Chapter 11: Mendel and the Gene Idea – Study Notes

Chapter 11: Mendel and the Gene Idea

11.1 Mendel's Scientific Approach to Inheritance

Gregor Mendel, an Austrian monk, discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. His work established the foundation for modern genetics.

• Model Organism: Mendel chose peas due to their many varieties with distinct features (characters) and traits (e.g., purple or white flowers).

• Controlled Mating: Mendel could strictly control which plants mated, ensuring reliable results.

Crossing Pea Plants

Mendel's experimental technique involved crossing true-breeding pea plants and observing the inheritance of traits.

• Removed stamens from a purple flower and transferred pollen from a white flower.

• Hybridization produced F1 offspring (first filial generation).

• Self-pollination or cross-pollination of F1 hybrids produced F2 generation.

Mendel’s Experimental, Quantitative Approach

Mendel tracked only characters that occurred in two distinct alternative forms (e.g., white or purple flower) and used true-breeding varieties.

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

The Law of Segregation

Mendel observed that traits such as flower color segregate during gamete formation, resulting in a 3:1 ratio in the F2 generation.

• Dominant trait: Purple flower

• Recessive trait: White flower

• Heritable factor is now called a gene.

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

Mendel studied seven characters, each with two forms. The F2 ratios for dominant to recessive traits were approximately 3:1.

Character	Dominant Trait	Recessive Trait	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 explained the 3:1 inheritance pattern with four related concepts:

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

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

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

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

Mendel's Segregation Model

The segregation model accounts for the 3:1 ratio observed in the F2 generation. Punnett squares can be 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, yellow).

• 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 breeding it with a recessive homozygote.

• Can reveal whether the organism is homozygous dominant or heterozygous.a

The Law of Independent Assortment

Alleles of different genes assort independently during gamete formation, producing new combinations of traits.

• Identified by following two characters at the same time via a dihybrid cross.

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

• Genes close together on the same chromosome tend to be inherited together (dependent assortment).

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 may deviate from simple Mendelian patterns in several situations:

• Alleles are not completely dominant or recessive.

• Gene has two or more alleles.

• Single gene produces multiple phenotypes.

Degrees of Dominance

• Complete dominance: Heterozygote and dominant homozygote are indistinguishable.

• 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

Most genes exist in populations in more than two allelic forms. Example: ABO blood group in humans.

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 one gene affects multiple phenotypic traits. Examples include cystic fibrosis and sickle-cell disease.

Epistasis

Epistasis is when a gene at one locus alters the phenotypic expression of a gene at a second locus. Example: Labrador retriever coat color depends on two genes.

• One gene determines pigment color (B for black, b for brown).

• Another gene determines pigment deposition (E for color, e for no color).

Polygenic Inheritance

Quantitative characters vary along a continuum and are usually controlled by multiple genes (polygenic inheritance).

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

Comparing Pleiotropy to Polygenic Inheritance

• Pleiotropy: One gene affects many traits.

• Polygenic: Many genes affect one trait.

Multifactorial Traits

Some characters are influenced by both genetic and environmental factors. Example: hydrangea flower color varies with 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 and ethical concerns.

Pedigree Analysis

Pedigrees are family trees that track the inheritance of traits across generations. They help predict future offspring traits, especially for disabling or deadly diseases.

Recessively Inherited Disorders

• Thousands of genetic disorders are inherited as simple recessive traits.

• Disorders only show up in individuals homozygous for the allele; carriers are heterozygous and phenotypically normal.

The Behavior of Recessive Alleles

• If a recessive allele causing disease is rare, the chance of two carriers mating is low.

• Consanguineous matings increase the chance of two carriers mating.

Examples of Recessive Disorders

• Cystic Fibrosis: Most common lethal genetic disease in the US; defective chloride transport channels cause symptoms.

• Sickle-Cell Disease: Caused by a single amino acid substitution in hemoglobin; leads to sickle-shaped red blood cells and various symptoms.

• Albinism: Lack of pigment due to recessive alleles.

Dominantly Inherited Disorders

• Dominant alleles causing lethal diseases are rare.

• Huntington’s Disease: Degenerative nervous system disorder; symptoms appear after reproductive age.

Multifactorial Disorders

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

• Lifestyle significantly affects phenotype, especially for cardiovascular health.

Genetic Counseling

Genetic counselors provide information to parents concerned about family history for specific diseases, using Mendelian genetics as a foundation.