Mendelian Genetics

Introduction

Mendelian genetics is the study of how traits are inherited from one generation to the next, based on the principles established by Gregor Mendel. This topic covers the fundamental laws of inheritance, key genetic terminology, and the use of Punnett squares to predict genetic outcomes.

Key Genetic Terms

• Gene: A segment of DNA that codes for a specific trait. Genes are inherited from parents and determine various characteristics in an organism.

• Allele: Different forms of a gene found at the same locus on homologous chromosomes. Each individual inherits two alleles for each gene, one from each parent.

• Dominant Allele: An allele that is expressed in the phenotype even if only one copy is present. Represented by a capital letter (e.g., A).

• Recessive Allele: An allele that is only expressed in the phenotype if two copies are present. Represented by a lowercase letter (e.g., a).

• Genotype: The genetic makeup of an organism; the combination of alleles present (e.g., AA, Aa, or aa).

• Phenotype: The observable physical or physiological traits of an organism, determined by its genotype (e.g., flower color, seed shape).

• Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa).

• Heterozygous: Having two different alleles for a particular gene (e.g., Aa).

• Diploid: Having two sets of chromosomes (and thus two alleles for each gene), typical of somatic cells in animals and plants.

• Gamete: A reproductive cell (sperm or egg) that carries only one allele for each gene due to meiosis.

Mendel’s Laws of Inheritance

• Law of Segregation: During gamete formation, the two alleles for a gene separate (segregate) so that each gamete receives only one allele. This explains why offspring inherit one allele from each parent.

  • Example: A plant with genotype Aa produces gametes with either A or a allele.

• Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome.

  • Example: The inheritance of seed color does not affect the inheritance of seed shape in pea plants.

Punnett Squares

Punnett squares are diagrams used to predict the possible genotypes and phenotypes of offspring from a genetic cross. They are especially useful for visualizing monohybrid (single trait) and dihybrid (two traits) crosses.

• Monohybrid Cross: Examines the inheritance of one trait (e.g., flower color).

• Dihybrid Cross: Examines the inheritance of two traits simultaneously (e.g., seed color and seed shape).

Example: Monohybrid Cross

Crossing two heterozygous pea plants (Yy x Yy) for seed color:

Genotypic ratio: 1 YY : 2 Yy : 1 yy Phenotypic ratio: 3 yellow : 1 green (assuming yellow is dominant)

Example: Dihybrid Cross

Crossing two heterozygous plants for two traits (YyRr x YyRr):

The phenotypic ratio for a dihybrid cross with independent assortment is typically 9:3:3:1.

Comparison of Key Terms

Formulas and Equations

• Probability of a genotype or phenotype:

• Number of possible gamete combinations (for n heterozygous gene pairs):

Applications

• Predicting the likelihood of inheriting genetic diseases.

• Understanding patterns of inheritance in plants and animals.

• Breeding programs in agriculture and animal husbandry.

Additional info: The notes reference practice problems and using Punnett squares, which are essential skills for mastering Mendelian genetics. Students are encouraged to practice both monohybrid and dihybrid crosses to reinforce these concepts.