Modes of Inheritance

Autosomal recessive inheritance

Two copies for each trait exist in the genome. An animal derives one of each of these
copies from the father and one from the mother. A recessive allele is overwritten by it's
dominant counterpart. Therefore, autosomal recessive inherited diseases only affect
animals which received two mutated alleles. Father and mother of the affected animal
have to carry the recessive allele, but could be healthy themselves.
Three possible genotypes exist:
1. Genotype N/N (homozygous healthy): This animal does not inherit the mutation and
contracts the disease with very low probability. The animal will never pass a mutated
allele to it's offspring.
2. Genotype N/mut (heterozygous carrier): This animal posses one copy of the mutated
allele with a very low probability to contract the disease. The offspring of the carrier
inherit the mutated allele with 50% probability. Therefore, breeding should only take
place with homozygous healthy animals.
3. Genotype mut/mut (homozygous affected): Carrying two mutated copies of the allele,
this animal will most probably exhibit the disease. Additionally, 100% of it's offspring
inherit the mutated allele. Only breeding to a mutation free animal could be tolerated.
With 50% probability carrier animals inherit the mutated allele to their offspring. Breeding
two carriers results in 25% of the offspring being affected by the disease.
In healthy populations, carrier animals enlarge the gene pool and should not be excluded
from breeding categorical. Never the less, breeding of carriers with other carriers or even
affected animals should strictly be avoided.

Autosomal dominant inheritance

Two copies for each trait exist in the genome. An animal derives one of each of these
copies from the father and one from the mother. A dominant allele overwrites it's recessive
counterpart. Therefore, autosomal dominant inherited diseases affect animals which
received one mutated allele. Either father or mother of the affected animal have to carry
the mutated allele and are affected by the disease themselves.
Three possible genotypes exist:
1. Genotype n/n (homozygous healthy): This animal does not inherit the mutation and
contract the disease with very low probability. The animal will never pass a mutated
allele to it's offspring.
2. Genotype n/Mut (heterozygous affected): This animal posses one copy of the mutated
allele with an increased probability to contract the disease. The offspring of the carrier
inherit the mutated allele with 50% probability.
3. Genotype Mut/Mut (homozygous affected): Carrying two mutated copies of the allele,
this animal will most probably exhibit the disease or die at very young age. Additionally,
100% of it's offspring inherit the mutated allele. As both parents have to be affected
animals, this genotype is very rare.
Autosomal dominant traits often increase the risk to contract the inherited disease. Those
changes in probability for the disease to occur are called variable penetrance of the
mutation. Therefore, carriers of the mutation and affected animals do not exhibit the
disease by 100%.
As in some diseases symptoms just develop in old age, the necessity arises to test the
animal for the mutation before breeding.

X-chromosomal or gonosomal recessive inheritance

Two copies for each trait exist in the genome. For gonosomal coupled traits, an animal
derives an X- or Y-chromosome from the father and one of two X-chromosomes from it's
mother. The mutated allele resides on the X-chromosome only.
Five possible genotypes exist:
Female animals:
1. Genotype xN/xN (homozygous healthy female): This animal does not inherit the
mutation and contracts the disease with very low probability. The animal will never pass
a mutated allele to it's offspring.
2. Genotype xN/xmut (heterozygous carrier female): This animal posses one copy of the
mutated allele with a very low probability to contract the disease. The offspring of the
carrier inherits the mutated allele with 50% probability.
3. Genotype mut/mut (homozygous affected female): Carrying two mutated copies of the
allele, this animal will most probably exhibit the disease. Additionally, 100% of it's
offspring inherit the mutated allele.
Male animals:
4. Genotype xN/y (healthy male): This animal does not inherit the mutation and contracts
the disease with very low probability. The animal will never pass a mutated allele to it's
offspring.
5. Genotype xmut/y (affected male): Carrying one mutated copie of the allele on it's only
X-chromosome, this animal will most probably exhibit the disease. Additionally, 100%
of it's female offspring inherit the mutated allele.
For an x-chromosomal recessive trait, male carriers of the mutated allele are affected by
the disease and inherit the mutation to 100% of their female offspring. Their sons can only
obtain the mutated allele from the mothers side.
The inheritance for female animals follows an autosomal recessive trait. Female carriers
do not exhibit symptoms, but pass the mutation with 50% probability to all of their
offspring, which results in 50% of the males being directly affected.

Mutations and phenotypical expression

The following factors describe the difference between the presence of a mutation and the
occurance of clinical symptoms of the corresponding disease:


Penetrance
The term penetrance indicates the proportion of individuals carrying a particular variant
(allele) of a gene, that actually express the associated phenotype. There is a distinction
between the complete penetrance ( if the allele is present, the associated phenotype is
generally expressed) and the incomplete or reduced pentetrance (some individuals will not
express the associated phenotype even though they carry the allele).

Expressivity
The term expressivity describes the extent of the expression of a genetic caused trait at the
single individual.