Genetics deals with how a few characteristics are transferred from generation to generation, i.e. heredity, or inheritance. Just like the majority of living organisms, human beings exhibit variation.
If you consider roughly any characteristic, you will discover differences between different people or other animals or plants in a population. There are two forms of variation:continuous and discontinuous variation.
Features that show continuous variation differ in a general way, with a wide range, and a lot of intermediate values between the extremes.
As a matter of fact, if you consider a large enough model from a population, perhaps plotting frequency as a histogram or as a frequency polygon, you would discover that the majority of the values are close to the average (mean), and farthest range of values are essentially to a certain extent rare.
Height is an example of a continuously variation in so far as you take into consideration a constant sample, for instance a huge number of people of a specific age and sex.
It is normally difficult to give a straightforward explanation of the genetic basis for these continuously variation due to the fact that they result from a combination of genetic factors in addition to environmental influences.
Characteristics that exhibit discontinuous variation fall into a few very different classes. The capability to roll the tongue, and blood groups, are examples of discontinuous variation.
These characteristics can be explained much more simply by straightforward rules of genetics and are less likely to be affected by other factors.
Human physical appearance is referred to as the outward phenotype or look of human beings. There are countless variations in human phenotypes, though society minimizes the variation to different categories.
Physical appearances of humans, especially those characters which are known as crucial for physical beauty, are believed by anthropologists to considerably affect the development of personality and social relations. Humans are extremely sensitive to their physical appearance.
A few differences in human appearance are genetic, others are due to age, lifestyle or disease, and many are the result of personal beautification.
Humans are dispersed across the globe with exception of Antarctica, and form an extremely erratic species. In adults, average weight differs from around 40 kilos for the smallest and major lightly built tropical people to around 80 kilos for the heavier northern peoples.
Size as well differs between the sexes, the sexual dimorphism in humans is more pronounced that that of chimpanzees, but less the sort of dimorphism found in gorillas.
The colouration of skin, hair and eyes as well differs greatly, with darker pigmentation authority in tropical climates and lighter in Polar Regions.
Factors Affecting Physical Appearance
A lot of factors are considered pertinent in relation to the physical appearance of humans.
• Genetic, ethnic affiliation, geographical ancestry
• Height, body weight, skin tone, body hair, sexual organs, moles, birthmarks, freckles, hair color, hair texture, eye color, eye shape, nose shape example nasal bridge, ears shape- example earlobes and body shape.
• Body deformations, mutilations and other variations like amputations, scars, burns and wounds
Causes of variation
A few of the characteristics possessed by an individual in a population can be said to be inherited. This means that they obtained from the past generation. These characteristics are transferred from generation to generation in a rather conventional way, due to sexual reproduction.
Sexual reproduction as well introduces an atom of unpredictability, so that variation is brought about in a population.
These two approximately contradictory factors: reliable inheritance of characteristics from parents, and variation that exist within a population – are indispensable to the perception of the process of evolution.
Three examples of physiological variations that exist between human being are:
a)Ability to roll tongue
There are two classes of tongue-rolling ability:
Rollers and non-rollers.
b) Ability to taste phenylthiocarbamide (PTC)
C) Blood groups ABO classifications
There are 4 classes of blood group: A, B, O and AB.
The ABO blood group system
The antigens for the ABO system are a group of glycoproteins. Frankly attached to the red cell membrane is a protein.
At a definite segment of the protein is bonded a type of 5-carbon sugar, fucose. These fucose sugar molecules are known to as the H antigen, and interact with an antiserum known as anti-H.
The production of antigen H is controlled by a detached locus from that of the ABO blood group, but antigen H is closely linked with the ABO system.
The majority of people who possess an allele for blood type O have antigen H, and ought to more correctly be classified as blood type H. Therefore, the most correct way to explain this blood group system is the ABH system.
There are some individuals, who do not have the H antigen and just possess a naked protein chain hanging aloof their red cells. This is as well known as the Bombay blood type.
The frequency of the Bombay allele is anywhere around .0066, so homozygotes are very rare. This allele is normally represented as the normal H allele is dominant to it.
The allele for blood group A makes it possible for another sugar to be attached to the antigen H, fucose, and sugar molecule. This attached sugar is N-acetylgalactosamine (NAG), and is the A antigen.
The allele for blood group B makes a molecule of simple galactose sugar to be attached to the fucose molecule. This is the B antigen. The O allele causes the H antigen to remain unmodified.
People who are AA homozygotes or AO heterozygotes have mainly the A antigen, with typically a little free H antigen. In fact there are 4 dissimilar A alleles, the variations that exists between them are poorly understood, but which appear to vary primarily in the amount of H antigen that gets converted to A antigen.
So, they make differences in the strengths of the antigen-antiserum reactions to occur.
Nevertheless, specific antisera can be made to at least few of the 4 different blood groups subtypes, so there ought to be a number of differences in the actual antigen as well.
Most non-African populations have only A1 and A2 alleles, but Africans can also have Aint and Abantu alleles.
People who are BB or BO have more often than not the B antigen, with a little free H antigen. There is no chief variability in the B blood type.
People who are AB heterozygotes possess both the A and B antigens. Every one of the cell will possess more or less half of its H antigens customized into A antigens and about half customized into B antigens.
This showcases the phenomenon of codominance. Neither A nor B is dominant to the other, so the products of both alleles can exist in a heterozygote. Both A and B are dominant to O.
An individual does not usually make antibodies to any antigen which he or she personally has. The galactose and fucose sugars are widespread enough in nature, especially in disease carrying organisms, so people produce antibodies to these sugars if they are not part of their personal antigen system.
Consequently, everybody who is not blood group A will make anti-A antibodies. Everyone who is not blood group B will make anti-B antibodies.
Approximately nobody produces anti-H antibodies, but you can extract an anti-H antiserum from the seeds of the widespread gorse plant.
The major significance of the ABH blood group system is in blood matching for transfusions. If the donor and recipient are not matched in terms of their ABH blood types then the antibodies in the recipient’s plasma may result to an agglutination reaction of the red cells from the donor.
This is a serious situation for the patient. Noteworthy though is that people with blood type AB possess no antibodies in their plasma, so they can in fact receive blood from anybody. This is why they are known as universal recipients.
Normally, the small amount of antibodies introduced with the plasma from the donor’s blood doesn’t lead to a very severe reaction in the recipient to cause any problems, even though it is still better to match blood types exactly.
Noteworthy as well is the fact that blood type O individuals have no A or B antigen. This means that nobody’s antibodies can agglutinate their cells. This is why type O people are frequently called universal donors, although you may infrequently have problems with the antibodies in the plasma of type O blood.
A lot of other serum proteins, red cell proteins, blood groups, and antigen systems exist. What we have described here is a few of the main well known ones.
A Gene Mutation is an extremely rare occurrence really. A mutation in a single inheritable characteristic (gene) is normally less likely than one in a million, but immediately it has happened, it might be passed on to the next generation, along the same lines as other inherited characteristics.
Nevertheless, not every individual carrying mutation survives; the majority of them have been found to be harmful, so that the organisms carrying them are at a disadvantage. In the wild, that type of organism is not likely to survive.
However, a few valuable mutations confer an advantage, and others are neutral. They are of no advantage or disadvantage – in the slightest till there is a few reason for selection of adapted types to take place.
This may be a different reason for variation within a population. In fact, some variable forms resulting from mutation that are beneficial can spread through a population by natural selection, and this might have the eventual effect of altering a population to a great extent that it varies from its original form – leading to the evolution of a fresh species.
Chromosome mutations may as well lead to an alteration in the number of chromosomes included into the sex cells. A child produced as a consequence may possess, for an example, an extra chromosome, or an extra part of a chromosome affixed to the normal set.
Down’s syndrome results in a child who possesses 47 chromosomes instead of the normal 46 per cell.