Due to the events in the meiosis a sperm cell and an oocyte contains 50 % of the DNA of the parent (a natural law in the transmission from DNA of parents to offspring) and contains a unique combination of chromosomes of the parent (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 implies that each animal receives half of its chromosomes, half of it 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. But the fact 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. 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.
Definition
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 |
Thus relatives share similar DNA. The average percentage they share is clear, but without further knowledge of their DNA (genotypes) or their phenotypes, it is not clear which part of the DNA, which alleles they share.
2.7.1 mt- and Y-DNA
The mitochondria in the cells descend from the mitochondria in the oocyte of the mother. They contain DNA and form a maternal extra-nuclear inheritance. This DNA is called mt-DNA and can be used to trace female genetic lines in pedigrees.
They Y-chromosome is only passed from father to son. The DNA on this chromosome is called Y-DNA, it shows no crossing over and can be used to trace male genetic lines in pedigrees.