Horse Genetics. Ernest Bailey

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СКАЧАТЬ variant may be additive, depending on how the phenotype is measured.

       Co-dominance among alleles

      Co-dominance is a lot like partial dominance in that when the gene is present it is expressed. However, it does not have an impact on its co-dominant allelic partner. Blood group markers and SNPs are co-dominant. Their presence can be detected whenever they are present, but they do not have an effect on the expression of their alleles. Most of us are familiar with the ABO human blood groups. The ABO blood group is encoded by a single locus. One of the alleles makes the A blood group substance. Another makes the B blood group substance. When the A factor is present it is detected. When the B factor is present it is detected. The detection of the A phenotype has nothing to do with the detection of B. When neither A nor B are present, they are not detected, and the individual is referred to as type O.

      However, the ABO system is also a good example of the nature of recessive and dominance interactions. Since there is no O protein (O is a null allele) there is no way to distinguish a homozygote for A (genotype A/A) from a heterozygote for A and O (genotype A/O). As a result, the allele for O is recessive to the allele for A. Therefore, the alleles for A and B are co-dominant but the allele for O is recessive.

      In summary, dominance, recessive, partial dominance, and co-dominance are descriptions of the interactions among alleles.

       Expected ratios, statistical tests and alternative models

      Learning to predict and recognize simple 1:1, 1:2:1, 3:1, and 2:1 trait ratios among offspring is extremely useful for building genetic models of trait transmission. These ratios may be complicated, as we have seen, by the interactions of more than one gene, and may not be obvious unless large numbers of offspring are available. Statistical testing (e.g. the chi-square test) may be needed to determine whether the observed ratios match the expected ratios. (Consult a basic genetics or statistics text for information on how to apply such tests.)

      If the data obtained do not match expected ratios, then alternative proposals need to be considered, such as:

      1. The trait is genetic, but the hypothesized transmission mechanism is incorrect (e.g. more than one gene may be involved).

      2. The trait is produced by environmental influences, not genes.

      3. The gene shows “reduced penetrance” (i.e. the phenotype is modified by environment or other gene combinations, so that the effect of the mutant gene is not easily recognized).

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