Which one of the following statement is false with respect to Mendelian inheritance?
C. Gregor Johann Mendel was a monk who was interested in horticulture and botany. He studied garden peas and proposed ‘laws’ of inheritance. The first law is the law of uniformity. According to this law, if two plants that differ in just one trait (black and white) are crossed, then the resulting hybrids will be uniform in the chosen trait (either black or white, not blue). This is not entirely true as later geneticists demonstrated intermediate phenotypes resulting from codominant heterozygous expression. The second law is the principle of segregation. It states that for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent to an offspring. Which allele in a parent’s pair of alleles is inherited is a matter of pure chance. For example if there are two alleles with one determining white colour and one determining black colour in the first generation, then these two alleles segregate and only one of them from each parent could be passed on to the second generation. This was later proved to be true by studying chromosomes during cell division. The third principle is the principle of independent assortment. It states that different pairs of alleles are passed to offspring independently of each other. The result is that new combinations of genes present in neither parent are possible. As a very simplistic example, if a man with blue eyes and brown hair fathers a child with a woman with brown eyes and black hair, their offspring can have blue eyes and black hair. The inheritance of blue eyes does not take brown hair ‘with it’; these traits are independently assorted. Thus Mendelian principles are applicable to human genetics as well. Note that all traits studied using Mendelian genetics refer to categorical, all-or-none, traits, that is black vs. brown, blue vs. brown, tall vs. short, etc. It does not apply with the same simplicity to dimensional traits such as IQ or blood pressure.
Reference:
Which of the following patterns of inheritance can skip generations and can affect individuals with unaffected parents?
B. Autosomal recessive traits skip generations and may ‘catch families unaware’. Assuming a good degree of penetrance, an autosomal dominant pattern affects all generations. Mitochondrial diseases will affect all generations but via maternal inheritance. X-linked recessive disorders can skip generations but not X-linked dominant. Autosomal recessive diseases are clinically expressed only in homozygous states. Most commonly, the homozygote is produced by the union of two heterozygous parents (carriers) who themselves will be unaware of harbouring such an allele. The recurrence risk in children born to such parents is 25%. If an affected homozygote marries a heterozygote the recurrence risk is 50%. Consanguinity (union between relatives) increases the likelihood of inheriting autosomal recessive diseases as related parents may have both inherited carrier status for the same disease from their common ancestor.
Which one of the following best describes a substitution mutation?
B. Mutation is a sudden, permanent, and heritable change in the DNA sequence. Changes in DNA may be transcribed to mRNA and translated to proteins, leading to disease expression. Point mutation refers to a single base change in DNA. Point mutations are usually substitutions, where one base is replaced by another. It is termed a transition if a purine is replaced by another purine or a pyrimidine replaced by another pyrimidine (e.g. A to G). It is called a transversion if a purine is replaced by a pyrimidine or vice versa (e.g. A to T). According to the effect on the triplet code, mutations could be a frame shift or in-frame. In frame-shift mutations, the deletion or insertion is not in multiples of three codons, for example a five-base deletion mutation. This leads to a shift in the triplet reading frame with variable results. In-frame mutation refers to changes occurring in multiples of three bases, with no disturbances in the reading frame. According to the effect of a mutation on the protein product, mutations could be silent, mis-sense, or nonsense. A silent mutation causes no change in the protein product—this is possible because a single amino acid is often coded by more than one triplet sequence. In a silent mutation one triplet sequence is replaced by a different sequence but without changing the amino acid sequence. In a mis-sense mutation, the new mutant codon specifies a different amino acid with variable effects on the final protein product, for example in haemophilia and sickle cell anaemia. In a nonsense mutation the new codon is UUA, UGA, or UAG, which signal ‘stop’ to the amino acid sequence, resulting in a non-functional protein. Point substitutions do not shift the reading frame; they often occur in non-coding regions and go unnoticed. Even in coding regions they are often silent or mis-sense mutations.
Fragile sites present in human chromosomes are demonstrated using deprivation of which one of the following components used in DNA synthesis?
A. Thymidine and folate deprivation are used in the demonstration of fragile sites in chromosomes. Cytogenetic techniques now available for the direct molecular identification of such fragile sites. Fragile-X syndrome, Huntington’s disease, spinal muscular disease, and myotonic dystrophy are some of the disorders associated with fragile, trinucleotide expansions in chromosomes.
Which of the following is not an established candidate endophenotype for schizophrenia?
E. Duration of untreated psychosis (DUP) is not an endophenotype. Working memory defects, information processing defects such as prepulse inhibition, smooth pursuit defects, glial cell changes, and certain other putative neurocognitive markers are termed probable endophenotypes for schizophrenia. To be an endophenotype, a characteristic must be observable independent of clinical state and must be measurable in relatives at a higher degree than in the general population. By definition, DUP cannot be measured in those who are not having psychosis.
Click to expand