Mendel’s principles of inheritance

Our understanding of how inherited traits are passed between generations comes from principles first proposed by Gregor Mendel in 1866. Mendel worked on pea plants, but his principles apply to traits in plants and animals – they can explain how we inherit our eye colour, hair colour and even tongue-rolling ability.

Gregor Mendel

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Gregor Mendel (1822–1884) is known as the father of genetics. He proposed the key laws of genetics from this work on inheritance of traits in peas in 1866.

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Inheritance in pea plants

Mendel followed the inheritance of 7 traits in pea plants (Pisum sativum). He chose traits that had 2 forms:

• Pea shape (round or wrinkled)

• Pea colour (yellow or green)

• Flower colour (purple or white)

• Flower position (terminal or axial)

• Plant height (tall or short)

• Pod shape (inflated or constricted)

• Pod colour (yellow or green).

Mendel began with pure-breeding pea plants because they always produced progeny with the same characteristics as the parent plant. Mendel cross-bred these pea plants and recorded the traits of their progeny over several generations.

Read more about Mendel’s experiments.

Mendel’s principles of inheritance

Key principles of genetics were developed from Mendel’s studies on peas.

1. Fundamental theory of heredity

Mendel found that paired pea traits were either dominant or recessive. When pure-bred parent plants were cross-bred, dominant traits were always seen in the progeny, whereas recessive traits were hidden until the first-generation (F1) hybrid plants were left to self-pollinate. Mendel counted the number of second-generation (F2) progeny with dominant or recessive traits and found a 3:1 ratio of dominant to recessive traits. He concluded that traits were not blended but remained distinct in subsequent generations, which was contrary to scientific opinion at the time.

Mendel didn’t know about genes or discover genes, but he did speculate that there were 2 factors for each basic trait and that 1 factor was inherited from each parent.

We now know that Mendel’s inheritance factors are genes, or more specifically alleles – different variants of the same gene. In today’s genetic language, a pure-breeding pea plant line is a homozygote – it has 2 identical copies of the same allele. An F1 cross-bred pea plant is a heterozygote – it has 2 different alleles.

2. Principle of segregation

Inheritance of a single trait in peas

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Mendel followed the inheritance of 7 pea traits. Dominant traits, like round peas, appeared in the first-generation hybrids (F1), whereas recessive traits, like wrinkled peas, were masked. However, recessive traits reappeared in the second generation (F2). Each individual carries a pair of factors for each trait, and they separate from each other during fertilisation. This is the basis of Mendel’s principle of segregation.

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Mendel proposed that, during reproduction, the inherited factors must separate into reproductive cells. He had observed that allowing hybrid pea plants to self-pollinate resulted in progeny that looked different from their parents. Separation occurs during meiosis when the alleles of each gene segregate into individual reproductive cells (eggs and sperm in animals, or pollen and ova in plants).

3. Principle of independent assortment

Mendel observed that, when peas with more than one trait were crossed, the progeny did not always match the parents. This is because different traits are inherited independently – this is the principle of independent assortment. For example, he cross-bred pea plants with round, yellow seeds and plants with wrinkled, green seeds. Only the dominant traits (yellow and round) appeared in the F1 progeny, but all combinations of trait were seen in the self-pollinated F2 progeny. The traits were present in a 9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green).

Exceptions to Mendel’s rules

Inheritance of multiple traits in peas

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Mendel cross-bred plants with 2 or more traits and found that each trait was inherited independently of the other and produced its own 3:1 ratio. For example, a plant with round, yellow seeds crossed with a plant with wrinkled green seeds gives a ratio of 9:3:3:1. This is the basis for Mendel’s principle of independent assortment.

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There are some exceptions to Mendel’s principles, which have been discovered as our knowledge of genes and inheritance has increased. The principle of independent assortment doesn’t apply if the genes are close together (or linked) on a chromosome. Also, alleles do not always interact in a standard dominant/recessive way, particularly if they are codominant or have differences in expressivity or penetrance.