INTRODUCTION. Heredity is the totality of characteristics transmitted from the parents to the offspring. The transmission of genetic characteristics is a function of the CHROMOSOMES and GENES (q.q.v.). The exact mechanism of hereditary transmission was not known until the discovery of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). It is now believed that the molecules of DNA serve as a kind of code whose built-in program influences the developing tissues through the agency of RNA, which serves in turn as a messanger for DNA. CHEMICAL BASIS OF INHERITANCE. In the nucleus of every cell are the chromosomes containing the genes composed of segments of deoxyribonucleic acid (DNA) within which genetic information is stored. There are nitrogenous base, a sugar molecule (deoxyribose), and a phosphate molecule in each nucleotide as DNA is made up of two polynucleotide chains, twisted together to form a double helix. The nitrogenous bases in DNA are adenine and guanine ( purines) and cytosine and thymine (pyrimidines). A purine in one chain always pairs with a pyrimidines on the other chain, specific base pairing: guanine in one chain always pairs with cytosine in the other chain and adenine always pairs with thymine. The details cannot be given here, but, in short, the primary action of the gene is to synthesise protein by various combinations of 20 different amino acids. Genetic information is stored within the DNA molecule in the form of a triplet code such that a sequence of a three bases specifies the structure of one amino acid. FROM: PRINCIPLES AND PRACTICE OF MEDICINE. UNIFACTORIAL INHERITANCE A primary error in the DNA code paves the way for disease or disorder because of the defects of a single gene. Mendelian laws reveal about the unifactorial inheritance. It can be accurately predicted on theoretical grounds and genetic counselling, therefore, is straightforward. Chromosome on which the abnormal or (mutant) gene is situated or the nature of the trait itself explains. An autosomal trait is determined by a gene situated on an autosome which may be either dominant or recessive. In the same way, a trait is determined by a gene and it is sex-linked. AUTOSOMAL DOMINANT INHERITANCE. Dominant trait is manifested in the in the heterozygote. It does mean that an autosomal dominant trait possesses both the mutant gene and the normal gene. It is necessary for the dominant to be manifested in heterozygote if there is only one mutant gene. The disorder may be common in homozygotes ( i.e. a double dose of mutant gene). In case, if the disorder is rare, then, the individuals. are almost always heterozygotes as the normal and abnormal genes are called alleles, i. e. the alternative forms of the same gene. Heterozygotes are usually perfectly healthy. An autosomal dominant trait may be the outcome of one generation to the next in a family which is minimally affected or more commonly due to a new mutation. A person is rendered infertile or do not survive to reach reproductive age in diseasrs which are severe. In such conditions, the disease becomes extinct. The individuals who suffer from chronic renal failure have no symptoms or signs of the disease until early middle life and, thus, run the risk of transmitting the trait to their offspring as they have already had their family. In such condition, if an individual whose spouse is a normal person, there is a 1 in 2 chance of producing an affected individual. In smaller size of offspring there may be perfectly healthy, half, or by chance all affected individuals. In some cases, the individuals may carry a mutant gene and are unaffected, it is called a non-penetrant gene. The phenomenon of dominant mutant traits appear to have “skipped” generations in cwrtain cases. The variation in expression of a mutant gene is a modifying influence of other genes and from environmental factors. AUTOSOMAL RECESSIVE INHERITANCE. Recessive traits are maanifest only in the homozygous state, in those who possess a double dose of the mutant gene. If a spouse of a homozygote is also the homozygote for the same recessive trait, the offspring would be affected. In general, a rare recessive gene found in a homozygous would have healthy generation past and future. In autosomal recessive inheritance affected individuals cannot be traced from one generation to the next if more than one individual of a family is affected they are usually sibs. SEX-LINKED INHERITANCE. Genes situated on either of the. sex chrosomes are sex-linked traits. The genes situated on X or Y chromosomes are different sex-linked traits. Y linkage of a gene would affect only males. According to researchers all known sex-linked conditions are due to genes on X chromosome. These conditions are dominant or recessive with autosomal traits. ENVIRONMENT. Environment is totality of, or any aspect of, physical and social phenomena that surround or affect an individual organism or part of an organism. Psychologists recognize several types of environment:1. the postnatal environment which the environment after birth; 2. the prenatal environment or the environment of the individual before birth; 3. the cellular environment consisting of the blood and electrolytic fluids and other cells that surround any given cell; 4. the internal environment, or the environment of the cells inside the body. Environmental psychology is the branch of psychology concerned with the relationship between behavior and the environment in which it occurs, with special emphasis on the variables of space, architecture, crowding, and noise. Environmental psychology is also the field of psychology focusing on the effects of environmental setting on an individual’s feelings and behavior. Perceptions of environment. The individual’s perceptions are accurate or not it is ascertained. An individual may be unduly sensitive to glances or chance remarks. A person may suffer from illusions, misinterpretations of sensations arising from real stimuli. Hallucinations are sense perceptions. Delusions are false beliefs as these occur from the source outside.