The dog (Canis Familiaris) has 78 (39 pairs) Chromosomes in total. Chromosomes are separate strands of genes, contained in the nucleus of a cell - and normally appear in corresponding pairs.

Out of these 78 Chromosomes, two are the sex Chromosomes i.e. Gamete Chromosomes (sex chromosomes “X” and “Y”), the remainder are referred to as Autosomes.

Autosomal inheritance refers to the mode of inheritance where the gene is carried on a Chromosome that is not sex-linked i.e. not on the "X" or "Y" Chromosome.

In order to create a new life, the Autosomal cells (none sex Chromosomes) divide by the process of Mitosis. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. At the same time the sex cells divide by a process called Meiosis so that the father (carrying "XY") offers either an "X" or a "Y" to his offspring and the mother (carrying "XX") gives one "X" to her offspring.

Cell division is a two step process that reduces the Chromosome number by half, when the sperm and egg cells unite at conception each contributes half of the total number of Chromosomes to the new offspring and the resulting embryo will have the usual 39 pairs (and not double the amount i.e. not 39 from each parent).

Therefore, when we are looking at diseases carried on Autosomes were are referring to diseases which are not carried on the sex Chromosomes. There are two forms of Autosomal genes, "Dominant" and "Recessive.” "Dominant" genes are genes which, when present on a chromosome, pass on a certain physical characteristic, even when the gene is present in only one copy.

"Recessive" genes are genes that do not produce a physical characteristic when they are paired with a "Dominant" gene, but produces their effect only when there are two identical copies of it.

In the case of prcd-PRA and FN, these diseases are caused by Autosomal "Recessive" genes. The "Recessive" gene is carrying a mutant copy of DNA. In order to produce a dog affected with a disease caused by a "Recessive" gene the dog MUST inherit one mutant "Recessive" gene from each parent - making a total of two copies of the mutant gene.

Table showing how "Recessive" genes interact with "Dominant" genes

Dominant genes are depicted with a capital letter "P" (for PRA) and Recessive genes are depicted with a lowercase letter "p".

The offspring of each combination are shown as grey for "Normal/Clear", lilac for "Carrier" and purple for "Affected".

	P P
	P P
P P	PP	PP
P P	PP	PP

Results of two "Normal/Clear" parents - all offspring "Normal/Clear" (above)

	p p
	p p
P P	Pp	Pp
P P	Pp	Pp

Results of one "Normal/Clear" parent and one "Affected" parent - all offspring "Carrier" (above)

	P p
	P p
P p	PP	pP
P p	PP	pP

Results of two "Carrier" parents - 50%* of the offspring "Carrier" and 50%* "Normal/Clear" (above)

	p p
	p p
p p	pp	pp
p p	pp	pp

Results of two "Affected" parents - all offspring "Affected" (above)

	p p
	p p
P p	Pp	Pp
P p	pp	pp

Results of one "Affected" parent and one "Carrier" parent - 50%* of the offspring "Carriers" and 50%* "Affected" (above)

*Please note, the above expected results are based on Mendelian inheritance and the percentages are based on probability.

With Mendelian inheritance, there is the "Law of Independent Assortment.” In independent assortment, the chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes.

Because of the “Law of Independent Assortment” and because ratios are calculated in percentages, unless the same two parents produce 100 offspring together (probably impossible!!) the difference between expected outcomes and actual results can vary hugely due to the small sample size i.e. a litter containing 6 pups as opposed to 100 puppies!

If you mate, a "clear" to a "carrier" the expected ratio of "clear" to "carrier" offspring is 50/50, however it is possible that the entire litter could be "carriers"! Mate the same parents again and the next litter could be 25% "clear" and 75% "carriers,” mate the parents again and the entire litter could be "clear"! The inheritance of genes is a random process and in a small litter, all could be "carriers" or all could be "clear" (when mating "clear" to "carrier").

Because of the unpredictable outcomes of mating a 'carrier' to a 'clear' partner, no assumptions regarding the genetic status of the offspring should be made, any dog/bitch who has a 'carrier' parent could be a 'carrier' themselves and should be DNA tested to confirm its genetic status BEFORE being bred.