Mendel's Experiments and Laws of Inheritance

Mendel's Experiments and Laws of Inheritance

• Gregor Mendel, the "father of genetics," studied inheritance patterns using pea plants.
• He discovered fundamental principles of heredity via controlled crosses of pea plants with distinct traits, like flower color and seed shape.
• Mendel observed how traits are passed from one generation to the next through genes.

Key Terminology in Mendelian Genetics

• True-breeding: Organisms that consistently produce offspring with the same traits when self-pollinated; homozygous for those traits.
• P generation: The parental generation in a cross.
• F1 generation: The first generation of offspring from a cross.
• F2 generation: The second generation of offspring from a cross.
• Homozygous: Having two identical alleles for a particular gene (e.g., BB or bb).
• Heterozygous: Having two different alleles for a particular gene (e.g., Bb).
• Carriers: Heterozygous individuals who carry a recessive allele but don't express the trait.
• Phenotype: Observable physical traits.
• Genotype: The genetic makeup, including the alleles an organism possesses.

Mendel's Experimental Process

• Mendel meticulously selected and cross-pollinated pea plants, carefully controlling which plants were crossed to observe inheritance patterns over generations.

Mendel's First Experiment

• Mendel crossed true-breeding purple-flowered plants with true-breeding white-flowered plants.
• All F1 offspring displayed purple flowers.
• This demonstrated dominance of purple flower color over white flower color.

Mendel's Law of Segregation

• During gamete formation, the two alleles for a gene separate randomly, so each gamete receives only one allele per trait.
• Each offspring inherits one allele for each trait from each parent.

Punnett Squares

• Punnett squares are tools used to predict the possible genotypes and phenotypes of offspring from genetic crosses.
• Example: Pp x Pp cross yields 75% purple flowered plants (Pp & PP) and 25% white flowered plants (pp).

Complete Dominance

• One allele (dominant) completely masks the expression of the other (recessive) allele in a heterozygote.

Test Cross

• A test cross involves crossing an organism with the dominant phenotype and an unknown genotype with a homozygous recessive organism to determine the unknown genotype.

Human Single-Gene Traits

• Examples of human traits controlled by single genes include eye color, widow's peak, freckles, dimples, color blindness, cystic fibrosis, and Huntington's disease.

Mendel's Dihybrid Crosses

• Mendel performed dihybrid crosses to assess if alleles for different traits were inherited together or independently.
• He crossed contrasting pairs of traits (e.g., seed color and seed shape).

Mendel's Law of Independent Assortment

• Alleles for different traits separate independently during gamete formation.
• Inheritance of one trait doesn't influence inheritance of another.

Meiosis and Mendel's Laws

• During meiosis, homologous chromosomes separate, ensuring only one allele from each parent is passed to offspring (Segregation).
• Random alignment of homologous chromosomes during meiosis results in independent assortment of alleles.

Other Inheritance Patterns

• Incomplete Dominance: Neither allele is completely dominant; heterozygots display a blended phenotype.
• Multiple Alleles: A trait controlled by multiple possible alleles in a population.
• Codominance: Both alleles are expressed equally in the heterozygote.
• Epistasis: One gene influences the expression of another.
• Polygenic Inheritance: Traits influenced by multiple genes.
• Pleiotropy: A single gene affecting multiple seemingly unrelated traits.
• Environment & Phenotype: Environmental factors often affect phenotype expression.

Sex-Linked Inheritance

• Sex chromosomes (X and Y) determine biological sex. Females (XX), males (XY).
• Sperm carries X or Y, determining sex of offspring.
• Recessive sex-linked traits are more often seen in males.