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Scientists Create Healthy Mice With Same-Sex Parents (bbc.com)
Researchers at the Chinese Academy of Sciences were able to make baby mice with two moms and no dad. "The aim of the Chinese researchers was to work out which rules of reproduction they needed to break to make baby mice from same-sex parents," reports the BBC. "That in turn helps understand why the rules are so important." From the report: It was easier with double mums. The researchers took an egg from one mouse and a special type of cell -- a haploid embryonic stem cell -- from another. Both contained only half the required genetic instructions or DNA, but just bringing them together wasn't enough. The researchers had to use a technology called gene editing to delete three sets of genetic instructions to make them compatible (more on that later). The double-dad approach was slightly more complicated. It took a sperm, a male haploid embryonic stem cell, an egg that had all of its own genetic information removed and the deletion of seven genes to make it all work.
The reason we need to have sex is because our DNA -- our genetic code -- behaves differently depending on whether it comes from mum or dad, the study in Cell Stem Cell suggests. And without a female copy and a male copy our whole development gets thrown out of whack. It's called genomic imprinting with parts of the DNA in sperm and parts of the DNA in eggs getting different "stamps" that alter how they work. The bits of DNA carrying these stamps were the ones the researchers had to delete in order to make the baby mice viable.
The reason we need to have sex is because our DNA -- our genetic code -- behaves differently depending on whether it comes from mum or dad, the study in Cell Stem Cell suggests. And without a female copy and a male copy our whole development gets thrown out of whack. It's called genomic imprinting with parts of the DNA in sperm and parts of the DNA in eggs getting different "stamps" that alter how they work. The bits of DNA carrying these stamps were the ones the researchers had to delete in order to make the baby mice viable.
Almost right.
Our chromosomes exist in pairs, and we have 23 pairs of them.
On one particular pair, women have two X chromosomes and men have an XY combination. Thus, the most obvious genetic difference between our sexes exists only on one chromosome, which is why in common parliance we refer to "the X chromosome" or "the Y chromosome". But again, in both cases these are part of a pair.
When reproducing, both parents give one (pretty much randomly selected) of each pair of their own chromosomes to the child.
Meaning that women always give an X sex chromosome to their child, and men give 50% X and 50% Y.
Thus, two women will indeed always give two X chromosomes, leading to a girl. Two men will have 25% female children, 50% male children and 25% children with YY chromosomes. A quick google shows that pretty much no-one knows what that would be like. It is not impossible that it would not be viable.