This article explains the results of Gregor Mendel’s science experiments with the common pea plant- experiments that he conducted at St Thomas’s Monastery in Brno, the Czech Republic, between 1855 and 1863
After eight years of carrying out experiments and 29 000 pea plants later he finally reached conclusions about the heredity of the common pea.
In Gregor Mendel’s Pea Plants (Part 1) we learn about his early life and how he planned his investigations.
In this article we focus our attention on a single trait whose heredity he researched- the heredity of purple and white flowers.
More specifically we look at how variations in flower color are passed onto any offspring and which variation in the ‘flower color’ trait any offspring inherits.
We use 20th century terminology to explain the results of Gregor Mendel’s 19th century science experiments….. in the same way that this 20th century Andy Warhol style ‘pop art’ painting takes its inspiration from a contemporary 19th century photo of Mendel.
Before starting his experiments Mendel grew pea plants that were ‘true breeding’.
A ‘true breeding’ pea plant is one that produces the same trait after it has self pollinated. (for an explanation of ‘self pollination’ read Gregor Mendel’s Pea Plants -Part 1)
After it self pollinates a ‘true breeding’ pea plant with purple flowers always produces offspring with purple flowers….
…and a ‘true breeding’ plant with white flowers always produces offspring with white flowers.
Mendel took one ‘true breeding’ purple flowering plant and cross bred it with a ‘true breeding’ white flowering plant. He called this generation of plants the ‘Parent’ Generation (‘P’ generation).
These ‘P’ generation plants were all identical in nearly every respect apart from differences to a single trait- the color of the flowers.
He called the offspring of the true breeding purple and white flowered plants the ‘Filius 1’ or ‘F1’ generation. (Filius is Latin for ‘son’)
Mendel noticed that the F1 ‘hybrid’ offspring the ‘P’ generation plants all had purple flowers! (A ‘hybrid’ is offspring resulting from ‘cross breeding’.)
Next Mendel used self pollination techniques to breed from the F1 generation of offspring. He then planted the F2 generation seeds and grew the ‘F2’ generation of flowers.
Mendel recorded that this F2 generation produced, on average, three purple flowers to every white flower- a ratio of 3:1.
Mendel carried out identical experiments over three generations (P, F1 and F2) for the other six traits he chose to research. (see Gregor Mendel’s Pea Plants -Part 1 for details of those other six traits)
He found that, in every case, one variation of a trait disappeared in the F1 generation, only to reappear in F2 generation.
Take the tall pea plant/ short pea plant variations as another example….
In the F1 generation all pea plants had long stems…
Following the ‘self pollination’ of the F1 generation plants, the F2 generation produced, on average, three long stemmed plants to every short stemmed plant. (ratio of 3:1)
Based on his observations of the offspring of cross bred and self pollinating pea plants, Mendel developed his theory of heredity.
These are the main conclusions of his science experiments:
There are different variations of genes called alleles.-There are purple and white variations of ‘flower color’ genes; green and yellow variations of pod color genes; round and wrinkled variations of ‘seed shape’ genes etc
2) Visible (dominant) and hidden (recessive) variations
In the case of the cross bred purple and white flowered plants, the purple flower allele is visible (or dominant) while the white flower allele is hidden (or recessive). (Read Explaining Genetic Inheritance for a better understanding of dominant and recessive alleles)
In the diagram below the genotypes for each color of flower are shown. ‘Upper case P’ is the dominant purple allele and ‘lower case p’ is the recessive allele. You can see in generation F1 that where a dominant allele (P) is combined with a recessive allele (p) the white flower variation remains hidden.
The dominant variation of the gene (dominant allele ‘P’) always shows, even if an individual only has one copy of that allele.
The recessive variation of the gene ( recessive allele ‘p’) never shows when an individual has one recessive allele and one dominant allele.
A recessive allele only shows if the individual has two copies of it. In the ‘parent’ generation the white flower has two copies of the white flower genotype, so the ‘phenotype’ (what is visible) is white.
3) Homozyogous and heterozygous alleles
If an individual has a ‘PP’ or a’pp’ genotype the alleles are said to be ‘homozygous’. If an individual has a mixed Pp genotype the alleles are said to be ‘heterozygous’. In the diagram below you can see which individuals have homozygous alleles and which have heterozygous alleles;
4) Reaching conclusions about which variations of traits are dominant and which are recessive
Mendel concluded the following about the seven traits he investigated;
The long plant stem is the dominant trait; the short plant stem is the recessive trait.
5) Inheritance of alleles from parents
For every trait a pea plant inherits two alleles, one from each parent. Mendel noted that offspring could inherit a dominant or recessive variation from either parent. In the case of the dominant variation, it did not matter if it was the male pollen grain or female egg that supplied that variation.
6) Variations in traits are inherited by organisms during the process of egg fertilisation.
When the nucleus of the male pollen grain fuses with the egg, the egg is fertilized. It is only when this happens that the organism receives its ‘genetic instructions’ telling it which alleles it will inherit.
A fertilized egg (or ‘zygote’) only inherits one allele for each trait from each parent.This is because ‘reproductive cells’, which includes the nucleus of a grain of pollen and an egg, only pass on one allele for each trait.
We call reproductive cells, which have one allele for each trait, ‘gametes’. Gametes are further explained below:
This diagram shows how the egg of a true breeding purple plant is fertilized by the nucleus of the pollen of a true breeding white plant, producing a ‘zygote’ or fertilized egg.
Mendel planted the seeds of the self pollinating F1 generation, all of which had identical heterozygous ‘Pp’ genotypes.
In the F2 generation he bred a mixture of both homozygous (upper case PP/ lower case pp) and heterozygous genotypes (Pp and Pp)
7) Anthers and ovaries produce only one copy of each allele
Remember that ‘gametes’ (mature reproductive cells which including male pollen nucleii and female eggs) carry only one allele of a trait.
The male gamete (nucleus of a grain of pollen) is produced in a plant’s anther, and the female gamete (the egg or ‘ovum’) is produced in the plant’s ovaries.
So when a plant self pollinates, each gamete passes on one allele of a trait to the fertilized egg (‘zygote’) In the F 1 generation there are, on average, four possible outcomes.
8) Inheritance of one trait will not affect the inheritance of another
Variations in traits are inherited randomly and independently of each other.
A pea plant will not necessarily inherit all dominant ‘genotypes’ such as long stems, purple flowers in ‘axial’ positions and unripe pods colored green…..
….just as a pea plant will not necessarily inherit all recessive ‘genotypes’ including short stems, white flowers in terminal positions and unripened pea pods colored yellow.
Pea plants are much more likely to inherit a random mix of dominant…
The 1960’s pop artist Andy Warhol is credited with saying, “in the future, everyone will be world-famous for 15 minutes”.
Gregor Mendel has far more than fifteen minutes of fame- his legacy being to establish the science of genetics. Mendel’s findings into the nature of heredity go a long way to explaining the complex universal laws of inheritance in plants, animals and human beings.