Mendelian Genetics: Principles, Crosses, and Extensions

Mendelian Genetics: Principles, Crosses, and Extensions

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, focusing on how traits are inherited through generations. Gregor Mendel's experiments with pea plants revealed predictable patterns of inheritance, which are explained by the behavior of genes and chromosomes.

Monohybrid Crosses and Mendel's Experiments

Homozygous vs. Heterozygous

• Homozygous: An organism with two identical alleles for a gene (e.g., RR or rr).

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

Monohybrid Cross

• Involves a single trait (e.g., seed shape: round vs. wrinkled).

• Parental generation (P): RR (homozygous dominant) × rr (homozygous recessive).

• First filial generation (F1): All offspring are heterozygous (Rr).

Gametes and Punnett Squares

• Gametes: Sex cells carrying one allele for each gene.

• Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring.

Offspring of Monohybrid Cross

• F1 generation: All heterozygous (Rr).

• F2 generation (crossing F1 × F1): Genotypic ratio 1:2:1 (RR:Rr:rr), phenotypic ratio 3:1 (dominant:recessive).

Example:

• Cross between two heterozygotes (Rr × Rr):

	R	r
R	RR	Rr
r	Rr	rr

Dihybrid Crosses and the Principle of Independent Assortment

Dihybrid Cross

• Examines inheritance of two traits simultaneously (e.g., seed shape and seed color).

• Parental generation: RRYY × rryy.

• F1 generation: All RrYy (heterozygous for both traits).

Principle of Independent Assortment

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

• F2 generation shows a phenotypic ratio of 9:3:3:1.

Number of Gametes

• For n heterozygous gene pairs, number of possible gametes = $2^n$.

Extensions to Mendelian Genetics

Sex Determination (Nettie Stevens)

• Sex determined by sex chromosomes (e.g., XX in females, XY in males).

• Y chromosome determines maleness in many species.

Sex-Linked Inheritance

• Genes located on sex chromosomes (X or Y).

• X-linked recessive traits more common in males (e.g., color blindness).

Linked Genes and Linkage

• Linked genes are located close together on the same chromosome and tend to be inherited together.

• Linkage can be broken by crossing over during meiosis.

Crossing Over and Genetic Mapping

• Crossing over exchanges genetic material between homologous chromosomes, creating recombinant gametes.

• Frequency of recombination is used to map gene positions (1 map unit = 1% recombination frequency).

Summary Table: Key Ratios and Concepts

Type of Cross	Genotypic Ratio	Phenotypic Ratio
Monohybrid (F2)	1:2:1	3:1
Dihybrid (F2)	9:3:3:1	9:3:3:1

Glossary of Key Terms

• Allele: Alternative form of a gene.

• Genotype: Genetic makeup of an organism.

• Phenotype: Observable traits of an organism.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Dominant: Trait expressed in heterozygote.

• Recessive: Trait masked in heterozygote.

Additional info:

• These notes expand on the outline by providing definitions, explanations, and examples for each key concept in Mendelian genetics, including extensions such as sex-linked inheritance and genetic mapping.