Mendel's Experiments

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Gregor Johann Mendel was the first person who found fundamental rules about the heredity of traits. He worked with two kinds of peas, one sort round, the other wrinkeled. He first made shure that his experiments are reproducible, by crossing peas from the same kind and seeing whether their filial peas are again of the same kind.

Mendel1.png

Then he crossed the round and the wrinkled peas. Interestingly, the result was not $50\%$ round $50\%$ wrinkled, as one might expects. One trait, namely the wrinkled was lost. He then went on and self-crossed the resulting round pea with itself. What he found was, that some of the peas were round, but the wrinkled trait was back again.

'F' for filial. F(0) first generation; F(1) second Generation ...

Mendel also counted how many round and how many wrinkled peas resulted from the experiment. The experimental data suggested that the ration between round and wrinkled peas was about $3 : 1$. Mendel searched for an explanation of the experimental data and formed the following model.

Mendel3.png

The peas of $F(0)$ carry eather the information for round (R) and wrinkled (r). Today we know that this Information is stored in genes. Crossing the beans results in peas that carry both, the information (R) and (r) but only one trait is acctually observed, so one trait is dominant over the other. Crossing peas from the $F(1)$ generation leads to (RR), (Rr) and (rr) peas in the $F(2)$ generation. Notice, each pea contains always two genes. This could be just a copy of the same gene or the alternative form (e.g. r is the alternative form of R) called allele (from greek meaning 'other'). All possible allel combinations that follow from self-crossing of the $F(1)$ generation are

\(R\) \(r\)
\(R\) \(RR\) \(Rr\)
\(r\) \(Rr\) \(rr\)

Since (R) is dominant, this model explains the ratio of $3:1$ of round to wrinkled. This method also works out more traits, so that Mendel was able to make predictions about the abundance of traits in subsequent crossings.

Definitions

Gene factor of inheritance controlling trait
Allele alternative forms of a gene [Rr,Aa,+,m (mutant)]
Phenotype appearance (round, wrinkled)
Genotype pair of allels carried by individuals
Homozygote individual with a copy of the same alleles (rr,RR)
Heterozygote individual with different alleles (e.g. Rr)
Dominant phenotype one (in our example R) is dominant over phenotype two (r) if the $F(1)$ generation shows phenotype one
Recessive phenotype one (in our example r) is recessive if the $F(1)$ generation shows phenotype two (R)



Video Lecture:

  • MIT: Eric Lander, Robert Weinberg - Introduction to Biology Lecture 6 [1]