Genetics: Patterns of Inheritance

Introduction to Mendelian Genetics

Genetics is the study of heredity and the variation of inherited characteristics. Gregor Mendel’s experiments with pea plants established the foundational principles of inheritance, which explain how traits are passed from one generation to the next.

Mendel’s Experiments and Contributions

• Experimental Design: Mendel cross-fertilized pea plants and observed seven distinct characters, each with two contrasting traits.

• Key Discovery: Mendel found that inheritable factors (now called genes) retain their individuality across generations.

Genes, Alleles, and Traits

• Gene: A segment of DNA that codes for a specific trait (e.g., stem height).

• Allele: Alternative versions of a gene (e.g., tall or dwarf for stem height).

Genotype and Phenotype

• Genotype: The genetic makeup of an organism (e.g., TT, Tt, or tt).

• Phenotype: The observable physical trait (e.g., tall or short plants).

Homozygous and Heterozygous

• Homozygous: Both alleles for a gene are identical (e.g., TT or tt).

• Heterozygous: The alleles for a gene are different (e.g., Tt).

Dominant and Recessive Alleles

• Dominant Allele: Expressed in the phenotype even if only one copy is present (represented by uppercase letters, e.g., T).

• Recessive Allele: Expressed only when two copies are present (represented by lowercase letters, e.g., t).

Mendel’s Law of Segregation

During gamete formation, the two alleles for a gene separate so that each gamete carries only one allele for each gene. This explains why offspring inherit one allele from each parent.

Hybridization and Generational Terminology

• Hybridization: Cross-fertilization between different varieties (e.g., tall x short plants), producing hybrids.

• P Generation: Parental generation.

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

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

Monogenic and Polygenic Traits

• Monogenic Trait: Controlled by a single gene (e.g., pea plant height).

• Polygenic Trait: Controlled by multiple genes (e.g., human skin color).

Punnett Squares and Mendelian Ratios

Punnett squares are used to predict the probability of offspring genotypes and phenotypes based on parental alleles. For a monohybrid cross (Tt x Tt), the F2 generation shows a 3:1 ratio of dominant to recessive phenotypes.

• Genotypic ratio: 1 TT : 2 Tt : 1 tt

• Phenotypic ratio: 3 tall : 1 short

Test Cross

A test cross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The offspring phenotypes reveal the unknown genotype.

Mendel’s Principle of Independent Assortment

Alleles of different genes assort independently during gamete formation. This means the inheritance of one trait does not affect the inheritance of another, provided the genes are on different chromosomes.

Rules of Probability in Genetics

• Rule of Multiplication: The probability of two independent events occurring together is the product of their individual probabilities.

• Rule of Addition: The probability of an event that can occur in multiple ways is the sum of the probabilities for each way.

Example: Probability of aabbcc offspring from AaBbCc x AaBbCc parents is .

Dihybrid Crosses

A dihybrid cross involves parents differing in two traits. The F2 generation typically shows a 9:3:3:1 phenotypic ratio, demonstrating independent assortment.

Non-Mendelian Inheritance

Incomplete Dominance

In incomplete dominance, the heterozygote displays a phenotype intermediate between the two homozygotes (e.g., red x white flowers produce pink offspring).

Codominance

In codominance, both alleles are fully expressed in the heterozygote (e.g., AB blood type in humans).

Pedigree Analysis

Pedigrees are diagrams that track the inheritance of traits across generations in families. They are useful for determining genotypes and predicting genetic disorders.

Genetic Disorders

Autosomal Recessive Disorders

• Most genetic disorders are recessive and occur when both parents are carriers (heterozygous).

• Offspring have a 25% chance of inheriting the disorder if both parents are carriers.

• Example: Cystic Fibrosis

Inbreeding

Inbreeding increases the probability of offspring inheriting recessive disorders due to increased homozygosity.

Autosomal Dominant Disorders

• Dominant disorders are usually expressed in heterozygotes; homozygous dominant individuals often do not survive.

• Offspring have a 50% chance of inheriting the disorder if one parent is affected.

• Examples: Achondroplasia, Huntington’s Disease, Polydactyly (extra fingers or toes)

Sex-Linked Disorders

Some disorders, such as hemophilia, are inherited on the sex chromosomes and can be traced through family pedigrees.

Prenatal Genetic Testing

Amniocentesis

• Performed between weeks 14-20 of pregnancy.

• Amniotic fluid is extracted and fetal cells are cultured for genetic and chromosomal analysis.

• Used to detect disorders such as Down syndrome and Tay-Sachs disease.

• Risks include bleeding, miscarriage, or premature birth (about 1%).

Chorionic Villus Sampling (CVS)

• Sample of chorionic villi is taken from the placenta as early as the 8th week of pregnancy.

• Allows for rapid karyotyping and biochemical testing.

• Risks include bleeding, miscarriage, or premature birth (about 2%).

Summary Table: Key Genetic Concepts