Due to the events in the meiosis a sperm cell and an oocyte contains 50 % of the DNA of the animal producing the sperm cell or oocyte (an important fact in the transmission from DNA of parents to offspring). This 50% of the DNA is comprised in an unique combination of chromosomes of the animal (random process in the transmission of DNA between generations). After the fertilization of an oocyte with a sperm cell the nucleus of the zygote contains again chromosomes in twofold grouped in pairs. This process implies that each animal receives half of its chromosomes (or half of its genetic value) from its sire and half of its dam. Thus the genetic relationship between an animal and each of its parents is 0.5. This is called the additive genetic relationship.
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DefinitionThe additive The additive genetic relationship between two animals is the amount of DNA they share due to the fact that they are related |
A few additive genetic relationships are presented below:
Relationship | Percentage of similar DNA they share |
Parent-son or daughter | 50 |
Grandparent-grandchild | 25 |
Great-grandparents- great-grandchild | 12.5 |
Full brother(s) – full sister(s) | 50 |
Half-brother(s) – half-sister(s) | 25 |
It is important to stress that each sperm cell and each oocyte from a parent contains a unique combination of the chromosomes of the parent causes that the offspring of the same combination of a sire and a dam (full sibs) still show differences in traits, due to the Mendelian sampling effect. The additive genetic relationship of full brothers and full sisters (full sibs) is 0.5, because on average they share 50 % of the DNA of their parents.
The additive genetic relationship between two related animals is of great importance in animal breeding. Because, for example, the additive genetic relationship between a sire and a daughter is 0.5, they share 50 per cent of their DNA. This means that the traits of a sire have a predictive value for the traits of a daughter. The reverse is also true: the traits of the daughter maybe used to calculate the breeding value of her sire. Of course the heritability of the traits play in this respect a crucial role. For traits with a high heritability the additive genetic relationship between two animals has more impact in breeding than for traits with a low heritability. For instance, height of withers (with a high heritability of 0.6) of a mare is a good predictor of the height of withers of her daughter. However, the success of insemination has a low heritability of 0.1. This means that the predictive value of the insemination success of the mare for the insemination success of her daughter is low, despite that the additive relationship between the mare and her daughter is 0.5. Therefore, the The pedigree of an animal is very informative when you study genetic relationships between animals.
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DefinitionA pedigree A pedigree is the set of known parent-offspring relationships in a population, often displayed as a family tree diagram. This can be used to derive the relationships among animals in a population |
Below you can find an example of a pedigree of a mare. This is a print of an official studbook as a certificate of the Dutch Studbook for horses (KWPN).
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