleles.

Genetic Vocabulary

• Homozygous: Organism with two identical alleles for a character (can be dominant or recessive).

• Heterozygous: Organism with two different alleles for a gene; not true-breeding.

• Phenotype: Physical appearance (e.g., purple or yellow flowers).

• Genotype: Genetic makeup (e.g., PP, Pp, pp).

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a recessive homozygote.

• Application: Reveals whether the dominant phenotype is homozygous or heterozygous.

The Law of Independent Assortment

This law states that alleles of different genes assort independently during gamete formation, producing new combinations of traits.

• Monohybrid Cross: Follows a single character.

• Dihybrid Cross: Follows two characters simultaneously, revealing independent assortment.

• Chromosomal Basis: Applies to genes on different chromosomes or far apart on the same chromosome.

Comparing Segregation and Independent Assortment

• Law of Segregation: Deals with one gene's alleles splitting into different gametes.

• Law of Independent Assortment: Deals with how alleles of different genes mix and match independently.

Complex Forms of Inheritance

Inheritance patterns may deviate from Mendelian genetics in certain cases:

• Alleles are not completely dominant or recessive.

• Genes have more than two alleles.

• Single genes produce multiple phenotypes.

Degrees of Dominance

• Complete Dominance: Heterozygote and dominant homozygote are indistinguishable in phenotype.

• Incomplete Dominance: Heterozygote phenotype is intermediate between the two parental varieties.

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways.

Multiple Alleles

Some genes exist in more than two allelic forms within a population. The ABO blood group in humans is a classic example.

Allele	Carbohydrate
IA	A
IB	B
i	none

Genotype	Phenotype (Blood Group)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

Pleiotropy

Pleiotropy occurs when a single gene affects multiple phenotypic traits. Many hereditary diseases, such as cystic fibrosis and sickle-cell disease, are caused by pleiotropic alleles.

Epistasis

Epistasis is when a gene at one locus alters the phenotypic expression of a gene at another locus. For example, in Labrador retrievers, coat color depends on two genes: one for pigment color and another for pigment deposition.

Polygenic Inheritance

Polygenic inheritance involves the additive effect of two or more genes on a single phenotype, resulting in continuous variation. Examples include human height, skin color, and eye color.

Comparing Pleiotropy and Polygenic Traits

• Pleiotropy: One gene affects many traits.

• Polygenic: Many genes affect one trait.

Multifactorial Inheritance

Some traits are influenced by both genetic and environmental factors. For example, hydrangea flower color is affected by soil pH.

• Acidic soil: blue flowers

• Alkaline soil: pink flowers

• Neutral soil: purple flowers

Human Traits and Mendelian Patterns

Many human traits follow Mendelian inheritance, but humans are not ideal subjects for genetic research due to long generation times, few offspring, and ethical concerns.

Pedigree Analysis

Pedigrees are family trees that track the inheritance of traits across generations. They are used to predict future offspring and assess the risk of genetic disorders.

Recessively Inherited Disorders

Genetic disorders inherited as recessive traits only manifest in individuals homozygous for the allele. Carriers are heterozygous and phenotypically normal.

The Behavior of Recessive Alleles

• Rare recessive alleles have a low chance of meeting in a population.

• Consanguineous matings increase the risk of recessive disorders.

Examples of Recessive Disorders

• Cystic Fibrosis: Defective chloride transport channels cause mucus buildup and nutrient absorption issues.

• Sickle-Cell Disease: Abnormal hemoglobin causes red blood cells to deform, leading to various health problems. Heterozygotes are less susceptible to malaria.

• Albinism: Lack of pigment due to recessive alleles.

Dominantly Inherited Disorders

• Dominant alleles causing lethal diseases are rare and often result in early death.

• Huntington's Disease: Degenerative nervous system disorder with late onset, caused by a dominant allele.

Multifactorial Disorders

Many diseases have both genetic and environmental components, such as heart disease, diabetes, cancer, alcoholism, and mental illness. Lifestyle choices significantly affect phenotypic outcomes.

Genetic Counseling

Genetic counselors use Mendelian genetics to provide information to parents concerned about hereditary diseases.

Additional info: For further review, see the recommended Amoeba Sisters video on Mendelian genetics.