A mother may love her children just as much but her genetics do not.
Women and men differ genetically in only one chromosome out of 46. For each A person has 23 pairs of chromosomes in the genome. For women, pair number 23 is two identical chromosomes named X, meaning women are XX. In the men, pair number 23 contains one X chromosome, as in the women, and one Y. The Y is responsible for all the difference.
Males will always get Y from their father and X from their mother. Females will always get X from dad and X from mom. This difference between males and females also affects the heredity of diseases. The better known cases fall under the definition of "holding on to X". In this situation, a diseased recessive (dominant) gene is found on the X chromosome. In the male, there will not be another copy of the gene and therefore the disease will manifest itself. In females, it is necessary for both copies of the X chromosome to contain the disease for it to manifest. That is, for a female to be sick, her father must be sick and her mother must be a carrier. Carriers are females who have a defective copy and a healthy copy and appear healthy since the healthy copy is dominant. A famous case of such a disease is Hemophilia of the royal families of Europe.
Another difference in heredity between males and females is found outside the nucleus. In each of the cells of our body there is another genome that originates from an event that occurred billions of years ago at a time when all living creatures on earth were composed of only one cell. Some cells would feed on other cells by ingesting them. Normally, the ingested cell would undergo digestion, but in one case, this did not happen. This time, the cells made an alliance. Over time, the engulfed cell transferred most of its genes into the host cell's genome, leaving only the genes necessary for energy production. You guessed it - the swallowed cell became aMitochondrion (singular) – An organ produces energy that allows its host to evolve and become more complex. The partnership with mitochondria (plural) forms the basis of all complex life on earth, animals, plants and fungi.
Bottom line, all animals basically have two genomes - the main one in the nucleus, and a much smaller genome in the mitochondria. The two genomes work together and influence each other. in known humans Hereditary diseases arising from mitochondrial problems, such as a type of muscular dystrophy, And there are those who are already looking for solutions to the problem. The heritability of these two genomes, however, is significantly different. The nuclear genome is a combination of genes from both parents, but the mitochondrial genome comes from the mother only. This asymmetry is the source of the "mother's curse".
The mitochondrial genes in the female will pass to the next generation through the egg, but those in the male are an evolutionary dead end since the part where they are in the sperm cell remains outside the egg during fertilization. In females, these genes can respond to the pressures of natural selection, the beneficial ones will remain, the harmful ones will disappear. But these pressures are hidden in males - they all disappear. Some genes work in a similar way in males and females, but not all genes are so equal - some favor one partner while they burden the other. If mitochondrial genes change in a way that affects females, they will be swept out - after all, females are their ticket to the next generation. But if the genes change so that they harm males, that's fine, after all, men are a dead end. Over time, according to evolutionary theory, the mitochondrial genome is expected to accumulate changes that harm males.
On May 13, 2011 it was published Article in Science magazine who for the first time brings an example that matches the predictions of evolutionary theory regarding the curse. The researchers took the favorite pet of geneticists - fruit flies (Drosophila Drosophila melanogaster). They modified the flies to create five genetic lines whose nuclear genomes are the same but whose mitochondrial genomes are different. The flies were of course sterilized to clean them of bacterial contamination and raised in identical environments.
Previous studies have shown that the mitochondrial genome can influence the nuclear genome, influencing which genes will be turned on and which will be turned off. If the maternal curse really exists, we would expect to see a greater influence of mitochondrial genes on nuclear genes in males than in females. Indeed that is the case. Replacing the mitochondrial genome affected the activity of only 7 genes in the female flies. In the male flies, the same replacement affected 1,172 genes, about 10% of the total number of genes.
Many of these genes were genes that act in the testes or seminal glands, affecting the health, fertility and success of males but not females. It is possible that these differences are not harmful, but this probability is low since most changes in gene activity are negative.
Thanks to their mitochondrial inheritance, the male flies probably carry an extra heavy burden that their sisters are exempt from. One of the genetic lines of the flies in the experiment, for example, was completely sterile when it received replacement mitochondria, but fertile when it carried its normal mitochondria. This information joins previous studies that have found that mitochondrial genes in humans, rabbits and other species may play an important role in sperm quality and male fertility.
The male genome does not go over this injury in silence and fights it with the only weapon it has - the Y chromosome. The Y chromosome is indeed one of the smaller chromosomes, but the genes on it affect and regulate gene activity throughout the genome, and also on genes that affect mitochondrial activity. How exactly this effect works we still do not know.
It is possible that this small group of genes is waging a species war that we are only now beginning to understand.