Basics of Genetics: what makes every living thing how it is

One of biological molecule is the molecule of inheritance – DNA. Let us see in detail how this molecule is able to pass on our features or traits. For now, however, we need to talk a little about the structure of the molecule and where it is situated in cells. The figure shows how chromosomes, genes and DNA are related.

Chromosomes are found in the nucleus of a cell and are made from the DNA, which is bound with histone proteins to form the chromosomes.
A gene is a section of DNA (and therefore a section of a chromosome) that determines a particular feature, for example, earlobe attachment in humans. Whether or not your earlobes hang free or are attached is determined by a single gene. However, there are two versions of this gene. One version says ‘be attached’ and the other says ‘don’t be attached – be free’.

Different versions of the same gene are called alleles.Because humans reproduce sexually, we receive half of our chromosomes from one parent and half from the other. And the two sets are very similar. In our cells we have 46 chromosomes; 23 are paternal in origin (came from our fathers) and 23 are maternal in origin (came from our mothers). And when they get together, they form pairs called homologous pairs. These homologous pairs have genes controlling the same features in the same position – or locus – on the chromosome. However, the alleles may not be the same.

Gene 1 has two alleles P and P. Both chromosomes have the dominant P allele (we say that the individual is homozygous for the dominant allele). Gene 2 has two alleles, a and a. Both chromosomes have the recessive allele a (homozygous for the recessive allele). Gene 3 has alleles B and b. One chromosome has the dominant allele, B, whilst the other has the recessive allele, b. We say that the individual is heterozygous for this particular gene.

The alleles of a particular gene possessed by an individual are its genotype (for that feature). Th actual result of that genotype (whether or not the earlobes are attached, for example) is the phenotype (for that feature). If you know the genotypes of two parents, you can make predictions about the type and proportions of offspring they will have in relation to a particular feature. You can also throw that into reverse: if you know proportions of offspring showing certain versions of a feature, you can often work out the genotypes of the parents.

How do we predict ratios in a monohybrid cross?

First, let us make clear what we are talking about. A monohybrid cross is a genetic cross or breeding situation that relates to just one trait or feature. Th ‘father’ of genetics, the man who discovered the rules by which genes are inherited, was the Austrian monk Gregor Mendel. Living and experimenting in a monastery in Brno, Mendel experimented with pea plants and was able to deduce the rules of inheritance from his results.

He noticed that pea plants exhibited ‘contrasting characteristics’. For example, the plants were either tall or short, had purple flowers or white flowers. Now not all tall pea plants are exactly the same height, neither are all dwarf ones. But you wouldn’t mistake a tall plant for a dwarf one. And obviously, you wouldn’t mistake a purple-flowered plant for a white-flowered one. This was the key that enabled Mendel to experiment successfully – there were never any medium-height pea plants or plants with pale purple flowers. Figure below shows the seven contrasting characteristics that Mendel used in his experiments.

Gene a section of DNA that determines a specifi feature histone the core of a chromosome around which the chromosome’s DNA is wrapped chromosome a long strand of DNA on which a large number of genes is stored allele a version of a gene that determines a particular trait.

Homologous pairs the chromosomes in a eukaryotic cell usually come in pairs called homologous pairs. Each of the chromosomes in a homologous pair have corresponding genes that together determine the same trait locus (plural loci) the position of a particular gene on a chromosome homozygous an organism is homozygous for a particular gene if it has the same allele for that gene on each of the chromosomes in the homologous pair heterozygous an organism is heterozygous for a particular gene if it has different alleles for that gene on each of the chromosomes in the relevant homologous pair genotype a genotype describes the pair of alleles for a particular gene possessed by a organism phenotype a phenotype describes the trait or traits determined by a particular genotype

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