Using common pea plants, Pisum sativum, Mendel created what's known as Mendelian genetics.

Credit: Wikimedia

Facts box

Who: Gregor Mendel

When: 1866

Where: Austria

Topics: biology, genetics, inheritance

Gregor Mendel first published his groundbreaking work in genetics in 1866, but it went unrecognised until 1900. Using common pea plants, Pisum sativum, which are easy to grow in large numbers and have both male and female reproductive organs, Mendel observed a number of traits that were characterised by one of two forms, including either purple or white flowers, wrinkled or round seeds and yellow or green pods.

From this research into what's now known as Mendelian genetics, Mendel created two laws still used to describe basic genetic inheritance today: the law of segregation and the law of independent assortment.

Gregor Mendel was an Austrian scientist and an Augustinian monk who taught natural science to high school students. Before his theory was accepted, many scientists believed in blended inheritance; the idea that two alleles would blend together to form a completely new allele. In the early 20th century, English geneticist Reginald Punnett created the Punnett Square, a simple graphical representation of genetics.

Law of segregation

According to the law of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring.

It is now known that this separation occurs during the formation of gametes. Gametes, or sex cells, are produced by halving the chromosomes of the male or female so that when conception occurs, the resulting fertilised oocyte will have a full set of chromosomes. The separation of these chromosomes, and thus the inheritance of a particular allele, is completely a matter of chance.

This is why, in Mendelian genetics, breeding a pure wrinkled seed pea plant, rr, with a pure round seed pea plant, RR, will produce four possible combinations of their alleles: RR, Rr, Rr and rr. These combinations produce the typical Mendelian 3 dominant: 1 recessive phenotype ratio.

Law of independent assortment

According to Mendel's law of independent assortment, alleles for different traits will be passed on from each parent independently of each other, which could result in completely new combinations of traits than seen in the parents.

For example, in pea plants, the inheritance of wrinkled seeds over round seeds doesn't make it more likely that the plant will inherit purple flowers instead of white flowers.

Mendel created this law by observing the inheritance of more than one trait in pea plants. To do this he bred two plants in a dihybrid cross, meaning plants with pure lines in two traits. For example, for round seeds and purple flowers as the dominant traits and wrinkled seeds and white flowers as the recessive traits, the genotypes for the plants would be written much like this: RRPP and rrpp.

This law only holds true for genes that aren't linked, like those found in Mendel's pea plants. Scientists now know that non-linked genes are separated by a large distance on the chromosome, or occur on separate chromosomes. Traits with alleles close together on the same chromosome are considered to be linked, for example the genes affecting eye colour and wing length in fruit flies.

Mendelian genetics key words

Allele - An allele is one of a pair, or series, of genes signifying a particular trait. Using a Mendelian example, the gene for round seeds in pea plants is one allele and the gene for wrinkled seeds is its pair. An allele can either be dominant, typified by a capital letter; or recessive, typified by the lower case version of the dominant trait's letter. Recessive genes are only expressed when there are no dominant alleles present that control the same trait.

Pure line - In Mendel's experiments he created what is known as a 'pure line', a population of plants that always shows a particular trait. When written with the dominant or recessive symbols, the pure line for the dominant round seeds would be RR and the recessive wrinkled seed line would be rr.

Phenotype - This is the trait that can be seen, for example round or wrinkled seeds; purple or white flowers; or green or yellow pods. When a pure recessive line is bred with a pure dominant line the ratio of dominant phenotypes to recessive phenotypes is 3:1.

Genotype - This is the exact combination of alleles found in each plant. In a combination of two alleles, round seeds or R or wrinkled seeds r, there are three possible genotypes: RR, Rr and rr. Both the RR and Rr genotypes would produce the dominant round seed trait. When a pure recessive line is bred with a pure dominant line the ratio of possible genotypes is 1RR:2Rr:1rr.

Homozygous - Homozygous is the term that describes pure lines, where two of the same alleles are combined. In the case of wrinkled versus round seeds, the genotypes RR and rr are homozygous.

Heterozygous - Heterozygous is the term used to describe the mixed allele combinations produced. In the case of wrinkled versus round seeds, the genotype Rr is heterozygous.