Understanding Mendel's Laws of Inheritance

Mendel's Experiments with Pea Plants

Mendelian genetics refers to the laws of inheritance discovered by Gregor Mendel through his work with pea plants. He studied specific characters, including:

• Seed color (Yellow vs. Green)
• Seed surface (Smooth vs. Wrinkled - implied, though not explicitly stated in the original text)

In his first cross, Mendel bred purebred yellow-seeded plants with purebred green-seeded plants. The resulting first filial generation (F1) consisted of 100% plants with yellow seeds.

Conclusions from Initial Crosses

• Each individual carries two hereditary factors (now called alleles) for each character, one inherited from each parent.
• In an individual carrying alleles for both yellow and green seeds, the yellow allele dominates over the green allele (which is recessive).
• The individuals of the F1 generation are not purebred but are heterozygous (possessing one dominant and one recessive allele).

Mendel's Laws of Inheritance

1. Law of Uniformity

   When two purebred individuals (homozygous) differing in a specific character are crossed, all individuals of the first filial generation (F1) are identical in phenotype and genotype (heterozygous).
   Example: NN (Yellow) x nn (Green) = Nn, Nn, Nn, Nn (All Yellow phenotype)

2. Law of Segregation

   The two alleles for a character separate (segregate) during gamete formation (meiosis) and are distributed to different gametes. They recombine during fertilization.
   Example: Heterozygous cross Nn x Nn. Gametes produced by each parent: N and n. Possible offspring genotypes: NN, Nn, Nn, nn.

3. Law of Independent Assortment

   Alleles for different characters are inherited independently of each other. The genes determining different traits combine freely and are transmitted independently during gamete formation (assuming they are on different chromosomes).
   Example: Dihybrid cross involving plant height (N=normal, n=dwarf) and flower color (V=purple, v=white).
   Parental cross: NNvv x nnVV
   F1 generation: All NnVv (Normal height, Purple flowers)
   F1 cross: NnVv x NnVv
   Gametes produced by each F1 parent: NV, Nv, nV, nv.
   (Offspring combinations are typically shown using a Punnett square).

Inheritance Examples

Blood Group Inheritance

In human ABO blood groups, alleles A and B are dominant over allele O. Alleles A and B are codominant with each other (both are expressed if present). For a person to have blood group O, they must be homozygous recessive (OO). The Rh factor is another trait; Rh positive (Rh+) is dominant over Rh negative (Rh-).

Key Genetic Definitions

• Gene: A segment of DNA that determines a specific character or trait.
• Species: A group of organisms that can reproduce with each other and produce fertile offspring.
• Dominance: A relationship between alleles where one allele (dominant) masks the expression of another allele (recessive) at the same locus.
• Codominance: A condition where neither of two alleles of a gene is dominant or recessive; both affect the phenotype.
• Dominant Allele: An allele that expresses its phenotypic effect even when heterozygous with a recessive allele.
• Recessive Allele: An allele whose phenotypic effect is only expressed in the homozygous state (when two copies are present).
• Hereditary Characters: Traits passed down from parents to offspring through genes.
• Acquired Characters: Traits obtained during an organism's life due to environmental influences or living conditions; they are not inherited.
• Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
• Genotype: The genetic makeup of an organism; the specific set of alleles it possesses.
• Phenotype: The observable physical or biochemical characteristics of an organism, resulting from the interaction of its genotype and the environment.