BackChapter 14-Mendel and the Gene: Principles of Inheritance and Extensions
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Chapter 14: Mendel and the Gene
The Garden Pea as a Model Organism
Gregor Mendel used the garden pea (Pisum sativum) as the first model organism in genetics, laying the foundation for the chromosome theory of inheritance. Mendel 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 ideal for studying inheritance patterns.
Key Terms in Mendelian Genetics
Mendelian genetics uses specific terminology to describe inheritance:
Gene: A hereditary unit that determines a trait.
Allele: Different versions of a gene.
Dominant allele: Expressed in the phenotype when present.
Recessive allele: Masked by the dominant allele in heterozygotes.
Homozygous: Two identical alleles for a gene.
Heterozygous: Two different alleles for a gene.
Genotype: Genetic makeup of an organism.
Phenotype: Observable traits of an organism.
Mendel’s Monohybrid Crosses and the Principle of Segregation
Mendel performed monohybrid crosses to study inheritance of a single trait. He developed the principle of segregation, stating that two members of each gene pair segregate into different gametes during egg and sperm formation. Mendel used letters to represent alleles (e.g., R for dominant, r for recessive).
Each parent contributes one allele to offspring.
Segregation occurs during meiosis.
Mendel’s Dihybrid Crosses and the Principle of Independent Assortment
Mendel’s dihybrid crosses examined inheritance of two traits simultaneously. The principle of independent assortment states that alleles of different genes are transmitted independently of one another. A Punnett square predicts four possible phenotypes in a 9:3:3:1 ratio for dihybrid crosses.
Genes located on different chromosomes assort independently.
Independent assortment occurs during meiosis.
Testcrosses
A testcross involves crossing a parent with a dominant phenotype (unknown genotype) with a parent that contributes only recessive alleles. The offspring’s phenotypes reveal the unknown genotype. Mendel used testcrosses to confirm the principle of independent assortment.
Meiosis Explains Mendel’s Principles
Meiosis is the cellular process that explains Mendel’s principles:
Segregation: Homologous chromosomes separate, distributing alleles into gametes.
Independent assortment: Nonhomologous chromosomes assort independently, explaining the transmission of different genes.
Extending Mendel’s Rules
Linkage and Crossing Over
Linkage refers to the tendency of genes located on the same chromosome to be inherited together. However, crossing over during meiosis can separate linked genes, especially if they are far apart. The frequency of recombinant offspring estimates the distance between genes, allowing construction of genetic maps.
Genes farther apart are more likely to cross over.
Genetic maps show relative positions of genes.
Multiple Allelism
Some genes have more than two alleles in a population (multiple allelism). For example, the human ABO blood group has three common alleles, each coding for a different enzyme that modifies red blood cell membranes.
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.
Incomplete Dominance
In incomplete dominance, heterozygotes have an intermediate phenotype. For example, crossing pure-line red and white flowered plants produces pink flowers in the offspring.
Environmental Effects on Phenotype
Phenotypes are often influenced by environmental factors such as sunlight, water, and soil. Genes and environment interact to shape the phenotype.
Quantitative Traits
Traits that vary continuously (quantitative traits) are controlled by multiple genes and environmental factors. These traits often show a normal distribution (bell-shaped curve) in populations.
Applying Mendel’s Rules to Human Inheritance
Pedigrees and Modes of Transmission
A pedigree is a family tree used to study inheritance patterns in humans. The mode of transmission describes whether a trait is autosomal or sex-linked and the type of dominance involved. Pedigrees help determine if a trait is caused by a dominant or recessive allele and whether it is located on a sex chromosome or autosome.
Autosomal recessive: Trait appears in offspring of unaffected parents.
Autosomal dominant: Trait appears in every generation.
X-linked recessive: Males are more frequently affected; trait skips generations.
X-linked dominant: Males and females equally likely to be affected; trait does not skip generations.
Summary Tables
Tables in Mendelian genetics summarize key terms, ratios from crosses, and inheritance patterns. They are essential for comparing and classifying genetic outcomes.
Term | Definition |
|---|---|
Gene | Unit of heredity |
Allele | Variant of a gene |
Dominant | Expressed in heterozygote |
Recessive | Masked in heterozygote |
Homozygous | Identical alleles |
Heterozygous | Different alleles |
Cross Type | Phenotypic Ratio |
|---|---|
Monohybrid | 3:1 |
Dihybrid | 9:3:3:1 |
Key Equations
Probability calculations in Mendelian genetics:
Probability of a genotype:
Expected ratio for monohybrid cross:
Expected ratio for dihybrid cross:
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