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.

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.

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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!