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GENETICS AND HEREDITY

We’ve talked a lot about chromosomes. Now it’s time to talk about GENES.
In a way, that’s what we’ve been working out way up to.
And in a way, we’ve been doing it backwards compared to the way it worked out historically.
Your book gives a nice sketch of the history.

We know about chromosomes, and how the behave.
We know they occur in pairs.
We know they house genes, which consist of DNA.
We know what happens in meiosis and fertilization.

But nobody knew all that 150 years ago, and nobody knew how it was that traits are inherited.
Certainly, Darwin didn’t know anything about genes when he
wrote the Origin of Species in 1859.
In fact, what makes this even more fascinating is that these two
ideas, evolution by natural selection, and the concept of genes,
are ABSOLUTELY intertwined with each other, yet they essentially arose independently.
It was only in the early to mid 20th century that biologists put them both together and saw how important one was for the other.
Actually, there’s a third and a fourth strand at work too.
Our modern concept of genetics was made possible by continued elaboration of the cell theory, and the structure and behavior of
chromosomes.
In fact, our concept of genetics results from a convergence of different evidence that is mutually consistent.
That’s the way science works. The same with evolution, ecology, etc.

Of course, the father of genetics was an Austrian monk called Gregor Mendel, who had what every good scientist has:
CURIOSITY ABOUT HOW THE NATURAL WORLD WORKS.

And in particular, he wanted to know how inheritence works.

So, he chose to investigate it with garden peas.
Good choice. Why? What is it about pea plants that made them useful to Mendel?

He was also a very good scientist. Why?

Mendel investigated a number of obvious ‘traits’ of pea plants to follow.

He did a lot of crosses between plants that bred true for those traits.

          short vs tall
        yellow seed vs green seed
        round vs wrinkled seed

Numbers a key in his success: he was one of the first quantitative biologists.
Mendel dealt in probabilities!! Statistics!

Through his crosses Mendel established certain principles of inheritence.

For example, one version of a trait is often DOMINANT over another, RECESSIVE version.
In other words, one version masked another.
Individuals carry pairs of these versions, or as we now call them, ALLELES.
Even though a dominant allele can mask a recessive one, the latter is still there and segregates during each the production of
gametes.
Alleles then recombine during fertilization, and the recessive trait can reappear (if two recessive alleles are present).

Let’s look at a simple cross.
Cross a plant that's tall
    with a plant that's short. Both plants are pure genetically for this trait, or HOMOZYGOUS.
   LL x ll
        >>>> Again, think PAIRS!!! DIPLOID AND HAPLOID!!! THINK GAMETES!!!
Tall is dominant over short.
   Fig. 13.10
All the plants that result from this cross are tall
   That’s their PHENOTYPE.
   And they’re all HYBRIDS. Ll - that’s their GENOTYPE. They're HETEROZYGOUS
        HOW DID THIS HAPPEN? AGAIN, THINK MEIOSIS

Now, cross hybrid plants with itself or another hybrid plant.
    Ll x Ll
Each plant produces two types of gametes b/c the genes for length
    segregate.
If the gametes are equally viable, and equally fertile, then have equal chances of contributing to the next
    generation.

Soooooooooo.....
Statistically, 25% of next generation will be LL, 50% will be Ll, and 25% will be ll.
    Those are their GENOTYPES.

But, the PHENOTYPE of Ll is the same as that of LL because L is dominant.
THE PHENOTYPE IS....TALL
Soooooooooo......
75% of next generation is tall and 25% is short.
Also see that the recessive trait reappears, as long as their are two recessive genes.

Another principle to emerge from Mendel’s experiments:
   INDEPENDENT ASSORTMENT.
The factors, or elements, or genes as we now call them, for different traits act independently of each other
    during formation of gametes.
E.g., genes for seed shape and flower color assort independently, as if they’re separate entitities that aren’t
    connected to each other, or linked.
That’s evident in a DIHYBRID CROSS.   (Fig. 13.12).
Notice that the genes for each trait segregate, and recombine, independently of each other.
   Again, INDEPENDENT ASSORTMENT.
Again, this behavior was only made sense when we realized that chromosomes behave the same way
   during MEIOSIS.

Back down to earth.....
These rules are relatively simplistic.
Most traits aren’t governed this way. They’re more complex.
E.g. Many traits aren’t ‘all or none’, but quantitative.
Show a range or distribution in phenotype . E.g. fruit size, number of kernels on a cob.

Some traits show incomplete dominance - e.g. flower color in some species. - Red, white and PINK.
Genes for different traits are often LINKED, on the same chromosome. This changes expected frequencies.

Many traits are not determined by just one gene, but by several or many. They're polygenic.
    E.g., a pigment make require the action of several enzymes in sequence, each encoded by a separate gene.

Also remember, CROSSING OVER occurs during meiosis.
So, you do get some reassortment or traits. In other words, predicted percentages of genotypes in offspring
    may be off.
More crossing over with greater distance between genes.
   If at opposite ends of chromosome, genes on same chromosome can behave as if they aren't; i.e. unlinked. <>Finally....
Transposable elements, or jumping genes. What are they and what do they do?
    Who was Barbara McClintock?