Biologists have discovered certain genes (the basic unit of genetic material found on the chromosomes) that behave very differently depending on whether they are passed down to offspring from the father or the mother. These genes, called imprinted genes, are chemically altered in cells that give rise to eggs and sperm.These alterations result in dramatically different properties. In the imprinted genes that have been most fully studied, the female alters the gene so that certain proteins are not produced.The protein remains active in the father’s genes.Researchers have posed numerous theories to explain the evolution of imprinted genes. Three of the theories are presented below. Competing Parental Interest Theory Some biologists think that imprinted genes evolved in a battle between the sexes to determine the size of offspring. It is to the genetic advantage of the female to rear a number of offspring, all of which will pass along her genetic material. Consequently, while she wants each offspring to be healthy, she does not want them to be so large that the strain of feeding and/or delivering them would jeopardize her ability to bear future babies. Conversely, it is to the genetic advantage of males in non-monogamous species (species that do not always mate for life) to have the mother expend as much of her resources as possible to ensure the health of his offspring. He is not concerned with her ability to bear future offspring, since these will not necessarily be fathered by him (and, therefore, will not be transmitting his genetic material). Hence,imprinted genes have developed in this parentaltug-of-war. Normally, each offspring receives one copy of an imprinted gene from the father and one from the mother. The changes that the parents make in their genes result in an offspring that is smaller than the male would like and larger than the female would like. Anti-Cancer Theory This theory holds that imprinted genes evolved to prevent cancer. The genes have been found in the placenta (an organ that develops to nourish a growing fetus). Placental tissue grows and burrows into the uterus, where the fetus develops. The ability to grow and invade tissues is also seen in aggressive cancers. Imprinted genes might have developed to ensure that the potentially dangerous placenta will not develop if there is no fetus to nourish. The female might inactivate certain growth genes in her eggs,while the sperm kept them turned on. If no fertilization took place, the growth would not occur. If a sperm did join the egg, the male’s gene would ensure that the protein developed. Protein Control Theory A third group of biologists holds that imprinted genes developed to ensure the precise regulation of certain proteins. Genes do their work by initiating the production of different proteins.Some proteins involved in the growth of embryos may need to be regulated with great precision to ensure the healthy development of the offspring. Proponents of the protein control theory suggest that this careful regulation might be easier if only one parent is involved. Thus,one parent might turn off such genes, leaving the regulation to the other. 1.Which of the following experimental findings poses the most serious difficulties for proponents of the anti-cancer theory?