The role of Chromosome translocations in (Fertility of) Breeding Stock Cattle.
The genetic blueprint of any organism is reflected in it’s genetic or DNA genome.
The DNA is organised into cellular structures called chromosomes of which each living organism has a unique number. These chromosomes consist of two haploid sets of chromosomes, one inherited from the mother and one from the father.
These haploid sets of chromosomes are found in the germ-cells (sperm and ova) and when fertilised ovum is formed.
In the case of cattle the diploid number of chromosomes is 60, with cows having two X-chromosomes and bulls having one X- and one Y-chromosome. By viewing a karyotype of an animal a picture of the chromosomes is obtained and any apparent chromosomes may be identified. A translocation occurs when two chromosomes fuse to form one single chromosome. In the case of cattle such animals will have 59 chromosomes and not 60. As no genetic material is lost during such a fusion, carriers of such a translocation are completely normal and can achieve similar standards to normal non-carrier cattle.
The most common and well documented known translocation in cattle is the translocation between chromosome 1 and 29, also known as the 1/29 translocation. This translocation has been documented in several breeds of cattle including: Limousine, Blonde D’Aquitan, Charolais, Ramagnola, Norwegian Red and White, Simmentaller and Brown Swiss.
Based on the laws of independent segregation a carrier of the translocation has a 50% chance of transmitting the translocation to it’s offspring. At the same time there is a 50% chance that the offspring will not have the translocation and when crossed with other non-carriers of the translocation this genetic entity is eliminated out of the particular bloodline.
Theoretically, however, carriers of the 1/29 translocation will have a lower calving percentage compared to non-carrier stock. The cause can be illustrated as follows:
50% of the possible gametes formed during fertilisation will have an unbalance genetic material content and will not develop after fertilisation. This usually results in a very early miscarriage and cows will usually come on heat within three to four weeks again.
It should be stressed that, although the 1/29 translocation causes a slightly lower calving percentage, animals carrying the translocation are completely normal and do not influence stock quality, growth or development. Should siblings carry a translocation, consideration may be given to commercialising the siblings instead of use as registered stock.