Patterns of Inheritance: Classical Genetics Study Guide

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Patterns of Inheritance

Classical Genetics Terms

Understanding the terminology of classical genetics is essential for analyzing inheritance patterns. Below are key definitions:

Character: A heritable feature or property, such as flower color or seed shape.
Trait: A specific variant of a character, e.g., purple flowers or round seeds.
Gene: A unit of heredity; a segment of DNA that encodes information for a specific character.
Allele: Alternative forms of a gene found at the same locus; e.g., A and a.
Genotype: The genetic makeup of an organism; the combination of alleles present.
Phenotype: The observable physical or physiological traits of an organism, determined by its genotype.
Dominant Allele: An allele that masks the expression of another allele at the same locus; represented by a capital letter (e.g., A).
Recessive Allele: An allele whose expression is masked by a dominant allele; represented by a lowercase letter (e.g., a).
True Breeding: Organisms that, when self-fertilized, produce offspring identical to themselves for a given trait.

Simple Traits vs. Complex Traits

Traits can be classified based on their genetic basis:

Simple Trait: Controlled by a single gene with clear dominant and recessive alleles. Example: Pea plant flower color.
Complex Trait: Influenced by multiple genes (polygenic) and often environmental factors. Example: Human height.

Monohybrid and Dihybrid Crosses

Genetic crosses allow prediction of offspring genotypes and phenotypes:

Monohybrid Cross: Involves one gene with two alleles. Example: AA x aa.
Dihybrid Cross: Involves two genes, each with two alleles. Example: AaBb x AaBb.

Typical ratios for Mendelian inheritance:

Monohybrid Cross (heterozygotes): Phenotype ratio 3:1; Genotype ratio 1:2:1.
Dihybrid Cross (heterozygotes): Phenotype ratio 9:3:3:1.

Example: Crossing Aa x Aa yields genotypes AA, Aa, aa in a 1:2:1 ratio.

Patterns of Inheritance

Different genetic mechanisms affect inheritance patterns:

Classical (Mendelian): Traits follow dominant/recessive rules.
Sex Linked: Genes located on sex chromosomes (e.g., X-linked traits). Males (XY) are more likely to express recessive X-linked traits.
Incomplete Dominance: Heterozygotes show intermediate phenotype. Example: Red and white flowers produce pink offspring.
Co-Dominance: Both alleles are fully expressed in heterozygotes. Example: Blood type AB.
Pleiotropy: One gene affects multiple traits. Example: Sickle cell gene affects blood cell shape and other symptoms.

Linkage and Linkage Analysis

Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as linkage.

Linkage: The tendency of genes on the same chromosome to be inherited together due to their physical proximity.
Linkage Analysis: A method used to map genes to specific loci by studying recombination frequencies in offspring.

Example: If two genes are linked, the expected Mendelian ratio may not be observed; instead, parental combinations are more frequent than recombinant types.

Sample Table: Patterns of Inheritance Comparison

Pattern	Genotype	Phenotype	Example
Classical (Mendelian)	AA, Aa, aa	Dominant or recessive trait	Purple vs. white flowers
Sex Linked	XAXa, XaY	Trait often seen in males	Hemophilia
Incomplete Dominance	RR, RW, WW	Intermediate phenotype	Pink flowers
Co-Dominance	IAIB	Both traits expressed	AB blood type
Pleiotropy	Single gene	Multiple traits affected	Sickle cell disease

Key Equations

Probability of genotype in monohybrid cross:

Probability of phenotype in monohybrid cross:

Dihybrid cross phenotype ratio:

Recombination frequency (linkage analysis):

Additional info: Academic context and examples were added to clarify definitions and inheritance patterns.