2. BASIC GENETICS OF HUMAN DISEASES

Gregor Mendel founded the basic rules of genetics in 1865. He started his experiments using the plant Pisum sativum. But later it was found that these rules applied also on animals including man.  Mendelian rules explain how the traits are inherited throughout the different generations. He established that a pair of genetic factors now known as alleles controls each character. These factors segregate during gamete formation, and reunite during embryo formation.  It is now known that these factors are located on rod-like structures, which are the chromosomes. Each individual is characterized by having a constant chromosome number in the somatic cell, which is known as the diploid number (2n). Gametes, which are produced by the meiotic cell division, contain half the chromosome number, which is the haploid number (n). For example in man his diploid number is 2n = 46 chromosome, while his haploid number is n = 23. The sexually reproduced individuals contain special chromosomes that determine the type of sex, these are the sex chromosomes, and the rest of the chromosomes are known as autosomes.  In female, the two sex chromosomes are similar and designated as XX, while in male they are different and designated as XY. Normally the Y-chromosomes do not carry genes.

The physical position of a gene on a chromosome is the locus. Each gene has two forms that are the alleles located at the same locus on both of the homologous pair of chromosomes. When there are two identical alleles at a locus a person is known as homozygous, when the two alleles are different, the person is heterozygous. The genetic constitution is the genotype. The expression of the genotype is the phenotype. The two alleles are present in two forms, one is known as the dominant allele and the other is known as the recessive allele. Any individual could have only a pair of these alleles. The dominant allele masks the expression of the recessive allele, when present in the heterozygous condition.

The genes located on the autosomes are autosomal while the genes located on the sex chromosomes as sex-linked genes. Because females have two X-chromosomes and male have only one the inheritance of characteristics that are X-linked is different from that of chromosomes determined by the other 22 autosome pairs. Therefore, Mendelian inheritance shows one of three patterns, autosomal dominant, autosomal recessive or sex X-linked (which may be dominant or recessive)