Between 1856 and 1863
Gregor Mendel an Austrian monk grew and studied peas
That probably doesn't sound that exciting, but what he learned about genetics changed everything. Let's look at what he did.
Mendel bred pea plants looking for many different traits like color,
wrinkly or round seeds... or tall or short plants among other things.
Once he had a purebred trait he would run an experiment
Mendel crossed two purebred traits. We'll look at green versus yellow colored peas
When the offspring grew he found that all of the peas were yellow. He couldn't understand why the green trait disappeared
Why didn't the peas have a 50/50 chance of being green or yellow?
To explore this further he took two peas from the first filial or F1 generation, that was just made, and he bred them together.
Yes, this is inbreeding, but they're peas so no one seems to mind
When the second filial generation arrived-- F2
75% of the peas were yellow but 25% were green
The green peas seemed to disappear in the F1, but magically reappear in the F2 generation
What could be happening?
Mendel decided that there were factors which we now call genes. The genes could be either dominant or recessive.
And dominant alleles or traits mask recessive alleles. In this case the yellow is dominant and the green is recessive
So the purebred parents made offspring that had green and yellow alleles
but only the yellow showed because it was dominant and when the F1 generation bred,
there was a chance that they could both give the recessive allele to the offspring so the green peas showed up again
if you could look at the chromosomes in the pea plants you would find the gene for seed color.
A gene is a section of DNA that controls a trait
The different seed colors have different alleles which are variations of a gene like green or yellow.
Allele is a fun word to say and it's actually even more fun to write in cursive, but you can try that next time you're bored
Now you get two alleles for every gene
Because you have a set of chromosomes from mom and a set of chromosomes from dad and the same is true for plants
When you write alleles,
dominant alleles get capitalized letters and recessive alleles are lowercase if a dominant and recessive allele are together, it's called heterozygous
"hetero-" means different and the alleles are different
so that makes sense. If the alleles are the same and dominant we call that homozygous dominant homo means same
If the alleles are the same and recessive we call them homozygous recessive. The letters we choose here are arbitrary
but it's good to use letters that are very different when you view capital and lowercase versions otherwise it can be confusing
So letters like O aren't good, but letters like Y, T or R are great
In pea pods, green is dominant over yellow. If these are the genotypes or allele combinations, what are the phenotypes?
Having just one dominant allele of the gene will make it green in this case. So there are two green pods and one yellow pod
Now this is pretty simple, but let's try thinking about this in terms of making gametes
Assuming tallness is dominant. What would the genotype look like for a man who's homozygous for tall? And what are the possible gamete combinations?
Now because tall is dominant this would be a homozygous dominant genotype. Two capital letters
Again, you can choose any two letters you like, but I'm going to use T for tall
when gametes form only half of the DNA is in each cell so
Only one allele can go to each of the sperm that are formed
There's only one possible gamete combination in this case capital T the tallness gene, but let's try another case
What would the genotype look like for a woman who's heterozygous for tall? And what are the possible gaming combinations?
Now heterozygous means different alleles, so this will be written with a capital and a lowercase t
this also means that there are two possible Gamete combinations an
Egg with a big T for the tallness gene or an egg with a little t for short gene
Heterozygous traits will have more variation in their offspring
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