Mendelian Genetics: Principles, Patterns, and Extensions

Mendelian Genetics (Chapter 14)

Introduction to Mendelian Genetics

Mendelian genetics is the study of how traits are inherited from one generation to the next, based on the pioneering work of Gregor Mendel. Mendel's experiments with pea plants established the foundational laws of inheritance, which remain central to modern genetics.

• Gregor Mendel: An Austrian monk known as the "Father of Genetics" for his systematic study of inheritance patterns in pea plants.

• Key Contributions: Formulated the laws of segregation and independent assortment before the discovery of chromosomes, DNA, or meiosis.

Why Pea Plants?

Mendel chose garden peas (Pisum sativum) for his experiments due to their suitability for genetic studies.

• Advantages:

  • Readily available and easy to grow

  • Exhibit many distinct varieties (traits)

  • Short generation time

  • Ability to self-fertilize or be cross-fertilized

• Experimental Scale: Mendel conducted experiments on over 30,000 pea plants, analyzing several characteristics to develop his hypotheses.

Structure of a Pea Flower

The pea flower contains both male and female reproductive organs, allowing for controlled breeding experiments.

• Stamen: Male sex organ, produces pollen

• Carpel: Female sex organ, contains ovules

• Pea plants can self-fertilize or be cross-pollinated by transferring pollen between flowers.

Mendel's Experimental Design

Mendel studied traits that appeared in two distinct forms (e.g., purple vs. white flowers, round vs. wrinkled seeds). He performed controlled crosses to observe inheritance patterns.

• P Generation: True-breeding parents with contrasting traits

• F1 Generation: Offspring of the P generation, all showing the dominant trait

• F2 Generation: Offspring of self- or cross-fertilized F1 plants, showing a 3:1 ratio of dominant to recessive phenotypes

Key Mendelian Principles

Principle of Dominance

When two different alleles are present, one (the dominant allele) masks the expression of the other (the recessive allele).

• Dominant Trait: Expressed in the F1 generation

• Recessive Trait: Masked in the F1 generation, reappears in F2

Law of Segregation

Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.

• Explains the 3:1 ratio in F2 generation

• Supported by meiosis, where homologous chromosomes separate

Equation:

$\text{Genotype ratio in F}_2 = 1 \text{(homozygous dominant)} : 2 \text{(heterozygous)} : 1 \text{(homozygous recessive)}$

Law of Independent Assortment

Alleles of different genes assort independently of one another during gamete formation, leading to genetic variation.

• Observed in dihybrid crosses (crosses involving two traits)

• Results in a 9:3:3:1 phenotypic ratio in the F2 generation

Equation:

$\text{Phenotypic ratio in F}_2 \text{ (dihybrid cross)} = 9:3:3:1$

Types of Crosses

• Monohybrid Cross: Involves one trait (e.g., flower color)

• Dihybrid Cross: Involves two traits (e.g., seed color and shape)

• Test Cross: Cross between an individual with a dominant phenotype and a homozygous recessive individual to determine genotype

Punnett Squares

Punnett squares are tools used to predict the genotypic and phenotypic outcomes of genetic crosses.

• Monohybrid Example (Pp x Pp):

	P	p
P	PP	Pp
p	Pp	pp

• Genotype ratio: 1 PP : 2 Pp : 1 pp

• Phenotype ratio: 3 purple : 1 white (if P = purple, p = white)

Genotype and Phenotype

• Genotype: The genetic makeup of an organism (e.g., PP, Pp, pp)

• Phenotype: The observable physical or physiological traits (e.g., purple or white flowers)

• Homozygous: Two identical alleles (PP or pp)

• Heterozygous: Two different alleles (Pp)

Homologous Chromosomes and Alleles

Genes are located on chromosomes, and each gene may have different versions called alleles.

• Homologous Chromosomes: Chromosome pairs with the same genes in the same order, but possibly different alleles

• Alleles: Alternative forms of a gene (e.g., P and p for flower color)

Extensions of Mendelian Genetics

Not all traits follow simple Mendelian inheritance. Some show more complex patterns:

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

• Codominance: Both alleles are fully expressed in heterozygotes (e.g., MN blood group in humans)

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

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

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

• Polygenic Inheritance: Multiple genes contribute to a single trait, resulting in continuous variation (e.g., skin color, height)

Human Genetic Disorders

Some human diseases are inherited in Mendelian patterns:

• Autosomal Recessive Disorders: Require two copies of the mutant allele (e.g., cystic fibrosis, sickle cell anemia)

• Autosomal Dominant Disorders: Only one copy of the mutant allele is needed (e.g., Huntington's disease, familial hypercholesterolemia)

Pedigree Analysis

Pedigrees are family trees used to track inheritance patterns of traits across generations.

• Symbols: Squares for males, circles for females; shaded for affected individuals

• Used to determine mode of inheritance and predict genotypes

Summary Table: Key Mendelian Concepts

Concept	Definition	Example
Dominance	One allele masks the effect of another	Purple flower color in peas
Segregation	Alleles separate during gamete formation	3:1 ratio in F2 generation
Independent Assortment	Genes for different traits assort independently	9:3:3:1 ratio in dihybrid cross
Incomplete Dominance	Heterozygote shows intermediate phenotype	Pink flowers from red x white
Codominance	Both alleles expressed equally	MN blood group
Polygenic Inheritance	Multiple genes affect one trait	Human height, skin color

Additional info: These notes synthesize and expand upon the provided lecture slides and images, ensuring a comprehensive overview of Mendelian genetics and its extensions for college-level biology students.