Mendelian Genetics: Principles, Patterns, and Extensions

Mendelian Basics

Genetic Vocabulary

Understanding key genetic terms is essential for studying inheritance patterns.

• Homozygote: An organism with two identical alleles for a gene (e.g., PP or pp).

• Heterozygote: An organism with two different alleles for a gene (e.g., Pp).

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

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

Example: In pea plants, both PP and Pp genotypes produce purple flowers, but only pp produces white flowers.

Mendel's Experimental, Quantitative Approach

Gregor Mendel used pea plants to study inheritance, focusing on characters and traits.

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• Mendel used true-breeding varieties and performed hybridization experiments.

Experimental Setup

• P generation: True-breeding parents.

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Offspring from self- or cross-pollination of F1 hybrids.

The Blending Hypothesis

Historically, heredity was thought to be a blending of parental traits. Mendel's experiments disproved this by showing discrete inheritance patterns.

Mendel's Experiments and Traits

• Crosses between true-breeding plants produced F1 hybrids with only the dominant trait.

• F2 generation showed a 3:1 ratio of dominant to recessive traits.

• Mendel observed similar patterns in six other pea plant characters.

Character	Dominant Trait	Recessive Trait
Flower Color	Purple	White
Seed Shape	Round	Wrinkled
Seed Color	Yellow	Green
Pod Shape	Inflated	Constricted
Pod Color	Green	Yellow
Flower Position	Axial	Terminal
Stem Length	Tall	Dwarf

Mendel's Model: Four Related Components

Component 1: Alleles

Alternative versions of genes (alleles) account for variations in inherited characters. Each gene resides at a specific locus on a chromosome.

Component 2: Inheritance of Alleles

For each character, an organism inherits two alleles, one from each parent. These alleles may be identical (homozygous) or different (heterozygous).

Component 3: Dominance

If two alleles differ, the dominant allele determines the organism's appearance, while the recessive allele has no noticeable effect.

Component 4: Law of Segregation

The two alleles for a heritable character separate during gamete formation and end up in different gametes.

• This corresponds to the distribution of homologous chromosomes during meiosis.

Applying Mendel's Model

Punnett Squares and Genotype Ratios

Punnett squares are used to predict the possible combinations of alleles in offspring.

• Dominant alleles are represented by capital letters (e.g., P), recessive alleles by lowercase letters (e.g., p).

The Testcross

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 mystery parent must be heterozygous.

Mendelian Laws and Probability

Law of Segregation

Alleles segregate independently during gamete formation.

Law of Independent Assortment

Each pair of alleles segregates independently of other pairs during gamete formation. This law applies only to genes on different chromosomes or those far apart on the same chromosome.

Probability Rules

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

• Addition Rule: The probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

Example: For an Aa x Aa cross, the probability of an offspring being AA is $\frac{1}{4}$, Aa is $\frac{1}{2}$, and aa is $\frac{1}{4}$.

Beyond Mendel: Extensions and Complexity

Extending Mendelian Genetics for a Single Gene

Inheritance patterns may deviate from simple Mendelian rules in several ways:

• Alleles are not completely dominant or recessive (incomplete dominance, codominance).

• A gene has more than two alleles (multiple alleles).

• A gene produces multiple phenotypes (pleiotropy).

Degrees of Dominance

• Complete dominance: Heterozygote phenotype is identical to dominant homozygote.

• Incomplete dominance: Heterozygote phenotype is intermediate between both homozygotes.

• Codominance: Both alleles are expressed in heterozygotes.

Relationship Between Dominance and Phenotype

• Dominant alleles may code for functional enzymes, while recessive alleles may code for nonfunctional forms, affecting traits such as seed shape.

• Tay-Sachs disease: At the organismal level, the allele is recessive; at the biochemical level, the phenotype is incompletely dominant; at the molecular level, alleles are codominant.

Frequency of Dominant Alleles

• Dominant alleles are not necessarily more common than recessive alleles.

• Polydactyly: A condition caused by a dominant allele but is rare in the population.

Multiple Alleles

• Most genes exist in populations in more than two allelic forms.

• ABO blood group: Determined by three alleles (IA, IB, i).

Allele	Carbohydrate
IA	A
IB	B
i	none

Pleiotropy

Most genes have multiple phenotypic effects, a property called pleiotropy.

• Pleiotropic alleles are responsible for multiple symptoms of hereditary diseases (e.g., cystic fibrosis, sickle-cell disease).

Summary Table: Relationships Among Alleles

Relationship	Description	Example
Complete dominance	Heterozygous phenotype same as homozygous dominant	PP (purple flowers)
Incomplete dominance	Heterozygous phenotype intermediate	CRCW (pink flowers)
Codominance	Both phenotypes expressed	IAIB (AB blood group)
Multiple alleles	More than two alleles in population	IA, IB, i (ABO blood group)
Pleiotropy	One gene affects multiple traits	Sickle-cell disease

Extending Mendelian Genetics for Two or More Genes

Epistasis and Polygenic Inheritance

• Epistasis: One gene affects the phenotype of another due to interaction of their gene products.

• Polygenic inheritance: Multiple genes independently affect a single trait.

Relationship	Description	Example
Epistasis	Phenotype expression of one gene affects another	Coat color in mice
Polygenic inheritance	Single phenotype affected by multiple genes	Skin color in humans

Poll Questions: Application of Concepts

Poll Question 1

Haploid cells may undergo mitosis but not meiosis because homologous chromosomes cannot pair.

Poll Question 2

Normal gametes produced from one meiotic event each have the same chromosome number but are not genetically identical to each other.

Poll Question 3

Genotype determination from offspring ratios: Tall stems and axial flowers crossed with dwarf stems and terminal flowers. The previously unknown genotype is TtAa.