Gregor Mendel’s pea plants

Gregor Mendel
Gregor Mendel (Photo credit: Wikipedia)

Gregor Mendel was a Monk, who lived during the 1800s. He grew pea plants, and using different characteristics, he worked out the basics of how characteristics are passed on from one generation to the other. He used the pod colour, petal colour, seed shape and other such characteristics. He did not know about genetics or DNA at the time, but he still managed to worked out how the traits from parents are passed down to their offspring.

Firstly, there are a number of different combinations of alleles. Alleles are different versions of the same gene. So for example, blue eyes, brown eyes, green eyes are all different alleles for the eye colour gene.  You receive one alleles from your mother, and the other from your father. If a person has a recessive feature, such as blue eyes, then both of their alleles have to be the same. If a person has a dominant feature, such as brown eyes, then they only need to receive one. Dominant alleles will always be a visible characteristic when present. Recessive features will only be visible if they are the only allele present.

When you have two recessive alleles, you have homozgous recessive alleles; but when you have two dominant alleles you have homozgous dominant alleles. If you have one of each, then they are just heterozgous. This is because a heterozgous combination will always have a dominant and recessive allele, so overall it will always be a dominant characteristic.

If a person gains one allele for blue eyes, and one for brown eyes, then they will have brown eyes. However, in their genes, they still have the genetic coding for blue eyes, which they may pass down to their children. This is exactly what Mendel had started to discover hundreds of years ago. Now there is a difference between the genetic make up of a person, and their resulting physical traits. These are known as the genotype and phenotype. The genotype is the genetic make up of a person. A person’s genotype may code for blue eyes and brown eyes. A person can pass on any of characteristic of their genotype to their children, even if it is not  physical trait they have themselves.

Screen Shot 2013-09-29 at 10.20.25On the other hand a phenotype is the physical characteristics of a person, and how they actually look. So it is the eye colour you can see, the hair colour you can see, the height and weight of the person. The phenotype is what Mendel was working with, but he realised that there was something more than just what you can see. He took two plants, one homozgous dominant (with a green coloured pod), and the other homozgous recessive (with a yellow coloured pod). The green plant had the alleles GG, since it was homozgous. The capital ‘G’ tells us that it is the dominant trait. Conversely the yellow plant had the alleles gg, and the lower case tells us that it is a recessive trait.

Mendel bred these two plants together, and the result he got were nothing unusual. If you look the left, you can see the punnett square, which shows the results of the breeding. You can see that he got 4 green plants, and no yellow plants. This was because one of the parents could only pass on a dominant green allele (G), so all of the plants had to have a green allele. This was nothing special – Mendel had one green plant, so producing 4 more green plants seemed to be completely normal. These offspring are referred to as the first filial or F1 generation. In other words, they are the first generation of offspring from the two parents.

Screen Shot 2013-09-29 at 10.27.39However when Mendel bred two of the heterozgous Gg plants, he found something quite surprising. He actually got 3 green plants and 1 yellow plant. Now considering both of the parents were green, Mendel didn’t expect to see the return of the yellow plant. In fact, he realised that the appearance of the pea plant is only half the story. He suggested that there must be some way of passing down the ability to make a yellow plant, from the original parent. The plant to the right show the results.

Mendel himself came up with two different laws. These were the laws of independent assortment and segregation. The law of segregation states that there is a 50-50 chance to the characteristic each plant will pass down to it’s offspring. He didn’t know about alleles or genes, but he said, that each plant would give two possible characteristics, and there is an even chance of either being passed on to the offspring. On the other hand, the law of independent assortment says that one characteristic does not affect another characteristic. In other words our eyes colour is not at all affected by our hair colour – each characteristic is independently selected, based on the combination of genetics.

Through all of this important and advanced work, Mendel was hailed the father of genetics. He was ahead of his time because he managed to work out a lot, with only very basic knowledge. In fact he gave us a starting point to finding the DNA and genes which actually allow everything to happen. Mendel had worked out everything, he just didn’t know what caused it.

Sources

Andersen Paul, “Mendelian Genetics” BozemannScience Published: July 20th 2011 Retrieved from: http://www.youtube.com/watch?v=NWqgZUnJdAY

Toole Glenn, Toole Susan “Studying inheritance” AQA Biology A2 Chapter 8.1 Pages 112-113 ISBN 9780784798131

Toole Gleen, Toole Susan “Monohybrid inheritance” AQA Biology A2 Chapter 8.2 Pages 116-117 ISBN 9780784798131

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