Genetics and Inheritance Patterns

Patterns of Inheritance

Mendel's Experiments

• Gregor Mendel (1822-1884) conducted groundbreaking work in genetics from a monastery garden in the Czech Republic.
• Key Points of Mendel's Research:
  • Started studying pea plants in 1856.
  • Chose pea plants due to their ability to self-pollinate, short generation time, large offspring numbers, and true breeding characteristics.

Key Vocabulary

• Character: A heritable feature (e.g., flower color, eye color).
• Trait: Each variant of a character (e.g., purple or white flowers).

Mendel's Observations

• Monohybrid Crosses:
  • For instance, crossing purple and white flower traits resulted in offspring (F1 Generation) that were all purple.
  • When F1 plants were self-pollinated, F2 Generation exhibited a 3:1 ratio of purple to white flowers.
• Dominant and Recessive Traits:
  • Purple flower color (dominant) masked the expression of white flower color (recessive).
  • Publication of his work in 1866 emphasized the inheritance of discrete "heritable factors" (now termed genes).

Patterns of Variation

• Continuous Variation: Characteristics show a range (e.g., human height).
• Discontinuous Variation: Characteristics appear in distinct forms (e.g., flower colors).

Laws of Inheritance

Learning Goals

• Examine the relationship between genotypes and phenotypes.
• Learn to use Punnett squares for predicting inheritance outcomes.
• Understand Mendel's laws of segregation and independent assortment.
• Explore methods of test crosses.

Genetics Terms

• Gene: Mendel's heritable factor governing a character (e.g., flower color).
• Alleles: Alternative versions of genes that determine traits (e.g., purple and white flower alleles).
• Genotype: The allelic makeup of an organism (e.g., TT, Tt, or tt).
• Phenotype: The expressed traits of an organism (e.g., purple or white flowers).
• Dominant Allele: Expressed if present; represented by uppercase letters.
• Recessive Allele: Masked by dominant allele; represented by lowercase letters.

Types of Genotypes

• Homozygous: Both alleles are the same (e.g., TT or tt).
• Heterozygous: Two different alleles (e.g., Tt).

Types of Crosses

• Monohybrid Cross: Parents differ in one trait (e.g., flower color).
• Dihybrid Cross: Parents differ in two traits.
• Punnett Square: A tool to predict genotype and phenotype ratios from genetic crosses.

Rules of Probability in Genetics

• Rule of Multiplication: Used to find the probability of multiple independent events occurring together.
• Rule of Addition: Used when considering the chance of an event occurring in various ways.

Mendel's Laws

Law of Segregation

• Three key rules:
  1. Each individual possesses two alleles for any character (one from each parent).
  2. Dominant alleles mask recessive alleles; homozygous dominance and heterozygosity show identical phenotypes.
  3. During gamete formation, alleles segregate so that each gamete carries one allele.

Law of Independent Assortment

• States that alleles for different traits segregate independently during gamete formation, applies to genes on different chromosomes.

Genetic Examples

• Frequency of Dominant Traits: Dominant traits are not necessarily more common in populations (e.g., polydactyly).
• Test Cross: Used to determine if an organism with a dominant phenotype is homozygous or heterozygous by crossing it with a homozygous recessive.

Extensions of the Laws of Inheritance

Advanced Concepts

Incomplete Dominance

• Heterozygote displays a phenotype that is intermediate (e.g., flower colors).

Codominance

• Both alleles are expressed (e.g., ABO blood types).

Multiple Alleles

• Genes exist in more than two variants (e.g., ABO blood group).

Polygenic Inheritance

• A character influenced by multiple genes leads to continuous traits (e.g., skin color).

Sex-Linked Genes

• Located on sex chromosomes, causing males (XY) to express traits with only one copy (hemizygous). Females (XX) require two copies for recessive traits.

Human Sex-Linked Disorders

• Commonly affect males due to the presence of only one X chromosome. Notable examples include hemophilia, color blindness, and Duchenne muscular dystrophy.

Summary

• Mendel proposed that alleles behave in dominant and recessive patterns, segregating into gametes independently. The existence of codominance and incomplete dominance exemplifies that phenomena can vary beyond simple Mendelian inheritance.
• Understanding these principles is crucial in genetics, facilitating predictions about inheritance patterns and traits.