Gregor Mendel and his work

GREGOR MENDEL 1822-1884

'The father of heredity'

Gregor Mendel was an Austrian monk who taught natural science to high school students (see fig. 1). However, he also played a vital role in the history of genetics. His research and now famous experiments helped future scientists understand the laws of heredity. At the time, Mendel's work was unvalued by many scientists and it was rediscovered in the 20th century. In 1900, people saw a connection between meiosis and Mendel's work.

This founder of heredity knew nothing of chromosomes, DNA or genes, but discovered patterns in his results which led to his powerful findings.

His love of nature drew him in to researching the effects of the enviroment on plants, but instead he came along to identify how and why plants obtained irregular or regular characterists. Most of Mendels work was allocated to the garden pea, the Pisum, however he also did genetic work with other plants and honey bees.

Figure 1: Image of Gregor Mendel

Mendel was the first person to analyse and look into the charcterists of generations and thus came to be referred as the father of heredity. The famous pea expriment was the beginning of Mendels work and he started by growing a number of different variaties. These variaties consisted of different coloured seeds, different seed shapes, different pod colours, different pod shapes, different coloured flowers, different positioned flowers and different stem heights (see fig. 2). He grew these for several years to insure that their offspring were always the same. He did this by allowing the plants to self pollinate and ended up with the pure breeding plants that he wanted. These plants were used as the parents in his next step therefore he named this first generation the P generation (P for parental). His second step was when he cross polinated two different varities of the P gerenation that had distinct features. The offspring was named the F1 generation (F for filial, meaning brothers and sisters). The third step involed the F1 generation self pollinating and the offspring were named the F2 generation.

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Mendel believed an organism has two alternate forms of a trait. These are now known as alleles, and an organism can have two alleles either the same or one of each form. The two identical alleles of a gene is known as homozygous and if the offspring contained one of each it would be known as heterozygous. Mendel indicated this by using letters in upper or lower case to represent the alleles. For example homozygous would be AA or aa and heterozygous would be Aa. He used upper case to signify that the organism was 'dominant' for the triat, and lower case to signify that the organism was 'recessive' for the trait. To state that an allele is dominant means that it is expressed in the organism wether or not its other allele is identical. And an allele which is overridden if paired with a dominant one is known as recessive.

From the pea expriment where Mendel grew an estimated 28 000 pea plants over eight years, he noticed a paticular trend; The F1 generation always had offspring exposing only one of the distinct characterist that had been inherited (parental trait). The F2 generation always had offspring exposing both parental traits. Mendel described the frequent trait in F1 generation as dominant and if the trait overlept the F1 generation and be in in the F2 generation it was called recessive.

The trait reappeared and disappered in the hybrid plants, and Mendel mathematically drew his conclusions about what was happening deep inside the cell. He found a ratio of 3:1 when he crossed monohybrid with monhybrid (Aa x Aa). Mendel then went onto cross plants which differed in more than one trait and again found a predictable ratio. This was 9:3:3:1 when he crossed dihybrid with dihybrid (AaBb x AaBb). This confirmed that traits were passed indivdually to the offspring and were not linked in any other way.

The expriments showed both monohybrid inheritance and dihybrid inheritance. Monohybrid inheritance is the pattern of inheritance of a charcterist influenced by a single pair of alleles. Dihybrid inheritance is the study of the inheritance of two charcterists that are determined by genes on different non homolgous chromosomes. Mendel showed monohybrid crosses when he exprimented with just one characterist from the element, such as tallness and shortess of the stem, and self pollinated the two. He then showed the dihybrid cross when he used two different charcterists in his expriment, such as a yellow seed with round seed and a green seed with a wrinkled seed. Predicting the results of a dihybrid cross is much more complicated than predicting the outcome of a monhybrid cross. This is because all possible cominations of the two alleles for the two triats in each parent gametes, and then all the possible gamete combinations at fertilisation. Below a dihybrid cross is illistrated more simply;

For a dihybrid cross - the chance that 2 independent events will occur together is the product of their chances of occuring separately.

From his findings, Mendel discovered a distinction between genotype and phenotype that is still applied in studies of genetics. Genetype is defined as the genetic make-up of an indivdual, whereas phenotype is the pysical expression of the indivduals genotype.

By observing the number and types of phenotypes in the offspring plants, Mendel created the rules of inheritance known as Mendels law. In his first law he said that each character of a diploid organism is controlled by a pair of alleles, and from this pair of alleles only one can be represented in a gamete. The modern version of this law has been identified as the law of segregation, which states that members of each pair of alleles of a gene seperate when gametes are produced in meiosis. In Mendels second law, he said either of the alleles in a pair (A or a) can combine with either of another pair (B or b). The modern version for this second law now stands as the law of independent assortment, which states that pairs of alleles separate independently of each other during the gamete formation.

In 1866 Mendel published his work on heredity in the Journal of the Brno Natural History Society. It had absolutely no affect and his complex and detailed work was not understood by anyone of the time.

The findings of Mendels expriments and his laws basically laid the foundations for the science of genetics. Along with Mendels findings and improved microscopes of the 20th century, chromosomes and DNA were discovered. Mendel did not live to see how much of an impact his work had on the world. This new science of heredity and variation was named 'genetics' by a British sciencist, and later Mendels 'elements' were known as genes.

This essay not only reveals Mendels work into the theories of heredity, but also allows us to aknowlegde that Gregor Mendel gave future scienctists the chance to learn how to manipiulate genes. The molecular basis of inheritance is now better understood, thanks to him.

Word count:

1285

References:

Revise A2 Biology, John Parker, Letts