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The idea of mutation first originated from the observations of a Dutch botanist Hugo de Vries (1880) on variations in plants of Oenothera lamarckiana. The mutation can be defined as sudden, stable discontinuous and inheritable variations which appear in organism due to permanent change in their genotype. Mutation is mainly of two types :
(1) Spontaneous mutations : Mutation have been occurring in nature without a known cause is called spontaneous mutation.
(2) Induced mutation : When numerous physical and chemical agents are used to increase the frequency of mutations, they are called induced mutations.
Gene mutations
Gene or point mutations are stable changes in genes i.e. DNA chain. Many times a change in a gene or nucleotide pair does not produce detectable mutation. Thus the point or gene mutation mean the process by which new alleles of a gene are produced. The gene mutation are of following types :
Tautomerism : The changed pairing qualities of the bases (pairing of purine with purine and pyrimidine with pyrimidine) are due to phenomenon called tautomerism.
Tautomeres are the alternate forms of bases and are produced by rearrangements of electrons and protons in the molecules.
Substitutions (Replacements) : These are gene mutations where one or more nitrogenous base pair are changed with others. It may be further of three sub types :
(1) Transition : In transition, a purine (adenine or guanine) or a pyrimidine (cytosine or thymine or uracil) in triplet code of DNA or mRNA is replaced by its type i.e. a purine replaces purine and pyrimidine replaces pyrimidine.
\[GC\to AT\text{ }or\text{ }AT\to GC\]
(2) Transversion : Transversion are substitution gene mutation in which a purine (adenine or guanine) is replaced by pyrimidine (thymine or cytosine) or vice versa.
\[GC\to CG\text{ }or\text{ }TA~~~~,~~~~~AT\to TA\text{ }or\text{ }CG\]
(3) Frame shift mutations : In this type of mutations addition or deletion of single nitrogenous base takes place. None of the codon remains in the same original position and the reading of genetic code is shifted laterally either in the forward or backward direction.
Chromosomal mutation or aberrations
A gene mutation normally alters the information conveyed by a gene, it alters the message. On the other hand, chromosomal mutation only alters the number or position of existing genes. They may involve a modification in the morphology of chromosome or a change in number of chromosomes.
(1) Morphological aberrations of chromosomes
Deletion or deficiency : Sometimes a segment of chromosome break off and get lost. If a terminal segment of a chromosome is lost, it is called deficiency. Deficiency generally proves lethal or semilethal. If intercalary segment is lost it is termed deletion.
Deletion occurs during pairing in meiosis. For example in human babies deletion of a segment of chromosome number 5 causes a disease called cri-du-chat syndrome (the baby cries like more...
More than two alternative forms (alleles) of a gene in a population occupying the same locus on a chromosome or its homologue are known as multiple alleles.
Characteristics of multiple allelism
(a) There are more than two alleles of the same genes.
(b) All multiple alleles occupy the corresponding loci in the homologous chromosomes.
(c) A chromosome or a gamete has only one allele of the group.
(d) Any one individual contains only two of the different alleles of a gene, one on each chromosome of the homologous pair carrying that gene.
(e) Multiple alleles express different alternative of a single trait.
(f) Different alleles may show codominance, dominance-recessive behaviour or incomplete dominance among themselves.
(g) Multiple alleles confirm to the Mendelian pattern of inheritance.
Examples of multiple allelism : A well known example of a trait determined by multiple alleles is the blood groups in man and skin colour. Other example are eye colour in Drosophila, colour of wheat kernel, corolla length in Nicotiana, Coat colour in Cattle etc.
Blood groups in man
Blood proteins : According to Karl landsteiner (1900) a Nobel prize winner, blood contains two types of proteinous substances due to which agglutinations occurs.
(1) Agglutinogen or antigen : It is a protein found on the cell membrane of RBC’s.
(2) Agglutinin or antibody : This the other proteinous substance, found in the plasma of the blood.
Whenever the blood of a person receives the foreign proteins (antigen) his blood plasma starts forming the antibodies in order to neutralize the foreign antigens.
Agglutinations : Two types of antigens are found on the surface of red blood corpuscles of man, antigen A and B. To react against these antigens two types of antibodies are found in the blood plasma which are accordingly known as antibody – anti-A or a and anti-B or b. Agglutination takes place only when antigen A and antibody a occur together or antigen B and antibody b are present in the blood.
Under such condition antibody a reacts with antigen A and makes it highly sticky. Similarly antigen B in presence of antibody b become highly sticky with the result RBC’s containing these antigens clump to form a bunch causing blockage of the capillaries. Agglutination in blood is therefore antigen-antibody reaction.
Types of blood groups
ABO blood group : Landsteiner divided human population into four groups based on the presence of antigens found in their red blood corpuscles. Each group represented a blood group. Thus there are four types of blood groups viz. A, B, AB and O. He observed that there was a reciprocal relationship between antigen and antibody according to which a person has antibodies for those antigens which he does not possess.
Blood groups of man with antigen and antibodies
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In such a pedigree analysis a person who is the beginner of the family history is called proband. It is called propositus, if male and poposita, if female. The children of such parents are known as sibs or siblings. So a family is constituted by such parents and their siblings. Sometimes, a very large family is formed as a result of interconnected marriages. Such a circle of large persons interconnected is called Kindred. 
Protenor type of sex determination in Grasshopper
Haploid-diploid mechanism of sex determination
Hymenopterous insects, such as bees, wasps, saw flies, and ants, show a unique phenomenon in which an unfertilized egg develops into a male and a fertilized egg develops into a female. Therefore, the female is diploid (2N), and the male is haploid (N). eggs are formed by meiosis and sperms by mitosis. Fertilization restores the diploid number of chromosomes in the zygote which gives rise to the female. If the egg is not fertilized, it will still develop but into a male. Thus, the sex is determined by the number of chromosomes.
In honeybee, the quality of food determines whether a diploid larva will become a fertile queen or a sterile worker female. A larva fed on royal jelly, a secretion from the mouth of nursing workers, grows
