The following points highlight the top three techniques used to study the genetics of human traits. The techniques are: 1. Pedigree Analysis 2. Amniocentesis 3. Twins.
Technique # 1. Pedigree Analysis:
Since, in human beings controlled crosses cannot be made, so human geneticists have to resort to a scrutiny of established matings in the hope that informative matings have been made by chance. The scrutiny of established matings is called pedigree analysis. A member of a family who first comes to the attention of a geneticist is called the propositus. Usually the phenotype of the propositus is exceptional in some way—for example, a dwarf.
The investigator then traces the history of the character shown to be interesting in the propositus back through the history of family, and a family tree or pedigree or pedigree chart is drawn up using certain standard symbols.
A family pedigree chart conventionally has circles for female individuals and squares for male individuals. A marriage is indicated by a horizontal bar connecting a circle and square and the symbols for offspring are shown suspended from a line drawn perpendicular to the marriage bar.
Individuals on the same line are from the same generation, and individuals in each generation are numbered in sequence-(such as III, II2, II3, and so on). The un-shaded circles or squares designate people who are normal for the character being studied, while solid black (or shaded) squares or circles depict “affected” individual.
Heterozygotes customarily are designated by colouring half of the symbol blacks. Carrier of a sex-linked recessive gene is designated by a black dot in the middle of the symbol. Occasionally, an arrow pointing at a particular affected individual is added to indicate that this is the person who brought the trait to the geneticists, attention.
Example:
A case of polydactyly, the occurrence of extra fingers, in man can be considered as an example for pedigree analysis.
Here, we have a marriage between a polydactylous man and a normal woman (generation I). They reproduce three children, a polydactylous daughter, a polydactylous son, and a normal son (generation II).
The first and third individuals of II generation, each marry normal (pentadactylous) persons, their children are shown in generation III. From the analysis of pedigree of polydactyly, one fact becomes clear that polydactylous trait is dominant over the normal pentadactylous trait which is recessive.
How pedigree charts are read?
Many human diseases and other exceptional conditions are determined by simple Mendelian recessive alleles. There are certain clues in the pedigree that must be sought. Characteristically, the diseased condition appears in progeny of unaffected parents. Furthermore, two affected individuals cannot have an unaffected child.
Quite often such recessive alleles are revealed by consanguineous matings-for example, cousin marriages. This is particularly true of rare conditions where chance matings of heterozygotes are expected to be extremely rare, e.g., albinism, Tay-Sachs disease, cystic fibrosis and phenylketonuria (PKU).
There are also examples of exceptional conditions caused by dominant alleles. Once again, there are some simple rules to follow to ascertain from pedigrees, a condition caused by a dominant allele: the condition typically occurs in every generation; unaffected individuals never transmit the condition to their offspring; two affected parents may have unaffected children; and the condition is passed, on average, to one-half of the children of an affected individual.
Some examples of abnormal conditions caused by dominant alleles in humans are achondroplasia (a kind of dwarfism). Huntington’s chorea and brachydactyly (very short fingers).
Technique # 2. Amniocentesis:
Prenatal (= occurring before birth) screening of babies for gross chromosomal aberrations (such as polyploidy, aneuploidy, deletions, translocations, etc.), as well as, sex prediction, is now possible by the technique of amniocentesis.
Amniocentesis involves removing amniotic fluid by the help of a hypodermic needle from the amnion (“the bag of water”) surrounding the foetus in uterus; culturing the foetal cells contained within the fluid; and screening the cultured cells for karyotype, enzyme production and restriction site pattern analysis of its DNA.
At least 35 genetic diseases have been detected in human beings by this technique. If a disease is detected by this method, abortion of such a foetus can then be recommended.
For karyotype preparation, foetal cells found in amniotic fluid can be cultured in vitro in a highly nutritive solution, treated with colchicine to stop mitosis at metaphase, subjected to a hypotonic solution to cause the cells to swell and scatter the chromosome, placed on a slide, stained and photographed under a microscope.
Individual chromosomes are then cut from the resulting photograph and matched as homologous pairs to form an ideogram. Further, a combination of amniocentesis with analysis of restriction enzyme fragments now enables scientists to do prenatal screening for sickle cell anaemia.
The single nucleotide substitution in the gene for the beta haemoglobin chain eliminates a restriction site in that gene. As a result, a larger fragment is produced upon enzyme digestion of DNA obtained from cultured cells from the amniotic fluid. This diagnosis cannot be made until several months after birth, as the gene is inactive before that time.
Technique # 3. Twins:
Normally, a woman gives birth to only one young at a time, but sometimes more than one child is born to a woman at the same time. The delivery of more than one baby by a mother is called multiple births.
Most commonly in the case of multiple births, the number of births are two and the individuals who are born in such a way are called twins. In case of multiple births, the number of infants may be three (triplets), four (quadruplets), five (quintuplets) or even more (e.g., eleven).
Twins are of two basic types:
1. Identical or Monozygotic Twins (MZ twins):
Twins having no variability in their traits are called identical twins. They are produced from a single fertilized ovum. Sometimes, the two blastomeres resulting from the first cleavage (mitosis) of the fertilized egg or zygote separate from each other and develop into independent embryos (hence they are also called monozygotic twins).
Since, the two embryos have arisen by mitosis, they have the same chromosome sets in their body cells. Thus, identical twins possess identical genes and they are like one person walking around in two bodies. These twins belong to the same sex. The situation is the same for identical triplets and other multiplets, but they are quite rare in human beings.
Sometimes, identical twins fail to separate completely from each other, they are called Siamese twins.
2. Fraternal or Dizygotic Twins (DZ twins):
Non-identical or fraternal twins are produced by simultaneous fertilization of two separate ova by two separate sperms. Two foetuses so obtained from two zygotes are called dizygotic twins.
They contain different sets of genes and are like any other brothers and sisters, but just happen to grow simultaneously in the same uterus. So fraternal twins differ from each other in their traits, development and often even sex.
Twins are useful in detecting the relative effectiveness of heredity and environment upon the expression of a disease or trait. The hereditary trait is most likely shown by both members of the identical twins.
If it is shown by one member of the identical twins, then the environmental factor is playing the major role since both possess the same genes. Very recently, studies of identical twins separated since birth revealed that genes (heredity) play a much greater role in determining behaviour than previously thought.
Although these twins were not raised together, they exhibited many similar behavioural traits. In any behavioural characteristic studied, genes are found to exert at least 50 per cent influence on the trait.
The conclusion is that genes play a major role in shaping almost any type of behaviour including alcoholism, criminality, intelligence (I.Q.), political attitudes, schizophrenia and sociability
Genetic Significance of Study of Twins:
Twin study has helped in understanding the effect of heredity and environment on the development of complex characters such as intelligence, talent, longevity, general health and personality and also behavioral characters.
The idea for twin study in human genetics was introduced by Galton. Muller, Freeman and Holzinger reared the two members of monozygotic twins separately under different environmental and social conditions. These MZ twins showed variation in their intelligence test.
It shows that environment plays an important role in the development of intelligence. However, the MZ twins reared under the same environmental and social conditions showed almost no difference in their intelligence. This proves that inheritance of intelligence has a genetic basis on which environment plays its role.