A race of complete female humanoids have DNA that is similar to humans but distinct enough to be consider a different species (think donkeys and horses). This species can mate with humans and other similar humanoids, but produce only daughters is there away to explain this biologically without the help of magic?
Male blobs/fetuses are fragile.
Even in human female moms, under adverse conditions, a miscarriage is more likely with a male embryo than with a female one.
Boys are 14% more likely to be born premature than girls, and even at the same gestational age boys have a higher risk of death and complications because girls develop faster in the womb, new global research shows.
Even with your miraculously compatible aliens, female embryos are viable, whereas male ones are simply not.
This could happen if male offspring weren't viable. In Earth animals, you see similar behavior in cross breeds between animals like lions and tigers, in which all male offspring are infertile. Certain genetic abnormalities that make mating impossible are present only in male hybrids.
Something similar could happen with your aliens, only more severe. If XX hybrids are viable offspring, but XY hybrids develop with fatal defects, such as severely diminished brains or incomplete hearts, all offspring would be female. In many cases, human bodies will miscarry if a fetus isn't viable, and your aliens likely do the same, so it might be that the aliens never even appear to become pregnant, unless there will be a female child.
This is just my basic understanding of genetics, but can't there just be a dominant chromosome?
Have the male be XY and the female be ZX or ZY, where Z is dominant and results in a female.
Males produce X or Y for mating
Females produce only Z no matter what
The resulting combination will always be ZX or ZY.
Biologically, something about the Z chromosome changes the production of your sex cells.
Say in one species, we'll call them Neanderthals for ease, all males have a gene on the Y chromosome, let's say it is vital to male foetal survival in their species' wombs, but is lethal to the fetus in other wombs.
When this species breeds with another, we'll call them HomoS for ease, with a HomoS mother (and Neanderthal father) all male fetuses will die, they all have a gene that kills them in foreign wombs. With a Neanderthal mother (and a HomoS father) all male fetuses will again die, they lack the gene crucial for foetal survival in a Neanderthal womb.
End result, only female babies from interbreeding.
In your example, the Neanderthals are your all female species, they can have babies with us, but our males lack a gene for foetal survival in their wombs.
There are real animals where this happens; it's called gynogenesis. However, in gynogenesis, no DNA is transferred from the male to the offspring so perhaps that doesn't fit your needs.
There's a paper here on the evolution of the species which might also be informative.
For simplicity, let's say the humans and the humanoids both descend from the same group of ancestors, who reproduced sexually like humans do. During their isolation from the humans, the humanoids transitioned from sexual reproduction to parthenogenesis.
In humans, the X chromosome carries genes that are essential to life. An embryo with no X chromosome can't survive long enough to develop into a baby. A human with only one X chromosome, on the other hand, can survive just fine. There's a lot of evolutionary pressure for genes on the X chromosome to function properly in people with only one X chromosome, because nearly half of all humans have only one X chromosome! If a human gene on the X chromosome mutates and loses its ability to function in people with only one X chromosome, the mutant version of the gene will be at a severe evolutionary disadvantage, and it will probably die out very quickly.
Among the humanoids, on the other hand, pretty much everyone has two X chromosomes. That means there's very little evolutionary pressure for genes on the X chromosome to function properly in people with only one X chromosome. If a humanoid gene on the X chromosome mutates and loses its ability to function in people with only one X chromosome, the mutant version of the gene won't have any serious disadvantage, and nothing will stop it from spreading through the population. Eventually, just by chance, the original version of the gene could disappear completely from the population, leaving only the mutant version. Let's imagine that this has happened several times, so the humanoid X chromosome carries a bunch of genes that only work properly in people with two X chromosomes. A humanoid embryo with a missing X chromosome will be very unlikely to survive and develop, just like a human embryo missing an ordinary chromosome.
Now, imagine a humanoid mates with a human carrying sex chromosomes XY, and an embryo starts to develop. If the embryo inherits the human's X chromosome, it will have two X chromosomes in total—one from the humanoid and one from the human. The genes on its X chromosomes have a good chance of functioning properly, because there are two X chromosomes present, and the genes on the human X chromosome would be able to function pretty well on their own anyway. The embryo can survive and develop.
If the embryo inherits the human's Y chromosome, on the other hand, it will only have on X chromosome—a humanoid X chromosome, which carries a bunch of genes that don't work in people with only one X chromosome. The embryo is very unlikely to survive.
This is exactly this situation you're looking for: humanoids and humans can mate and make embryos, and only the embryos with two X chromosomes can survive and develop into babies. When those babies grow up, however, they'll have a very weird power. The child of a humanoid and a human will typically carry two X chromosomes: one humanoid and one human. If the child grows up and mates with a human, they could make an embryo with both a human X chromosome and a human Y chromosome! A humanoid and a human can't have XY children, but they can have XY grandchildren.
Simple - the aliens' bodies see Y chromosomes in sperm as unwanted intruders, so the immune system kills off all of the Y-chromosome-carrying sperm before fertilization can happen. The sperm with X chromosomes wouldn't be attacked, because the aliens have X chromosomes and thus those sperm aren't seem as hostile.
Make the aliens cousins to humans. More closely related than we are to the great apes but more distantly related than any 2 normal humans.
Have the aliens carry a mutation on their X chromosome which causes complete androgen insensitivity syndrome. Lets call their version the "Ẍ chromosome"
Any ẌX or ẌY individual will develop as female.
Hand wave that in their own population they have a different version of the Y chromosome (lets call it the Ч chromosome) with mutations that create alternate versions of testosterone and other androgens which still work in their own population.
Result: Any human mating with one of these aliens will have only phenotypically female children.
Alien mother, human father:
ẌX: phenotypically female, viable but may have trouble with the next generation.
ẌY: phenotypically female but infertile.
Human mother, alien father :
XẌ:phenotypically female, viable but may have trouble with the next generation.
XЧ: probably non viable but if not then still probably phenotypically female but infertile.
Many of the Answers are pre-supposing the mammalian mechanism in which sex is determined genetically and the genes are controlled by a "Y" chromosome.
That is not a universal thing. One of the lifeforms I cohabitate with does not exhibit sexual dimorphism, but I had a blood test done to determine his (as it turns out) sex at an early age. The test showed the absence of the female-specific W chromosome (males are ZZ, females ZW).
Many animals don't have sex determined by inheritance, but by incubation conditions (e.g. temperature).
I've seen commercials for a movie coming soon, a sequel to Finding Nemo. But why does the father Amphiprioninae still have a male voice? He should have become female by now, as they are sequential hermaphrodites.
So, you could have a large number of models to choose from for controlling the sex of the species, some of which would easily work as you want. If sex is determined by conditions not genes, then the mix caused by the hybrid might always be female. You don’t have to work-around the diploid inheritance of maleness as a trait, when you can simply not use that as a thing!
An explanation (see this answer) could be that the humanoid species have evolved mechanisms that identify some sequences/structure as alien body.
If one of these sequence is constantly found inside the Y chromosome, the chromosome is destructed, leading to cell (embryo) apoptosis.
Similarly, in place of a sequence/structure detection, it can be a mechanism that ckecks genetic integrity of the embryo, and the Y chromosome is detected as a mistake, leading to apoptosis.
In both of these case, human × humanoid lead to female embryo that are not touched by the protection mechanism, while male embryo meet apoptosis in the early stage of development.
These two phenomenoms are found in many documented biological cases. (marker scale is typical of immune system, and embryo genetic control is a very controlled task)
The female aliens have antibodies that react to fragments of Y chromosome in their blood streams, rejecting a male embyro.
This could happen very early in the gestation, and the embyro would be absorbed / discarded with the menstrual cycle, instead of causing a miscarriage.
You could use the genetics of some species of rodents (voles and lemmings) as a basis to create INDIVIDUALS who can only have daughters. Then think of a reason why only those ones are visiting the humans.
The voles have gene/s on the X chromosome which basically say "don't pay any attention to those other genes which are telling you to be male."
So voles without the Don't Pay Attention (DPA) gene are XX (female) or XY (male). (Sorry I have no idea what the official name of the gene is). Voles with the Don't Pay Attention gene (X) are XX (female) and XY (female).
Those XY females can only have daughters. If they supply a X to their offspring it has the DPA gene on it, so the kid is female, regardless of whether dad supplied an X or a Y. Ova carrying just a Y gene from mum are non-viable.
So if only XY females, visit Earth, they'll only have daughters. This solution has the downside that there are males of this species hanging out elsewhere in the galaxy. But it has the upside that your females will have the correct, erm, anatomy to mate with Earth guys.
'Cos if they've been an all-female species since dinosaurs ruled the Earth, why would they still have the Tab A fits into Slot B anatomy? :-)
A few options:
The aliens reproduce by cloning - a baby is born, identical to her mother. The process can be triggered by a full moon, or whatever, or by sexual act with a man.
The alien embryo cord is attached inside the embryo, through the vagina. Male embryo won’t survive.
Maybe for sone reason human-alien match would always produce a chimera with female organs only.
Man-killing, hyperfeminizing mitochondrial DNA
The only way that a "species" like this could exist is if it was a close genetic relative to humans - the idea of aliens interbreeding with humans is scientifically unfeasible. While there are many good examples here of how a mutation could render a parent incapable of producing male offspring - generally through rejection - they fail to take into account the issue of genetic recombination. Since each newborn would only get 50% of her genes from her mother, there would only be a 25% - 50% chance of inheriting the mutant gene each generation, depending on the dominance/recessive quality of the gene.
However, if the gene was housed in the mitochondrial DNA, things would be different. Unlike nuclear DNA, mitochondria are passed directly from the mother to child. If the mitochondria produced a chemical that reacted badly with something on the Y chromosome, it could kill male children, but be passed to female offspring 100% of the time.
Since this is clearly a detrimental trait, it would have to convey some kind of reproductive benefit to the child as well in order to persist. It is possible that the "man-killing" mutation could be preceded by an earlier mitochondrial mutation that triggered "hyperfeminization", making the women more appealing to men, but rendering male children infertile. A new mutation that kills off the infertile males before they start to develop would be beneficial to this lineage, since less energy would be wasted on producing them. Since mitochondrial DNA does not recombine, any mutation a female lineage picks up is retained as long as the women in that lineage continue to reproduce.
So basically, you would have a race of human women who could only produce daughters. Given the nature of human society, it follows that there would be a social stigma attached to them, and it is also likely that they would end up forming their own fringe society as a result, although they would never be able to completely detach from non-mutant humans since they would need them to reproduce. Eventually they would come to be thought of as a different "species" even though they really aren't.