Cccc h cc ch cc eumelanin phaecomelanin full brown c

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CC/Cc h /Cc ch /Cc eumelanin + phaecomelanin = full brown c is a defective allele that produces neither pigment cc X X = albino c ch produces a partial defect in pigmentation c ch c ch /c ch c h /c ch c eumelanin very low = chinchilla (gray) c h is an example of a temperature-sensitive allele in which the pigment (black) is only expressed in the cooler regions of the body (ears, nose, tail) c h c h /c h c eumelanin X = himalayan Similar types of temperature-sensitive alleles have been found in species of domestic animals, such as the Siamese cat (Figure 4.10). Alleles of the ABO blood group can be dominant, recessive, or codominant (Figure 4.11). Human blood groups are an example of a multiple allele system consisting of three alleles: I A , I B and i . The i allele is recessive to both I A and I B ; I A and I B are codominant. The I A allele results in production of A antigen on the surface of red blood cells. The I B allele results in production of B antigen on the surface of red blood cells. The i allele results in production of neither A nor B on the surface of red blood cells.
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6 An individual can have only two alleles (one on each of the two homologous chromosomes on which they are located), so there are six possible genotypes; because of codominance, there are four possible phenotypes: Genotype Cell Surface Antigen(s) Phenotype ii None Type O I A i A only Type A I A I A A only Type A I B i B only Type B I B I B B only Type B I A I B A and B Type AB The inheritance pattern of X-linked genes can be revealed by reciprocal crosses. Mammalian females have two X chromosomes; mammalian males have only one. Any gene on the X, known as a “X-linked gene”, will therefore only be present in one copy in male cells. Duchenne muscular dystrophy (DMD) is a disease caused by an X-linked recessive allele. Males are therefore more likely to get DMD due to having only one possible copy of the gene associated with DMD. In a pedigree (Figure 4.12), multiple males will likely be affected, while very few females will be affected. A reciprocal cross reverses the sexes and phenotypes to identify if the gene under study is X-linked, because X-linked genes will not behave the same in reciprocal crosses (Figure 4.13). Genes located on mammalian sex chromosomes can be transmitted in an X-linked, Y-linked, or a pseudoautosomal pattern. A sex-linked gene is found on one, but not both, of the sex chromosomes (X and Y). Because there are many more X-linked genes than Y-linked genes, the terms “sex- linked” and “X-linked” have become synonymous. The term hemizygous describes the single copy of X-linked genes in males. Holandric genes are those located on the Y chromosome. A small group of genes, including Mic2 , are located on both the X and Y chromosomes and are classified as showing pseudoautosomal inheritance (Figure 4.14).
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7 Some traits are influenced by the sex of the individual.
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  • Spring '08
  • SAXENA
  • Genetics, red blood cells, Sex linkage

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