In the above discussion, we noticed that when male sex is heterogametic (XX♀, XY♂ or XX♀, XO♂), X-linked genes (more than one hundred genes) are subject to dosage compensation. In contrast to this, when the female sex is heterogametic (ZZ ♂, ZW ♀), as in birds, butterflies and moths, Z-linked genes are apparently not dosage compensated. Similar situation exists in some reptiles and. amphibians, where female heterogamety is predominant. A study of the absence of dosage compensation in these heterogametic females led to the following conclusions : (i) Genes which, require dosage compensation are primarily those that control morphogenesis and the prospective body plan, (ii) The products of these genes are ' required in disomic doses especially during oogenesis and early embryonic development, (iii) During oogenesis itself, heterogametic females synthesize and store morphogenetically essential gene products, including those encoded by Z-linked genes, in large quantities, (iv) Abundance of these gene products in the egg and their persistence relatively late in embryogenesis enables heterogametic females to overcome the monosomic state of the Z-chromosome in ZW embryos. Whenever the females are heterogametic, they have megalecithal eggs containing several thousand times more maternal RNA and other maternal messages than eggs of organisms, where male is the heterogametic sex.

Among amphibians, one example of female heterogametic sex (ZW) is Xenopus laevis, an organism extensively utilized for genetic studies. Its egg is 4000 times the size of egg in mouse or human, and it contains 10,000 times as much stored mRNA, proteins, lipids and other constituents. In XX females (e.g., mice, humans, etc.), this need for higher activity of X-linked genes is satisfied by the ' XX chromosome constitution of the zygote. In ZW females, the inadequate gene dosage of Z-linked genes is overcome in two ways : (i) increased synthesis and storage of the products of maternally acting genes; (ii) a shift in the timing of action of zygotic genes, such that they become effectively 'maternal genes' in early stages. These differences between organisms having female heterogamety and those having male heterogamety allow survival of heterogametic females without dosage compensation.

Genetic Imprinting Genetic imprinting is a phenomenon, which involves differences in expression of genes inherited from mother and father. In other words, the chromosomes in the sperm and egg are differentially imprinted or marked as having come from the father or the mother, so that some maternally inherited genes and other paternally inherited genes are differentially expressed in the progeny. Any epigenetic changes in the germ cells, such as the inactivation of an X-chromosome, are reversed or altered during or just before meiosis, which leads to the production of sperms and eggs. After this reversal of epigenetic changes, the eggs and sperms are differentially imprinted. As a consequence of this, even if a chromosome in a sperm happens to have been inherited from the fathers’ mother, it would still bear the male imprint (Fig. 17.24). Such imprinting has been found to be crucial to normal development in mice and other mammals, because it silences and activates different sets of genes in the maternal and paternal chromosomes. These differences in paternal and maternal chromosomes complement each other in directing normal development. This differentiation or reprogramming of the genome is believed to be achieved through changes in methylation patterns. In mice, differential methylation of paternal and maternal chromosomes has actually been demonstrated. A case of genetic imprinting has also been shown in human patients suffering with a disease known as Beckwith Wiedermann Syndrome (BWS), where both copies of a region on the short arm of chromosome 11 (Up 15.5)were inherited from the father. This feature is described as paternal disomy. A similar situation has been shown involving paternal disomy for mouse chromosome 7 (homologous to human chromosome 11), which led to production of abnormally large embryos, a condition analogous to BWS in humans. This is attributed to differential paternal expression (imprinting) of a gene Igf2(insulin like growth factor-2) in mouse, which is believed to be homologous to BWS-gene in humans.

Fig. 17.24. Genetic imprinting of chromosomes, so that they are identifiable as having been donated to an individual (a) by its father (dotted screen) or (b) by its mother (solid black). This imprinting may persist for many cell generations, but disappears in germ cells during meiosis.