Genetic Variation, Mendelian Genetics, and Extensions of Mendelian Inheritance

Sources of Genetic Variation

Mechanisms Generating Variation in Meiosis

Genetic variation is essential for evolution and adaptation. Meiosis introduces variation through three main mechanisms:

• Independent Assortment: During metaphase I of meiosis, homologous chromosomes align randomly at the metaphase plate. This random orientation leads to a variety of possible genetic combinations in gametes.

• Recombination at Synapsis (Crossing Over): Homologous chromosomes exchange genetic material at points called chiasmata during prophase I, resulting in new allele combinations.

• Fertilization: The fusion of two gametes (egg and sperm) is a random event, further increasing genetic diversity in offspring.

Foundations of Mendelian Genetics

Gregor Mendel and the Laws of Inheritance

Gregor Mendel, through his experiments with pea plants, established the basic principles of heredity. He demonstrated that traits are inherited as discrete units (genes), not through blending.

• Blending Hypothesis: The outdated idea that parental traits mix to form intermediate offspring. Mendel's experiments disproved this, showing that traits can reappear in later generations.

Mendel’s Experimental Approach

• True-Breeding Plants: Mendel started with plants that consistently produced the same trait over generations.

• Controlled Crosses: He removed stamens to prevent self-fertilization and used pollen from other plants to fertilize flowers artificially.

Mendel’s Hypothesis and Laws

Four Components of Mendelian Genetics

1. Alternative Versions of Genes (Alleles): Genes can exist in different forms called alleles, which account for variations in inherited traits.

2. Inheritance of Alleles: Each organism inherits two alleles for each gene, one from each parent.

3. Dominance: If two alleles differ, the dominant allele determines the phenotype, while the recessive allele is masked.

4. Law of Segregation: The two alleles for a gene segregate during gamete formation, so each gamete carries only one allele for each gene.

Genotype vs. Phenotype

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, or aa).

• Phenotype: The observable traits resulting from the genotype (e.g., purple or white flowers).

Homozygous and Heterozygous

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

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

Punnett Squares and Probability

Punnett squares are used to predict the possible genotypes and phenotypes of offspring from a genetic cross. Probability rules (such as the multiplication rule) are applied to determine the likelihood of specific outcomes.

Extensions of Mendelian Genetics

Incomplete Dominance

In incomplete dominance, the heterozygote displays a phenotype intermediate between the two homozygotes. This is not blending inheritance, as the original traits can reappear in later generations.

Codominance and Multiple Alleles

In codominance, both alleles in a heterozygote are fully expressed, as seen in human ABO blood groups. Multiple alleles mean that more than two forms of a gene exist in the population.

Polygenic Inheritance

Polygenic inheritance occurs when two or more genes affect a single phenotype, often resulting in continuous variation (e.g., skin color, height).

Epistasis

Epistasis is when the expression of one gene is affected by another gene. For example, in mice, one gene determines pigment color, while another gene controls pigment deposition. If the deposition gene is homozygous recessive, the mouse is albino regardless of the pigment gene.

Environmental Effects on Phenotype

Environmental factors such as nutrition, temperature, and pH can influence the expression of genetic traits. For example, the coat color of Himalayan rabbits depends on temperature, and hydrangea flower color depends on soil pH.

Human Genetic Disorders

Recessive Disorders

• Sickle Cell Anemia: Caused by a mutation in the hemoglobin gene, leading to abnormal red blood cell shape and resistance to malaria in heterozygotes.

• Cystic Fibrosis: Caused by a recessive allele affecting chloride channels, leading to mucus buildup and respiratory issues.

Dominant Disorders

• Achondroplasia (Dwarfism): A dominant allele causes dwarfism, but the allele is rare in the population.

• Huntington’s Disease: A dominant allele on chromosome 4 causes a fatal neurodegenerative disorder, typically manifesting in mid-adulthood.

Genetic Counseling and Probability

Understanding inheritance patterns allows for genetic counseling and risk assessment for inherited disorders. Probability calculations help predict the likelihood of offspring inheriting specific traits or disorders.

Summary Table: Types of Inheritance

Type of Inheritance	Description	Example
Complete Dominance	Dominant allele masks recessive allele in heterozygote	Purple vs. white pea flowers
Incomplete Dominance	Heterozygote has intermediate phenotype	Pink snapdragon flowers
Codominance	Both alleles fully expressed in heterozygote	ABO blood groups
Polygenic Inheritance	Multiple genes contribute additively to phenotype	Human skin color
Epistasis	One gene affects expression of another gene	Coat color in mice