While it is true that mitochondria are predominantly inherited from the female and also that the female’s size can have an impact on offspring size, the mitochondria themselves do not play any role in size inheritance.
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Yes. Next question.
Okay, so, more seriously, polygenetic means that multiple genes are involved in the phenotype. So polygenetic traits are heritable, it is just a matter of whether you get all or some or none of the gene alleles involved in the phenotype.
Simple example - blood types. Everyone knows A, B, AB, and o and most people also know the ‘positive’ and ‘negative’ that goes with those, e.g., A-neg, AB-pos, etc. That ‘positive’ / ‘negative’ denotation is for Rh-factor, so technically A-neg is better written out as ARh- and AB-pos would be ABRh+. But in addition to those two fairly well known blood groups, there are another thirty-six groupings: Duffy, Kell, Kidd, Lewis, M, N…
Thus, blood type is polygenetic.
Now… Say you are A/Rh+/D-/K+/Ja/Lb/M-/N+
What are the odds that your offspring will share the identical blood type as you?
That is going to depend on two things:
- Your genotype
- Your mate and their genotype
Let us assign your genotype as: Ao/Rr/dd/KK/Jj/ll/mm/Nn
And your mate as: BB/rr/dd/kk/jj/Ll/MM/nn
Right away we can see that you can never have an offspring with your same blood type because your mate being BB means that all your offspring will be either phenotypically AB or B (also, your mate is MM and you are mm, so all of your offspring will be phenotypically M+)
Apply this same logic then to size. Say you need all eight of those genes to be homozygous to have an 8000g animal: AA/RR/dd/KK/JJ/ll/mm/NN
But… each gene pair, depending on inheritance pattern, still plays some small role in size. In this manner, even if your offspring does not get the perfect combination of gene pairs, it is still possible to get some of the right pairs and thus have some influence on size. So maybe they only hit 4000g instead of 8000g
And it should be at least somewhat obvious that pairing two phenotypically similar animals should then play a role in increasing the frequency of the genes behind those phenotypes coming together to maintain the phenotype.
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This is more a correlation than a causation thing. None of my Mojaves have ever broken 1500g for example