Mendel and the Gene Idea: Principles of Inheritance (Section 14.1)

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Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the fundamental principles of inheritance, which explain how traits are passed from parents to offspring. These principles form the basis of classical genetics and are essential for understanding heredity in all organisms.

Mendel’s Laws: The foundational rules describing inheritance patterns.
Punnett Squares: A tool for predicting the outcome of genetic crosses.
Law of Independent Assortment: Explains how different genes independently separate from one another during gamete formation.

Mendel’s Model of Inheritance

Genes and Alleles

Mendel proposed that traits are determined by discrete units of inheritance, now known as genes. Each gene can exist in different forms called alleles.

Gene: A segment of DNA that encodes information for a specific trait.
Allele: An alternative form of a gene found at the same locus on homologous chromosomes.
Each organism inherits two alleles for each gene, one from each parent.

Homozygous and Heterozygous

Homozygous: Both alleles for a gene are the same (e.g., PP or pp).
Heterozygous: The two alleles for a gene are different (e.g., Pp).

Dominant and Recessive Alleles

For each gene, one allele may be dominant and the other recessive:

Dominant allele: Expressed in the phenotype even if only one copy is present (e.g., P for purple flowers).
Recessive allele: Expressed in the phenotype only when two copies are present (e.g., p for white flowers).
Dominant alleles mask the effect of recessive alleles in heterozygotes.

Law of Segregation

The Law of Segregation states that the two alleles for a gene separate during gamete formation (meiosis), so each gamete carries only one allele for each gene.

Gametes from a true-breeding parent (homozygous) will all carry the same allele.
At fertilization, alleles from each parent pair up again in the offspring.

Genotype and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is the observable trait.

Genotype	Phenotype
PP (homozygous dominant)	Purple
Pp (heterozygous)	Purple
pp (homozygous recessive)	White

Each letter in the genotype represents an allele inherited from a parent.
The combination of alleles determines the phenotype.

Punnett Squares

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

Each box represents a possible genotype of offspring.
Gametes from each parent are placed along the top and side of the square.

Biological Mechanisms of Inheritance

Meiosis and Fertilization

Meiosis is the process by which gametes (sperm and egg) are formed, reducing the chromosome number by half. Fertilization restores the diploid number by combining gametes from two parents.

Humans have 46 chromosomes (23 pairs).
Meiosis I: Homologous chromosomes separate.
Meiosis II: Sister chromatids separate.
Each gamete receives one chromosome from each pair.
Fertilization unites sperm and egg, re-establishing pairs of homologous chromosomes.

Law of Independent Assortment

Principle and Application

The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation. This means the inheritance of one trait generally does not affect the inheritance of another, provided the genes are on different chromosomes.

Explains genetic variation in offspring.
Tested using dihybrid crosses (crosses involving two traits).

Dihybrid Cross Example

Consider two traits in pea plants: seed color (yellow, Y, is dominant to green, y) and seed shape (round, R, is dominant to wrinkled, r).

True-breeding yellow, round (YYRR) crossed with true-breeding green, wrinkled (yyrr) produces F1 hybrids (YyRr).
Self-pollination of F1 (YyRr x YyRr) produces offspring with a phenotypic ratio of 9:3:3:1 (yellow round : yellow wrinkled : green round : green wrinkled).

Phenotype	Genotype(s)	Expected Ratio
Yellow, round	YYRR, YYRr, YyRR, YyRr	9
Yellow, wrinkled	YYrr, Yyrr	3
Green, round	yyRR, yyRr	3
Green, wrinkled	yyrr	1

Key Terms and Concepts

Gene: Unit of heredity; segment of DNA.
Allele: Alternative form of a gene.
Homozygous: Two identical alleles for a gene.
Heterozygous: Two different alleles for a gene.
Dominant: Allele that determines phenotype in heterozygotes.
Recessive: Allele masked by dominant in heterozygotes.
Genotype: Genetic makeup (e.g., PP, Pp, pp).
Phenotype: Observable trait (e.g., purple or white flowers).
Punnett Square: Diagram to predict genetic crosses.
Law of Segregation: Alleles separate during gamete formation.
Law of Independent Assortment: Genes on different chromosomes assort independently.

Formulas and Equations

Probability of a genotype in a monohybrid cross:

Phenotypic ratio in a monohybrid cross (heterozygotes):

(dominant:recessive)

Phenotypic ratio in a dihybrid cross (heterozygotes):

Example Applications

Monohybrid Cross: Crossing two heterozygous purple-flowered pea plants (Pp x Pp) yields offspring with a 3:1 ratio of purple to white flowers.
Dihybrid Cross: Crossing two plants heterozygous for seed color and shape (YyRr x YyRr) yields a 9:3:3:1 phenotypic ratio.

Summary Table: Key Genetic Terms

Term	Definition	Example
Gene	Unit of heredity	Flower color gene
Allele	Alternative form of a gene	P (purple), p (white)
Homozygous	Two identical alleles	PP or pp
Heterozygous	Two different alleles	Pp
Genotype	Genetic makeup	Pp
Phenotype	Observable trait	Purple flowers

Additional info: These notes expand on the provided slides by including definitions, examples, and context for Mendel's laws, Punnett squares, and the chromosomal basis of inheritance, ensuring a self-contained study guide for exam preparation.