I'm aware of the usual definition of Haldane's rule, which says that, if the offspring of inter-specific hybrid offspring are sterile, then the sterile offspring will be of the heterogametic sex. That is, for example, females in birds, and males in mammals. But I'm a bit confused. This implies that non-sterile hybrids are also produced, which rarely happens.

So, I have two questions, please: 1. Does Haldane's Rule really mean that only offspring of the heterogametic sex are produced?

And 2. If so, please account for the exceptions. Crosses between Golden Pheasants and Lady Amherst's Pheasants regularly produce female as well as male offspring, as do crosses between Ring-necked Doves and Barbary Doves. Thanks!

  • $\begingroup$ XY and ZW are the heterogametes - males in mammals, females in birds $\endgroup$
    – rg255
    Commented Sep 21, 2016 at 23:04
  • $\begingroup$ Yep sorry - wrote it the wrong way around $\endgroup$
    – Katrina
    Commented Sep 26, 2016 at 13:50

2 Answers 2


I am detecting a bit of confusion in you wording of Haldane's rule. Haldane's rule makes predictions on the fertility of offspring from an inter-specific cross, not the sex-ratio of the offspring from the cross.

"if inter-specific hybrid offspring are sterile, then the sterile offspring will be of the heterogametic sex."

In inter-specific crosses there is not an effect on sex-ratio of the resulting offspring, but a effect on the ability of the F1 offspring to reproduce. For the offspring, those of the heterogametic sex, males in mammals (XY) and females in birds (ZW) will be sterile while the homogametic offspring will not be affected by hybrid sterility.

  • $\begingroup$ Thanks. But that's what I don't quite understand: virtually all mules, for example, are females. Virtually all canary mules are male. And virtually all hybrids are sterile, including the mules of both. It seems to be the opposite of Haldane's rule in both cases. Sorry for my late reply, by the way - I didn't realise I had answers. $\endgroup$
    – Katrina
    Commented Sep 26, 2016 at 13:59
  • $\begingroup$ With these mule examples, it is important to consider how distantly related horse and donkey are. Haldan's rule could be acting, but there are likely other effects on top of it causing a different result. Many 'miscommunications' could mess up development when the diverged horse and donkey genomes are mixed. Haldane's rule is illustrated more clearly with closely related sub-species or species because there are fewer incompatibilities which would affect development. The effects much stronger in the XY or ZW hybrid offspring. $\endgroup$ Commented Sep 27, 2016 at 15:44
  • $\begingroup$ Many thanks. It seems there are still a lot of unanswered questions relating to hybrids. Interesting stuff. $\endgroup$
    – Katrina
    Commented Sep 29, 2016 at 8:44

As already stated Haldane's rule doesn't always apply to odds of sex being born, it usually applies to the odds of that hybrid being fertile based off of whatever sex it actually was. Haldane's rule can also apply to the odds of a sex being born at all, though this situation is run into less often. So in short, no the vast vast majority of the answer to your first question is no, it doesn't mean that only one sex is produced. The majority of the time when Halden's law applies it only affects fertility of the offspring not the production of them.

Let me get into more specifics though, to help clear things up better. I think I detect two main conceptual mistakes which may be leading to your other confusion, so let me address them and see if it answers all your confusion. At least one, maybe both, seem related to the way you quoted Haldane's law, so let me first quote a slightly more accurate version of the law, straight form wikipedia:

When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex (heterogametic sex)

Halden's rule doesn't apply to all hybrids

The law starts by saying "when in f1 offspring one of 3 situations occur it will be the heterozygous sex. I think the first confusion is simply an attempt to apply this rule a little too broadly instead of limiting it to these categories. In your example of the Pheasant's it appears that both sexes of hybrids are fertile. Since neither sex is rare or sterile Haldane's rule simply doesn't apply, the precondition isn't met so you can't say anything about they heterozygnous sex.

In the case of the doves, well I'm not an expert at this but as far as I can tell Barbary and ringneck are two names for the same dove so I'm not sure that is a hybrid at all. I may just be misunderstanding dove nomenclature/classifications here, but in any case it looks like most doves hybrids are fertile so if I had to guess Haldane's rule doesn't apply in this case either.

However, just because your specific examples may not apply that doesn't necessarily invalidate the entire question. It is certainly possible species where Halden's rule applies where children of both sexes are born, in fact this is usually the case. Thus let's get to the other confusion.

Haldane's rule applies when one sex meets listed conditions

One confusion you seem to have is that the rule seems to imply that fertile hybrids exist despite the version of the rule you quoted specifically saying that halden's rule applies when hybrids are sterile. That's because the actual rule applies only to one sex, meaning that one sex will be infertile and the other sex fertile. If both sexes are infertile, as with the mule, Haldne's rule doesn't actually apply since it's not a trait unique to one sex(except when it does...I'll come back to that).

When precondition X applies the heterozygnous species demonstrates that condition

The law states that if offspring are "absent, rare, or sterile" that sex is the heterozygnous one. Those are three conditions any of which can apply, this doesn't mean all the conditions apply. The most common examples of Halden's law your likely to run into is where one sex is sterile, in which case that sex is sterile, that doesn't mean it has to be sterile and rare and absent. For that matter in cases where one sex is rare it's possible that the sex will be fertile on the odd example it's born despite being rare.

So back to what I said at first, most often Halden's law means two sexes that are equally common, but where one sex is sterile when it's born. The best example that I can think of right now is with bovine hybrids, where cattle were often crossed with other bovines to get the 'best' bovine to raise for meat was common. Crossing bison with Cattle produced more resiliant and better animals that were easier to raise, the only problem was that males were born infertile while females were fertile. This resulted in an inability to keep hybrid breeding populations without males to mate and thus keeping a 'feral' breeding pool of bison cows to mate with cattle bulls; which was a problem since bison weren't domesticated and keeping breeding stock was an issue. Thus a very common hybrid ran into issues due to Haldane's rule, keep this issue in mind, I'm coming back to this example later for my last point.

It is true that some hybrids have only one sex common due to Haldne's rule, I can't think of an example right now but there are many, just none exist in the common big/domesticated mammals I'm most aware of. Usually if a sex is rare that sex is likely to also be infertile, the issues that cause the sex to be rare will usually also make it infertile. However, as I understand the genetics it's entirely possible for a species to have one sex be rare, but that sex be fertile when it is produced; in fact id love an example if anyone can suggest one!

Laws are really just rule's of thumb

You don't want to apply this, or really any genetics law, too exactly or biology is going to decide to throw you a curve ball. Because of the oddities of the ways genes are combined, and even the possibility of random mutation, many 'impossible' situations still come up. Look at mules, thought to be infertile and yet a half dozen fertile mules have been documented at least, in fact most common 'sterile' hybrids I know of have examples of fertile hybrids being produced.

Which brings us back to the beefalo example above. Remember keeping bison cows for breeding stock was a major problem for them? Well that problem solved itself when a fertile male beefalo was born. Even though they are almost always infertile this male was fertile, as were both sexes of his offspring. This was due to random chance (I don't know if it was mutation or just the combination of the genes specifically), but a rule was broken by genetics. Thanks to this we were able to start up hybrid breeding populations of beefalo, all decedents of this male, beefalo are the most common bovines raised for meet now of days!

The point being go in expecting any rules to be bent in genetics at time. There are exceptions to most of these rules, life finds a way yadda yadda.

Hope all that rambling gave you a better idea of how the rule applies.

  • $\begingroup$ Many thanks for taking so much time to answer this. Apologies for not replying sooner - I have only just found the answers via Twitter, which I rarely use $\endgroup$
    – Katrina
    Commented Sep 26, 2016 at 14:13
  • $\begingroup$ Apologies also for confusion over Ring-necked Dove - I'd intended to write [Eurasian] Collared Dove but was distracted. Husband has bred some of these and they do appear to be sterile and occur in both sexes. Certainly canary hybrids with finches are virtually all sterile, and virtually all males. I had recalled reading an explanation of this in Tim Birkhead's book The Red Canary which, (as far as I remember) attributed this sex bias to Haldane's Law - hence my confusion. Many thanks, once again. $\endgroup$
    – Katrina
    Commented Sep 26, 2016 at 14:20
  • 1
    $\begingroup$ Thanks very much also for the information about beefalo - that was all very new to me and useful information. $\endgroup$
    – Katrina
    Commented Sep 26, 2016 at 14:27

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