Mendel’s Laws of Inheritance – Notes and Q & A

Mendel’s Laws of Inheritance

In 1865 Gregor Mendel proposed three main laws of inheritance based on his experiments with garden pea plants. Before you understand these laws you must understand the key terms used.

Terms Used in Mendel’s Laws

• A gene is the overall unit of inheritance (e.g., the gene for plant height or eye color). An individual organism inherits a pair of alleles for each gene—one allele coming from the mother and one allele coming from the father.
• Alleles: Alternative or slightly different forms of the same gene. (e.g., T or t)
• Dominant Trait: An allele or physical characteristic that is expressed and dominates the recessive form when present. (e.g., Tall (T), round seed (R))
• Recessive Trait: An allele or physical characteristic that is masked or hidden when a dominant allele is present. (e.g., Dwarf (t), wrinkled seed (r))
• Homozygous: Having a pair of identical alleles for a specific gene (e.g., TT or tt).
• Heterozygous: Having two different alleles for a specific gene (e.g., Tt).
• Phenotype: The observable physical appearance of an organism determined by its alleles. (e.g., tall plant or dwarf plant)
• Genotype: The genetic makeup or specific combination of alleles possessed by an organism. (e.g., Tt (Heterozygous tall) or TT (Homozygous tall))
• Monohybrid Cross: A breeding experiment that tracks the inheritance of a single character. For example: purple flowers ( PP x PP or PP x pp or PP x Pp or Pp x Pp or Pp x pp or pp x pp  ), similarly for round seeds ( RR x RR or RR x Rr or Rr x rr or rrxrr…….)
• Dihybrid Cross: A breeding experiment that tracks the inheritance of two separate pairs of contrasting characters simultaneously. For example: Cross between heterozygous round and yellow seed plants RrYy x RrYy  or  Cross between homozygous tall with purple flowers and heterozygous tall with purple flowers TTPP x TtPp

Mendel’s Laws of Inheritance

1. Law of Dominance
This law states that in a heterozygous pair of alleles (different alleles, like Tt), the dominant allele will mask or suppress the expression of the recessive allele. For example: A plant with the genotype Tt will grow up Tall because the dominant allele (T) completely masks the recessive allele (t).

2. Law of Segregation
Allele pairs separate during gamete (sex cell) formation so that each gamete receives only one allele for each trait. This is also known as the law of purity of gametes. For example: A heterozygous tall plant (Tt) splits its alleles during reproduction. Half of its gametes will carry the T allele, and the other half will carry the t allele.

3. Law of Independent Assortment
This law applies to a dihybrid cross (where two different traits are studied at the same time). Alleles for different traits are passed on to offspring independently of each other. The inheritance of one trait (like seed color) does not influence the inheritance of another trait (like plant height) during gamete formation. For example: Cross between a Round Yellow (RRYY) plant with a Wrinkled Green (rryy) plant creates completely new combinations such as Round Green and Wrinkled Yellow.