Mendel and the Gene Idea: Principles of Inheritance and Human Genetics

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: He could strictly control mating between plants, ensuring reliable results.

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 allowed Mendel to control fertilization.

• P Generation: True-breeding parents.

• F1 Generation: First filial generation, all showed the dominant trait.

• F2 Generation: Produced by self-pollination or cross-pollination of F1 hybrids, revealed the recessive trait.

Mendel's Experimental, Quantitative Approach

Mendel tracked only characters that occurred in two distinct forms and used true-breeding varieties to ensure consistency.

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

The Law of Segregation

Mendel observed that traits could be hidden in one generation and reappear in the next, leading to the law of segregation.

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

• Recessive Trait: The trait that is masked in the F1 but reappears in the F2 generation (e.g., white flowers).

• 3:1 Ratio: In the F2 generation, the dominant to recessive trait ratio is approximately 3:1.

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

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

Mendel studied seven different characters, each with two contrasting traits. The F2 ratios consistently approximated 3:1 for dominant to recessive traits.

Character	Dominant Trait	Recessive Trait	F2 Ratio
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, based on four key concepts:

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

2. Inheritance: For each character, an organism inherits two alleles, one from each parent. These alleles may be identical (homozygous) or different (heterozygous).

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

4. Law of Segregation: The two alleles for a heritable character separate during gamete formation and end up in different gametes.

Mendel's Segregation Model

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

• Dominant allele: Represented by a capital letter (e.g., P).

• Recessive allele: Represented by a lowercase letter (e.g., p).

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 homozygous recessive individual.

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

The Law of Independent Assortment

Mendel's second law states that alleles of different genes assort independently during gamete formation.

• Monohybrid Cross: Involves a single character.

• Dihybrid Cross: Involves two characters; determines if traits are inherited together or independently.

• Independent Assortment: Applies to genes on different chromosomes or far apart on the same chromosome.

• Dependent Assortment: Genes close together on the same chromosome tend to be inherited together.

Comparing Laws

• Law of Segregation: Deals with a single trait; alleles separate into different gametes.

• Law of Independent Assortment: Applies to multiple traits; alleles for different genes sort independently.

Complex Forms of Inheritance

Inheritance patterns may deviate from simple Mendelian genetics in several ways:

• Alleles may not be completely dominant or recessive.

• Genes may have more than two alleles.

• A single gene may produce multiple phenotypes.

Degrees of Dominance

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

• Incomplete Dominance: F1 hybrids have a phenotype intermediate between the two parental varieties.

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

Multiple Alleles

Many genes exist in populations in more than two allelic forms. 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 a second locus. For example, in Labrador retrievers, coat color depends on two genes: one for pigment color and one for pigment deposition.

Polygenic Inheritance

Quantitative characters vary along a continuum and are usually governed by polygenic inheritance, where two or more genes have an additive effect on a single phenotype.

• Examples: Human height, skin color, eye color.

Comparing Pleiotropy and Polygenic Traits

• Pleiotropy: One gene affects many traits.

• Polygenic: Many genes affect one trait.

Multifactorial Traits

Some characters 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 Mendelian Traits

Many human traits follow Mendelian patterns, 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 useful for predicting future offspring and identifying carriers of genetic diseases.

Recessively Inherited Disorders

• Disorders only show up in individuals homozygous for the allele.

• Carriers are heterozygous and phenotypically normal but can transmit the allele.

Examples of Recessive Disorders

• Cystic Fibrosis: Defective chloride transport channels; symptoms include mucus buildup and abnormal nutrient absorption.

• Sickle-Cell Disease: Abnormal hemoglobin causes red blood cells to deform; symptoms include pain, organ damage, and resistance to malaria in heterozygotes.

• Albinism: Lack of pigment due to recessive alleles.

Dominantly Inherited Disorders

• Dominant alleles causing lethal diseases are rare and often result in death before reproductive age.

• Huntington's Disease: Degenerative nervous system disorder; symptoms appear after age 35-45.

Multifactorial Disorders

• Many diseases have both genetic and environmental components (e.g., heart disease, diabetes, cancer, alcoholism, mental illness).

• Lifestyle can significantly affect phenotype.

Genetic Counseling

Genetic counselors provide information to parents concerned about family history of specific diseases, helping them understand risks and inheritance patterns.

Key Equations

• Probability of genotype in F2 generation: $P( ext{genotype}) = ext{frequency of allele combinations}$

• Phenotypic ratio for monohybrid cross: $3:1$ (dominant:recessive)

• Genotypic ratio for monohybrid cross: $1:2:1$ (homozygous dominant : heterozygous : homozygous recessive)

Additional info: These notes expand on the original slides by providing definitions, examples, and context for each concept, ensuring a comprehensive study guide for exam preparation.