Heredity
How does a new human being arise out of a fertilized ovum? Why do children look like their parents? Why do brothers and sisters resemble each other? The secret is stored in the genes that are inherited from the parents. Genes encode instructions that define our traits. Where are these genes?
In most organisms, the genetic information is distributed onto a few very long DNA threads. In humans, there are 46 DNA threads in each cell of the body. For lack of space – they can reach a length of up to two meter – the DNA threads are wound around a protein framework. A DNA thread together with its protein framework is called chromosome. Every chromosome consists of teo chromatides.
A human being has 46 chromosomes. Of these, two are alike in shape and composition (an exception are the gonosomes x and y, that differ in form and size),the term 23 chromosome pairs is also used.
The only exceptions are the sperm and the ovum – altogether, these only contain 23 chromosomes.
When parents conceive a child, the father's sperm cell fuses with the mother' egg cell – thus, a cell with 46 chromosomes (23 chromosome pairs) emerges again. This cell will divide, ultimately developing into a child. In order that germ cells with only 23 instead of 46 chromosomes are emerging, the cell is dividing in a special way, the meiosis. In the first phase of meiosis, the chromosomes of similar type from father and mother accumulate in pairs, subsequently they get divided. In this process formerly maternal or paternal hereditary dispositions are randomly mixed. With 23 pairs of chromosomes the number of possibilities is 2 23 or 8,388,608 possible combinations! Additionally, the chromosomes can interchange DNA-fragments among each other resulting in chromosomes with fragments of the paternal and the maternal chromosomes. This process, called homologous recombination or Crossover, aids in increasing genetic diversity by producing novel genetic combinations. In the second phase of meiosis the two chromatides of each chromosome are separated, subsequently the cells divide. The resulting four germ cells have one chromatide of each type. After the meiosis, the gamets are ready for fertilization.
During a meiosis there also happen errors with the dividing of the chromosomes, for example if a chromosome pair is not divided and one germ cell with only one and another germ cell with 3 chromosomes arises. A well known example is the down syndrome, that is caused by the existence of an additional chromosome 21.
Since the parents contribute chromosomes randomly to each child, every child receives a different combination of chromosomes. As a result, every child will inherit a unique combination of traits. Whether or not we have inherited mom’s slender build or dad’s tendency towards early baldness or whether grandpa’s short-sightedness has overleaped a generation, it is not until the fusion of the sperm and the egg cell that it is determined which genetic information we have received from previous generations.
The first person, who investigated the mechanisms of heredity and therefore is the originator of classic genetics was the Augustinian Gregor Mendel, who was interested in natural sciences. He formulated Mendel’s laws that describe the heredity of traits that are determined by only one gene, like the colour of pea seeds or the human blood type.
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