Saturday, November 25, 2006

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.



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 knwon 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.



[Calico_cat_Phoebe is from Free Software Foundation.]

13 comments:

  1. Ah yes. Weekend cat blogging, Moran-style.

    Thanks! I happen to know a Calico. Most people think she's cuter!

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  2. A couple of pesky questions from a non-biologist:

    Does random deactivation of one X in humans mean that in a very minor sense, all women are chimeras?

    And what about tortoise shell cats? Are they all female?

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  3. Another pesky question:

    What controls the stripes on tabby cats?

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  4. Pesky question #4:

    What about 2-color cats? I've mostly seen black-and-white ones, but I used to be the pet human and devoted servant of one (male) who was orange-and-white, and I've also seen lots of tabby-and-white ones.

    Pesky question #5:

    Why are 2-color and tabby cat markings so symmetrical?

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  5. Yes, all women are chimeras. It explains a lot .... :-)

    Tortoise shell cats are the same as Calico cats.

    I don't know anything more about the genetics of coat color in cats. Perhaps someone else will respond.

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  6. Darn, I had to study from your book, too (got an A+ though so it ended well...LOL)

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  9. [You know how stuff just pops into your head at 3 am? I just realized that my next-to-last post had a major boneheaded goof in it - and so did the last one. So I'm trying again.]

    "Yes, all women are chimeras. It explains a lot"

    =LOL= And all this time I thought it was just because I'm bipolar. Just one more way in which my Two-Headed Monster avatar fits, I guess. =grin=

    OK. Leaving aside the autosomes for the moment (pesky autosomes, they complicate everything! =another grin=):

    In effect, we women end up with half-and-half "mom X's" and "dad X's," while men have only "mom X's." If we have half our working X's from each parent and men have all their X's from mom (on top of which there's far less genetic information on the Y than the X, AFAIK at least), could it be said that in developmental terms we're "related" to our moms and dads to a very slightly different degree (or at least in a different way) than our brothers are?

    And what happens to the Y chromosome as males develop? Does it stay active alongside the X?

    Am I confused enough yet? =yikes!= and =grin= simultaneously (There goes my Two-Headedness again!) Well, I did say at the outset that I'm not a biologist! I'd better quit before I get any further behind!

    "Tortoise shell cats are the same as Calico cats."

    That's what I thought, but I wasn't 100% sure. I don't recall ever having seen a male tortie. I've known some real sweetheart tortie ladies though - a gray tortie once graciously condescended to allow me and my (now former) Significant Other to share her living space.

    Come to think of it, the last cat who adopted us was gray-and-tan too - a light calico who evidently got fed up with living in another apartment with about 6 other cats, some of whom were meaner than dirt when it came to others of their species.

    She walked into our place one day while we were moving stuff in, curled up in the sunny spot on top of the dining-room radiator and zonked out, and that was that. She'd been an indoor-outdoor critter all her life, but we could never get her to even think about going outside and possibly meeting up with one of the meanies.

    Fortunately, her former pet human was OK with it!

    "I don't know anything more about the genetics of coat color in cats. Perhaps someone else will respond."

    I hope so. I've been fascinated by genetics ever since I was a little kid. I discovered Mendel's experiments one day while looking through my dad's college biology textbook. I must have been about 7 at the time, but 45+ years later I still remember that first "wow, that's so neat!" moment.

    (Back to those pesky complicated autosomes: I'm the oldest of 4 blood siblings. Except for the fact that 3 of us have brown hair, we look almost nothing alike. My mom says, "You all look like you came from the adoption agency!")

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  10. I've been carrying a link to this as a draft for months and only now got around to publishing it. For some reason it doesn't appear in "Links to this post" - so here it is: "Sandwalk on Calico Cats".

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  11. I have a calico cat too. And it's female.But My friend at squidoo has a male calico. here are my lens about calico cat My pet

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  12. I don't understand much about genetics and biochemistry, but I know that calico cats are very cute and adorable. :)

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  13. Specifying and sustaining pigmentation patterns in domestic and wild cats
    http://www.ncbi.nlm.nih.gov/pubmed/22997338

    Genetics of Pigmentation in Dogs and Cats
    http://www.annualreviews.org/doi/full/10.1146/annurev-animal-031412-103659

    Both articles are free.

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