Mendelian Inheritance and Beyond
Epistasis and gene interaction......beyond single single inheritance.
This webpage was last updated on February 13, 2004 by Sheila Schmutz, Ph.D. firstname.lastname@example.org
Gregor Mendel chose a small set of traits in peas to develop his principles of genetics. There has been considerable debate since whether he studied only these traits or chose to build his founding principles using the data from only these traits.
His principles have now stood the test of time. However there are additional concepts about inheritance that need to be added to cover other traits. Some concepts will probably still be developed as our understanding continues to build on the foundation his principles set.
Autosomal Dominant Traits are those in which a single copy of an allele is enough for the trait to be expressed or shown in the phenotype of the animal. One common example is Polled or the lack of horns in cattle. For a further discussion of polled & horned visit another page in this series.
There is substantial evidence to suggest that some animals with dominant traits are the result of new mutations. Polled cattle have arisen as new mutations in several different breeds. Certain genes seem to be more prone to mutation than others.
As males age, they continue to produce sperm but their DNA repair mechanism deteriorates. In humans there are several studies showing that men over 55 are more prone to having children with new dominant mutations. What age would that happen in cattle? 11? 12? No beef bulls would be in natural service at that age, but some famous dairy bulls are collected into that age range.
Some autosomal dominant traits cause deleterious effects when they occur in the homozygous state.......when a calf has 2 copies of the allele instead of just one. One example is dwarfism in Dexter cattle. Note that a red dwarf Dexter is in the foreground and a full height animal is in the background. The heterozygous dwarf carrier cattle are short-legged with normal size bodies but the homozygous dwarf fetuses do not survive. Sometimes such traits are called Homozygous Lethals.
Several dominant traits do not have absolutely equivalent phenotypic expression. This concept is called variable expressivity. An example of such a trait is the belt pattern in Belted Galloway cattle. The photos above show that not all belts are the same size or shape. To be registered as a Belted Galloway in Canada, the belt must be continuous around the entire animal but not overlap onto the legs.
Autosomal recessive traits require that the calf have 2 copies of the trait to express the phenotype. Since polled is dominant in cattle, horned is recessive. This gene has only 2 alleles........horned or polled.
Other genes have multiple alleles. Alpha-mannosidosis is a disease in Angus and Galloway cattle. There are 2 different mutations that can cause this recessive disease. An animal that has one copy of each of the 2 different recessive deleterious alleles can be called a compound heterozygote. Such an affected calf is not strictly a homozygote because it has two different alleles but in this case, each allele leads to an improperly made alpha-mannosidase enzyme molecule and so the calf is still sick.
Autosomal recessive diseases are usually severe and more consistent in symptoms than are autosomal dominant diseases. Often such diseases are due to a lack of a critical enzyme or an improperly formed enzyme that can not perform its normal function.
Another example of such a recessive disease is Beta-mannosidosis. This disease occurred in Salers cattle but a concerted DNA testing program implemented by the Canadian Salers Association has virtually eradicated this problem in Canada. Calves with Beta-mannosidosis could not stand or suckle because they had no grey matter in their brains. This calf shows a slightly domed forehead.
|Autosomal recessive diseases occur more frequently when there is inbreeding or linebreeding. Technically linebreeding is a special type of inbreeding in which a desired ancestor is re-introduced over and over to build a line. Many ranchers and farmers use the term linebreeding because they believe it has less negative connotations than inbreeding does. Any animal suspected of carrying an autosomal recessive disease should not be inbred. One could even say it should not be bred at all. DNA testing has allowed the suspicion to be confirmed or not for a few diseases but there are many where the gene and/or mutation are not known and therefore no test is available. Hairlessness in Hereford cattle is one of the traits where the causative gene is not known.|
Occasionally a gene has two alleles and when the calf is a heterozygote a different phenotype is produced than that of either homozygote. This is often called co-dominant inheritance. The classic text book example of this in cattle is roan. This roan Shorhorn heifer has 2 different alleles at the Mast Cell Growth Factor (MGF or KITLG) gene. One homozygote would be white and the other would be red in Shorthorn cattle. In Belgian Blue cattle the homozygotes would be white or black.
Sex-linked recessive traits occur more commonly in males than females. Since males have only a single X chromosome, a recessive trait on the X chromosome has no normal allele on another X chromosome to produce a normal protein. In females, two copies of the deleterious allele, one of each X chromosome is required for the animal to be affected. This is rare because most males that have X linked diseases are not bred.
An example of an X-linked disease in cattle is a form of hairlessness with tooth problems, called "anhidrotic ectodermal dysplasia". Affected cattle have little hair, few sweat glands and no incisors as shown in the photo at the left. This disorder has been seen in male Holstein cattle primarily.
Sex-Linked Dominant Inheritance
Sex linked-dominant traits seem to be more rare than sex-linked recessive traits. They should be considered more deleterious because most are male lethal. An example of an x-linked dominant trait in cattle is Streaked Hairlessness in Holsteins. This disorder causes streaks of missing hair, especially on the flanks. Males which inherit this allele die in utero.
Holandric is the term used to describe genes carried on the Y chromosome in mammals. Since only males have a Y chromosome, the traits inherited in this manner will be exhibited only by males. The Y chromosome is a very small chromosome and some of it does not seem to harbor any genes. In cattle, the only genes that we know are carried on the Y are those relating to the primary sex characteristics. Although few, these are very important phenotypic features!
A few traits are not caused by genes on the X or the Y chromosome but nevertheless occur in only one sex of animals. Antlers in deer in such a trait. Only bucks, never does have antlers. We could say that milk yield is a trait expressed by only cows. Milk yield was recently shown to be affected by the DGAT gene on cattle chromosome 14. Both bulls and cows carry the DGAT1 gene and therefore both contribute an allele to their daughters which affects the quantity of milk that daughter produces.
Sex-influenced inheritance is a pattern of inheritance in which the sex hormones of the animal affect the expression of a trait by the heterozygotes. Both homozygotes are unaffected and express the trait irrelevant of the hormones produced.
Scurs are a sex-influenced trait. Scurs are further complicated because only heterozygous polled cattle can have scurs. In females 2 copies of the scur allele are needed for the cow or heifer to have scurs. In males a single copy of the scur allele is sufficient to produce scurs. Some people say a sex-influenced trait is dominant in males and recessive in females. I prefer not to use that expression because to me a sex-influenced trait is NOT dominant or recessive.......this is a separate inheritance pattern.
The crossbred bull at the left has scurs and through DNA testing has been shown to have a single scur allele. For a further discussion of scurs & polled visit another page in this series. The gene for scurs in on cattle chromosome 19.
photo by John Potter
One form of gene interaction is when specific alleles at two or more alleles are required for a trait to be expressed. An example of this is dun brown coat color in Dexter cattle. Brown is a color produced by a type of eumelanin pigment. For eumelanin to be produced cattle must have at least one ED allele at the MC1R gene. Then for cattle to be dun brown, they must be homozygous for the b allele at the TYRP1 gene. Therefore dun brown is a recessive trait but it requires a dominant allele at an interactive gene.
Sometimes one gene affects the expression of another gene by over-riding it. This is called epistasis. This could be because one epistatic gene makes or does not make a product essential for the function of other hypostatic genes. It could be because one gene causes a trait that would "mask" the expression of another trait.
|Albinism is the absence of pigment in an animal.....note Snowdrop's colorless hooves and pink nose and eyes. Braunvieh cattle have occasionally been albino since at least 1933. In 2002 the mutation causing albinsim was found in the tyrosinase gene, the same gene that causes albinism in many other animals (although not all albinism is caused by this gene). Calves, such as Snowdrop make only half of the tyrosinase enzyme and stop prematurely, instead of making the complete enzyme. Since this enzyme is required for other genes later in the pigmentation pathway to make pigment, none is made. Albinism is therefore an epistatic trait. The albino genotype is espistatic to the the ED allele of the MC1R gene which causes black pigment to be made. The albino genotype is also epistatic to the agouti allele that probably causes the normal pigmentation color and pattern in Braunvieh cattle.|
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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