Mendel and the Gene: Foundations and Extensions of Mendelian Genetics

Chapter 14: Mendel and the Gene

The Garden Pea as a Model Organism

Gregor Mendel established the foundation for modern genetics 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 matings. These features made peas an ideal model organism for studying inheritance.

Key Terms in Mendelian Genetics

Mendelian genetics uses specific terminology to describe inheritance patterns:

• Gene: A hereditary factor that determines a particular trait.

• Allele: Different versions of a gene.

• Genotype: The genetic makeup of an organism.

• Phenotype: Observable characteristics of an organism.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Dominant allele: Expressed in the phenotype when present.

• Recessive allele: Expressed only when two copies are present.

Mendel’s Monohybrid Crosses and the Principle of Segregation

Mendel’s monohybrid crosses involved parents differing in a single trait. He discovered that two members of each gene pair segregate during gamete formation, so each gamete carries only one allele for each gene. This is known as the principle of segregation.

• Dominant and recessive alleles are represented by uppercase and lowercase letters, respectively (e.g., R for round, r for wrinkled).

• Each parent contributes one allele to the offspring.

Mendel’s Dihybrid Crosses and the Principle of Independent Assortment

By examining two traits simultaneously (dihybrid crosses), Mendel found that alleles of different genes assort independently during gamete formation. This is the principle of independent assortment.

• Predicted phenotypic ratio for a dihybrid cross is 9:3:3:1.

• Alleles for different traits are transmitted independently if they are on different chromosomes.

Testcrosses

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The phenotypes of the offspring reveal the unknown genotype.

Chromosomal Basis of Mendel’s Principles

Mendel’s principles are explained by the behavior of chromosomes during meiosis:

• Genes located on different nonhomologous chromosomes assort independently.

• Segregation of alleles occurs during the separation of homologous chromosomes.

Extending Mendel’s Rules

Not all traits follow simple Mendelian inheritance. Extensions include:

• Linkage: Genes located close together on the same chromosome tend to be inherited together.

• Crossing Over: Genes far apart on the same chromosome can be separated by crossing over, producing recombinant offspring. The frequency of recombination can be used to create genetic maps showing the relative positions of genes.

Multiple Alleles and Codominance

Some genes have more than two alleles (multiple allelism). For example, the human ABO blood group is determined by three alleles (IA, IB, and i).

• Codominance: Both alleles are expressed in the phenotype of heterozygotes (e.g., AB blood type).

Incomplete Dominance

In incomplete dominance, heterozygotes have an intermediate phenotype between the two homozygotes. For example, crossing red-flowered and white-flowered plants produces pink-flowered offspring.

Environmental Effects on Phenotype

Phenotype is often influenced by both genotype and environment. Mendel controlled environmental variables in his experiments, but in nature, factors such as sunlight, water, and soil can affect trait expression.

Quantitative Traits

Some traits, called quantitative traits, show continuous variation and are influenced by multiple genes (polygenic inheritance). These traits often display a normal distribution in populations.

Human Inheritance and Pedigree Analysis

Inheritance patterns in humans are studied using pedigrees, which are family trees that track traits across generations. Modes of transmission include:

• Autosomal dominant

• Autosomal recessive

• Sex-linked (X-linked) dominant

• Sex-linked (X-linked) recessive

Pedigrees help determine whether a trait is dominant or recessive and whether it is autosomal or sex-linked.

Summary Table: Key Terms in Mendelian Genetics

Term	Definition
Gene	Hereditary factor that determines a trait
Allele	Alternative form of a gene
Genotype	Genetic makeup of an organism
Phenotype	Observable characteristics
Homozygous	Two identical alleles
Heterozygous	Two different alleles
Dominant	Allele expressed in phenotype
Recessive	Allele masked in heterozygote

Summary Table: Mendel’s Monohybrid Cross Results

Parental Genotypes	F1 Phenotype	F2 Phenotype Ratio
RR x rr	All round	3 round : 1 wrinkled
YY x yy	All yellow	3 yellow : 1 green
SS x ss	All smooth	3 smooth : 1 constricted
TT x tt	All tall	3 tall : 1 short
PP x pp	All purple	3 purple : 1 white
AA x aa	All axial	3 axial : 1 terminal

Additional info: The above tables are reconstructed based on standard Mendelian genetics and the context provided in the source material.