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Principles of Inheritance — Part 1: Mendel’s laws

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Lesson

Principles of Inheritance

Part 1 — how characters pass from parents to offspring, as Mendel first worked out.

In this lesson you will

Use the terms of genetics correctly
State Mendel’s principles and the reasons for his success
Work out monohybrid and dihybrid crosses, back crosses and test crosses with Punnett squares

Concept

Terms of genetics

A clear vocabulary makes crosses easy to describe.

Table of genetic terms and their meanings
Term	Meaning
Gene (factor)	Unit of inheritance that controls a character
Alleles	Alternative forms of a gene, e.g. T (tall) and t (dwarf)
Homologous chromosomes	A pair of chromosomes with the same genes in the same positions
Autosomes and sex chromosomes	Chromosomes not involved in sex determination, and those that are (X, Y)
Genotype	Genetic make-up, e.g. TT, Tt, tt
Phenotype	Observable character, e.g. tall
Homozygous / heterozygous	Two identical alleles (TT, tt) / two different alleles (Tt)

Concept

Why Mendel succeeded

Mendel chose the garden pea and worked carefully.

Table of reasons for Mendel’s success
Reason	Why it mattered
Pea plants	Easy to grow, short life cycle, naturally self-pollinating with true-breeding lines, and easy to cross artificially
Seven pairs of contrasting characters	Clear either/or traits such as tall/dwarf, round/wrinkled seeds
One or two characters at a time	Kept results simple to interpret
Large samples and statistics	Ratios became reliable; records kept carefully
Luck	The characters he chose were on different chromosomes or far apart, so they assorted independently