Mendel and the Gene: Principles of Inheritance and Extensions

Chapter 14: Mendel and the Gene

The Garden Pea as a Model Organism

Gregor Mendel established the foundation for the chromosome theory of inheritance through experiments with garden peas (Pisum sativum). He selected peas because they were inexpensive, easy to grow, had a short generation time, produced many seeds, and allowed controlled mating. These features made peas an ideal model organism for genetic studies.

Key Terms in Mendelian Genetics

Mendelian genetics uses specific terminology to describe inheritance patterns. Understanding these terms is essential for interpreting genetic crosses and results.

Term	Definition
Gene	A hereditary unit that determines a trait
Allele	Different versions of a gene
Dominant	An allele that masks the effect of a recessive allele
Recessive	An allele whose effect is masked by a dominant allele
Homozygous	Having two identical alleles for a gene
Heterozygous	Having two different alleles for a gene
Genotype	The genetic makeup of an organism
Phenotype	The observable traits of an organism

Monohybrid Crosses and the Principle of Segregation

Mendel's monohybrid crosses involved tracking a single trait. He discovered the principle of segregation, which states that two members of each gene pair separate into different gametes during egg and sperm formation. This explains why offspring inherit one allele from each parent.

• Key Point: Each parent contributes one allele for each gene.

• Example: R (dominant for round seed) and r (recessive for wrinkled seed).

Dihybrid Crosses and the Principle of Independent Assortment

Mendel extended his experiments to two traits (dihybrid crosses). He found that alleles of different genes are transmitted independently, leading to the principle of independent assortment. This means the inheritance of one trait does not affect the inheritance of another if the genes are on different chromosomes.

• Key Point: Four possible phenotypes appear in a 9:3:3:1 ratio in the F2 generation.

• Example: Seed shape and seed color.

Testcrosses

A testcross is used to determine the genotype of a parent with a dominant phenotype by crossing it with a parent that contributes only recessive alleles. The phenotypes of the offspring reveal the unknown genotype.

• Key Point: Testcrosses confirm predictions about inheritance and independent assortment.

Meiosis and Mendel’s Principles

Meiosis explains Mendel’s principles. Genes located on different nonhomologous chromosomes assort independently during meiosis, accounting for the principle of independent assortment. The principle of segregation is explained by the separation of homologous chromosomes.

• Key Point: Chromosome behavior during meiosis underlies Mendelian inheritance.

Extending Mendel’s Rules

Researchers found that some traits do not follow Mendel’s rules. These include linkage, multiple alleles, codominance, and incomplete dominance.

Linkage and Crossing Over

Genes located close together on the same chromosome tend to be inherited together (linkage). However, crossing over during meiosis can separate linked genes. The frequency of recombinant offspring estimates the distance between genes, allowing construction of genetic maps.

• Key Point: Genes farther apart are more likely to cross over.

• Example: Mapping genes in fruit flies.

Multiple Allelism

Some genes have more than two alleles in a population (multiple allelism). For example, the human ABO blood group gene has three common alleles, each coding for a different enzyme that modifies red blood cell membranes.

• Key Point: Multiple alleles increase genetic diversity.

• Example: ABO blood types.

Codominance

Codominance occurs when both alleles in a heterozygote are fully expressed. In the ABO blood group, individuals with genotype IAIB express both A and B antigens.

• Key Point: Both alleles contribute equally to the phenotype.

Incomplete Dominance

Incomplete dominance is when heterozygotes have an intermediate phenotype. For example, crossing red-flowered and white-flowered plants produces pink-flowered offspring.

• Key Point: Neither allele is completely dominant; the phenotype is blended.

Environmental Effects on Phenotype

Mendel controlled environmental variables in his experiments, but most phenotypes are influenced by both genes and environment. Environmental effects include factors like sunlight, water, and soil, which interact with genetic factors to shape phenotype.

• Key Point: Phenotype is the result of both genetic and environmental influences.

Quantitative Traits

Traits that vary continuously, such as height or skin color, are called quantitative traits. These are influenced by many genes and often show a normal distribution in populations.

• Key Point: Quantitative traits are polygenic and do not fall into discrete categories.

Applying Mendel’s Rules to Human Inheritance

Human traits can be analyzed using pedigrees to determine their mode of transmission (autosomal or sex-linked, dominant or recessive). Pedigrees help identify whether a trait is caused by a single gene and its inheritance pattern.

• Key Point: Pedigrees are essential for studying inheritance in humans.

X-Linked Recessive Traits

X-linked recessive traits, such as red-green color blindness, are more common in males. The trait is never passed from father to son, and carrier females can transmit the trait to their sons.

X-Linked Dominant Traits

X-linked dominant traits affect both males and females equally. Affected males always have affected daughters, and the trait does not skip generations.

Summary Table: Mendelian Inheritance Patterns

Pattern	Key Features	Example
Autosomal Dominant	Appears in every generation; affected individuals have at least one affected parent	Huntington's disease
Autosomal Recessive	Can skip generations; affected individuals may have unaffected parents	Cystic fibrosis
X-Linked Recessive	More common in males; trait never passed father to son	Red-green color blindness
X-Linked Dominant	Affects both sexes; does not skip generations	Hypophosphatemia

----------------------------------------