Genetics & Heredity

Why do children resemble their parents? Why do some traits skip a generation? Genetics answers these questions by studying how traits are passed from parents to offspring. The foundation was laid by Gregor Mendel through his famous experiments with pea plants.

Key Terms

• Gene: A unit of heredity; a segment of DNA that codes for a trait
• Allele: Different forms of the same gene (e.g., T for tall, t for short)
• Genotype: The genetic makeup (e.g., TT, Tt, tt)
• Phenotype: The physical expression (e.g., tall or short)
• Homozygous: Both alleles are the same (TT or tt)
• Heterozygous: Alleles are different (Tt)
• Dominant: The allele that is expressed in heterozygous condition (T)
• Recessive: The allele masked in heterozygous condition (t); expressed only when homozygous (tt)

Mendel's Laws

Law of Segregation: During gamete formation, the two alleles for each trait separate, so each gamete carries only one allele.

Law of Independent Assortment: Genes for different traits are inherited independently of each other (when on different chromosomes).

Monohybrid Cross

A cross involving one trait. Example: Tall (TT) x Short (tt)

F1 generation: All Tt (tall) -- all heterozygous

F2 generation (Tt x Tt):

Using a Punnett square:

| | T | t | |---|---|---| | T | TT | Tt | | t | Tt | tt |

• Genotypic ratio: 1 TT : 2 Tt : 1 tt
• Phenotypic ratio: 3 Tall : 1 Short

Dihybrid Cross

A cross involving two traits. Example: Round Yellow (RRYY) x Wrinkled Green (rryy)

F1: All RrYy (round yellow)

F2 (RrYy x RrYy): 9:3:3:1 phenotypic ratio

• 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

Worked Example

In pea plants, purple flowers (P) are dominant over white flowers (p). Cross two heterozygous plants (Pp x Pp). What fraction of offspring will have white flowers?

Punnett Square:

| | P | p | |---|---|---| | P | PP | Pp | | p | Pp | pp |

Answer: 1 out of 4 offspring (25%) will have white flowers (pp).

Incomplete Dominance

Sometimes neither allele is completely dominant. In snapdragon flowers:

• RR = Red, rr = White, Rr = Pink (blending)
• F2 ratio: 1 Red : 2 Pink : 1 White

Nepal Connection

Understanding genetics helps Nepali farmers improve crop varieties through selective breeding. The Nepal Agricultural Research Council uses genetic principles to develop disease-resistant rice and wheat varieties suited to different altitudes.

Key Takeaways

• Mendel's laws explain how traits are inherited
• Monohybrid cross gives 3:1 phenotypic ratio in F2
• Dihybrid cross gives 9:3:3:1 ratio in F2
• Punnett squares are a systematic way to predict offspring genotypes and phenotypes