Mendel and the Gene Idea: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

This chapter explores the foundational principles of inheritance as discovered by Gregor Mendel. It covers key terminology, Mendel's laws, genetic crosses, and exceptions to classic Mendelian patterns.

Key Terms and Concepts

• Hybridization: Mating or crossing of two true-breeding varieties.

• P Generation: Parental generation in a genetic cross.

• F1 Generation: First filial generation, offspring of the P generation.

• F2 Generation: Second filial generation, offspring of the F1 generation.

• Dominant Allele: Determines the organism's appearance when present.

• Recessive Allele: Has no noticeable effect on appearance unless homozygous.

• Genotype: Genetic makeup of an organism (e.g., AA, Aa, aa).

• Phenotype: Observable traits of an organism.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

Mendel’s Laws of Inheritance

Law of Segregation

States that two alleles for a heritable character segregate during gamete formation and end up in different gametes.

• Each gamete receives only one allele from each gene pair.

• Explains the 3:1 ratio observed in F2 generation of monohybrid crosses.

Law of Independent Assortment

Each pair of alleles segregates independently of other pairs during gamete formation.

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

• Explains the 9:3:3:1 ratio in dihybrid crosses.

Genetic Crosses and Probability

• Punnett Square: Diagram used to predict the outcome of a genetic cross.

• Testcross: Breeding an organism of unknown genotype with a homozygous recessive individual to determine genotype.

• Monohybrid Cross: Cross between individuals heterozygous for one character.

• Dihybrid Cross: Cross between individuals heterozygous for two characters.

Rules of Probability

• Multiplication Rule: Probability of two independent events occurring together is the product of their probabilities.

• Addition Rule: Probability of any one of two or more mutually exclusive events is calculated by adding their probabilities.

Example: Probability of YyRr offspring in a dihybrid cross:

$P(YyRr) = P(Yy) \times P(Rr) = \frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

Exceptions to Mendelian Inheritance

• Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., red and white flowers produce pink offspring).

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways (e.g., MN blood group in humans).

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood groups).

• Pleiotropy: One gene affects multiple phenotypic traits (e.g., sickle-cell disease).

• Epistasis: One gene affects the expression of another gene.

• Polygenic Inheritance: Multiple genes independently affect a single trait (e.g., skin color).

Relationship Between Genotype and Phenotype

• Dominance does not always mean the dominant allele is more common or better.

• Phenotype can be influenced by environmental factors as well as genotype.

• Carriers are heterozygous individuals who carry a recessive allele but do not express it.

Pedigree Analysis

Pedigrees are family trees that describe the interrelationships of parents and children across generations. They are used to track inheritance patterns of traits and predict the probability of genetic disorders.

• Squares represent males; circles represent females.

• Shaded symbols indicate individuals expressing the trait.

• Half-shaded symbols indicate carriers.

Examples of Genetic Disorders

• Recessive Disorders: Cystic fibrosis, sickle-cell disease, Tay-Sachs disease.

• Dominant Disorders: Huntington’s disease, achondroplasia.

• Recessive disorders often appear only in individuals homozygous for the allele.

• Dominant disorders can appear in heterozygotes.

Sample Table: Comparison of Dominance Relationships

Type	Genotype	Phenotype	Example
Complete Dominance	AA, Aa	Dominant phenotype	Purple flower in peas
Incomplete Dominance	CRCR, CRCW, CWCW	Intermediate (pink)	Snapdragon flower color
Codominance	IAIB	Both phenotypes expressed	MN blood group

How to Solve Genetics Problems

• Identify genotypes and phenotypes of parents.

• Determine possible gametes.

• Use Punnett squares to predict offspring ratios.

• Apply rules of probability for complex crosses.

Summary

• Mendel’s principles form the basis of classical genetics.

• Inheritance patterns can be predicted using laws of segregation and independent assortment.

• Exceptions to Mendelian inheritance include incomplete dominance, codominance, and polygenic traits.

• Pedigree analysis is essential for tracking genetic disorders in families.