Mendel and the Gene Idea: Patterns and Extensions of Inheritance chptr. 14

Mendel and the Gene Idea

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with pea plants led to the discovery of fundamental laws of inheritance, which explain the transmission of genetic information.

Historical Context and Mendel's Experiments

Pre-Mendelian Theories of Inheritance

• Blending Hypothesis: Proposed that offspring are a uniform blend of parental traits. This model could not explain traits that skip generations or the persistence of variation.

• Particulate Hypothesis: Mendel's alternative, suggesting that traits are inherited as discrete units (genes) that remain intact across generations.

Gregor Mendel and the Pea Plant Experiments

• Mendel, trained in science and a monk, began his experiments in 1857 using Pisum sativum (pea plants).

• Pea plants were ideal due to their many heritable characters, short generation time, and controlled fertilization.

Experimental Design: Self- and Cross-Pollination

• Self-pollination: Pollen from a flower fertilizes ovules of the same flower.

• Cross-pollination: Pollen from one plant fertilizes ovules of another, allowing Mendel to control parental traits.

True-Breeding and Hybrid Crosses

• True-breeding plants: Always produce offspring with the same trait when self-fertilized.

• P Generation: Parental, true-breeding plants.

• F1 Generation: First filial generation, hybrids of P generation.

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

Mendel's Laws of Inheritance

Law of Segregation

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

• Each individual inherits two alleles, one from each parent.

• If alleles differ, the dominant allele determines the phenotype.

• Separation occurs during meiosis (homologous chromosomes segregate).

Punnett Squares and Genetic Vocabulary

• Punnett Square: Tool to predict offspring genotypes and phenotypes.

• Genotype: Genetic makeup (e.g., PP, Pp, pp).

• Phenotype: Observable trait (e.g., purple or white flowers).

• Homozygous: Two identical alleles (PP or pp).

• Heterozygous: Two different alleles (Pp).

Testcross

A testcross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

• If all offspring show the dominant trait, the parent is homozygous dominant.

• If offspring show a 1:1 ratio, the parent is heterozygous.

Law of Independent Assortment

The Law of Independent Assortment states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome.

• Explained using dihybrid crosses (e.g., seed color and seed shape).

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

Extensions of Mendelian Genetics

Degrees of Dominance

• Complete Dominance: Heterozygote is indistinguishable from dominant homozygote.

• Incomplete Dominance: Heterozygote phenotype is intermediate between the two homozygotes (e.g., red x white = pink flowers).

• Codominance: Both alleles are fully expressed in the heterozygote (e.g., human blood types).

Multiple Alleles

• Most genes have more than two alleles in the population.

• Example: ABO blood group system (IA, IB, i).

Pleiotropy

• One gene affects multiple phenotypic traits.

• Example: Sickle cell anemia affects hemoglobin, red blood cell shape, and malaria resistance.

Polygenic Inheritance

• Multiple genes contribute to a single trait, resulting in continuous variation (quantitative characters).

• Examples: Skin color, height.

Epistasis

• One gene affects the expression of another gene.

• Example: Coat color in Labrador retrievers (B/b for color, E/e for pigment expression).

Environmental Effects on Phenotype

• Phenotype can be influenced by environmental factors such as temperature, nutrition, and sunlight.

• Examples: Siamese cat fur color, human height, flower color in different soils.

Human Genetics and Inheritance Patterns

Recessively Inherited Disorders

• Caused by homozygous recessive alleles (e.g., cystic fibrosis, Tay-Sachs disease, sickle cell anemia).

• Heterozygotes are typically healthy carriers.

• Consanguineous mating increases the risk of recessive disorders.

Dominantly Inherited Disorders

• Caused by a single dominant allele (e.g., achondroplasia, Huntington's disease).

• Homozygous dominant may be lethal (as in achondroplasia).

Pedigree Analysis

• Pedigrees are diagrams that trace inheritance patterns across generations, useful for studying human genetics.

Genetic Testing and Counseling

• Carrier testing, preimplantation genetic diagnosis, fetal testing (amniocentesis, CVS), and newborn screening are used to detect genetic disorders.

• Examples: Tay-Sachs, sickle cell disease, cystic fibrosis, phenylketonuria (PKU).

Summary Table: Mendelian vs. Non-Mendelian Inheritance

Inheritance Pattern	Description	Example
Complete Dominance	Dominant allele masks recessive	Purple vs. white pea flowers
Incomplete Dominance	Heterozygote is intermediate	Red x white = pink flowers
Codominance	Both alleles expressed equally	ABO blood types
Multiple Alleles	More than two alleles for a gene	ABO blood types
Pleiotropy	One gene, multiple effects	Sickle cell anemia
Polygenic Inheritance	Many genes, one trait	Skin color, height
Epistasis	One gene affects another's expression	Lab coat color

Key Equations and Ratios

• Monohybrid cross phenotypic ratio: $3:1$

• Dihybrid cross phenotypic ratio: $9:3:3:1$

• Probability of genotype (Punnett square): $P = \frac{\text{number of specific genotype}}{\text{total offspring}}$