Patterns of Inheritance

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical inheritance, describing how traits are passed from parents to offspring. Gregor Mendel's experiments with pea plants established key principles that are still used to solve genetics problems today.

• Mendel’s Unique Approach: Mendel used controlled crosses and statistical analysis, focusing on discrete traits.

• Character vs. Trait: A character is a general feature (e.g., flower color), while traits are specific forms (e.g., purple or white flowers).

Mendel’s Experiments and Conclusions

Mendel’s experiments involved crossing true-breeding plants and analyzing the resulting generations.

• True-breeding: Plants that consistently produce offspring with the same trait.

• P-generation: Parental generation, true-breeding for a trait.

• F1 generation: First filial generation, all showed the dominant trait.

• F2 generation: Second filial generation, showed a 3:1 ratio of dominant to recessive traits.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

Traits and Alleles

Alleles are different versions of a gene, and their combination determines the genotype and phenotype.

• Allele: Variant form of a gene.

• Locus: The physical location of a gene on a chromosome.

• Diploid: Two alleles per character.

• Haploid: One allele per character.

• Meiosis: Alleles segregate so each gamete receives one allele per gene.

Solving Monohybrid Genetics Problems

Monohybrid crosses involve a single character. The steps to solve these problems include writing allele keys, determining parent genotypes, gametes, and using Punnett squares.

• Genotype: Genetic makeup (e.g., PP, Pp, pp).

• Phenotype: Observable trait (e.g., purple or white flowers).

Punnett Square Example

Crossing a homozygous purple (PP) with a white (pp) plant:

• Genotype ratio:

• Phenotype ratio: Purple : White

Dihybrid Crosses and Independent Assortment

Dihybrid crosses track two characters simultaneously and demonstrate the law of independent assortment.

• Dihybrid: Two characters (e.g., seed color and shape).

• Genotype Example: (diploid parent), (haploid gamete).

• Foil Technique: Used to determine possible gametes from a parent with genotype .

Dihybrid Cross Example

Crossing :

• All offspring are (heterozygous for both traits).

Crossing :

• Phenotype ratio: 9:3:3:1 (yellow-round : yellow-wrinkled : green-round : green-wrinkled)

Rules of Probability in Genetics

Probability rules help predict the likelihood of specific genotypes and phenotypes in offspring.

• Multiplication Rule: Probability of independent events occurring together is the product of their probabilities.

• Addition Rule: Probability of either event occurring is the sum of their probabilities.

Pedigree Analysis

Pedigrees are diagrams that show inheritance patterns across generations, useful for tracking traits in families.

• Symbols: Squares = males, circles = females, shaded = affected.

• Generation Number: Top row is generation I.

• Affected Individuals: Usually shaded.

Pedigree Practice Problem 1: Tay Sachs Disease

Tay Sachs is a recessive disorder. The pedigree shows affected individuals in different generations.

• Genotype of affected individual in generation 1: Homozygous recessive.

• Genotype of female in generation II with children: Carrier or unaffected, depending on offspring.

Pedigree Practice Problem 2: Achondroplasia Dwarfism

Achondroplasia is a dominant disorder. The pedigree shows affected individuals in multiple generations.

• Genotypes of labeled individuals: 1, 2, and 3 can be determined based on their affected status and family relationships.

Determining Dominant vs. Recessive Inheritance in Pedigrees

Dominant traits appear in every generation, while recessive traits may skip generations. Careful analysis of shaded and unshaded individuals helps determine the inheritance pattern.

Beyond Mendel: Extensions of Inheritance

Some traits do not follow simple Mendelian rules. The following table summarizes key extensions:

Additional info: Pedigree analysis is a critical tool in medical genetics for identifying carriers and predicting disease risk. Probability calculations are essential for predicting outcomes in both monohybrid and dihybrid crosses.