Mendelian Genetics and Extensions: Study Notes for General Biology

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Mendelian Genetics

Introduction to Genetics

Genetics is the study of heredity and variation in organisms. Gregor Mendel, often called the "father of genetics," performed pioneering experiments with pea plants that established the basic principles of inheritance.

Key Terms: Gene (heritable unit), Allele (variant form of a gene), Genotype (genetic makeup), Phenotype (observable traits).
Model Organisms: Pea plants, fruit flies, mice, and others are commonly used in genetic studies due to their short generation times and ease of breeding.

Gregor Mendel portrait Common model organisms in genetics

Mendel’s Experiments with Pea Plants

Mendel chose pea plants for their distinct traits and ability to self- or cross-pollinate. He tracked inheritance patterns by controlling fertilization and observing offspring.

Advantages of Pea Plants: Many varieties, true-breeding strains, controlled mating, easy cross-pollination, many offspring, short generation time.
Experimental Technique: Mendel crossed plants with different traits and observed the resulting generations.

Mendel's cross-pollination technique with pea plants

Mendel’s Model of Inheritance

Mendel proposed that invisible "factors" (now known as genes) determine traits. Each organism inherits two alleles for each gene, one from each parent. The dominant allele masks the effect of the recessive allele.

Law of Segregation: The two alleles for a trait separate during gamete formation (meiosis).
Law of Independent Assortment: Alleles for different genes assort independently during gamete formation.

Mendel's pea plant generations and trait ratios Punnett square for Mendelian inheritance

Punnett Squares and Genetic Ratios

Punnett squares are used to predict the probability of offspring genotypes and phenotypes from parental crosses.

Monohybrid Cross: Cross between parents differing at one gene locus. Typical F2 ratio is 3:1 for dominant:recessive phenotype.
Genotype Ratio: F2 generation shows a 1:2:1 ratio (homozygous dominant : heterozygous : homozygous recessive).

Punnett square for F2 generation

Phenotype vs. Genotype

The phenotype is the observable trait, while the genotype is the genetic makeup. Homozygous individuals have two identical alleles; heterozygous individuals have two different alleles.

Example: In pea plants, purple flowers (P) are dominant to white (p). Genotypes PP and Pp produce purple flowers; pp produces white.

Phenotype and genotype ratios in pea plants

Law of Independent Assortment

This law states that alleles of different genes are distributed independently during gamete formation. Dihybrid crosses illustrate this principle, producing a 9:3:3:1 phenotypic ratio in the F2 generation.

Example: Crossing plants with two traits (seed color and shape) demonstrates independent assortment.

Dihybrid cross and independent assortment

Extensions of Mendelian Genetics

Many traits do not follow simple Mendelian patterns. Variations include degrees of dominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance.

Complete Dominance: Heterozygote phenotype is identical to homozygous dominant.
Incomplete Dominance: Heterozygote phenotype is intermediate between homozygous dominant and recessive.
Codominance: Both alleles are fully expressed in the heterozygote.
Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood group).
Pleiotropy: One gene affects multiple traits (e.g., sickle-cell disease).
Epistasis: One gene affects the expression of another gene.
Polygenic Inheritance: Multiple genes contribute to a single trait (e.g., skin color).

Degrees of dominance: dominant, recessive, codominant, incomplete dominance Polydactyly, a dominant trait ABO blood group alleles and phenotypes Pleiotropy: single gene affects multiple traits Sickle cell disease: normal and sickled red blood cells Epistasis: Labrador retriever coat color Polygenic inheritance: skin color Hydrangea flower color affected by environment

Human Genetics

Some human traits follow Mendelian rules, but humans are not ideal for classic genetic experiments due to ethical and practical reasons. Most human genetic data comes from pedigree analysis and correlational studies.

Pedigree Analysis: Used to track inheritance of traits in families.
Genetic Counseling: Helps individuals and couples understand risks of genetic disorders.
Example: Widow’s peak and albinism are traits studied in human genetics.

Summary Table: Relationships Among Alleles and Genes

The following table summarizes key relationships among alleles and genes, with examples:

Relationship	Description	Example
Complete dominance	Heterozygous phenotype same as homozygous dominant	PP, Pp (pea plant flower color)
Incomplete dominance	Heterozygous phenotype intermediate	CRCR, CRCW, CWCW (snapdragon flower color)
Codominance	Both phenotypes expressed in heterozygotes	IAIB (ABO blood group)
Multiple alleles	More than two alleles for a gene	IA, IB, i (ABO blood group)
Pleiotropy	One gene affects multiple traits	Sickle-cell disease
Epistasis	One gene affects expression of another	Labrador retriever coat color
Polygenic inheritance	Multiple genes affect one trait	Skin color

Key Equations

Probability of genotype in monohybrid cross: for each homozygote, for heterozygote
Phenotypic ratio in dihybrid cross:

Nature vs. Nurture

Phenotype is shaped by both genetics and environment. For example, hydrangea flower color depends on soil pH, illustrating gene-environment interaction.

Hydrangea flower color affected by environment

Additional info: Academic context was added to clarify definitions, examples, and relationships among genetic concepts. The notes are structured to provide a comprehensive overview suitable for exam preparation in General Biology.