MadSci Network: Genetics

Re: what's the chance of someone having fine caucassian hair if theyre1/3 black

Date: Tue Jun 3 17:33:27 2003
Posted By: Jeff Buzby, Scientist, CHOC Research Institute
Area of science: Genetics
ID: 1053639661.Ge

Dear Shay,

In general, curly hair is considered to be an incompletely dominant trait, as exemplified in the "Incomplete Dominance" section of this tutorial on The Role of a Genes, from Okla. Panhandle St. Univ. Although it is difficult to assess your particular genotype without knowing more information, such as your mother's and your father's ethnic backgrounds, some general predictions are possible based on the phenotypic characteristics that you describe, as well as the incompletely dominant expression of curly hair. If you're confused by some of the genetic terminology, please see The Role of a Genes for definitions of homozygous, heterozygous, genotype, phenotype, & allele.

If curly hair were a completely dominant trait, individuals who were either homozygous or heterozygous for curly hair would have the same curly phenotype, so those individuals who were "carriers" of a heterozygous straight hair gene would not be phenotypically detectable. However, wavy hair is an intermediate phenotype that allows those individuals heterozygous for incompletely dominant curly hair to be detectable. The phenotype that you describe for your own hair is very much like what would be expected of someone heterozygous for curly hair, which would be a less curly, wavy type of hair. Although this can't be concluded with certainty from the info. given, if you actually are heterozygous for curly hair, you would be a "carrier" of the recessive gene for straight hair. So your offspring would have a 50% chance for straight hair if their father had straight hair. You might be able to see how this works out if you substitute "c" for "C" with one of the parents in The Role of a Genes Punnett Square for Curly Hair. A heterozygous parent would be "C - c", as shown, whereas a homozygous, straight-haired parent would be "c - c". In addition, even if your children's hair was wavy, it might not be as curly as yours, since other genetic factors often influence the intermediate expression, characteristic of incomplete dominance.

However, this prediction is further complicated by the nature of African curly hair. As described in the Hair Types section of the Genotype -> Phenotype Classroom Booklet, from The Woodrow Wilson Biology Inst., the primary factor that creates curly hair is the amount of disulfide cross-linking between its structural keratin protein molecules. But another contributing factor is the shape of the hair follicle. As described in the Ethnic Differences section of the Hair Biology tutorial from, "Most African Americans have very curly, flattened hair follicles in their skin that produce the highly coiled hair." This contributes to the "coarse" characteristic that you describe for your hair. I couldn't find any info. on the phenotypic dominance or the relative genetic contribution of hair follicle structure/shape vs. keratin cross-linking, with respect to hair curliness. However, assuming that the "flattened hair follicles" that generate "coarse", "highly coiled hair" are also a dominant trait, you would most likely be heterozygous for that, as well, since you could not have inherited this allele from your mother, who has "fine", straight hair. So your offspring would also have a 50% chance for "fine hair" if their father had "fine hair".

Hair color is a more complicated, polygenic trait, meaning that it is governed by multiple genes, as described in the Hair Color section of the Genotype -> Phenotype Classroom Booklet. This online tutorial also has a nice section specifically discussing the genetics of Red Hair. Coincidently, it is also an incompletely dominant trait, but it "is complicated by the fact that dark pigment, controlled by the many hair color genes, may mask or hide the red color. The darker the brown, the less the red shows through, although more shows with (GG) [homozygous] than with (Gg) [heterozygous]. As the hair becomes lighter in color, more red shows through." So unless you can detect a reddish tint in your hair color, there is no easy way to know with certainty if you've inherited any red hair (G) alleles. However, if your mother's hair is "a more intense red", which sounds like it might be the case from your description, she is probably homozygous (GG), in which case you would have had to inherit one red hair (G) allele from her, so you are probably heterozygous for red hair, too, unless you also inherited another from your father. Assuming that you are most likely heterozygous for the red hair (G) allele, your offspring would have a 50:50 chance of inheriting this allele by the same logic as applied to the inheritance of curly hair as a heterozygous "carrier". However, whether or not the offspring would express a red hair phenotype would also depend upon their inheritance of the darker pigment genes, which could mask its expression.

In summary, I would predict that you are most likely heterozygous for both straight & red hair, so there should be some possibility that your offspring could have both straight & red hair. The probability for straight hair is the most predictable & should be 50% if the father has straight hair, as described above. However, even if the father was also heterozygous with wavy hair, there would still be a 25% chance of your offspring inheriting straight hair, as shown directly in The Role of a Genes Punnett Square for Curly Hair. But if the father had curly hair, there would only be a 50:50 chance of your offspring inheriting either curly or wavy hair, but no chance for straight hair. The unknown factor here arises if the father has "coarse" hair, too, since we don't know if there is any way to phenotypically distinguish homozygous from heterozygous hair follicle structure/shape. If he were heterozygous, there should be a 25% chance of having offspring with "fine" hair, as opposed to no chance if he were homozygous. To further complicate any predicted outcome, the relative phenotypes of those individuals with coarse- wavy vs. fine-wavy vs. coarse-straight are not clear. The probability for your offspring inheriting red hair is also complicated by the potential, unknown contribution of dark pigment from the other, polygenic alleles, as described in the Hair Color section of the Genotype -> Phenotype Classroom Booklet, once again. This potential contribution would be reduced if the father had very light colored hair, but there would still be some probability for them to inherit enough dark pigment-expressing genes from you to obscure the red coloration. You can try using Punnett Squares to visualize some of these genetic crosses if you're starting to get a feel for them.

All of this genetic info. might be a bit confusing for you, but I hope you can appreciate the rationale for predicting that there should at least be some likelihood for you to "pass on that gene [for straight hair] from [your] mom, or possibly even the red color of her hair as well". Thanks for the interesting question,

Jeff Buzby, Ph.D.
CHOC Research Institute
MadSci Genetics Network

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