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 through experiments with garden pea plants (Pisum sativum).

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

• Significance: Mendel’s work showed that both sexes contribute equally to offspring and that variations among family members are due to the combination and segregation of heritable factors (genes).

• Evolutionary Context: Mendel’s model is compatible with evolution, as environmental selection acts on trait combinations, favoring those that enhance reproductive success.

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 meticulous records and applied mathematical probability to interpret his results.

• Particulate Theory of Inheritance: Traits are determined by discrete units (genes) that are inherited independently.

One-Trait Inheritance

Mendel studied inheritance of single traits (e.g., pod color) through controlled crosses.

• 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 plants with yellow pod plants produced all green pods in F1, but yellow pods reappeared in F2.

Punnett Square

• Definition: A diagram that shows all possible combinations of alleles from egg and sperm, predicting offspring genotypes and phenotypes.

• Application: Used to visualize Mendel’s results and expected ratios (e.g., 3:1 green to yellow pods in F2).

Mendel’s Interpretation

• Key Insight: The 3:1 ratio in F2 is possible only if each parent carries two copies of each gene (alleles), which segregate during gamete formation.

• Random Fertilization: All possible gamete combinations occur at fertilization.

Mendel’s First Law: Law of Segregation

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

• These factors segregate during gamete formation, so each gamete receives only one allele from each pair.

• Fertilization restores the pair of alleles in the offspring.

One-Trait Testcross

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

• Method: Cross the individual with a homozygous recessive plant.

• Results: If any offspring show the recessive trait, the tested parent is heterozygous; if all show the dominant trait, the parent is homozygous dominant.

The Modern Interpretation of Mendel’s Work

• Chromosomal Theory of Inheritance: Genes are located on chromosomes, which segregate during meiosis, explaining Mendel’s laws.

• Gene Locus: The specific location of a gene on a chromosome.

• Homologous Chromosomes: Each carries one allele for each gene pair.

Alleles

• Definition: Alternative forms of a gene.

• Dominant vs. Recessive: Dominant alleles mask the expression of recessive alleles.

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

Genotype Versus Phenotype

• Genotype: The genetic makeup (allele combination) of an individual (e.g., AA, Aa, aa).

• Phenotype: The observable physical or physiological traits determined by the genotype.

• 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

Mendel also studied inheritance of two traits simultaneously (dihybrid crosses).

• Example: Crossing tall, green pod plants (TTGG) with short, yellow pod plants (ttgg).

• Results: F1 plants showed both dominant traits; F2 generation revealed four phenotypes if alleles segregate independently.

Mendel’s Second Law: Law of Independent Assortment

• Each pair of alleles segregates independently of other pairs during gamete formation.

• All possible combinations of alleles can occur in gametes.

• Expected F2 Phenotypic Ratio: 9:3:3:1 in a dihybrid cross.

Two-Trait Testcross

• Model Organism: Fruit fly (Drosophila melanogaster).

• Purpose: To determine genotype of an individual showing dominant phenotypes for two traits by crossing with a double recessive individual.

• Result: A 1:1:1:1 ratio in offspring indicates the tested parent is heterozygous for both traits (dihybrid).

Mendel’s Laws and Probability

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

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

• Example: Probability of flipping two tails in a row: $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

10.2 Mendel’s Laws Apply to Humans

Mendelian inheritance patterns are observed in human genetics, often studied using pedigrees.

• Pedigree: A chart showing family history for a particular trait.

• Symbols: Squares for males, circles for females, shaded for affected individuals, horizontal lines for unions, vertical lines for offspring.

• Genetic Counseling: Pedigrees help predict the likelihood of inheriting genetic disorders.

Pedigrees for Autosomal Disorders

• Autosomal Recessive Disorders: Affected individuals can have unaffected parents; heterozygous parents 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: Lack of enzyme to convert methemoglobin to hemoglobin; causes bluish skin but is relatively harmless.

• Cystic Fibrosis: Most common lethal genetic disorder among Caucasians in the U.S.; caused by defective chloride ion channels, leading to thick mucus.

• Alkaptonuria: Lack of homogentisate oxygenase; homogentisic acid accumulates, turning urine black when exposed to air.

• 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 degeneration of neurons, symptoms appear in middle age.

10.3 Beyond Mendel’s Laws

Some inheritance patterns do not follow simple Mendelian rules.

• Incomplete Dominance: Heterozygotes have an 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.

Multiple-Allele Traits

• More than two alleles exist for some genes (e.g., ABO blood group: IA, IB, i).

• Each individual has only two alleles, but multiple combinations are possible.

• IA and IB are codominant; both are expressed in type AB blood.

Polygenic Inheritance

• Traits are governed by two or more sets of alleles, each contributing additively to the phenotype (e.g., human height, skin color).

• Results in continuous variation, often forming a bell-shaped curve in populations.

• Multifactorial traits are influenced by both genes and environment (e.g., diabetes, allergies, cancer).

Environmental Influences

• Environmental factors can affect gene expression (e.g., UV exposure increases melanin production in skin).

Pleiotropy

• A single gene affects multiple traits (e.g., Marfan syndrome, sickle-cell anemia).

Linkage

• Genes located on the same chromosome tend to be inherited together (gene linkage).

• Linked genes do not assort independently unless crossing over occurs during meiosis.

10.4 Sex-Linked Inheritance

• Sex Chromosomes: Females are XX, males are XY; Y chromosome carries SRY gene for maleness.

• X-Linked Genes: Genes on the X chromosome; males are more likely to express X-linked recessive traits because they have only one X.

• Carrier: A female who has one copy of a recessive X-linked allele but does not express the trait.

Pedigree for Sex-Linked Disorders

• X-Linked Recessive Disorders: More common in males; affected males inherit the allele from their mother.

• X-Linked Dominant Disorders: Daughters of affected males always inherit the condition; affected females can pass it to both sons and daughters.

• Y-Linked Disorders: Only affect males and are passed from father to all sons.

Examples of X-Linked Recessive Disorders

• Color Blindness: Red-green color blindness affects about 8% of white males.

• Duchenne Muscular Dystrophy: Absence of dystrophin protein causes muscle degeneration.

Additional info: These principles are foundational for understanding human genetics, genetic disorders, and the molecular basis of inheritance, which are essential for Anatomy & Physiology students.