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Someone asked me:

If a striped cat is the result of a white and a black cat, why the result of a white woman and a black man is not a striped child?

I know the answer. I also know that the result of a black cat and a white one is not a striped cat. But I'm looking for a simple answer. I'm looking for an explanation without a genetics background required. I'm looking for a metaphor or something similar.

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    $\begingroup$ Perhaps metaphors should be sought on English.SE. I vote to close, as the question is not about the biological underpinnings. If it were, it is closable on homework grounds. $\endgroup$
    – AliceD ♦
    Commented Jun 30, 2015 at 12:58
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    $\begingroup$ I disagree with the close vote - it's a biological question. Asking for a metaphor (perhaps analogy is a better word) is just asking for a laymans answer. Though if you can't explain it simply I would question whether you know the answer, and why you would want the answer to not require genetics training @Farzad... $\endgroup$
    – rg255
    Commented Jun 30, 2015 at 13:24
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    $\begingroup$ @rg255 - I don't like the general tone, I don't like the fluff, and it is homework as there is no proof of any effort. $\endgroup$
    – AliceD ♦
    Commented Jun 30, 2015 at 13:28
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    $\begingroup$ I'm not sure I really understand. "If X then why not Y?" is completely answered by "not X". A complete answer to this question is "because that's not how you get striped cats". Everything else is "simple ways of giving basic genetics information to people". $\endgroup$
    – Resonating
    Commented Jun 30, 2015 at 13:32
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    $\begingroup$ I want an answer without genetics background required, because I want to explain it to the guy who asked me and he has no idea about genetics. I can explain it by the means of genes and alleles that the result of a pure black and a pure white cat (that's so rare) is whether a white or a black cat. But I just want to find a real-world analogue for it, and it's still science and it's still biology. @rg255 $\endgroup$
    – Farzad
    Commented Jun 30, 2015 at 14:30

2 Answers 2

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The reason that there are no stripes when black and white people get babies is the fact that the number of pigment cells is the same for black and white people. What is different is the amount of pigment produced, and it will be different in the kids as well (lighter than the dark parent, darker than the light parent) See references 1 and 2 for details on the melanocyte density. Skin color is a multigenic trait, so the genetic background of both parents and their effects on pigmentation come into full play here.

Besides this effect, humans do have stripes, although they are mostly not visible. They are called Blaschko's lines and look like below:

enter image description here

These develop from the migration pattern of the melanocytes (and their precursor cells) in the developing embryo. They can get visible in some rare genetic conditions or by diseases. See reference 3 for a more popular explanation and reference 4 for a review on this field.

References:

  1. The Regional Anatomy of the Human Integument with Special Reference to the Distribution of Hair Follicles, Sweat Glands and Melanocytes
  2. General Biology of Mammalian Pigmentation
  3. Blaschko’s Lines
  4. Lines of Blaschko.
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  • $\begingroup$ And aren't cat pigments controlled by one gene? $\endgroup$
    – SolarLunix
    Commented Jul 3, 2015 at 20:48
  • $\begingroup$ What about cats? $\endgroup$
    – TheMaskedCucumber
    Commented Jul 14, 2015 at 10:47
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Skin colour is determined by additive effects. That is, the amount of pigment is determined additively by the genes. For the basic genetics of it I will give a simple example, which is just illustrative and a gross oversimplification, it's probably much more complex (many loci all acting at once).

Imaginary example:

Suppose skin colour is affected by a single locus (at one position) within the DNA, which determines the amount of pigment produced. At this locus there are two alleles within a population, that is two versions of the DNA (e.g. ACGTCCATT and GACTTAACT are different DNA sequences). Allele one ($A_1$) produces a small amount of pigment, and allele 2 ($A_2$) produces a large amount. Thus people with $A_1 A_1$ genotype (people are diploid, i.e. they get a version from each parent) have light skin, $A_2 A_2$ genotypes have dark skin, and $A_1 A_2$ have an intermediate skin colour.

Analogy

This is analogous to mixing fruit juice with water - if you mix blackcurrant cordial with water the colour is determined by the proportions. If you put out two bottles, one with cordial and one with water, allow two people to randomly choose a bottle, and use their choices to mix a glass of drink, 50% of the time you will end up with 1/2 water mixed with 1/2 cordial, while 25% of the time just a glass full of water, and the remaining 25% of the time just a glass full of cordial.

Why stripes don't occur

Note: although stripes occur in animals including humans (see chris' answer), it's not I'm the way your question insinuates

For stripes to occur you would have to have structure in the skin with two different types of skin cell, where bands/stripes of type occur - alternating between expressing the allele inherited from the mother, and that inherited from the father. That would require structure in the skin and something called genomic imprinting so parent of origin could be a factor.

The analogy with the glass of cordial is what going on in each cell, for stripes you would have to have organised and alternating clusters of glasses (cells) which get their contents determined by one individuals choice of cordial, with the selector alternating with the cluster of glasses.

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  • $\begingroup$ Why would black and white bands require genetic imprinting? How do cats have black and white parts? $\endgroup$
    – TheMaskedCucumber
    Commented Jul 14, 2015 at 11:29
  • $\begingroup$ because the cells in the offspring would need to be able to differentiate between the paternally and maternally inherited allele to be able to organise them in to stripes $\endgroup$
    – rg255
    Commented Jul 14, 2015 at 11:50

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