Mendelian Genetics: Principles of Heredity

Introduction to Heredity and Mendel's Experiments

Genetics is the study of heredity, the process by which traits are passed from parents to offspring. Gregor Mendel, through his experiments with garden peas, established the foundational principles of heredity, which are now known as Mendelian genetics. Mendel's work disproved the "blending" hypothesis and supported the "particulate" hypothesis, demonstrating that traits are inherited as discrete units called genes.

Mendel's Experimental Approach

Mendel used pea plants for his experiments due to their short generation time, large number of offspring, and the ability to control mating. He tracked characters with two distinct forms and used true-breeding varieties to study inheritance patterns.

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

• Trait: Each variant for a character (e.g., purple or white flowers).

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

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of the P generation.

• F2 Generation: Offspring from self- or cross-pollination of F1 hybrids.

Mendel's Laws of Inheritance

The Law of Segregation

Mendel observed that when F1 hybrids self-pollinated, the F2 generation exhibited a 3:1 ratio of dominant to recessive traits. This led to the law of segregation, which states that two alleles for a heritable character separate during gamete formation and end up in different gametes.

• Dominant Trait: The trait that appears in the F1 generation (e.g., purple flowers).

• Recessive Trait: The trait masked in the F1 generation but reappears in F2 (e.g., white flowers).

• Alleles: Alternative versions of a gene.

• Locus: The specific location of a gene on a chromosome.

Genotype and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is its physical appearance. Homozygotes have two identical alleles; heterozygotes have two different alleles.

• Homozygous: Two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: Two different alleles for a gene (e.g., Pp).

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the mystery parent is heterozygous.

The Law of Independent Assortment

Mendel's second law states that each pair of alleles segregates independently during gamete formation. This law applies to genes on different chromosomes or those far apart on the same chromosome.

• Dihybrid Cross: Cross between F1 hybrids heterozygous for two characters.

• Independent Assortment: Alleles for different genes segregate independently.

Probability in Genetics

Rules of Probability

Mendelian inheritance follows the rules of probability. The multiplication rule calculates the probability of two independent events occurring together, while the addition rule calculates the probability of mutually exclusive events.

• Multiplication Rule:

• Addition Rule:

Extensions of Mendelian Genetics

Degrees of Dominance

Inheritance patterns may deviate from simple Mendelian genetics. Complete dominance occurs when the heterozygote and dominant homozygote are identical in phenotype. Incomplete dominance results in intermediate phenotypes, and codominance occurs when both alleles affect the phenotype in distinct ways.

• Complete Dominance: Heterozygote and dominant homozygote are phenotypically identical.

• Incomplete Dominance: F1 hybrids have intermediate phenotypes.

• Codominance: Both alleles are expressed in the phenotype.

Multiple Alleles and Pleiotropy

Most genes exist in populations in more than two allelic forms. Pleiotropy occurs when one gene affects multiple phenotypic traits.

• ABO Blood Groups: Determined by three alleles (IA, IB, i).

• Pleiotropy: One gene affects multiple traits (e.g., sickle-cell disease).

Epistasis and Polygenic Inheritance

Epistasis occurs when one gene affects the expression of another gene. Polygenic inheritance involves multiple genes independently affecting a single trait, often resulting in quantitative variation.

• Epistasis: Interaction between genes affects phenotype (e.g., coat color in Labradors).

• Polygenic Inheritance: Multiple genes contribute to a single trait (e.g., height, skin color).

Environmental Impact on Phenotype

The phenotype for a character can depend on both genotype and environment. Multifactorial traits are influenced by multiple genes and environmental factors.

• Multifactorial Traits: Traits influenced by genetic and environmental factors.

Human Genetics and Mendelian Patterns

Pedigree Analysis

Pedigrees are family trees that describe the inheritance of traits across generations. They are used to predict the probability of specific phenotypes in future offspring.

Recessively and Dominantly Inherited Disorders

Many genetic disorders are inherited in a recessive manner, requiring two copies of the allele for expression. Dominant disorders require only one copy. Examples include cystic fibrosis (recessive) and achondroplasia (dominant).

• Carrier: Heterozygous individual carrying a recessive allele but phenotypically normal.

• Consanguineous Matings: Increase the likelihood of inheriting rare recessive alleles.

Genetic Testing and Counseling

Genetic counselors use Mendelian genetics and probability rules to assess the risk of inherited diseases. Fetal and newborn testing can detect genetic disorders early, allowing for informed decisions and interventions.

• Amniocentesis: Removal and testing of fetal liquid for genetic disorders.

• Chorionic Villus Sampling (CVS): Removal and testing of placental tissue.

• Newborn Screening: Routine tests for genetic disorders at birth.

Summary Table: Mendel's F1 Crosses for Seven Pea Plant Characters

Key Equations

Probability of genotype in dihybrid cross: