Chromosome Considerations in Cattle......Fertility
Chromosome anomalies affecting fertility and abortion in cattle.
This webpage was last updated on February 2, 2003 by Sheila Schmutz firstname.lastname@example.org
Cattle have 60 chromosomes, 29 pair of autosomes and 1 pair of sex chromosomes. As in other mammals, males have an X and a Y chromosome and females have 2 X chromosomes. All of the autosomes are somewhat tear drop shaped, with the centromere at the end of the chromosome. The sex chromosomes have the centromere in the middle of the chromsome, with the X being much larger than the Y.
During gametogenesis the chromosomes undergo meiosis which results in each primary reproductive cell producing 4 sperm with 30 chromosomes each from a bull or 1 ovum plus 3 polar bodies from a cow. The bull can contribute either his X or Y chromosome to a sperm so it is the bull's contribution that determines the sex of the calf. The cow always contributes one of her X chromosomes.
One of the anomalies that affects fertility in cattle is called a Robertsonian translocation. This type of translocation was named after a person called Robertson from Scotland. Because cattle autosomes always have their centromere at the end, two chromosomes can fuse at the centromere and result in 1 larger bi-armed chromosome with a Robertsonian translocation. This also changes the chromosome number in cattle with this to 59 instead of 60. This fusion or Robertsonian translocation does not alter any genes, just alters the position of such genes. Therefore carriers of such translocations look perfectly normal.
The most common type of Robertsonian translocation in cattle is the t(1;29) which is a fusion of a chromosome 1 (the largest of the autosomes) with a chromosome 29 (the smallest of the chromosomes). This translocation has been shown to occur in most beef breeds which came from the European continent. It therefore is either very old or arose many times or most likely, both.
The second most common Robertsonian translocation in cattle is the t(14;20) which has primarily been seen only in Simmental cattle. It is very rare, occuring in less than 1% of Simmentals.
Although cattle with Robertsonian translocations are phenotypically normal, they have fertility "issues". One can imagine that dividing a cell with 59 chromosomes "in half" during meiosis will not work well for these animals. Instead of 30 chromosomes in every sperm or ovum, some will get only 29. The diagram below shows the various products of conception possible when one parent, shown as the cow in this case is carrying a t(1;29).
Unfortunately there is no easy way to look at a cow or bull and see a translocation in them. However one can have a chromosome test done, often called karyotyping, to determine if this problem exists. When more than 10% of the cows a bull services come in open or abort, it may be time to consider karyotyping the bull to see if he has a translocation. Most ranchers ship cows that abort or are open, but if this ever happens a second time, then it may be time to karyotype the cow.
When purchasing expensive semen from an A.I. unit, it is worthwhile to ask if the bull has a normal karyotype. Many A.I. stations karyotype bulls and have this information on file. A few still sell semen from bulls with translocations so it is not "safe" to assume that because a bull's semen is sold, its karyotype is normal. Some countries and/or some breed associations within some countries demand that imported animals, semen or embryos have to have normal karyotypes before the calves born in that country can be registered.
There are fewer and fewer laboratories that offer commercial karyotyping. Your vet will know of the nearest lab where this can be done. Blood is drawn and couriered to the lab because viable cells capable of further division are needed for karyotyping.
In addition to the abortion that results when cattle carry translocations, there are other types of chromosome errors that lead to loss during pregnancy. Studies in humans who also have a 9 month gestation, suggest that 50% of fetal loss occurs during the first trimester. Many cattle may resorb rather than physically abort such early pregnancy deaths. Therefore ranchers do not find these malformed products of conception unless they persist into late gestation. We have not received malformed fetuses for necropsy less than 5 months old. Many such pregnancy losses may be recorded as "open cow" during fall pregnancy testing by a visiting veterinarian, instead of an early abortion. Therefore the actual abortion rate experienced on a working ranch does not seem very high but is probaly much higher than the records reflect.
Spermatogenesis takes about 60 days in a bull and continues from puberty to relatively old age, although the absolute quantity of sperm may diminish. Oogenesis in a cow actually begins when she is a fetus herself and then is suspended until puberty. The process in not completed until fertilization when the 3rd polar body is shed. This long suspended meiotic process is probably what makes ova more prone to having numerical chromosome errors, than sperm. We think the error rate begins to exponentially climb after a cow is about 9 years old (in women this climb begins at 35 years).
During meiosis the chromosomes are meant to pair and then one of each pair goes into each gamete. When this fails to happen correctly it is called non-disjunction. This results in one gamete receiving 31 chromosomes and the other receiving 29 chromosomes - both incorrect numerically. Both with many many genes in incorrect number because each chromosome has hundreds of genes. The calf who develops from a sperm or ovum with 29 chromosomes has "monosomy", a single copy of one of its chromosomes instead of the normal 2. The calf who develops from a gamete with 31 chromosomes has "trisomy". Down Syndrome in humans is trisomy 21 - 3 copies of human chromosome 21. In cattle the calves with monsomy die very early in gestation and the calves with trisomy die a bit later in gestation. The calf at the right had an extra chromosome and was found in a cow sent for slaughter by the abattoir staff. Ranchers sometimes may see a very malformed calf abort and this is likely a fetus with trisomy.
Therefore cows over 9 years old run a slightly higher risk of abortion because they are more prone to non-disjunction and numerical chromosome problems. All heifers and cows run a risk of about 1% of such an event. As non-intuitive as it may seem, the more malformed an aborted fetus is, the less reason there is to cull either the cow or the bull. Malformations that affect multiple systems in the body and therefore multiple organs are probably caused by multiple genes being out of sync.......and this is usually due to a numerical error of the chromosomes.
Numerical errors also occur with the sex chromosomes. Most of these are not lethal but do affect fertility. A bull fetus that inherits only a Y chromosome from the sire and no X from his dam aborts. The X chromosome is large and carries many essential genes.
A heifer fetus that inherits only an X from its dam and no chromosome from its sire lives. Such a heifer is sterile. She will develop only streak gonads, not proper ovaries and will never ovulate. This condition is called Turner Syndrome.
A bull that inherits 2 X chromosomes and a Y from its sire is said to be 61,XXY and have Klinefelter's Syndrome. This bull with have small testicles and usually be sterile but is otherwise normal. The bull at the left had this problem. A sperm check should indicate that he is producing no sperm at all.
A heifer fetus that inherits 3 X chromosomes is said to have trisomy X with the karyotype 61,XXX. Such females can reproduce and the bull calf at the left was hers. At least such cows two have had both normal calves and Klinefelter sons. It is possible that they abort more often than a normal cow but there are not enough documented cases to be sure about this.
Freemartins are heifers that are born co-twin to a bull. Twinning is discussed in more detail under the section on Teratogenic Inheritance. Both twins in unlike sex twin pairs in cattle have the karyotype 60,XX/60,XY meaning that each has some male cells and some female cells. The bull calf is fertile although he might have a slightly lower sperm count than would a normal bull. The freemartin heifer is sterile.
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for further information contact:
Department of Animal and Poultry Science
University of Saskatchewan
Saskatoon, Canada S7N 5A8
phone: (306)966-4153 fax: (306)966-4151
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