Mendel's Laws of Inheritance

• Gregor Mendel's study of pea plants led to the development of two laws explaining trait inheritance in sexually reproducing organisms.
• Mendel referred to these laws as "unit factors," now known as genes.

Mendel's First Law: Law of Segregation

• Unit factors (genes) on chromosomes separate during reproduction.
• Organisms with two different copies of a gene produce diverse offspring.
• Distinct gene copies or chromosomes separate during meiosis.

Mendel's Second Law: Law of Independent Assortment

• Each gene is independent of other genes during sexual reproduction.
• Each genetic trait segregates separately from other genetic traits.
• Important for genetic variation within a species through different gene combinations.
• Explains offspring ratios resulting from genetic crosses.
• Provides a mathematical explanation for the inheritance of human traits controlled by multiple genes.

Discovery of the Second Law

• Both of Mendel's laws explain chromosome behavior during meiosis (gamete production).
• Mendel confirmed his first law and discovered his second by crossing pea plants with different traits
• Mendel's experiments included crossing pure-breeding strains of peas.
• Pure-breeding strains always produce offspring with 100% of a given trait when self-pollinated.
• Crossing a pure-breeding green seeded strain with a pure-breeding yellow seeded strain, always resulted in green seeded plants (the dominant trait).
• Crossing the green seeded offspring ("F1") with themselves resulted in 3/4 (75%) green seeded offspring and 1/4 (25%) yellow seeding offspring ("F2").
• Similar results came from crossing round seeded plants with wrinkled seeded plants.
• The first cross produced round seeded plants (dominant) in the F1 generation.
• Crossing these F1 strains resulted in 3/4 (75%) round and 1/4 (25%) wrinkled offspring.
• This experiment helped describe the Law of Segregation, as mentioned above.

Dihybrid Cross Example

• In these experiments, the round seeded, wrinkled seeded, green seeded, and yellow seeded plants were combined to produce a pure-breeding round green seeded plant and a pure-breeding wrinkled yellow seeded plant.
• Mendel's next experiments combined round, wrinkled, green, and yellow seeded plants.
• The pure-breeding round green seeded plant was crossed with a pure-breeding wrinkled yellow seeded plant.
• This cross demonstrates a "Dihybrid Cross".
• The dihybrid cross proved that seed shape and seed color segregate independently, proving Mendel's Second Law.
• Dihybrid crosses involve two traits, such as seed shape (round vs. wrinkled) and seed color (green vs. yellow).
• Dihybrid crosses produce expected genetic ratios of traits as if they were independent.
• Dihybrid result shows that two traits are independent of each other.

Meiosis and the Law of Independent Assortment

• During Metaphase I of Meiosis, each gene in a heterozygous state lines up in the middle of the cell.
• A chromosome carrying one version of a gene pairs with a chromosome carrying another version.
• The two chromosomes then separate.
• Half the resulting gametes get one version of the gene, and half get the other.
• The 3:1 round to wrinkled ratio in Punnett Squares results from meiosis in each parent.
• Both traits are shown to follow Mendel's Second Law.
• Both traits sort independently during meiosis.
• The results of this sorting leads to a dihybrid cross-ratio of 9:3:3:1.
• This 9:3:3:1 ratio is a combination of two 3:1 ratios.
• The combination of the two 3:1 ratios proves Mendel's Second Law of Independent Assortment
• Genes coding for both traits sort independently during meiosis.
• The 3:1 ratio of seed shape (Rr x Rr) combined with the 3:1 ratio of seed color (Gg x Gg) explains the 9:3:3:1 ratio.
• The 9:3:3:1 ratio is determined by multiplying the probabilities of the two independent 3:1 ratios together.

Gregor Mendel's laws explain trait inheritance in sexually reproducing organisms. The Law of Segregation states that genes separate during reproduction. The Law of Independent Assortment states that each gene is independent of other genes during sexual reproduction and explains offspring ratios.