When you meet a mother and her adult or teenage daughter it is often easy to spot the family resemblance. It is also relatively easy to see the difference that the environment and lifestyle contribute to the different family generations.
The mother’s body will reflect life of an earlier age, having given birth to a number of children and having dealt with society over time. The daughter born more recently will usually be a little taller reflecting better quality of health and nutrition and her face will not have as much life experience as the mother.
Nevertheless there will be certain characteristics, perhaps the body shape or the ears that look the same especially when taking into account the difference in age. The reasons for these similarities lie in the way that human beings replicate to form new human beings.
The basic unit of the human body is the cell and each person has about 100 trillion cells. These cells combine to form tissues like muscles, which in turn make up complex organs like the heart and stomach. Inside each cell there is a centre called the nucleus within which are found chromosomes. Under a microscope each chromosomal pair can be visualised like a long ladder forming a spiral staircase with a section between the rungs of the ladder being a single gene.
The human genome, the complete set of human genes, number about 21,000 genes. In each ladder one side is inherited from the mother and the other from the father and if the microscopic strand of DNA in each cell was to be unwound it would stretch to about two metres in length. These genes contain the instructions for protein production that determine how a cell will work. Further it is the number of chromosomes a species carry that differentiates them. For example, humans have 23 chromosome pairs — that is 46 chromosomes — while a monkey has 42 chromosomes, a type of rat has 102, the highest among mammals. A chicken for your information has 78 chromosomes, the same number as a dog. What does that make you think?
Among the 23 pairs of chromosomes there is a single pair that is mismatched, XX in females and XY in males so differentiating the sexes. During the creation of a new human being, the new DNA get shuffled so that even though the ladder created is the same length bits of the ladder in between will differ from one person to the next, except for the Y chromosome which being one often gets passed almost intact. So the Y chromosome in a family of males will differ mainly because of random mutation so a boy, his father and grandfather often have a strong resemblance.
Random mutations occur because when a chromosome has to divide it has to make an exact copy of itself. While doing so there are times when the odd segment is not done exactly and so there is an error in the new chromosome strand. Luckily in the two metre strand there are many sections that are duplicates so that if one section does not work another does the same work. However, in certain cases — especially where the other parent’s chromosome DNA also has an error — the combined new chromosome in an offspring can be defective leading to a hereditary condition, which can be minor such as a funny looking nose or major life-threatening condition.
Genes work throughout your life, as long as you are breathing new cells have to be made. However the most crucial part of their work is done during pregnancy when the sperm and egg meet to create an embryo, which grows into a foetus and eventually a baby occurs. Most genes that humans carry are exactly the same as every living thing. Genetically mammals are very close. It can be very difficult to differentiate the embryo of a rat from that of a human being as they go through the same stages of development; except that the human being has certain genes that express the ‘humanness’ of a human kidney compared to the ‘ratness’ of a rat kidney, given that the function is the same.
The human embryo during the sixth week of gestation has a distinct tail protruding from the trunk in the same position as animals, which have a tail. However by the eighth week, the tail disappears absorbed back into the tissues and the spinal cord seals itself. But there are some individuals where this process does not completely finish and so they are born with an appendage.
Medically there are several types of human tails. The more common variation is a persistent embryonic tail and consists mainly of soft fatty tissue that after birth can be surgically removed without any complications. There is however a variation that is a true tail, which contains muscle, blood vessels, and nerves and is covered with skin. Such a tail can be wagged. This type of tail is very, very rare in humans. Less than 50 cases have ever been fully documented and treatment is much more complicated. In the end the important point to note is that despite the occasional difference we are really much all the same.