What is the relationship between Mendel’s laws and Watson and Crick’s DNA model?

Answer

Dear Sandra,

In fact, there is no real connection between Mendel’s research and Watson and Crick’s DNA model. Mendel has done research on the law of the inheritance of traits. He has tried to understand how different forms of a trait are inherited based on crosses between parents with different forms of a trait and how many of which of these forms occur in the first and second generations. For example, a property is the color of a flower and the shapes are then, for example, white or purple. Based on the results of his research, he deduced that a trait corresponds to a factor (gene) of which there are different forms (alleles) and that each parent has 2 forms (alleles) of this gene. Each parent passes on one of these 2 alleles to the offspring through reproduction. Each allele ends up in a sex cell and the distribution of the 2 alleles over the sex cells happens independently (Mendel’s first law). This implies that half of a parent’s sex cells have one allele of the gene and half have the other allele. The alleles of the gene can be different in a parent (heterozygous) or sometimes the same (homozygous). Mendel’s 2nd law states that the mating of sex cells in fertilization is also coincidental: so any sex cell from the father can combine with any sex cell from the mother. This means that the recombination of the alleles of a gene from the mother and the father is accidental: so that all combinations of father and mother alleles have an equal chance. If a mother has 2 different alleles of a gene and the father also has 2 different alleles, which are also different from those of the mother, then there are 4 different combinations in the offspring and each of those combinations has an equal chance of occurring: 25%. As mentioned, Mendel drew up these laws on the basis of numbers of different forms of a trait in a progeny of the crossing of 2 parents. He knew nothing about DNA, although at the time there were suspicions that the cause for these properties might be contained in large molecules or particles that would be passed on from the parents to offspring.

The discovery of the DNA molecule happened a few years after Mendel published his laws of heredity: 1869. At that time, however, people did not yet know what the role of that DNA molecule was, how that molecule was chemically constructed and what its structure was like. That DNA was the carrier of hereditary properties was discovered step by step in the period after Mendel, but it was not until the early 1950s that Franklin unraveled the spatial structure of the DNA molecule as a double helix and shortly afterwards Watson and Crick developed the chemical found its structure. The double-helix DNA molecule is coiled around proteins in the cell and this protein-DNA complex forms a chromosome. Each chromosome has a partner chromosome, where the same genes are located (we speak of chromosome pairs or homologous chromosomes).

From your question I suspect that you think that each strand of the DNA double helix represents a chromosome, but that is not the case: if there is an allele of a gene on one strand of the DNA, this does NOT mean that the corresponding piece on the other strand of the double helix is ​​the other allele. The other allele is on the other chromosome of the chromosome pair and that has a different DNA double helix!!

So Mendel’s laws of heredity were certainly no reason to assume that the DNA molecule consists of a double helix with 2 DNA strands.