Mendel and the Gene Idea: Patterns and Complexities of Inheritance

Chapter 14: Mendel and the Gene Idea

Concept 14.1: Mendel's Scientific Approach and Laws of Inheritance

Gregor Mendel's experiments with pea plants established the foundational principles of genetics. His methodical approach led to the discovery of two key laws of inheritance, which explain how traits are transmitted from one generation to the next.

• True Breeding: Organisms that, when self-fertilized, produce offspring identical to themselves for a given trait.

• Hybridization: The mating, or crossing, of two true-breeding varieties with different traits.

• Monohybrid Cross: A cross between individuals heterozygous for a single character.

• P Generation: The parental generation in a genetic cross.

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

• F2 Generation: The second filial generation, produced by self-pollinating or crossing F1 individuals.

Mendel's Model: Four Components

1. Alternative Versions of Genes (Alleles): Genes can exist in different forms (alleles) that account for variations in inherited characters.

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

3. Dominance: If two alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

4. Law of Segregation: The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Mendel's Law of Segregation

Law of Segregation: During the formation of gametes, the two alleles for a gene separate so that each gamete receives only one allele.

Punnett Squares and Genetic Crosses

• Punnett Square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Egg and Sperm Genotypes: The rows and columns of a Punnett square represent the possible gametes (egg and sperm) from each parent.

• Calculating Frequencies: The frequency of each genotype and phenotype among offspring can be determined by counting the number of times each appears in the Punnett square and dividing by the total number of squares.

• Example: Crossing two heterozygotes (Aa x Aa) yields genotypes AA, Aa, and aa in a 1:2:1 ratio and phenotypes in a 3:1 ratio if A is dominant.

Key Genetic Terms

• Dominant vs. Recessive: Dominant alleles mask the effect of recessive alleles in heterozygotes.

• Heterozygous vs. Homozygous: Heterozygous individuals have two different alleles for a gene (e.g., Aa); homozygous individuals have two identical alleles (e.g., AA or aa).

• Genotype vs. Phenotype: Genotype refers to the genetic makeup (e.g., AA, Aa, aa); phenotype refers to the observable traits (e.g., purple or white flowers).

Dihybrid Crosses and the Law of Independent Assortment

• Dihybrid Cross: A cross between individuals heterozygous for two characters (e.g., AaBb x AaBb).

• Phenotypic Ratio: The F2 generation from a dihybrid cross typically shows a 9:3:3:1 ratio.

• Law of Independent Assortment: Each pair of alleles segregates independently of other pairs during gamete formation. This is explained by the random orientation of homologous chromosomes during meiosis I.

Concept 14.3: Complex Patterns of Inheritance

Not all inheritance patterns follow simple Mendelian rules. Some traits are influenced by multiple alleles, gene interactions, or environmental factors.

Incomplete Dominance

• Definition: The phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.

• Example: Crossing red-flowered (RR) and white-flowered (rr) snapdragons produces pink-flowered (Rr) offspring.

Complete Dominance, Incomplete Dominance, and Codominance

• Complete Dominance: Heterozygote phenotype is identical to that of the dominant homozygote.

• Incomplete Dominance: Heterozygote phenotype is intermediate between the two homozygotes.

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

ABO Blood System and Codominance

• Inheritance: The ABO blood group is determined by three alleles: IA, IB, and i.

• Codominance: Both IA and IB are expressed in individuals with AB blood type.

• Example: A person with genotype IAIB has both A and B antigens on red blood cells.

Pleiotropy and Epistasis

• Pleiotropy: A single gene affects multiple phenotypic traits (e.g., the gene for sickle-cell disease affects hemoglobin structure, anemia, and resistance to malaria).

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers, where one gene determines pigment color and another gene determines pigment deposition).

Polygenic Inheritance

• Definition: Multiple genes independently affect a single trait, resulting in continuous variation (e.g., human skin color, height).

• Explanation: The additive effect of two or more genes leads to a range of phenotypes rather than discrete categories.

Concept 14.4: Mendelian Patterns in Human Traits

Many human traits and disorders follow Mendelian inheritance patterns, though some are influenced by more complex genetic and environmental factors.

Examples of Mendelian Disorders

• Albinism: A recessive disorder characterized by a lack of melanin pigment in the skin, hair, and eyes.

• Cystic Fibrosis: A recessive disorder causing thick mucus buildup in the lungs and digestive tract due to defective chloride channels.

• Tay-Sachs Disease: A recessive disorder leading to the accumulation of lipids in brain cells, causing neurological impairment.

• Sickle-Cell Anemia: A recessive disorder where abnormal hemoglobin causes red blood cells to become sickle-shaped, leading to anemia and other complications.

Late-Acting Lethal Dominant Genes

• Example: Huntington's disease is a dominant genetic disorder that manifests later in life, often after reproductive age.

• Explanation: Because symptoms appear after individuals have had children, the allele can be passed to the next generation before its effects are known, allowing it to escape elimination by natural selection.

Summary Table: Key Genetic Terms and Examples

Term	Definition	Example
Dominant	Allele that determines phenotype in heterozygotes	Widow's peak hairline
Recessive	Allele masked in heterozygotes	Attached earlobes
Homozygous	Two identical alleles for a gene	AA or aa
Heterozygous	Two different alleles for a gene	Aa
Pleiotropy	One gene affects multiple traits	Sickle-cell disease
Epistasis	One gene affects expression of another	Labrador coat color
Polygenic Inheritance	Multiple genes affect one trait	Human skin color

Key Equations

• Probability of Offspring Genotype (Monohybrid Cross):

$P(AA) = P(A \, \, from \, \, sperm) \times P(A \, \, from \, \, egg)$

• Phenotypic Ratio (Monohybrid Cross):

$3:1 \, \, (dominant : recessive)$

• Phenotypic Ratio (Dihybrid Cross):

$9:3:3:1 \, \, (both \, \, dominant : dominant/recessive : recessive/dominant : both \, \, recessive)$

Additional info: This guide expands on the learning objectives by providing definitions, examples, and explanations for each key concept, ensuring a comprehensive understanding of Mendelian and non-Mendelian inheritance patterns.