A phenotypically indistinguishable disorder occurring in the absence of the genotype is called:
A. Consider an epileptic disease with generalized tonic–clonic seizures which could be caused by a genetic alteration. An identical generalized seizure could be a result of head injury that someone sustained. This is called as phenocopy. Here, phenotypic expression occurs in the absence of a genotype, due to a non-genetic reason. The term pleiotropy and the two types of heterogeneity—allelic and locus—are explained elsewhere in this chapter.
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Considering the Hardy–Weinberg equation in population genetics, which of the following assumptions are made?
E. Gene frequencies and genotype ratios in a randomly breeding population remain constant from generation to generation. This is known as the Hardy–Weinberg law. This holds true only if the population is randomly breeding. If mutations are occurring in two genes at different frequencies, then this does not hold true as gene frequencies would change. In addition, if members of one population breed with occasional immigrants from an adjacent population this will introduce new genes or alter existing gene frequencies in the population. This is called gene fl ow. Similarly, strong interbreeding can happen within members of local populations. If the population is small, the Hardy–Weinberg equilibrium may be violated. As random mating can be assured only if sufficient numbers of matings occur, this is not possible in a small population. In such cases, the frequency of an allele may begin to drift toward higher or lower values. This is called genetic drift; it is accidental and aimless and is not an adaptive genetic change as it does not guarantee that the new generations will be more fi t than the predecessors. In natural selection, certain alleles are positively selected and so their frequencies increase compared to other genes. This will result in failure of the Hardy–Weinberg equilibrium.
RNA is synthesized from DNA in most the normal circumstances.
Which of the following enzymes catalyses the synthesis of DNA from RNA?
C. RNA is produced from DNA via transcription in most eukaryotic cells. A reverse procedure, where DNA is produced from RNA, takes place in certain viruses, especially retroviruses, including HIV. This procedure is mediated by a reverse transcriptase enzyme. DNA ligase acts in sealing DNA ends together during DNA replication. RNA polymerase acts in transcription of RNA from DNA. Primase acts in the initiation of DNA synthesis by catalysing the synthesis of RNA primers; this is necessary because DNA polymerases cannot initiate DNA synthesis without the help of RNA primers. When DNA replication is complete, DNA polymerase 1 destroys the RNA primers. DNA polymerase 1 contains an exonuclease which helps in proof-reading activity during DNA synthesis.
Equatorial alignment of chromosomes takes place in which of the following stages of mitosis?
C. Mitosis takes place in six identifiable phases. During interphase a cell is at rest. The individual chromosomes are not visible and active growth takes place. During prophase, which is the first phase of mitosis, the chromosomal material doubles and the nuclear membrane is broken down. Centrioles and spindle fibers become visible. During metaphase the chromosomes are equatorially aligned. Each centromere is now attached to two spindle fibers coming from opposite poles. During anaphase, the chromosomes separate and travel to opposite poles. During telophase an indentation appears in the cellular membrane and cytokinesis is then completed.
Condensation of chromatin material, resulting in the production of sister chromatids, takes place in:
A. In prophase, condensation of the replicated chromosomal material leads to the formation of sister chromatids, still attached at the centromeres. Each chromosome has two short and two long chromatids, corresponding to the short and long arms of the chromosomes.
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