Genetic and environmental interaction.
This webpage was last updated on January 25, 2003 by Sheila Schmutz firstname.lastname@example.org
In terms of disease, multifactorial inheritance implies a genetic predisposition or susceptibility and an environmental trigger. Often the genetic predisposition is polygenic or due to several genes.
Many multifactorial disorders are of late onset, such as cancer. However some are congenital defects. The most common congenital defect in cattle is cleft lip. Cleft lip is usually considered a multifactorial disorder when it is the only anomaly but it is often one of several anomalies when cytogenetic abnormalities occur. Cleft lip can occur unilaterally as in this calf or bilaterally or in combination with a cleft palate. In early fetal development the face of all mammals looks quite similar to a fish with several slits. These gradually close between week 7 to 9 of gestation in cattle. Consider that all sheep have a cleft lip and that all dogs have open nostril slits on each side of their nose.
Development in utero is proceeding along a timecourse of a biological clock. When that timecourse is delayed by a virus or fever, the dam is unable to divert the normal amount of energy to fetal development and so it temporarily slows. When the "normal" time period for facial development is over, the face simply halts in whatever stage of completion it has reached and other parts of the fetus develop next. In some breeds with wider faces, the time to complete facial slit closure and development takes longer than in other breeds. The "predisoposition" to cleft lip caused by a wide face is due to many genes therefore.
Because multifactorial disorders require a genetic predisposition and an environmental trigger, the recurrence risk is lower than for disorders with single gene inheritance. The ballpark recurrence risk for a multifactorial condition is 5%. However disorders with a common trigger may approach 10%. The more close relatives with a particular multifacortial condition that an animal has, the higher its risk of producing an offspring with that condition. Inbreeding also increases the risk since the predisposing genes could then be coming from both sides of the pedigree.
Quantitative inheritance is used more commonly by animal scientists and multifactorial inheritance is used more commonly in the medical fields. Quantitative inheritance is a term that denotes polygenic inheritance and environmental interaction. Traits with quantitative inheritance include growth traits such as weaning weight, average daily gain, carcass weight, etc. These traits are obviously affected by the nutrition of the animal, the health of the animal, etc.
Quantitative inheritance is presumed to be "additive" however. In other words 3 or more genes contribute to the trait in an additive or cumulative manner. Although this may be true for some genes and traits, as discussed in the page on Mendelian and beyond, there are complex interactions of genes and many different patterns of allele interaction which do not fit that well with the mathematical approaches used when assuming quantitative inheritance is additive.
Although coat color may not be a "trait" which leaps to mind as an example of quantitative inheritance, coat color is a good example of many genes interacting to produce the final trait. Since we are beginning to know which genes are involved and the allele interactions and gene interactions for some of them, it becomes clear that additive inheritance is not appropriate for this trait. Sun bleaching is one environmental effect that can cause black coat colors to take on a coppery, almost red, hue. Zinc or copper deficiency in diets may exacerbate pigmentation problems in cattle because these metal ions are required by some of the enzymes in the pigmentation pathway, such as tyrosinase related protein 2 and tyrosinase.
One of the characteristics of meat quality which is used as part of the grading system in North America is the amount of fat, particularly the amount of marbling. One gene which has been shown to affect this is leptin and the allele interaction at this locus is additive. Obviously the feeding regime used to finish cattle will also affect the amount of fat laid down and other genes which are yet undetermined likely also play a role. Sometimes such a gene is said to be a gene of "major effect". This implies that it is not the only gene affecting a trait but that its effects are substantial.
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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