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.

Probability & Genetics

When introducing Mendelian genetics, I showed that the Mendel found a cross of a purebred dominant and a purebred recessive pea plant resulted in all dominant offspring.

By crossing two of those offspring, he received 3 dominant for every 1 recessive plant.  Random chance indicates that if you run that test a thousand times you’ll get a ratio of 3:1.  The more times you run the cross, the closer to 3:1 you’ll get.  Genetics is partially a lesson in simple probability.

In order to perform crosses by hand, we set up a Punnett square, which will allow us to work out the cross on paper.  Because showing your work is actually advantageous outside of homework and tests, we’ll do some samples.

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Mendel and His Peas – The Story of Genetics

Hi loyal readers, glad to be back after a long stretch.  I recently returned from a long vacation in the Mediterranean!  I visited places like Pompeii and the Colleseum in Rome, but more importantly, I made sure to eat plenty of gelato.  Before I left I was doing some consulting work so I’ve been involved in biology, but I’m happy to get a new biology post put together for you.

Back in the 1800′s, a handsome devil did work that later led him to be called “the father of modern genetics.”  His name was Gregor Mendel (and you clicked the link, so you know he was a past-day David Beckham).  Austrian-born (in what is now the Czech Republic), Mendel lived and worked in a monastery where he did the work that he’s now famous for.

Mendel’s famous work was with almost 30,000 pea plants.  He found by crossing a purebred dominant and a purebred recessive plant for a trait, all the offspring were dominant.  But by cross-pollinating those offspring with one another, resulted in a 3:1 ratio for plants showing the dominance versus the recessiveness.  He tested 7 traits in all, and each one showed the same 3:1 ratio.  Turns out that 3:1 ratio of dominance to recessiveness also indicates a 1:2:1 ratio, i.e. out of every 4 offspring there are 1 purebred dominant, 2 hybrid dominant, and 1 purebred recessive plants.

Now we’ve given these ratios names.  We refer to 3:1 as a phenotypic ratio.  The phenotype is the expression of the genes, for example being tall or short would be a phenotype.   The 1:2:1 ratio is just as important, and it’s called a genotypic ratio.  The genotype is what alleles are present, so being purebred tall, hybrid tall, or purebred short would be a genotype.

Different forms of a gene are called alleles and each parent  passes 1 allele to its offspring.  So if the options are TT = purebred tall, Tt = hybrid tall, tT = hybrid tall, and tt = purebred short, the presence of the t or the T would indicate the allele.  To tie it all together: being TT, Tt, or tt would be called the genotype and being tall or short would be the phenotype.

Go ahead and let that simmer for awhile, because the next post will have problems!


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