Patterns of Inheritance: Mendel’s Laws and Beyond

Chapter 10: Patterns of Inheritance

10.1 Mendel’s Laws

This section introduces Gregor Mendel’s foundational work on inheritance, which established the basic principles of genetics using garden pea plants (Pisum sativum).

• Gregor Mendel: Austrian monk who conducted experiments in the 1860s to understand how traits are inherited.

• Significance: Mendel’s model explained how variations arise and are passed on, supporting evolutionary theory.

• Key Insight: Both sexes contribute equally to offspring, but combinations of traits are tested by the environment, and those leading to reproductive success are inherited.

Mendel’s Experimental Procedure

• Model Organism: Garden pea (Pisum sativum), chosen for its ease of cultivation, short generation time, and ability to self- or cross-pollinate.

• True-Breeding Varieties: Mendel used plants that consistently produced offspring identical to themselves.

• Data Collection: Mendel kept detailed records and used mathematical probability to interpret results.

• Particulate Theory of Inheritance: Traits are determined by discrete units (now called genes).

Garden Pea Anatomy and Traits

• Flower Structure: Includes stigma, anther, and ovary. Fertilization occurs when pollen (sperm) from the anther reaches the ovary via the stigma, resulting in seeds.

• Cross-Pollination: Mendel manually transferred pollen to control parentage.

One-Trait Inheritance

One-trait (monohybrid) crosses track a single characteristic.

• P Generation: Parental generation (true-breeding).

• F1 Generation: First filial generation (offspring of P generation).

• F2 Generation: Second filial generation (offspring of F1 self-pollination).

• Example: Crossing green pod (dominant) with yellow pod (recessive) plants yields all green pods in F1. F2 shows a 3:1 ratio of green to yellow pods.

Punnett Square

• Definition: A diagram that shows all possible combinations of alleles from egg and sperm.

• Application: Used to predict offspring ratios. For a monohybrid cross, F2 generation shows 3/4 dominant phenotype, 1/4 recessive.

Mendel’s Interpretation

• 3:1 Ratio Explanation: Each parent has two copies (alleles) for each trait; these separate during gamete formation, and fertilization is random.

Mendel’s First Law: Law of Segregation

• Each individual has two factors (alleles) for each trait.

• Alleles segregate during gamete formation; each gamete receives one allele.

• Fertilization restores two alleles for each trait in offspring.

One-Trait Testcross

• Purpose: To determine if an individual with a dominant phenotype is homozygous or heterozygous.

• Method: Cross with a homozygous recessive individual.

• Results: 1:1 ratio indicates heterozygous; all dominant phenotype indicates homozygous dominant.

The Modern Interpretation of Mendel’s Work

• Chromosomal Theory of Inheritance: Genes are located on chromosomes, which segregate during meiosis.

• Gene Locus: Specific location of a gene on a chromosome.

• Meiosis: Explains segregation and independent assortment of alleles.

Alleles

• Definition: Alternative forms of a gene.

• Dominant vs. Recessive: Dominant alleles mask recessive alleles.

• Gene Locus: Alleles are found at the same locus on homologous chromosomes.

Genotype Versus Phenotype

• Genotype: The alleles an individual possesses (e.g., AA, Aa, aa).

• Phenotype: The observable trait (e.g., normal pigmentation, albinism).

• Homozygous: Two identical alleles (dominant or recessive).

• Heterozygous: Two different alleles.

Allele Combination	Genotype	Phenotype
AA	Homozygous dominant	Normal pigmentation
Aa	Heterozygous	Normal pigmentation
aa	Homozygous recessive	Albinism

Two-Trait Inheritance (Dihybrid Cross)

• Definition: Examines inheritance of two traits simultaneously.

• Example: Crossing TTGG (tall, green pods) with ttgg (short, yellow pods) yields F1 with both dominant traits.

• Possible Results: If alleles assort together, only two phenotypes; if independently, four phenotypes.

Mendel’s Second Law: Law of Independent Assortment

• Each pair of alleles segregates independently during gamete formation.

• All combinations of alleles are possible in gametes.

• Phenotypic Ratio: For a dihybrid cross, the expected ratio is 9:3:3:1.

Two-Trait Testcross

• Model Organism: Fruit fly (Drosophila melanogaster).

• Purpose: To determine genotype of an individual with dominant phenotypes for two traits.

• Method: Cross with an individual homozygous recessive for both traits.

• Result: 1:1:1:1 ratio indicates dihybrid parent.

Mendel’s Laws and Probability

• Punnett Squares: Assume each gamete contains one allele for each trait and that gametes combine randomly.

• Rule of Multiplication: Probability of two independent events both occurring is the product of their individual probabilities.

• Equation Example: Probability of two heads in two coin flips: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

10.2 Mendel’s Laws Apply to Humans

Genetic principles apply to human inheritance, often studied using pedigrees.

• Pedigree: Diagram showing family history for a trait. Males are squares, females are circles, shaded shapes indicate affected individuals.

• Genetic Counseling: Uses pedigree analysis to predict inheritance risks.

Pedigrees for Autosomal Disorders

• Autosomal Recessive Disorders: Affected children can have unaffected parents; heterozygotes are carriers.

• Autosomal Dominant Disorders: Affected individuals usually have at least one affected parent; unaffected parents do not pass on the disorder.

Examples of Autosomal Disorders

• Methemoglobinemia: Harmless disorder causing bluish skin due to inability to convert methemoglobin to hemoglobin.

• Cystic Fibrosis: Lethal autosomal recessive disorder; defective chloride ion channel causes thick mucus.

• Alkaptonuria: Lack of homogentisate oxygenase leads to black urine upon air exposure.

• Sickle-Cell Disease: Single base change in hemoglobin gene causes sickle-shaped red blood cells, leading to anemia and poor circulation.

• Huntington Disease: Autosomal dominant; progressive brain degeneration, symptoms appear in middle age.

10.3 Beyond Mendel’s Laws

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

• Example in Humans: Familial hypercholesterolemia—one mutated allele causes high cholesterol, two cause even higher levels.

Additional info:

• These principles are foundational for understanding genetic diseases, inheritance patterns, and the molecular basis of traits in Anatomy & Physiology.