Can anyone think of astronomical objects or phenomena that were discovered accidentally?
hi friends, I have not forgotten you all or my promise to return and write a bunch of interesting things, but uni is mad busy
good news is, I’m taking geo and astro and they’re giving me lots of ideas
stay tuned, i swear, good things will come soon
oh my god that’s it, we’re done, the previous post was officially the last in the Bio 101 series
I’ll compile all the posts and upload them in some fashion over the comings weeks
but in the meantime, I’m going to take a few days off, and then this blog will be back to regular cool science shit
any suggestions for articles, drop them in. if not, your science education is in my hands, so buckle up friends
The Plot Thickens
The two main principles that Mendel discovered—the law of segregation and the lat of independent assortment—were the foundation for our modern understanding of genetics. But what Mendel didn’t realise was that there are exceptions to these “laws”, and that inheritance is a whole lot more complicated than that.
It turns out, Mendel’s pea plants had relatively simple genes, always completely dominant or completely recessive. But some alleles can exhibit different degrees of dominance and recessiveness. In some genes, neither allele is completely dominant—this is called incomplete dominance. In some plants, like in snapdragons, the F1 generation produced from crossing a red with a white can actually have pink petals—because the flowers have less red pigment.
We have to be careful here, because it seems to provide evidence for “blending” genes, but it’s actually not. If the blending hypothesis were true, we wouldn’t be able to “recover” the original red or white trait from the pink plant, but we can. When F1 hybrids produce F2 offspring, they can actually be red, white or pink. This tells us that the alleles maintain their identity and aren’t mixed up.
Alleles can also be co-dominant, which means they affect the phenotype in separate and unique ways. For example, there are several alleles for human blood groups, commonly known as A, B, and O. But the A and B alleles are codominant, so when they appear on adjacent loci in an offspring, that offspring doesn’t have A blood type of B blood type—they have AB blood type.
Body image sourced from Wikimedia Commons
Probability in Genetics
So far we’ve been talking about simple ratios, but how can we mathematically figure out the probability or the percentage that an organism will inherit a trait?
Consider a cross between YyRr and YyRr. Using a Punnett square, we can deduce that for colour, ½ the offspring will be Yy, ¼ yy, and ¼ YY. The same probabilites apply for shape: ½ Rr, ¼ rr, and ¼ RR. (Remember Y=yellow, y=green, R=round, r=wrinkled.) But how do we then figure out how many will be round and yellow, or green and wrinkled?
The rules of probability, of course.
We can determine the probabilities of the genotypes by just multiplying the traits we want to know about together. For example, what’s the probability of the pea ending up yellow and wrinkled? Well, we know that ¾ of our offspring will be yellow, and ¼ will be wrinkled. ¾ x ¼ = 0.1875. So about 18.75% of our offspring will be both yellow and wrinkled. (If you don’t like fractions, just use 0.75x0.25.)
What about green and wrinkled? ¼ x ¼ = 0.0625. So 6.25% will be green and wrinkled. We can do the same for the others:
Yellow and round: ¾ x ¾ = 0.5625 = 56.25%
Green and round: ¼ x ¾ = 0.1275 = 18.75%
Adding all of these up gets you 100%, of course. It also gives us our ratio that we’ve talked about—9:3:3:1.
Further resources: Probability in Genetics: Multiplication and Addition Rules video
The Law of Independent Assortment
But all of the above only describe what happens when we follow one particular gene through the cross. Mendel also observed the ratios of different characters as they were crossed together—such as following colour and shape, knowing that round peas are dominant (R) and wrinkly peas are recessive (r). He wanted to know whether certain genes stayed together and were more commonly found with each other, generation after generation, or whether they went their own way—whether they’re inherited independently.
To figure this out, Mendel performed a dihybrid cross, crossing YYRR with yyrr. He found that in the F1 generation, all plants are YyRr, so they’re all yellow and round. But in the F2 generation, after self-pollination, you end up with a ratio of 9:3:3:1—9 parts yellow and round, three parts yellow and wrinkled, three parts green and round, one part green and wrinkled.
If the colour and shape were inherited together, you’d end up with a 3:1 ratio, 3 parts yellow and round, one part green and wrinkled. But because Mendel ended up with the other, more complicated ratio, he realised that the alleles couldn’t be inherited together—they were independently passed down, paying no attention to any of the other alleles. This is called Mendel’s Law of Independent Assortment.
Note that this law only applies to genes on different chromosomes—those on the same chromosome tend to be inherited along with nearby genes (remember it’s more probably that far-away genes are crossed over). Their inheritance are more complex than this law can predict.
Further resources: Educationportal Video
Ratios and Punnett Squares
One way to easily illustrate and predict the patterns of inheritance is using a Punnett square. Usually, we use a capital letter to indicate a dominant allele and a lowercase letter to indicate a recessive allele. For example, Yy would be a pea plant with one dominant yellow allele and one recessive green allele.
If an organism has two identical alleles for a particular gene (like, a pea with two yellow alleles), then they are called homozygous for that gene (i.e. YY). If they have two different alleles, they are called heterozygous for that gene (i.e. Yy). When you cross two organisms that are both homozygous for the same gene, then their offspring will always be homozygous for that gene too. This is called purebreeding, or breeding true.
When we cross organisms that are heterozygous for that gene, we get:
When we cross organisms that are homozygous but have different alleles like yellow true bred peas and green true bred peas, we get:
Let’s say we have a plant that has a yellow phenotype, but we don’t know what its genotype is—since yellow is dominant, it could be YY or Yy. How can we figure it out? We can perform what’s called a test cross: cross is with a plant that is homozygous but only recessive, like yy. The allele contributed by the yellow plant will therefore always determine the appearance of the offspring—and if all the offspring are yellow, we know the original plant had dominant YY alleles.
If the offspring are half yellow and half green, we know the original plant had Yy alleles.
Further resources: A Beginner’s Guide to Punnett Squares
Anonymous said: Quick, possibly dumb, question: how was Mendel sure that the P generation peas were pure-bred?
Nope, not dumb at all. Mendel knew the P generation was true-breeding because he bred it himself. Basically, he allowed a certain variety of pea plant to self-pollinate (which is what peas ordinarily do) for several generations until they were homozygous for a given trait.
He could test this—say he had a yellow pea and allowed it to self-pollinate. If it’s homozygous, it could only ever produce yellow offspring, not the ratios we’ll see later on.