Question 61#

Variation in the expected gene frequency in a population can be explained using all of the following except:

A. Gene drift

E. Increased death rate will not affect the number of genes or genotype distribution in a population directly. All the other options given can alter the Hardy–Weinberg equilibrium.

Reference:

1. Malats N and Calafell F. Basic glossary on genetic epidemiology. Journal of Epidemiology and Community Health 2003; 57: 480–482.

Question 62#

A study finds that monozygotic concordance of intelligence is 0.86 while dizygotic concordance is 0.61. The heritability of intelligence is given by:

A. 0.5

A. For continuous traits such as IQ, path analysis could be used in measuring heritability from concordance rates. The heritability h2 = 2(RMZ − RDZ). Here monozygotic concordance is 0.86 and dizygotic concordance is 0.61. Hence, heritability is given by 2 (0.86 − 0.61) = 2(0.25) = 0.5.

Reference:

1. McGuffin P et al., eds. Psychiatric Genetics and Genomics. Oxford University Press, 2002, p. 45.

Question 63#

All of the following show non-Mendelian inheritance except:

A. Leber’s optic neuropathy

E. Cystic fibrosis follows an autosomal recessive inheritance pattern. It is one of the most frequently occurring recessive gene mutation in Caucasian populations, with an estimated frequency of 1 in 30 carriers in the general population. Single-gene disorders usually follow the Mendelian pattern of inheritance; notable exceptions are mitochondrial diseases, trinucleotide expansion diseases, and genomic imprinting.

Reference:

1. Kumar PJ and Clark ML. Clinical Medicine, 6th edn. Elsevier, 2006, p. 185.

Question 64#

Which one of the following methods is used in the determination of environment versus genetic contribution to a phenotype?

A. Angoff method

C. Path analysis provides a diagrammatic approach to estimate the contribution of genetic and environmental factors in inheritance of a trait. The shared environment and shared genetic make-up are drawn to demonstrate the sources of resemblance between two siblings. Path coefficients are calculated for each connecting path between the sources and the siblings, and sum of these coefficients can provide a genetic correlation between the siblings.

Reference:

1. Gelder M et al., eds. New Oxford Textbook of Psychiatry. Oxford University Press, 2000, p. 237.

Question 65#

The proportion of the total phenotypic variance accounted for by additive gene effects is called:

A. Broad heritability

B. The relative influence of genetic factors in defining the variance in a trait is expressed as heritability. If this is defined as the proportion of the total phenotypic variance attributable to additive genetic variance, then it is known as narrow-sense heritability. Heritability is also sometimes used to describe the proportion of variance explained by the total genetic variance (additive and non-additive genetic variance); here it is called broad-sense heritability. Non-additive genetic influences include phenomena such as epistasis (gene–gene interaction) and dominance effects, where presence of one gene mitigates the expression of other gene. A twin pair is said to be concordant when both cotwins have the same disease expression (or both are disease free). The pair can be discordant if one of them harbours a disease while the other does not. Due to the higher degree of genetic similarity among monozygotic twins, one would expect higher concordance between monozygotic twins compared to dizygotic twins if the disease being studied has a significant genetic component.

Reference:

1. Gelder M et al., eds. New Oxford Textbook of Psychiatry. Oxford University Press, 2000, p. 235.