Mendel’s experiments and genetic terms

Understand the idea

Gregor Mendel (1822–1884), an Austrian monk, crossed pea plants and tracked height, flower colour, and seed texture. He showed that traits are discrete units (now called genes) that come in pairs (alleles) and are inherited by rules, not by blending.

The law of segregation: the two alleles for a trait separate when gametes form, so each gamete carries one. Offspring get one allele from each parent. Independent assortment: alleles for different traits segregate independently. Dominance: a dominant allele can hide a recessive allele in a heterozygote. F2 often shows about three dominant phenotypes to one recessive.

A gene is a DNA unit of inheritance. Alleles are its alternative forms. Genotype is the allele set (TT, Tt, tt). Phenotype is what you see (tall or short). Capital letters are dominant; lowercase recessive. Homozygous means both alleles the same; heterozygous means they differ. Chromosomes are the threads that carry genes.

What to remember

• Segregation, independent assortment, and dominance are Mendel’s core rules.
• Genotype = alleles; phenotype = expressed trait.
• Monohybrid F2 phenotype ≈ 3:1; genotype 1:2:1.

Recap

Mendel turned inheritance into countable units. Learn the three laws, the letter code, and the monohybrid ratios before you read genes and chromosomes in Chapter 7.