Hahaha, this question is a bit interesting, it is a good question, and the answer may surprise many people.
Answer: Yes, yes, the twins obtained from the same egg may be of the opposite sex! is valid even if it is limited to humans. Strong3strong half identical twins are obtained by two sperm entering the same egg cell.
1. The most indispensable thing in the biological world is the exception
But under normal circumstances, two sperm and a egg cell fused cell has three chromosomes and cannot develop into a fetus. But...
has an exception! The most indispensable thing in the biological world is the exception.
This pair of twins comes from the same two sperm and one egg cell , so they can neither be called identical twins from the same fertilized egg[1], nor can they be called fraternal twins from different fertilized eggs[2 ], only a new name—" semi-identical twin"[3]. (This is probably the reason for the Baidu Encyclopedia mentioned by the subject)
In 2019, a pair of missed brothers in Brisbane, Australia, was confirmed as the second half of identical twins in the world [4]. Although it has been discovered before [5], this time, it was the first case discovered in the mother's body.
The question is, how did it form?
2. The developmental process of semi-identical twins
Semi-identical twins meet two sperm and an egg cell, then divide and combine to form and develop into a normal diploid (2N).
Figure 1: The formation of semi-identical twins,Picture source [4][3]
There is only one set of chromosome group for both sperm pronucleus and egg pronucleus, denoted as N. The fusion of two sperm nuclei and one egg nucleus will result in three chromosome groups, denoted as 3N, each of the three nuclei undergoes a mitosis. The chromosome group doubles to 6N and is combined into 3 cells. The chromosome group in each cell is two normal, which is restored to 2N.
Figure 2: Interpretation of the principle of cell division of semi-identical twins, picture source Xiaohu paper
After the above process, the cells forcibly formed three diploid cells (2N), The genetic material of one cell comes from two sperms, and it cannot survive normally. The reason for the inability to survive can be explained by the knowledge of epigenetics. The genomic imprinting of mammals prevents the two sets of genes from the paternal parent from developing into normal individuals [6]. The other two cells have general chromosomes from the father, and half of the chromosomes are from the mother, and they can develop normally and get normal individuals.
Figure 3: Epigenetic genomic imprinting, image source [6]
3. Human sex is related to the Y chromosome, provided by sperm
As for sex, human sex determines genes (SRY) is located on the Y chromosome [7], so we can say that human sex determination is determined by chromosomes. Those with Y chromosomes are males in the biological sense, and those without Y chromosomes are females in the biological sense [8].
Figure 4: SRY gene PCR, image source [7]
Semi-identical twins and fraternal twins and ordinary siblings have the same sex determination, which can be the same sex or the opposite sex. A pair of semi-identical twins appearing in the literature [4] is a pair of opposite-sex twins.
Identical twins and fraternal twins are explained as follows.
Figure 5: Identical twins and fraternal twins, picture source [1]
Strictly speaking, identical twins have the same chromosomal composition, so there is no possibility of opposite sex; but if identical twins only refer to Is from the same egg cell, then the “semi-identical twins” mentioned in the above content can be regarded as a special case of the same ovum but the twins can be of the opposite sex.
In summary, the subject's question seems to make no sense. If you think about it carefully, it is really a good question!
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