PRINCIPLES OF INHERITANCE AND VARIATION
Genetics is a branch of biology which deals with principles of inheritance and its practices. Progeny resembling the parents in morphological and physiological features has attracted the attention of many biologists. Mendel was the first to study this phenomenon systematically. While studying the pattern of inheritance in pea plants of contrasting characters, Mendel proposed the principles of inheritance, which are today referred to as ‘Mendel’s Laws of Inheritance’. He proposed that the ‘factors’ (later named as genes) regulating the characters are found in pairs known as alleles. He observed that the expression of the characters in the offspring follow a definite pattern in different–first generations (F1), second (F2) and so on.
Some characters are dominant over others. The dominant characters are expressed when factors are in heterozygous condition (Law of Dominance). The recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous condition. A recessive character that was not expressed in heterozygous condition may be expressed again when it becomes homozygous. Hence, characters segregate while formation of gametes (Law of Segregation). Not all characters show true dominance. Some characters show incomplete, and some show co-dominance.
When Mendel studied the inheritance of two characters together, it was found that the factors independently assort and combine in all permutations and combinations (Law of Independent Assortment). Different combinations of gametes are theoretically represented in a square tabular form known as ‘Punnett Square’. The factors (now known as gene) on chromosomes regulating the characters are called the genotype and the physical expression of the characters is called phenotype. After knowing that the genes are located on the chromosomes, a good correlation was drawn between Mendel’s laws : segregation and assortment of chromosomes during meiosis. The Mendel’s laws were extended in the form of ‘Chromosomal Theory of Inheritance’. Later, it was found that Mendel’s law of independent assortment does not hold true for the genes that were located on the same chromosomes. These genes were called as ‘linked genes’. Closely located genes assorted together, and distantly located genes, due to recombination, assorted independently. Linkage maps, therefore, corresponded to arrangement of genes on a chromosome. Many genes were linked to sexes also, and called as sex-linked genes. The two sexes (male and female) were found to have a set of chromosomes which were common, and another set which was different. The chromosomes which were different in two sexes were named as sex chromosomes. The remaining set was named as autosomes. In humans, a normal female has 22 pairs of autosomes and a pair of sex chromosomes (XX). A male has 22 pairs of autosomes and a pair of sex chromosome as XY.
In chicken, sex chromosomes in male are ZZ, and in females are ZW. Mutation is defined as change in the genetic material. A point mutation is a change of a single base pair in DNA. Sickle-cell anaemia is caused due to change of one base in the gene coding for beta-chain of hemoglobin. Inheritable mutations can be studied by generating a pedigree of a family. Some mutations involve changes in whole set of chromosomes (polyploidy) or change in a subset of chromosome number (aneuploidy). This helped in understanding the mutational basis of genetic disorders. Down’s syndrome is due to trisomy of chromosome 21, where there is an extra copy of chromosome 21 and consequently the total number of chromosomes becomes 47. In Turner’s syndrome, one X chromosome is missing and the sex chromosome is as XO, and in Klinefelter’s syndrome, the condition is XXY. These can be easily studied by analysis of Karyotypes.
List of Topics
| Mendel’s Laws of Inheritance | Mendel’s Laws of Inheritance |
| Inheritance of One Gene | Law of Dominance |
| Law of Segregation | |
| · Incomplete Dominance | |
| · Co-dominance | |
| Inheritance of Two Genes | Law of Independent Assortment |
| Chromosomal Theory of Inheritance | |
| Linkage and Recombination | |
| Polygenic Inheritance | Polygenic Inheritance |
| Pleoitropy | Pleoitropy |
| Sex Determination | Sex Determination in Human |
| Sex Determination in Honey Bee | |
| Mutation | Mutation |
| Genetic Disorders | Pedigree Analysis |
| Mendlelian Disorders | |
| Chromosomal Disorders |