<P> For example, coat color in domestic cats is affected by a series of alleles of the TYR gene (which encodes the enzyme tyrosinase). The alleles C, c, c, and c (full colour, Burmese, Siamese, and albino, respectively) produce different levels of pigment and hence different levels of colour dilution . The C allele (full colour) is completely dominant over the last three and the c allele (albino) is completely recessive to the first three . </P> <P> In humans and other mammal species, sex is determined by two sex chromosomes called the X chromosome and the Y chromosome . Human females are typically XX; males are typically XY . The remaining pairs of chromosome are found in both sexes and are called autosomes; genetic traits due to loci on these chromosomes are described as autosomal, and may be dominant or recessive . Genetic traits on the X and Y chromosomes are called sex - linked, because they are linked to sex chromosomes, not because they are characteristic of one sex or the other . In practice, the term almost always refers to X-linked traits and a great many such traits (such as red - green colour vision deficiency) are not affected by sex . Females have two copies of every gene locus found on the X chromosome, just as for the autosomes, and the same dominance relationships apply . Males however have only one copy of each X chromosome gene locus, and are described as hemizygous for these genes . The Y chromosome is much smaller than the X, and contains a much smaller set of genes, including, but not limited to, those that influence' maleness', such as the SRY gene for testis determining factor . Dominance rules for sex - linked gene loci are determined by their behavior in the female: because the male has only one allele (except in the case of certain types of Y chromosome aneuploidy), that allele is always expressed regardless of whether it is dominant or recessive . </P> <P> Epistasis ("epi + stasis = to sit on top") is an interaction between alleles at two different gene loci that affect a single trait, which may sometimes resemble a dominance interaction between two different alleles at the same locus . Epistasis modifies the characteristic 9: 3: 3: 1 ratio expected for two non-epistatic genes . For two loci, 14 classes of epistatic interactions are recognized . As an example of recessive epistasis, one gene locus may determine whether a flower pigment is yellow (AA or Aa) or green (aa), while another locus determines whether the pigment is produced (BB or Bb) or not (bb). In a bb plant, the flowers will be white, irrespective of the genotype of the other locus as AA, Aa, or aa . The bb combination is not dominant to the A allele: rather, the B gene shows recessive epistasis to the A gene, because the B locus when homozygous for the recessive allele (bb) suppresses phenotypic expression of the A locus . In a cross between two AaBb plants, this produces a characteristic 9: 3: 4 ratio, in this case of yellow: green: white flowers . </P> <P> In dominant epistasis, one gene locus may determine yellow or green pigment as in the previous example: AA and Aa are yellow, and aa are green . A second locus determines whether a pigment precursor is produced (dd) or not (DD or Dd). Here, in a DD or Dd plant, the flowers will be colorless irrespective of the genotype at the A locus, because of the epistatic effect of the dominant D allele . Thus, in a cross between two AaDd plants, 3 / 4 of the plants will be colorless, and the yellow and green phenotypes are expressed only in dd plants . This produces a characteristic 12: 3: 1 ratio of white: yellow: green plants . </P>

What type of genetics relationship produces traits utilizing 3 or more genes