While it's true that hybrid offspring are often sterile when their parents have the same number of chromosomes but are different species, it's not the *number* of chromosomes that is the primary cause. It's the arrangement and function of those chromosomes that determines fertility.

Here's a breakdown of the reasons why hybrid offspring from different species are often sterile:

* Chromosomal Structure Differences: Even if the number of chromosomes is the same, the arrangement of genes on those chromosomes can be vastly different between species. During meiosis (the process of creating sex cells), the chromosomes need to pair up correctly to ensure each offspring receives a complete set of genes. When chromosomes from different species try to pair, they often don't align properly due to their structural differences. This leads to uneven distribution of genetic material and often results in non-functional gametes (eggs or sperm).

* Gene Expression Discrepancies: Even if the chromosomes pair somewhat correctly, different species have different regulatory elements controlling gene expression. These differences can disrupt the proper development of the hybrid offspring, leading to sterility or other abnormalities.

* Hybrid Inviability: In some cases, the genetic incompatibility is so severe that the hybrid offspring don't even survive to maturity. This is known as hybrid inviability.

Examples:

* Mules: A mule is a hybrid offspring of a male donkey and a female horse. Both donkeys and horses have 64 chromosomes, but the arrangement and gene expression are sufficiently different to make mules sterile.

* Liger and Tigon: Ligers (lion male x tiger female) and Tigons (tiger male x lion female) are both sterile hybrids.

Exceptions:

There are some exceptions to this rule:

* Closely Related Species: If two species are very closely related, their chromosomes might be sufficiently similar for fertile hybrids to occur.

* Hybridization with Polyploid Species: Some species are polyploid, meaning they have multiple sets of chromosomes. This can sometimes allow for fertile hybrids with other species.

In summary: It's not simply the number of chromosomes, but the *organization and function* of those chromosomes, along with differences in gene expression, that create the barriers to fertility in hybrid offspring from different species.