WHAT IS A MOSAIC?
There are several definitions of "mosaic"
1) a student or scholar of the book of Moses
2) decorative artwork made up of small pieces of stone or glass
3) cute trade name for a coat color pattern in chinchillas
4) individuals with 3 or more cell lines or having extra chromosomes. Another definition is an individual who has some cells with an abnormal or unusual genetic or chromosomal makeup while the rest of the cells in the body have the typical genetic or chromosomal constitution |
Well, if you decided this article was about #3, you are correct. If you had decided this article was about #4, you are also correct. Give yourself a gold star and go to the head of the class.
#4- individuals with 3 or more cell lines or having extra chromosomes are mosaics. All plants and animals that sexually reproduce have 2 cell lines -dear old mum and dad. So genetically normal individuals have 2 cells-one from each parent.
In the world of plant breeding, many trihybrids (3 cell lines) are prized garden plants or tasty seedless fruits and vegetables. Your seedless watermelon is a trihybrid. In the plant breeding world, cloning or vegetative reproduction is a common way to reproduce endless supplies of perfect plants, so being seedless and sterile is not a problem to plant breeders. And it is fairly easy to make trihybrids for commercial seed sales.
On the other hand, "mosaic" animals are not considered a good thing. Most people that have a Down Syndrome baby would prefer a normal baby. Down Syndrome is one of the more common chromosomal "mosaic" disorders found in humans and other animals. As karotyping has gotten cheaper (about $30 per animal), animal breeders and veterinaries (UC Davis is considered to be leading edge in this field) are studying the reproductive and performance problems in cats, dogs, and horses.
Yes, "mosaic" disorders occur in humans and other mammals. In the plant world, a trihybrid plant is considered desirable. In the animal breeding world, a trisomy is not wanted! Autosomal trisomy of chromosome 21 is the other name for Down Syndrome which must be the most feared thing for older child-bearing women. A Down baby has 3 chromosome 21s You can have other chromosomal mosaicism disorders. Trisomy 13 is Patau’s Syndrome. Extra fingers and major heart defects are the common symptoms of this "mosaic". Edward’s Syndrome is a trisomy of #18. Again you have heart defects, skeletal problems and other health problems .Similar situation has been found in older mares. They are also prone to producing foals with extra chromosomes at the 23, 26, 28 and 30 pairs.
Then you have the XX and XY chromosomal abnormalities. These moasics are well documented in humans, cats, horses and dogs. Needless to say, fertility is poor. The main reason these problems are found is the search for the poor reproductive performance causes. These types of mosaics are usually caused by poor "copy" instruction. Just think of a copymachine happily churning out too many extra copies of unwanted material.
In females, you can have XXX or even more XXXXXXXXXs. Usually the female is infertile. But in humans, there have been documented cases of successful conception and live births. Another weird thing is the multiple XXX is more common in women (or is it that more women want to have children and end up in a fertility clinic and get karotyped for "mosaic" abnormalities???). Something is strange here about the number of women who have XXXs. Could be that owners of mares and bitches try breeding them a couple of times and then give it up as a lost cause? And women keep on trying through the fertility clinics?
The male mammal has a few "mosaic" disorders of their own. If you have a male calico cat, you have a Klinefelter "mosaic". The only way you can have a male calico cat is that he is a XXY or any number of odd combinations XXYYs. One male calico was documented with over twenty extra XXYY chromosomes. It did not make him "oversexed" (bad pun!). He was sterile as a cement post. If you want to learn more about male calico cats and their curious world of Klinefelter mosaics, there is a very easy to read book, CATS ARE NOT PEAS by Laura Gould (1996 Springer Verlag).
Now take chimerism (or not as the case may be) Chimeric animals can bred in the lab. You take a sperm (1 cell line), the nucleous from a male or female(2nd cell line) and insert them into a zona pelluda from a another female (3rd cell line) to produce an baby with 3 parents , You can confuse the issue and implant the fertilized egg into a surragate mother and have 4 parents. Yes, this has been done for human babies. NOVA on PBS did a program on the brave or crazy world of human fertility treatments. There are human babies with 3-4 genetic donors. Yes, they have also done the same things for prized dairy and beef cattle. At the current moment, the horse industry does not permit the registry of these chimeric foals. A.I. is controversy enough!
Chimerism can occur naturally. It doesn’t need a testtube to happen. The sex chromosomes,X and Y can get mixed up rather seriously. Gonadal dysgenesis is a problem for females who are XX/ -X. If you have a normal pairing of XX, you are a normal fertile female. If you had an abnormal but still fertile double pairing of XX/XX, you could produce viable offspring. But if you have three Xs, you are infertile. In males, the problem can be XY/-Y or XX/XY chimerism. Needless to say, fertility is a problem.
As for "mosaic" chinchillas, I am never sure whether I should correct the person by saying "You really have a piebald" or extend my condolences and ask where they had the karotyping done. By using "mosaic" incorrectly to decribe a color pattern that is not caused by a chromosomal disorder discredits the chinchilla industry. No serious animal breeder, veterinary, medical docter, geneticist or infertility specialist would use the term "mosaic" to describe a coat color pattern unless it was an obvious symptom of chromsomal mosaicism.
Recommended reading on mosaics and other chromosomal disorders:
Bowling,Ann T. HORSE GENETICS CAB International 1996
Gould, Laura CATS ARE NOT PEAS, a calico history of genetics
Springer Verlag 1996
Home page for CHROMOSOMAL MOSAICISM www.medgen.ubc.ca
It describes the autosomal1-22 and XY "mosaic" disorders found in humans.
There are individual webpages for some of more common trisomy disorders found
in humans- Angelman syndrome, Down syndrome, Klinefelter syndrome, Mosaic Turner syndrome, Prader-Willi syndrome, Patau syndrome and Edward’s syndrome.
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