Mendelian Genetics and Extensions: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

This chapter explores the foundational principles of genetic inheritance as discovered by Gregor Mendel. It covers Mendel's experiments, the laws of segregation and independent assortment, and extends to more complex patterns of inheritance. Understanding these concepts is essential for analyzing how traits are passed from one generation to the next.

Mendel’s Monohybrid Crosses

Key Concepts and Terminology

• True-breeding: Organisms that, when self-fertilized, produce offspring identical to themselves for a given trait.

• P generation (Parental generation): The original pair of individuals crossed in a genetic experiment.

• F1 generation (First filial generation): The offspring resulting from a cross of the P generation.

• F2 generation (Second filial generation): The offspring resulting from self-fertilization or crossing of F1 individuals.

• Punnett square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Self-fertilization: Fertilization in which both gametes come from the same individual.

• Cross-fertilization: Fertilization between gametes from two different individuals.

Example: Crossing true-breeding pea plants with purple flowers (PP) and white flowers (pp) yields all purple F1 offspring (Pp). Self-fertilizing F1 plants produces an F2 generation with a 3:1 ratio of purple to white flowers.

Law of Segregation

Principle and Application

• Diploid somatic cells have two alleles for each gene; haploid gametes have one allele.

• During gamete formation, the two alleles for a gene segregate (separate) so that each gamete receives only one allele.

Equation:

$\text{Genotype ratio in F2 generation: } 1\ \text{(homozygous dominant)} : 2\ \text{(heterozygous)} : 1\ \text{(homozygous recessive)}$

Genetic Vocabulary

• Allele: Alternative forms of a gene.

• Dominant allele: Expressed in the phenotype even if only one copy is present.

• Recessive allele: Expressed only when two copies are present.

• Homozygous: Having two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: Having two different alleles for a gene (e.g., Pp).

• Phenotype: Observable traits of an organism.

• Genotype: Genetic makeup of an organism.

• Wild-type: The most common phenotype in a natural population.

Solving Genetics Problems with Probability

Rules of Probability

• Multiplication Rule: Probability that two independent events both occur is the product of their individual probabilities.

• Addition Rule: Probability that either of two mutually exclusive events occurs is the sum of their individual probabilities.

Example: Probability of getting two recessive alleles (pp) in a monohybrid cross: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

Testcross

Purpose and Method

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

• If any offspring display the recessive phenotype, the tested individual is heterozygous.

Law of Independent Assortment and Dihybrid Crosses

Principle and Application

• Alleles of different genes assort independently during gamete formation if the genes are on different chromosomes.

• Dihybrid cross: A cross between individuals heterozygous for two genes (e.g., YyRr × YyRr).

Phenotypic ratio in F2 generation: 9:3:3:1

Equation:

$\text{Phenotypic ratio: } 9\ \text{(both dominant)} : 3\ \text{(one dominant, one recessive)} : 3\ \text{(one recessive, one dominant)} : 1\ \text{(both recessive)}$

Extensions to Mendel's Laws

Non-Mendelian Inheritance Patterns

• Incomplete dominance: Heterozygotes have an intermediate phenotype (e.g., red × white flowers yield pink offspring).

• Codominance: Both alleles are fully expressed in heterozygotes (e.g., AB blood type).

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

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

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers).

• Polygenic inheritance: Multiple genes influence a single trait (e.g., skin color, height).

• Multifactorial traits: Traits influenced by both genetic and environmental factors.

• Sex-linked genes: Genes located on sex chromosomes (X or Y).

• Environmental effects: Phenotype can be influenced by environmental conditions (e.g., hydrangea flower color varies with soil pH).

Inheritance Patterns of Autosomal Disorders

Autosomal Recessive and Dominant Disorders

• Autosomal recessive disorders: Require two copies of the mutant allele for expression (e.g., cystic fibrosis).

• Autosomal dominant disorders: Only one copy of the mutant allele is needed for expression (e.g., Huntington's disease).

Carriers for Autosomal Recessive Disorders

• Carrier: An individual who is heterozygous for a recessive disorder allele and does not show symptoms but can pass the allele to offspring.

Pedigree Analysis

Determining Inheritance Patterns

• Pedigree: A diagram showing the occurrence of phenotypes in several generations of a family.

• Used to determine whether a trait is dominant or recessive, and to predict the probability of inheritance in future generations.

Sex-Linked Genes (from Chapter 15.2)

X-Linked and Y-Linked Inheritance

• X-linked genes: Genes located on the X chromosome; often show different inheritance patterns in males and females.

• Y-linked genes: Genes located on the Y chromosome; passed from father to son.

• X-linked genetics problems: Males (XY) are more likely to express recessive X-linked traits because they have only one X chromosome (e.g., color blindness, hemophilia).

Summary Table: Mendelian vs. Non-Mendelian Inheritance

Inheritance Pattern	Key Features	Example
Mendelian (Simple Dominance)	One allele is dominant over the other	Purple vs. white pea flowers
Incomplete Dominance	Heterozygote shows intermediate phenotype	Pink snapdragon flowers
Codominance	Both alleles expressed equally	AB blood type
Pleiotropy	One gene affects multiple traits	Sickle-cell disease
Epistasis	One gene affects expression of another	Labrador coat color
Polygenic Inheritance	Multiple genes affect one trait	Human skin color
Sex-linked Inheritance	Gene located on sex chromosome	Color blindness

Additional info: For more detailed examples and practice problems, refer to textbook sections 14.1–14.4 and 15.2 as indicated in the study guide.