Mendelian Genetics

Key Terms and Definitions

Mendelian genetics explores how traits are inherited from one generation to the next, based on the principles established by Gregor Mendel. Understanding key terminology is essential for mastering this topic.

• True breeding: Parents with a specific phenotype produce offspring only with the same phenotype, indicating genetic uniformity for the trait.

• Hybridization: The process of combining different alleles, often through cross-breeding individuals with distinct genetic backgrounds.

• Monohybrid cross: A genetic cross involving a single trait, where the parents differ in one gene.

Distinguishing Genetic Terms

Several pairs of terms are fundamental to understanding Mendelian inheritance:

• Dominant vs. Recessive:

  • Dominant allele: Masks the presence of a recessive allele in a heterozygous individual.

  • Recessive allele: Expressed only when two copies are present (homozygous recessive).

• Homozygous vs. Heterozygous:

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

  • Heterozygous: Two different alleles for a gene (e.g., Aa).

• Genotype vs. Phenotype:

  • Genotype: The genetic makeup of an organism (the alleles it carries).

  • Phenotype: The observable physical or physiological traits of an organism.

Gene Copies in a Zygote

After fertilization, a zygote contains two copies of each gene:

• One copy is inherited from the sperm cell (father).

• One copy is inherited from the egg cell (mother).

Using Probabilities to Predict Genetic Disorders

Geneticists use probability calculations to estimate the frequency of genetic disorders in populations. This involves analyzing the likelihood of specific genotypes, especially those that are homozygous recessive, which often result in genetic disorders.

• For a disorder caused by a recessive allele, both alleles must be recessive (homozygous recessive).

• Probability calculations help predict the proportion of affected individuals.

Complete Dominance

Complete dominance occurs when the dominant allele fully masks the effect of the recessive allele in heterozygous individuals.

• Only the dominant phenotype is observed in heterozygotes.

• Example: In pea plants, the allele for tallness (S) is dominant over the allele for shortness (s).

Punnett Square Crosses: Phenotypic and Genotypic Ratios

Punnett squares are used to predict the outcomes of genetic crosses. Below are examples of crosses and their resulting ratios:

• S/s Cross (Tall vs. Short):

  • Genotypic ratio: 2 SS : 1 Ss : 1 ss

  • Phenotypic ratio: 3 Tall : 1 Short

• A/a Cross (Black Hair vs. Red Hair):

  • Genotypic ratio: 2 Aa : 2 AA

  • Phenotypic ratio: All Black Hair (if A is dominant)

• B/b Cross (Normal vs. Sickle Cell):

  • Genotypic ratio: 2 Bb : 2 bb

  • Phenotypic ratio: 2 Normal : 2 Sickle Cell

Punnett Square Example:

Additional info: The above table shows a monohybrid cross for a single gene with two alleles.

Rule of Multiplication: Calculating Genotype Frequencies

The rule of multiplication is used to calculate the probability of a specific genotype resulting from a cross. Each gene's probability is multiplied together to find the overall chance.

• Example: For a cross PpYyRr x Ppyyrr, the probability of offspring being pp, yy, and rr is:

Additional info: If all three genes must be homozygous recessive, the probability is the product of the individual probabilities for each gene.

Summary Table: Key Genetic Terms