https://en.wikipedia.org/wiki/Pleiotropyhttps://en.wikipedia.org/wiki/EpistasisNot everyone is a geneticist. Please read about these two concepts and ask questions if there is too much foreign terminology for you.
The results of outbreeding can be disastrous for the descendants of such pairings. Many of the offspring will lack the interactions of genes that should normally result in positive traits in either parent group. The terms introduced here are important to understand.
If there is a list of traits in a genepool that require epistatic interactions to be expressed and a history of natural selection has removed other genes from many of the loci where those genes are found then what happens when pairings are made with individuals who do have other alleles at those loci?
The interbeeding of different species is not rare in nature. When the environments the parents occupy meet at the borders by a gradual transition the hybrids can even survive as transient groups that have a chance of breeding with the parent populations. Sometimes they can become separate species. Most of the time, especially when the niches the parents occupy change abruptly with little mixed borders, the offspring become genetic dead ends. This can distance the parent populations resulting in less overlap of the genepools.
The offspring and all future descendants may lack the necessary genes to express many of the parents' phenotypes thus lacking specialized characteristics. When the two groups of organisms are distant enough there may be too many mismatches to ever come together in significant numbers in the mixed descendants.
Consider a gene that affects multiple characteristics. Some may be relatively benign while others can be strongly selected for. When the strongly selected phenotypes that the gene affects cannot be expressed due to lacking epistatic genes required for the phenotype then the gene will only persist at random as the individuals with it do not have better survival nor reproductive success. This gene may also express other characteristics that are mildly selected against. It has only become fixed in the parent population due to the benefits that can only be utilized with all the interacting genes also present. It is not true then that an evenly mixed population of hybrids would maintain about 50% of the genes from both parent populations over time. Epistatic relationships would become lost or imbalanced. Some genes would, either by random drift or by selection through the other phenotypes they affect, become less common.
Allele is a term for any gene in a particular location in your DNA (locus) on a particular chromosome. So in simple terms if there were genes for short medium and tall and they all reside in the same location on a particular chromosome then they are alleles. You could only have two of them one from each parent.
DNA is a molecule that stores genetic information and is composed of nucleotides. In simple terms these form codons (sets of 3 nucleotides) that code for particular amino acids. Proteins are composed from these which can then construct many other types of molecules in an organisms body.
Gene is a coding instruction unit of DNA. In simple terms it can be said to encode one product (though there is much more complexity this definition is sufficient for understanding the concepts described here).
Genotype refers to the particular set of genes a single organism has and is commonly used to describe a particular subset of a genome being observed (Genome for all sets of genes)
Locus (pl. locii) is a term for the location of a gene in a particular chromosome (humans gametes have 23 chromosomes so 46 for a normal human)
Phenotype refers to the expressed trait of an organism. (uncommonly used Phenome describes the set of all phenotypes)
Understand genetics to understand life. Everyone should know some of the basics.
Individual traits vary. Groups of individuals can be classified based on common genes that occur in varying proportions compared to other groups.
Some genes are unique to some groups occurring at nearly 0% in other groups.