Genetics: The Study of Heredity

Read this article to learn about the genetics:- the study of heredity. It is appropriately regarded as the science that explains the similarities and differences among the related organisms.

The Blood Theory of Inheritance in Humans:

For many centuries, it was customary to explain inheritance in humans through blood theory. People used to believe that the children received blood from their parents, and it was the union of blood that led to the blending of characteristics.

That is how the terms ‘blood relations’, ‘blood will tell’, and ‘blood is thicker than water’ came into existence. They are still used, despite the fact that blood is no more involved in inheritance.

With the advances in genetics, the more appropriate terms should be as follows:

I. Gene relations in place of blood relations.

II. Genes will tell instead of blood will tell.

Brief History and Development of Genetics:

Genetics is relatively young, not even 150 years. The blood theory of inheritance was questioned in 1850s, based on the fact that the semen contained no blood. Thus, blood was not being transferred to the offspring. Then the big question was what was the hereditary substance.

Mendel’s experiments:

It was in 1866, an Austrian monk named Gregor Johann Mendel, for the first time reported the fundamental laws of inheritance. He conducted several experiments on the breeding patterns of pea plants. Mendel put forth the theory of transmissible factors which states that inheritance is controlled by certain factors passed from parents to offspring’s. His results were published in 1866 in an obscure journal Proceedings of the Society of Natural Sciences.

For about 35 years, the observations made by Mendel went unnoticed, and were almost forgotten. Two European botanists (Correns and Hugo de Vries) in 1900, independently and simultaneously rediscovered the theories of Mendel. The year 1900 is important as it marks the beginning the modern era of genetics.

The origin of the word gene:

In the early years of twentieth century, it was believed that the Mendel’s inheritance factors are very closely related to chromosomes (literally coloured bodies) of the cells. It was in 1920s, the term gene (derived from a Greek word gennan meaning to produce) was introduced by Willard Johannsen. Thus, gene replaced the earlier terms inheritance factor or inheritance unit.

Chemical basis of heredity:

There was a controversy for quite some time on the chemical basis of inheritance. There were two groups—the protein supporters and DNA supporters. It was in 1944, Avery and his associates presented convincing evidence that the chemical basis of heredity lies in DNA, and not in protein. Thus, DMA was finally identified as the genetic material. Its structure was elucidated in 1952 by Watson and Crick.

Basic Principles of Heredity in Humans:

The understanding of how genetic characteristics are passed on from one generation to the next is based on the principles developed by Mendel. As we know now, the human genome is organized into a diploid (2n) set of 46 chromosomes. They exist as 22 pairs of autosomes and one pair of sex chromosomes (XX/XY). During the course of meiosis, the chromosome number becomes haploid (n). Thus, haploid male and female gametes — sperm and oocyte respectively, are formed.

On fertilization of the oocyte by the sperm, the diploid status is restored. This becomes possible as the zygote receives one member of each chromosome pair from the father, and the other from the mother. As regards the sex chromosomes, the males have X and Y, while the females have XX. The sex of the child is determined by the father.

Monogenic and Polygenic Traits:

The genetic traits or characters are controlled by single genes or multiple genes. The changes in genes are associated with genetic diseases.

Monogenic disorders:

These are the single gene disease traits due to alterations in the corresponding gene e.g. Sickle-cell anemia, phenylketonuria. Inheritance of monogenic disorders usually follows the Mendelian pattern of inheritance.

Polygenic disorders:

The genetic traits conferred by more than on gene (i.e multiple genes), and the disorders associated with them are very important e.g. height, weight, skin colours, academic performance, blood pressure, aggressiveness, length of life.

Patterns of Inheritance:

The heredity is transmitted from parent to offspring as individual characters controlled by genes. The genes are linearly distributed on chromosomes at fixed positions called loci. A gene may have different forms referred to as alleles. Usually one allele is transferred from the father, and the other from the mother.

The allele is regarded as dominant if the trait is exhibited due to its presence. On the other hand, the allele is said to be recessive if its effect is masked by a dominant allele. The individuals are said to be homozygous if both the alleles are the same. When the alleles are different they are said to be heterozygous.

The pattern of inheritance of monogenic traits may occur in the following ways (Fig. 69.1).

1. Autosomal dominant

2. Autosomal recessive

3. Sex-linked.

1. Autosomal dominant inheritance:

A normal allele may be designated as a while an autosomal dominant disease allele as A (Fig. 69.1 A). The male with Aa genotype is an affected one while the female with aa is normal. Half of the genes from the affected male will carry the disease allele.

On mating, the male and female gametes are mixed in different combinations. The result is that half of the children will be heterozygous (Aa) and have the disease. Example of autosomal dominant inherited diseases are familial hypercholesterolemia, β-thalassemia, breast cancer genes.

2. Autosomal recessive inheritance:

In this case, the normal allele is designated as B while the disease-causing one is a (Fig. 69.1 B). The gametes of carrier male and carrier female (both with genotype Bb) get mixed. For these heterozygous carrier parents, there is one fourth chance of having an affected child. Cystic fibrosis, sickle-cell anemia and phenylketonuria are some good examples of autosomal recessive disorders.

3. Sex (X)-linked inheritance:

In the Fig. 69.1 C, sex-linked pattern of inheritance is depicted. A normal male (XY) and a carrier female (X^CY) will produce children wherein, half of the male children are affected while no female children is affected. This is due to the fact that the male children possess only one X chromosome, and there is no dominant allele to mark its effects (as is the case with females). Colour blindness and hemophilia are good examples of X-linked diseases.

A selected list of genetic disorders (monogenic traits) due to autosomal and sex-linked inheritance in humans is given in Table 69.1.

Genetic Diseases in Humans:

The pattern of inheritance and monogenic traits along with some of the associated disorders are described above (Table 69.1). Besides gene mutations, chromosomal abnormalities (aberrations) also result in genetic diseases.

Aneuploidy:

The presence of abnormal number of chromosomes within the cells is referred to as aneuploidy. The most common aneuploid condition is trisomy in which three copies of a particular chromosome are present in a cell instead of the normal two e.g. trisomy-21 causing Down’s syndrome-, trisomy-18 that results in Edward’s syndrome. These are the examples of autosomal aneuploidy. In case of sex-linked aneuploidy, the sex chromosomes occur as three copies, e.g. phenotypically male causing Klinefelter’s syndrome has XXY; trisomy-X is phenotically a female with XXX.

Selected examples of chromosomal disorders along the with the syndromes and their characteristic features are given in Table 69.2.

Eugenics:

Eugenics is a science of improving human race based on genetics. Improving the traits of plants and animals through breeding programmes has been in practice for centuries. Eugenics is a highly controversial subject due to social, ethical, and political reasons. The proponents of eugenics argue that people with desirable and good traits (good blood) should reproduce while those with undesirable characters (bad blood) should not.

The advocates of eugenics, however, do not force any policy, but they try to convince the people to perform their duty voluntarily. The object of eugenics is to limit the production of people who are unfit to live in the society.

Eugenics in Nazi Germany:

Germany developed its own eugenic programme during 1930s. A law on eugenic sterilization was passed in 1933. In a span of three years, compulsory sterilization was done on about 250,000 people, who allegedly suffered from hereditary disabilities, feeble mindedness, epilepsy, schizophrenia, blindness, physical deformities, and drug or alcohol addition. The German Government committed many atrocities in the name of racial purity. Other countries however do not support this kind of eugenics.