Mendelian Genetics and Patterns of Inheritance

Mendelian Genetics

Introduction to Mendel's Laws

Mendelian genetics describes the principles of heredity discovered by Gregor Mendel through his experiments with pea plants. These laws explain how traits are inherited from one generation to the next and form the foundation of classical genetics.

• Hippocrates' idea: Incorrectly suggested that 'pangenes' travel from each part of an organism's body to the eggs or sperm.

• Reproductive cells: Not composed of particles from somatic cells.

• Blending hypothesis: Incorrect; does not explain how traits disappear and reappear across generations.

Heredity and Traits

Heredity is the transmission of traits from one generation to the next. Mendel studied inheritance using garden peas, focusing on observable characteristics (phenotypes).

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

• Trait: Variant of a character (e.g., purple or white flowers).

• P generation: Parental generation.

• F1 generation: First filial generation, offspring of the P generation.

• F2 generation: Second filial generation, offspring of the F1 generation.

Genes and Alleles

Genes are units of heredity that exist in different versions called alleles. Each organism inherits two alleles for each gene, one from each parent.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Dominant allele: Determines the organism's appearance.

• Recessive allele: Has no noticeable effect when paired with a dominant allele.

Mendel's Laws

Mendel formulated two key laws of inheritance:

• Law of Segregation: The two alleles for a gene separate during gamete formation.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation.

Monohybrid and Dihybrid Crosses

Genetic crosses are used to study inheritance patterns:

• Monohybrid cross: Cross between two organisms heterozygous for one character.

• Dihybrid cross: Cross between two organisms heterozygous for two characters.

• Test cross: Mating between an individual of unknown genotype and a homozygous recessive individual to determine genotype.

Probability in Genetics

Probability calculations help predict the outcomes of genetic crosses.

• Rule of multiplication: Probability of two independent events both occurring is the product of their individual probabilities.

• Rule of addition: Probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities.

Extensions of Mendelian Genetics

Not all traits follow simple Mendelian inheritance. Several patterns extend Mendel's laws:

• Incomplete dominance: Heterozygotes show intermediate phenotypes.

• Codominance: Both alleles are expressed in heterozygotes (e.g., AB blood type).

• Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood group).

• Pleiotropy: One gene influences multiple traits.

• Polygenic inheritance: Multiple genes affect a single trait.

Human Genetics and Pedigrees

Pedigree analysis is used to study inheritance patterns in families. Many human traits follow Mendelian laws, but some are influenced by multiple genes and environmental factors.

• Carriers: Individuals who carry a recessive allele but do not show the trait.

• Sex-linked genes: Genes located on sex chromosomes, often show different inheritance patterns in males and females.

• X-linked recessive traits: More common in males; females must inherit two copies to express the trait.

Examples and Applications

• Sickle-cell disease: Example of pleiotropy; affects hemoglobin and causes multiple symptoms.

• Cystic fibrosis: Recessive genetic disorder affecting respiratory and digestive systems.

• ABO blood group: Controlled by three alleles; demonstrates codominance and multiple alleles.

Summary Table: Key Genetic Terms

Term	Definition	Example
Gene	Unit of heredity	Flower color gene in peas
Allele	Variant of a gene	Purple or white flower allele
Homozygous	Two identical alleles	PP or pp
Heterozygous	Two different alleles	Pp
Dominant	Allele that determines phenotype	P (purple)
Recessive	Allele masked by dominant	p (white)

Key Equations

• Probability of independent events: $P(A \text{ and } B) = P(A) \times P(B)$

• Probability of mutually exclusive events: $P(A \text{ or } B) = P(A) + P(B)$

Additional info: These notes expand on Mendel's laws and include examples of inheritance patterns, human genetic disorders, and extensions of classical genetics for a comprehensive study guide.