Patterns of Inheritance

Mendel and Pea Plants

Gregor Mendel's experiments with pea plants established the foundational principles of inheritance. He chose pea plants for their distinct characters and traits, such as flower color, which could be easily observed and categorized.

• Unique Aspects: Mendel used controlled crosses and quantitative analysis, focusing on one character at a time.

• Character Example: Flower color (character); purple and white (traits).

Mendel’s Experiments and Conclusions

Mendel performed crosses between true-breeding plants and observed 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 both traits in a 3:1 ratio.

• Surprise in F2: Reappearance of the recessive trait.

• Mendel’s Explanation: Traits are determined by discrete units (genes) that segregate during gamete formation.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during meiosis so each gamete receives one allele.

Traits and Alleles

Alleles are alternative forms of a gene found at the same locus on homologous chromosomes.

• Alleles: Variants of a gene (e.g., purple or white flower allele).

• Purple allele: Produces purple pigment.

• White allele: Produces no pigment.

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

• Diploids: Two alleles per character.

• Haploids: One allele per character.

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

Solving Heredity Problems: Monohybrid Crosses

Monohybrid crosses involve a single character and follow Mendel’s steps for solving genetics problems.

• Steps:

  1. Write allele and genotype keys.

  2. Determine genotypes of parents.

  3. Determine gametes produced by each parent.

  4. Show fertilization using a Punnett square.

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

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

Example: Monohybrid Cross

If a white flowering plant (pp) is crossed with a homozygous purple flowering plant (PP):

• Genotype ratio:

• Phenotype ratio: Purple: White

Dihybrid Crosses

Dihybrid crosses involve two characters and demonstrate the principle of independent assortment.

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

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

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

Example: Dihybrid Cross

Crossing :

• All offspring will be (heterozygous for both traits).

Example: F1 Dihybrid Cross

Crossing :

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

Independent Assortment

The law of independent assortment states that alleles of different genes assort independently during meiosis.

• Meiosis Phase: Metaphase I

• Hybrid Parent: Four types of gametes formed.

• F2 Phenotype Ratio: 9:3:3:1

Rules of Probability in Genetics

Probability rules are used to predict the likelihood of specific genotypes and phenotypes in offspring.

• Application: Used for complex crosses and multiple gene inheritance.

Pedigrees

Pedigrees are diagrams that show the inheritance patterns of traits across generations in families.

• Purpose: To track inheritance of traits and identify dominant or recessive patterns.

• Generation Number: Top row is generation I.

• Affected Individuals: Usually shaded.

Pedigree Practice Problem 1: Tay Sachs Disease

Tay Sachs disease is tracked in a pedigree showing three generations. Shaded individuals represent those affected by the trait.

• Dominant or Recessive: The trait is recessive, as it skips generations and appears in offspring of unaffected parents.

• Genotype of Affected Individual (Generation I): Homozygous recessive (e.g., tt).

• Genotype of Female in Generation II with Children: Likely heterozygous (carrier).

Pedigree Practice Problem 2: Achondroplasia Dwarfism

Achondroplasia dwarfism is tracked in a pedigree with three generations. Multiple individuals are shaded, indicating they are affected.

• Dominant or Recessive: The trait is dominant, as it appears in every generation.

• Genotypes of Individuals 1, 2, and 3:

  • 1: Heterozygous dominant (e.g., Dd)

  • 2: Heterozygous dominant (e.g., Dd)

  • 3: Homozygous recessive (e.g., dd)

Pedigree Summary

To determine if a pedigree shows a dominantly or recessively inherited trait:

• Dominant: Trait appears in every generation; affected individuals have at least one affected parent.

• Recessive: Trait may skip generations; affected individuals can have unaffected parents.

Beyond Mendel: Extensions of Mendelian Genetics

Some inheritance patterns do not follow simple Mendelian rules. These include:

Additional info: The notes provide a comprehensive overview of Mendelian genetics, including monohybrid and dihybrid crosses, probability, and pedigree analysis, as well as extensions beyond Mendel's original principles.