Mendelian Genetics

Cell & Molecular Biology

Introduction

• Focus on Mendelian Genetics

• The study of how traits are transmitted from parents to offspring.

Transmission of Traits

• Each parent cell has two alleles for each character.

• Alleles for flower colors: purple and white.

• During gamete formation, parental cells segregate alleles into gametes (sperm and eggs).

• Offspring inherit one allele from each parent, resulting in purple-flowered or white-flowered offspring.

Mendel's Discovery

• Gregor Mendel discovered the basic principles of heredity through experiments with garden peas.

• His approach led to the understanding of heritable features, known as characters. Examples:

  • Character: Flower color

  • Traits: Purple or white flowers.

• Characteristics of peas that made them ideal for study:

  • Availability of various varieties.

  • Short generation time.

  • High offspring numbers.

  • Ability to control mating (self-pollination or cross-pollination).

Mendel's Experimental Approach

True-Breeding Varieties

• Mendel focused on characters with two distinct forms.

• Started with true-breeding varieties (self-pollinating plants).

• Hybridization between true-breeding varieties produces:

  • P Generation: true-breeding parents.

  • F1 Generation: hybrid offspring.

  • F2 Generation: produced by self-pollination of F1 hybrids.

Inheritance Pattern Observations

• Mendel found a 3:1 phenotype ratio in the F2 generation:

  • Dominant trait: Purple flowers

  • Recessive trait: White flowers.

• The recessive trait is not lost; it reappears in later generations.

Mendel's Model of Inheritance

Four Concepts

1. Gene Variations: Different versions of genes (alleles) account for variations in inherited characters (e.g., purple vs. white flowers).

2. Inheritance of Alleles: Organisms inherit two alleles from their parents, which may be identical or different.

3. Dominant vs. Recessive Alleles: One allele may dominate appearance if they differ; dominant alleles mask recessive ones.

4. Law of Segregation: Alleles segregate during gamete formation, with each gamete receiving one allele.

   • Corresponds to meiosis where homologous chromosomes segregate.

Punnett Squares and Predictions

• Mendel's model accounts for the 3:1 ratio in F2 generation.

• Punnett squares can illustrate possible allele combinations.

Genetics Vocabulary

• Homozygote: organism with two identical alleles.

• Heterozygote: organism with two different alleles.

• Phenotype: physical appearance.

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

Testcross Method

• Used to determine genotype of an individual with a dominant phenotype by breeding with a homozygous recessive parent.

• If any offspring show recessive phenotype, the mystery parent is heterozygous.

Law of Independent Assortment

• Derived by observing two characters simultaneously through dihybrid crosses.

• Law states that allele pairs segregate independently during gamete formation.

• Applies to genes on different chromosomes or far apart on the same chromosome.

Probability in Genetics

• Probability principles apply to genetic predictions, similar to tossing coins.

• Multiplication Rule: Probability of multiple independent events occurring together.

• Addition Rule: Probability of one of two mutually exclusive events occurring.

Complex Genetics Problems

• Multicharacter crosses can be analyzed as independent monohybrid crosses.

Inheritance Patterns Beyond Mendel

• Not all traits follow simple Mendelian principles.

• Factors that complicate inheritance:

  • Alleles not completely dominant/recessive.

  • More than two alleles.

  • Genes producing multiple phenotypes.

Degrees of Dominance

• Complete Dominance: Phenotype of heterozygote is identical to dominant homozygote.

• Incomplete Dominance: Intermediate phenotype in hybrids.

• Codominance: Both alleles expressed distinctly in phenotype.

Pleiotropy and Epistasis

• Pleiotropy: One gene affects multiple traits.

• Epistasis: The expression of one gene influences another gene.

Polygenic Inheritance

• Quantitative characters show continuous variation and result from additive effects of multiple genes.

• Traits like height and skin pigmentation are polygenic.

Environmental Impact

• Phenotype can depend on environmental factors in addition to genotype.

• Multifactorial traits involve multiple genes and environmental influences.

Human Genetics

• Challenges in genetic research due to long generation times and few offspring produced.

• Pedigrees used to track traits through generations.

Recessive Alleles

• Recessive disorders appear only in homozygous individuals; carriers are often phenotypically normal.

• Example: Albinism caused by recessive allele.

Consanguineous Matings

• Increase chances of recessive disorders due to shared genetics among close relatives.