Question

In marsupials, the inactivated X chromosome is always the one contributed by the father. What is the result of this method of dosage compensation?

Select one:

a. Male marsupials express the X-linked genes inherited from the mother

b. Male marsupials can’t pass on their mother’s X chromosome

c. The maternal copy of the X chromosome doesn’t contribute to development

d. Female marsupials express the X-linked genes inherited from the mother

e. The X chromosome inherited from the father is lost early in development

The following frequencies of genotypes have been identified for the Inuit and Australian Aboriginal populations for the blood group antigens M and N.  

Inuit                             M/M 0.835   M/N 0.156   N/N 0.009     

Australian Aboriginal M/M 0.024   M/N 0.304   N/N 0.824

If the genotypic frequencies have been determined from 1000 individuals of each population, which population does not fit (at a 0.05 error probability) the null hypothesis of Hardy-Weinberg equilibrium for M and N if the cut-off value for 1 degree of freedom and a 0.05 probability is 3.84?

Select one:

a. only the Inuit population does not fit the null hypothesis of HW equilibrium

b. both populations fit the null hypothesis of HW equilibrium

c. both populations do not fit the null hypothesis of HW equilibrium

d. only the Australian Aboriginal population does not fit the null hypothesis of HW equilibrium

e. because the populations have such different allele frequencies, the calculation of HW equilibrium is not possible

Answer 1

FIRST PART ANSWER

Option a – Male marsupials have only one copy of X-chromosome which is obviously inherited from maternal side, they do not require dosage compensation. This is the wrong option.

Option b – Male marsupials have only one X-chromosome inherited from mother which is passed on to the next generation. This is the wrong option.

Option c – Maternal copy of X chromosome contribute to the development, as X chromosome from paternal side is inactivated. This is the wrong option.

Option d – Female marsupials have two copies of X-chromosomes (they require dosage compensation). They express the X-linked genes inherited from mother, paternal X-chromosome is inactivated. This is the right option.

Option e – X chromosome inherited from father is not lost during development, rather it is inherited in the inactivated form. This is the wrong option.

Organisms with XX/XY or XX/XO sex determination face the problem of equalising the activities (dosage compensation) of X-linked genes. In drosophila, males who have single copy of X chromosome have vigorous transcriptional activity (hyperactivation) to equalise the output with their XX females. In C. elegans, XX genotypes serve as both males and females (hermaphrodites), whereas XO individuals serve as males. Partial inactivation (hypoactivation) of X-linked genes occurs in both the X chromosomes in XX individuals. In human females, one of the X chromosomes is inactivated, it begins at a particular site called X inactivation center (XIC) and starts spreading towards the end of the chromosome on both sides of XIC. But all the genes in the inactivated X chromosome are not silenced; one such specific transcript from the gene XIST (X inactive specific transcript) is present within the XIC region. XIST codes for RNA which remains associated with the inactivated X chromosome. But how come XIST is activated in one X chromosome and inactivated in another. There comes the role of another protein TSIX (XIST backwards) which is transcribed just in the opposite direction of XIST from same sequence. TSIX transcript is also a non coding RNA which negatively regulates XIST expression.

Eutherian mammals and marsupials follow imprinted X chromosome inactivation, that is, the X chromosome inherited from father or from paternal side is inactivated. Marsupials do not have XIST gene homolog. It is predicted that the X chromosome in male marsupials become pre-silenced in the male germ-line during meiosis. The inactivated X chromosome is passed on to the females of F1 generation. This mechanism is risky and it puts all the burden on maternal X-chromosome, which is transcribed for expression of X-linked genes in marsupials.

SECOND PART ANSWER

Let me first state that it has been mentioned in the question that the total population number is 1000, however, the observed Australian aboriginal population does not equals 1000. It might be some mistake in the question.

Hardy Weinberg Equilibrium: Hardy-Weinberg equilibrium states the formula of predicting genotype frequencies from allele frequencies, found out independently by G.H Hardy and Wilhelm Weinberg. The main factor of Hardy Weinberg principle is random mating, and no selectional pressure (like migration or mutation) occurs in the population.

Chi square value (0.359<3.84) is less than 1 degree of freedom (3.84). Thus, we accept the null hypothesis that Inuit population maintains Hardy-Weinberg Equilibrium.

Chi square value (23.69>3.84) is more than 1 degree of freedom (3.84). Thus, we reject the null hypothesis that Australian aboriginal population does not maintain Hardy-Weinberg Equilibrium.

Option a – Inuit population fits the null hypothesis. The option is wrong.

Option b – Australian aboriginal population does not fit the null hypothesis. The option is wrong.

Option c – Inuit population fits the null hypothesis. The option is wrong.

Option d - Australian aboriginal population does not fit the null hypothesis. The option is right.

Option e – HW equilibrium calculation is possible. The option is wrong.