Modes of inheritance - why it is important that I understand them?
Purebred dogs as we know them today are the result of intensive selection over the last few hundred years, which from an evolutionary point of view represents a recent event. Each breed arose from a small number of founder animals selected by man based on their behavioural (e.g., guarding, hunting…) and morphological characteristics (e.g., body size and shape, coat colour, coat length…). With careful selection and inbreeding, desirable traits were fixed within each breed, but at the same time there was an unintentional fixation of undesirable traits such as genetic diseases, morphological anomalies, and various predispositions. Currently, more than 400 breeds of dogs are recognized, each of which is unique in terms of specific characteristics, but also has a specific burden in terms of genetic diseases.
With improvements in the field of genetics, many dog DNA tests have been developed. They enable us to confirm the cause of a certain genetic disease or trait. Once the result of the DNA test is known knowledge about the mode of inheritance is crucial for further breeding (eradication of the disease, breeding of the desired traits). The mode of inheritance can be simple (a trait is controlled by one mutation) or complex (a trait is controlled by several mutations and environmental factors). There are 5 simple modes of inheritance: autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked inheritance. There is also exclusive maternal inheritance, which is linked to mitochondrial DNA.
Autosomal recessive inheritance
This type of inheritance is characterized by the presence of healthy carriers, males and females being affected with equal frequency, and equivalent transmission to offspring from males and females. An autosomal recessive disease requires the presence of two mutated alleles for its expression. This type of inheritance is most frequent when talking about simple genetic diseases in dogs. Affected individuals usually have healthy parents (carriers). A higher frequency of the disease occurs in the case of inbreeding. If we breed two healthy carriers, we can statistically expect a quarter of the affected offspring. The main problem with this mode of inheritance is posed by healthy carriers, which can only be detected by DNA testing.
Autosomal dominant inheritance
This type of inheritance is characterized by the absence of healthy carriers, males and females being affected with equal frequency, and equivalent transmission to offspring from males and females. In autosomal dominant mode of inheritance one of the parents of the affected individual is also affected. An individual who has one affected and one healthy parent has a 50% chance of being affected. Most autosomal dominant diseases show incomplete penetrance, meaning that not all individuals carrying the mutation show signs of the disease. These individuals do not express their genotype but pass it on to their offspring. Because not all individuals with the mutation show signs of the disease, it is recommended to use DNA tests when planning mating. DNA test can detect all animals with the disease genotype.
X-linked recessive inheritance
X-linked inheritance differs from autosomal inheritance in that the causal mutation is located on the X sex chromosome. Consequently, transmission to offspring and the risk of developing the disease in males (having one X chromosome) and females (two X chromosomes) are not the same. In females, the mutation must be present in two copies to express the disease, and in males only in one copy because they have only one X chromosome. Consequently, X-linked recessive diseases affect males to a greater extent than females. Affected males usually have healthy parents where the female is a healthy carrier. Male puppies from a healthy carrier mother have a 50% risk of being affected. Rare affected females are the result of mating an affected male and a healthy female carrier.
X-linked dominant inheritance
X-linked dominant diseases and traits are caused by a dominant mutation located on the X chromosome. There are no healthy carriers, females and males with the mutation on the X chromosome are affected. Diseases with this type of inheritance affect females more often than males. A diseased male will transmit the disease to all his female offspring, and a diseased female to only half of her offspring (male or female).
Only males have the Y chromosome therefore Y-linked diseases and traits occur only in males, and only males can pass them on to their male offspring. This mode of inheritance has not yet been described in dogs.
The development of complex or polygenic hereditary diseases and traits is influenced by many genes. The number of these genes and the proportion of their influence on the development of the disease is usually unknown. Consequently, the disease occurs in litters randomly. It is very difficult to predict the percentage of affected offspring, despite knowing the clinical status of the parents. Complex hereditary diseases are heavily influenced by environmental factors that, along with genes, influence the development of a disease or trait. An example of a complex disease is hip dysplasia in which several genes that give the genetic component of the disease have been associated with the disease, and at the same time it has been found that environmental factors (food, physical activity) also have a great influence on the development of the disease.
Maternal / mitochondrial inheritance
Mitochondrial diseases and traits are transmitted exclusively by the mother. Their development is influenced by genes located on mitochondrial DNA. Unlike other DNA, mitochondrial DNA is not found in the cell nucleus but in special cellular organelles - mitochondria. During fertilization, only maternal mitochondria are transferred to the embryo, which is why mutations in mitochondrial DNA are transmitted only through the mother. This mode of inheritance is extremely rare.