Not Just Dominance & Recessiveness

One of the foundations of genetics is that some traits are dominant and some are recessive.  It’s a great way to start the discussion, but it’s not the whole story.  The most basic genetic options are dominant and recessive, but there is also co-dominance, incomplete dominance, and polygenic traits (plus more variations!) that account for lots of genetic variability.  I’ll briefly touch on these so you get a better understanding of just how quickly predicting genetic outcomes can get tricky.

In the case of co-dominance, there are actually 2 dominant phenotypes that can be displayed.  A better way to grasp this is to picture your alleles as being R = red and R’ = white.  If the genotype is RR, the flower is red. If the genotype is R’R’, the flower is white. But if the genotype is RR’,  the phenotype is actually both red and white!

Using red and white flowers again, I’ll move onto incomplete dominance.  In this case, when both alleles are present, neither one is really dominant over the other.  So the alleles would be designated as R = red and r = white.  When the genotype is RR, the flower is red and when the genotype is rr, the flower is white.  But, when the flower is heterozygous and has a genotype of Rr, the flower is pink!

Polygenic traits are traits you have that are actually composed of many genes.  For example, eye color isn’t one color or another, it can be: black, dark brown, light brown, hazel, green, blue, gray, amber or varying combinations.  Skin color is another excellent example of a polygenic trait in humans.

As a bonus, because I’m feeling particularly science-y today, I’ll tell you about sex-linked genes too.   Sex-linked dominance is the presence of a gene on the sex chromosomes.  Since the sex chromosomes in humans are designated as X and Y, any gene that sits on the X or Y is considered sex-linked.  Because females have 2 X chromosomes, they will have a different phenotypic ratio than males that have 1 X and 1 Y.  An example is some forms of color-blindness are located on the X chromosome, but it’s recessive.  So girls are less likely to have 2 copies of the recessive allele.  If a boy gets the recessive allele, he has the color-blindness, because there’s no copy on the Y chromosome to “overpower” his recessive allele on his X.  So the phenotypic ratio for girls is 3:1, but for guys, it’s 1:1–meaning there’s a 50/50 chance of having color-blindness, whereas, for girls it was only a 25% chance. In a sex-linked dominant scenario, girls would have a 75% chance and boys would still have a 50% chance.

It can all get a little confusing, but don’t worry, as along as you keep your Punnett  square handy, you should be able to work through any scenario.


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