Principles of Inheritance and Variation Test - 43

Thomas Hunt Morgan discovered mutation of white-eyed in fruitfly and was awarded Noble prize for this in 1933. Normally these flies have red colour eyes but he noticed that few male flies have white colour eyes. He immediately crossed these flies with wild flies i.e., red-eyed flies and observed that the F1 generation has only red-eyed flies but when he selfed these flies he saw the ratio is3:1. Thus, the correct option is B

Solution:

• Correct answer is option D.

• Explanation for the correct option

• Mutations are the sudden or induced change that caused the change in the DNA sequences. This leads to the change in the expression of the gene. So mutations are harmful most of the times. 
• Mutation is of two types on the basis of genotype of allele i.e., recessive and dominant.
  
• Recessive mutation examples-Cystic fibrosis, sickle cell anemia.
• Dominant mutation examples- Huntington's disease.

Meiosis I includes pairing of homologous chromosomes during prophase I and their segregation to opposite poles during anaphase I, this ensures that each gamete receives half the number of chromosomes as in somatic cell. Random fertilization of gametes during syngamy restores the somatic cell chromosome number in the zygote. Thus, reduction of chromosome number to half maintain the somatic chromosome number in the zygote which would otherwise multiply exponentially during each reproductive cycle. Reduction and syngamy are not mutations as there is no change in chemical structure of genes at a molecular level; options A and B are incorrect. The presence of additional chromosome, depending on a number of extra chromosomes, results in euploidy or aneuploidy. It is the chromosomal aberration, not the mutation; since, chromosomal aberrations and point mutations produce same phenotypic effects, the former is sometimes referred to as chromosomal mutations. Thus, the correct answer is option C.

Mutagens are physical/ chemical/ biological agents that can cause heritable changes in genetic material (mutation), for example, UV rays, X-rays, $$\gamma$$ rays, formaldehyde, etc. Oncogenes are the mutated form of genes that mostly govern the process of cell growth. But the mutated form disturbs cell division and cause cancer. These genes, directly or indirectly, control the rate of cell growth. For example, Bcl-2. that codes for apoptosis inhibitor but the mutated form causes B-cell cancers. A self-replicating independent hereditary gene that is present in the cytoplasm is called as plasma gene. So, the correct answer is B.

Solution :

Mutations are sudden, random changes in the structure of the gene that alter the gene product structurally as well as functionally; statement B is incorrect. They produce discrete/discontinuous variation which means that mutant allele expresses contrasting phenotype as compared to that of wild-type, for example, the mutation in allele for plant height produced altered allele, the wild type allele produces tall while the mutant produce dwarf plants. Statement A is correct. Since genes are the unit of heredity, mutations in genes are passed from one generation to next. Thus, parent transmits mutations to F$$_1$$ which in turn pass it to F$$_2$$ and so on; this makes statement C and D incorrect. The correct answer is A.

For the success of evolution, mutations must occur in germplasm chromosomes, germplasm genes or germplasm DNA. The mutations are physically expressed in the next generation. 

A son always inherits X-chromosome from his mother, hence, none of the paternal grandparent or father himself will contribute it to the grandson or son respectively. A daughter inherits the X-chromosome from both parents; thus, the mother had received one copy of X-chromosome from son's maternal grand father and and other from son's maternal grand mother. The mother can pass any copy of her X-chromosome to her son; thus, the son can get his X-chromosome from his maternal grand mother or maternal grand father. 

Mutations are changes in chemical structure of gene at the molecular level. It can cause large changes in the appearance of short-legged breed in Ancon sheep. But mutations can also cause small changes which are negligible. Similarly, nutritional mutant does not produce any altered phenotype due to mutation but there is change its nutritional requirement. Owing to genetic code redundancy and diploidy, some mutations are neutral and do not produce any changes at all. As we know that wobble base paring allows multiple codons, most of which differ in their base, to code for single amino acid. For example, serine is encoded by UCC, UCA, UCG, AGU, and AGC. Therefore, any mutation that changes the last letter of the codon of serine will have no effect. Diploids have two copies of a gene, the presence of one functional copy is sufficient to ignore the effects of the mutation in the second copy. Thus, mutations do not produce only large changes, it can be large, small, negligible or even no changes.