Every human body cell has the same genetic information, except for gametes or sexual cells (eggs and spermatozoids).
There are nuclei inside every cell of the organism, where the chromosomes are located and store information. The reason why skin cells don’t look like pancreatic cells is the complete information, that is to say, genes expressed by each cell are different and that puts emphasis on the difference between them.
To understand the difference between cells, it is important to distinguish between genotype and phenotype:
- Genotype refers to the group of genes (genetic information) one person’s cells are composed of. All cells have the same genetic information, except for sexual cells.
- Phenotype refers to the expression of genotype genes. Phenotype is what differentiates cells and refers to the ‘aspect’ of each cell. It’s a part of the global information that constitutes the genotype.
We can compare it to the behaviour of a person: people are complex and depending on the situation they will be loving, tough, relaxed, or angry. They will always be internally complex, though.
Each somatic cell, i.e. all except but sexual cells, is composed of duplicate information, that is to say, each cell is diploid. It is said that human body cells are composed of 23 chromosome pairs, plus the sexual cells: XX for females and XY for males.
Each chromosome is structurally equal to their partner. One chromosome comes from the father’s sperm and the other from the mother’s egg. The embryo will have 1 Y chromosome and 1’ chromosome. Both have the same shape but different contents.
Roughly, in each chromosome there are many compartments with information that governs a characteristic, such as eye colour. In the eye, one compartment might have blue chromosomes and the other another darker eye colour. Which one will be visible once the child is born? In this case, the dominant characteristic is the darker colour.
Genetic information from cells fuses during the formation of an embryo. For these gametes to be formed, genetic information has to be reduced by half, or else the embryo’s genetic information would grow exponentially.
With this reduction, each individual of the human species has the same genetic quantity. Half of it comes from the father and the other half from the mother.
The information in the sexual cells is reduced through a process called meiosis.
Meiosis is a process of cell division in which a cell with 23 pairs of chromosomes and one pair of sex chromosomes (XX or XY) comes to have four cells, each with half the genetic information, that is to say, the cell now has only one member of each chromosome pair and only one sex chromosome.
For example, if a male somatic cell has the following chromosome content: 1,1’,2,2’,3,3’,4,4’,5,5’,6,6’,7,7’,8,8’…23,23’,X,Y, spermatozoids born from it could be 1,2’,3’,4,5,6’…23’. If we choose one random member from each original pair of chromosomes, when it joins the scant information the oocyte contains, a new group of 23 pairs, each formed by a chromosome from the father and one from the mother will be created.
When an oocyte is being created, it originates from a diploid mother cell, somatic, with 23 Y chromosome pairs and one XX pair. When the creation and formation are completed, from each mother cell, Oogonium, four distinct eggs will emerge with half the information. Characteristics will no longer be governed by two chromosomes but by only one without a duplicate content.
This way, during fertilization, an egg with half the information meets one spermatozoid with only half the genetic information. The embryo will contain one 2 chromosome from the father and one 2’ chromosome from the mother (or one 2’ from the father and one 2 from the mother). For this reason, each feature is, once again, governed by two chromosomes with similar structures but different information originated from the man or the woman.