Partial reprint from Basepaws blog
Did you know that kittens in the same litter can have different fathers?
Cat siblings are genetically very different. Unless the breeding occurred in a controlled environment, kittens of the same litter can actually have different fathers. And even those kittens who have both of the same parents, have likely randomly inherited a number of different gene combinations. This means that while certain genetic traits will be shared by the siblings, every sibling will be genetically unique. This remains untrue only for identical twin cats.
Identical twins are cats who have developed from the same initial egg fertilized by one sperm. This happens when a fertilized egg divides early in the development and results in two different lineages of cells who will ultimately form the whole organisms. This means that the two identical cats will carry the same genetic information. Unlike human identical twins, it can be rather difficult to recognize cat twins though, because the color pattern genes can be expressed differently in two cats even when the genetic information is identical.
With that being said, unless two cats are identical twins (which cannot be determined based off appearance), the DNA test results of two cat littermates are expected to be different. While siblings will have a certain amount of DNA in common, the combination of all the possible genetic variants given by the parents will be unique for each individual. And again, multiple fathers are very common in cats (particularly stray cats), which increases genetic diversity in the litter.
Order Basepaws genetic testing kit for your cat here: https://www.catbehaviorsolutions.org/resources