Question

What are the two causes of variation?

Answer 1

Hint: Variations are the differences between the individuals of a species. It is mainly caused by the mutations or genetic recombination which takes place during sexual reproduction. Variations cause changes in the phenotype of the organism i.e. the change in genes will affect the way they are expressed.

Complete answer:
variations caused by:-
1. Mutations- Mutations are the changes in the nucleotide sequence of the genome of an organism. Mutations may result from errors during DNA, mitosis, or meiosis or other types of damage to DNA which are caused by exposure to radiation (formation of thymine dimers). Mutations may also be a consequence of insertion or deletion of segments of DNA due to mobile genetic elements known as transposons.
mutations are of following types-
point mutations- In this ,change takes place in only one nucleotide which may or may not change the phenotype as they can take place in three ways.
a. In silent mutation the change corresponds to the previous sequence and barely causes any change in the amino acid produced.
b. In missense mutation change takes place at one point which alters the sequence and some other amino acid is formed.
c. In nonsense mutations, change causes the codon to turn into a stop codon which terminates the amino acid chain. This causes shortening of the amino acid chain.
frameshift mutations, in which a change will cause change in the reading frame overall as the codons are read without punctuation. Amino acid sequence changes from the point of mutation. new and defective protein is formed.
2. Genetic recombination - It is the rearrangement of DNA sequences by the breaking and rejoining chromosome segments and the consequences of such rearrangements is the inheritance of new combinations of alleles in the offspring that carry recombinant chromosomes. It is a programmed feature of meiosis in sexual organisms.

Note:
It can be caused by some environmental stresses like exposure to radiation (X -rays, UV rays etc). All the sexually reproducing organisms, in the process of cell division, specifically meiosis, experience genetic recombination at the prophase stage.