Chapter 5 Principles Of Inheritance And Variation
- Genetics is a branch of biology which deals with principles of inheritance and its practices.
- Mendel’s Laws of Inheritance: Mendel proposed that the factors 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 the 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 express 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 genes) on chromosomes regulating the characters are called the genotype and the physical expression of the characters is called phenotype.
- 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 the ‘Linked genes’.
- Many genes were linked to the sexes also, and called ‘sex-linked genes’. The two sexes (male and females) 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 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 chromosomes as XY.
- Mutation is defined as the change in the genetic material. A point mutation is a change of a single base pair in DNA. Sickle-cell anemia 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 or change in a subset of chromosome number.
- Down’s syndrome: This happens due to the trisomy of chromosome 21, where there is an extra copy of chromosome 21 and consequently the total number of chromosomes becomes 47.
- Turner’s syndrome: This happens when one X chromosome is missing and the sex chromosome is X0.
- Klinefelter’s syndrome: This happens when the sex chromosome is XXY.