Calico Cats

There's been a discussion on talk.origins about calico cats—do they have to be female? The color pattern is an interesting combination of sex-linked genetics and epigenetics. Epigenetics is the inheritance of characteristics other than nuleotide sequence. In this case, it's inheritance of an inactivated X-chromosome.I used calico cats as an example in the Moran/Scrimgeour et al. textbook (1994) published by Neil Patterson/Prentice Hall. Here's an excerpt from that book.

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One X Chromosome Is Inactivated in Mammalian Females by Condensation into Heterochromatin
The DNA within polytene chromosome bands is condensed but nevertheless accessible to transcription factors. However, there are forms of chromatin known as heterochromatin, that are much more highly condensed. Constitutive heterochromatin refers to chromosomes or parts of chromosomes that are heterochromatic in all cells of a given species. Examples of constitutive heterochromatin can be found in every multicellular eukaryote and can take the form of entire chromosomes or parts of chromosomes. For example, some maize cells contain multiple copies of a small, heterochromatic chromosome called chromosome B. In addition, between one-fourth and one-third of all DNA in Drosophila is found in heterochromatic regions near the centromeres.
Condensation of chromatin is an effective mechanism of repressing eukaryotic gene expression and is best exemplified by the process of X-chromosome inactivation in mammalian females. The sex of a mammal is determined by the presence or absence of the male-specific Y chromosome. In humans, males normally have one X and one Y chromosome per somatic cell, whereas females normally have two X chromosomes per somatic cell. The X chromosome is quite large and contains a number of genes, most of which play no role in sex differences. Proper human development requires that only one X chromosome be fully active in each somatic cell of an adult. Thus, one of the X chromosomes in females is inactivated by condensation into heterochromatin (Figure 27.53). Such condensed chromosomes are known as sex-chromosome bodies or Barr bodies. X-chromosome inactivation is one example of the genetic phenomenon known as dosage compensation because it involves regulating the dosage of genes.

In human females, X-chromsome inactivation occurs very early in embryonic development, at about the 20-cell stage. Condensation of an X chromosome into heterochromatin appears to begin at a unique point, the xist gene, and proceed bidirectionally along the DNA. Inactivation is associated with extensive methylation of DNA. Once a specific X chromosome has been inactivated in a particular cell of the 20-cell embryo, the same X chromosome remains inactivated in all daughter cells descended from that presursor cell (Figure 27_54). In each human cell, either the maternal of paternal X chromosome can be inactivated.

The frequencies of maternal and paternal X chromosome inactivation vary among mammals. In female marsupials, for example, the paternal X chromosome appears to be preferentially inactivated. This observation indicates that the maternal and paternal chromosomes are not identical and can be distinguished in the developing embryo. However, in most other mammals, including humans, the X chromosome that is condensed appears to be selected more or less at random. As a result, some of the cells in the mature organism contain an active maternal X chromsome, and some contain an active paternal chromosome. Consequently, the organism is a mosaic composed of cells expressing different genetic information.
Sometime cells containing an active maternal X chromosome can be physically distinguished from those containing an active paternal X chromosome. An example of such a visible mosaic is the calico cat, which has patches of orange and black fur. Calico cats are always female if they have normal X chromosomes. The patchiness results from random inactivation of X chromosomes in female cats in which the X chromosome inherited from one parent carries the gene [allele] for orange fur and the X chromosome inherited from the other parent carries the gene [allele] for black fur. (The white fur on the underside is due to expression of an autosomal gene.)
Genetic mosaicism due to X-chromosome inactivation also occurs in human females. For example, the gene for glucose-6-phosphate dehydrogenease is located on the X chromosome. If each chromosome carries a different allele, patches of cells will contain either one isoform or the other, depending on which X chromosome is inactivated. The theory of X-chromosome inactivation was developed in large part by Mary Lyon, and the process is sometimes known as Lyonization.

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[Calico_cat_Phoebe is from Free Software Foundation.]