為何混種動物都不能生育？

因為通常兩種不同種的動物所交配出的混種動物。如公騾子和大部分母騾子生出來是沒有生殖能力。

因為馬與驢子的生殖細胞中的染色體數目不一樣，所以令生出來的騾的染色體不成對(63個)。因此，它的生殖細胞無法進行正常的減數分裂(meiosis)，因為多數的配子(gamete)都會有缺憾。

請參考以下維基的資料~~~

騾子是馬和驢的雜交種。公驢和母馬的基因更容易結合，所以大部分騾都是這樣雜交的。不過基因結合的幾率還是很小：有的馬用了6年時間才成功的交配並弄到驢子懷孕。公騾子和大部分母騾子生出來是沒有生殖能力。沒有生殖能力是因為染色體不成對（63個），生殖細胞無法進行正常的分裂（即減數分裂）。母騾有性功能，子宮可以可以懷胚胎，但是最困難的地方是使母驢懷孕。嚴格地說，公驢和母馬所產後代稱為馬騾（英文：mule）；公馬和母驢所產後代稱為驢騾（英文：hinny）。


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A barren of mules.

Genetics Donkeys and wild equids have different numbers of chromosomes. A donkey has 62 chromosomes; the zebra has between 44 and 62 (depending on species). In spite of this difference, viable hybrids are possible provided the gene combination in the hybrid allows for embryonic development to birth. A hybrid has a number of chromosomes somewhere in between. The chromosome difference makes female hybrids poorly fertile and male hybrids sterile due to a phenomenon called Haldane's Rule. The difference in chromosome number is most likely due to horses having 2 longer chromosomes that contain similar gene content that contain the same genes as 4 zebra chromosomes.
Common wisdom states that hybrids only occur when the zebra is the sire, but the Barbados hybrid demonstrates otherwise. Two other known zebra hinnies have been foaled but did not survive to adulthood. The rarity of zebra hinnies indicates that the smaller number of chromosomes has to be on the male side if a viable hybrid is to be produced.
Zeedonks are interspecific hybrids bred by mating together two species from within the same genus. The offspring have traits and characteristics of both parents. Zeedonks vary considerably depending on how the genes from each parent are expressed and how they interact.

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Colchester Zoo's zeedonk, named Shadow


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A zeedonk in South Africa

因為通常兩種不同種的動物所交配出的混種動物都會有缺憾。