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What are hereditary diseases?

Hereditary diseases are caused by genetic variations in the DNA, also called mutations, that can be passed from parents to their offspring.

Often these mutations are “hidden” in an animal’s genome and do not influence its health or appearence. These animals are then called “carriers”.

If two carriers are bred, affected offspring can be produced although both parents are clinically healthy.

 

A carrier cannot be identified just by appearance – only genetic testing will bring certainty.

 

The Modes of Inheritance

The following modes of inheritance are explained with examples from hereditary diseases in horses and dogs.  These modes apply to other diseases, traits (like Coat Color) and species as well.

 

Recommendations for breeding:

Suggested mating
Mating that should be avoided

 

Click on the mode of interest for detailed information.

Autosomal-recessive

→ Animals with only one copy of the mutation are clinically normal carriers. Individuals with two copies of the allele are affected.

Example with Cerebellar Abiotrophy (CA) in horses.

 

 

If a carrier is bred to a normal animal:

  • 100% of their offspring will be clinically normal carriers.

The offspring should be tested before breeding to identify carriers.

 

 

 

 

 

If two carriers are bred:

  • 50% of their offspring will be carriers.
  • 25% of their offspring will be normal.
  • 25% of their offspring will be affected.

→ mating with risk of affected offspring

 

 

 

 

 

Animals that are affected by an autosomal-recessive hereditary disease should not be used for breeding.

 

 

 

 

Autosomal-dominant

→ Animals with one or two copies of the mutation are affected.

Example with Maligne Hyperthermia (MH)

 

 

If a heterozygous-affected animal is bred to a normal animal:

  • 50% of their offspring will be normal.
  • 50% of their offspring will be affected.
 

 

 

 

 

 

If a homozygous-affected animal is bred to a normal animal:

  • 100% of their offspring will be affected.

In some hereditary diseases homozygous individuals may show a faster development of the disease or more severe symptoms.

 

 

 

 

 

 

Animals that are affected by an autosomal-dominant hereditary disease should not be used for breeding.

 

 

 

 

Incomplete autosomoal dominant/semidominant

→ Animals with one or two copies of the mutation are affected. Animals with only one copy are at lower risk to develop symptoms or show milder symptoms than animals with two copies of the mutation.

Example with Equine Myopathy/PSSM2 in horses.

 

If a heterozygous-affected animal (P/n) is bred to a normal animal (n/n):

  • 50% of the offspring will be affected (heterozygous).
  • 50% of the offspring will be normal.

Heterozygotes (P/n, here in pink color) are at a higher risk of developing symptoms or show milder symptoms than homozygous animals (P/P).

 

 

 

 

 

 

If a homozygous-affected animal (P/P) is bred to a normal animal:

  • 100% of the offspring will be affected (heterozygous).

Heterozygotes (P/n, here in pink color) are at a higher risk of developing symptoms or show milder symptoms than homozygous animals (P/P).

 

 

 

 

 

If a heterozygous-affected animal (P/n) is bred to another heterozygous-affected animal::

  • 100% of the offspring is affected. 50% each homozygous and heterozygous.

Heterozygotes (P/n, here in pink color) are at a higher risk of developing symptoms or show milder symptoms than homozygous animals (P/P).

 

 

 

 

 

Animals affected by an incomplete autosomal dominant/semidominant hereditary disease should only be used for breeding after careful consideration and with support and advice from genetic experts and a veterinarian. Please contact us for consultation.

 

 

 

X-chromosomal-recessive

→ The mutation is located on the X-chromosome. Females can be carriers (one copy of the allele) or affected (two copies of the allele). Any males with this mutation is affected since males  lack a second X-chromosome.

Example with Muscular Dystrophy (Duchenne-type, DMD)

 

If a carrier female is bred to a normal male:

  • 50% of each of their male and female offspring will be normal.
  • 50% of their female offspring will be carriers.
  • 50% of their male offspring will be affected.

Because of the risk of affected offspring, carrier females should not be used for breeding.

 

 

 

 

 

 

 

 

 

If a carrier female is bred to an affected male:

  • 50% of each of their male and female offspring will be affected.
  • 50% of their female offspring will be carriers.
  • 50% of their male offspring will be normal.
 

 

 

 

 

 

Animals that carry or are affected by a X-chromosomal-recessive hereditary disease should not be used for breeding.

 

 

 

 

X-chromosomal-dominant

→ The mutation is located on the X-chromosome. Females with one copy of the allele are affected. Because males are usually not viable if they have this mutation, homozygous females are not present. Males are affected and are usually aborted during pregnancy. If they are viable, they can only have one copy of this allele because they lack the second X-chromosome.

Example with Incontinentia Pigmenti (IP)

 

 

If an affected female is bred to a normal male:

  • 50% of each of their male and female offspring will be normal.
  • 50% of their female offspring will be affected.
  • 50% of their male offspring will be affected and are ususally not viable.
 

 

 

 

 

 

 

 

Animals that are affected by a X-chromosomal-dominant hereditary disease should not be used for breeding.

 

 

 

 

Some breeders hope to reduce the appearence of hereditary diseases by not breeding carrier animals. But this may cause an already limited gene pool to be further reduced. A very small breeding population can cause new mutations and thus more hereditary diseases. It is therefore very important not to eliminate carriers from breeding but instead to mate them only with genetic normal animals. Their offspring will not be affected and the breeding population can be stabilized.

The CAG GmbH will determine your animal’s genome and help you to optimize your breeding results while keeping the genetic variability. Please contact us if your questions remain unanswered.

 

 

 


Encyclopedia

Allele: A certain form of a gene is referred to as “allele”. Two alleles are present for each gene. Mutations in one or both of the alleles may cause some diseases or traits.

Autosomal: The mutation is located on any chromosome that is not considered as a sex chromosome which are involved in the sex determination process (X or Y).

Chromosome: Is located in the nucleus of a cell and is built of tightly packed DNA. The exact number of chromosomes varies between different species.

DNA: Individual genetic information serving as bauplan for a creature’s development and functionality. It’s present as chromosomes in each living cell.

Dominant: One mutated allele is sufficient to cause a hereditary disease or a trait to be visible. Animals with one affected allele are referred to as “heterozygots” and animals with two affected alleles as “homozygots”.

Genes: Functional units of the DNA that contain the code for specific proteines, enzymes and much more. The amount of genes varies between different species. A horse has 23.000 genes.

Genome: The whole genetic information within one cell or the whole organism (sum of all genes).

Genotype: An individual’s varying allele combinations. The genotype is pictured as a code (see the examples for the modes of inheritance). The code for a healthy or “normal” animal is (N/N) or (n/n), a mutation, when present, is shown with the abbreviation of its associated disease, e.g. (N/ca) or (MH/n).

Mutation: A change in the sequence of the nuclear bases are called “mutation”. They can occur during proliferation (cell division) or be induced by environmental factors, such as UV rays, chemicals etc.). Often these mutations do not show any impact on a being’s health. Only if the mutation happens to be within an important coding gene it may cause some health issues or visible changes.

Nucleobases: DNA components defining cellular functionality and an individual’s appearence through their specific order. Variations in their sequence are called “mutations”.

Phenotype: Appearence or physical traits visible (e.g. reduced hair growth).

Recessive: A disease or trait only occuring if both alleles show the specific mutation.

X-chromosomal: The mutation is located on the X-chromosome. Diseases or traits with this mode of inheritance develop differently in males and females because males lack the second X-chromosome.

 

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