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.

• Genotype: The genetic makeup of an organism, describing the alleles present.

• True-breeding: Organisms that produce offspring of the same variety when they self-pollinate.

Example: In pea plants, both PP and Pp genotypes produce purple flowers (same phenotype), but their genotypes differ.

Mendel's Experimental, Quantitative Approach

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 chose varieties with distinct alternative forms and started with true-breeding plants.

• Hybridization: Mating two contrasting, true-breeding varieties.

Experimental Setup and Generations

• 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 explained by the "blending" hypothesis, which Mendel disproved by showing that traits can reappear in later generations.

• F1 hybrids from true-breeding parents were all purple, not a blend.

• F2 generation showed a 3:1 ratio of purple to white flowers.

Mendel's Model and Laws

Four Related Components

Mendel developed a model to explain the 3:1 inheritance pattern observed in F2 offspring.

1. Alleles: Alternative versions of genes account for variations in inherited characters. Each gene resides at a specific locus on a chromosome.

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

3. Dominance: If alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

4. Law of Segregation: The two alleles for a heritable character separate during gamete formation and end up in different gametes.

Example: In flower color, the purple allele is dominant over the white allele.

Punnett Squares and Testcrosses

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

• Testcross: Breeding an individual with a dominant phenotype with a homozygous recessive individual to determine genotype.

Increasing Complexity with Mendel's Peas

Monohybrid and Dihybrid Crosses

• Monohybrid cross: Follows a single character; F1 offspring are heterozygous for one character.

• Dihybrid cross: Follows two characters; F1 offspring are heterozygous for both characters.

The Law of Independent Assortment

Using dihybrid crosses, Mendel developed the law of independent assortment.

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

• This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.

Probability Laws in Mendelian Inheritance

• 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 probabilities.

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

Beyond Mendel

Extending Mendelian Genetics for a Single Gene

Inheritance of characters by a single gene may deviate from simple Mendelian patterns 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 and dominant homozygote are identical in phenotype.

• Incomplete dominance: Heterozygote phenotype is intermediate between the two homozygotes.

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways.

Relationship Between Dominance and Phenotype

• Dominant allele codes for a functional enzyme; recessive allele codes for a defective enzyme.

• 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: Caused by a dominant allele, but is rare in the population.

Multiple Alleles

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

• Example: ABO blood group in humans is determined by three alleles: $I^A$, $I^B$, and $i$.

Allele	Carbohydrate
IA	A
IB	B
i	none

Pleiotropy

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

• Example: Pleiotropic alleles are responsible for multiple symptoms of hereditary diseases such as cystic fibrosis and sickle-cell disease.

Relationship among alleles of a single gene	Description	Example
Complete dominance	Heterozygous phenotype same as that of homozygous dominant	PP (purple flowers)
Incomplete dominance	Heterozygous phenotype intermediate between two homozygotes	CRCW (pink flowers)
Codominance	Both phenotypes expressed in heterozygotes	IAIB (AB blood group)
Multiple alleles	Some genes have more than two alleles	IA, IB, i (ABO blood group)
Pleiotropy	One gene affects multiple phenotypic characters	Sickle-cell disease

Extending Mendelian Genetics for Two or More Genes

• 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 among genes	Description	Example
Epistasis	Phenotype expression of one gene affects the expression of another	Coat color in mice
Polygenic inheritance	Single phenotypic character affected by two or more genes	Skin color in humans

Poll Questions (Review)

• Why can't haploid cells undergo meiosis? Answer: Homologous chromosomes cannot pair.

• Normal gametes produced from one meiotic event: Answer: Each has the same chromosome number.

• Determining unknown genotype in a dihybrid cross: Use offspring ratios to infer genotype.

Additional info:

• These notes cover topics from Ch. 14 (Mendel and the Gene Idea), Ch. 15 (Chromosomal Basis of Inheritance), and Ch. 16 (Molecular Basis of Inheritance) in a General Biology course.