"Facts do not cease to exist because they are ignored ." --Aldous Huxley

I. More on Mendel

1. Mendel's work was forgotten during his lifetime, but it was rediscovered by several scientists in 1900.
2. Mendel's discoveries -- that "genes" come in pairs and are "shuffled" independently -- seemed to match what had been discovered about meiosis!
   1. Remember that chromosomes come in pairs and are "shuffled" at meiosis. . .
   2. Just as each parent contributes one of each chromosome to each offspring, each parent contributes one of each allele to each offspring. . .
   3. This made a stronger circumstantial case that genes were carried on chromosomes!
   4. In the 1910s, Thomas Hunt Morgan and his students at Columbia University, studying genetics in fruit flies, worked out that:
      1. Genes aren't the same as chromosomes. (Fruit flies have thousands of genes but only four pairs of chromosomes.)
      2. But genes are carried on chromosomes. Genes are, physically, parts of chromosomes. Keep this in mind. . .
   5. Some years after Mendel's work was rediscovered, a British geneticist named R. H. Punnett developed a simple way of working out the expected frequencies of any cross. This is the Punnett square.
   6. This rather colorful 2x2 Punnett square shows how to predict the results of Mendel's Pp x Pp cross:
      

      Three out of every four offspring from this cross will have purple flowers.

   7. In a similar way, with a 4x4 Punnett square, we can work out a cross between organisms with two traits of interest (a dihybrid cross). You will see this in lecture and lab. . .

1. When Mendel's work was rediscovered, and other scientists began doing similar crosses to Mendel's, they discovered some features of inheritance that Mendel hadn't.
   1. When a dominant allele doesn't completely mask a recessive allele, we have incomplete dominance .
      1. EXAMPLE: If you cross red snapdragon flowers (RR) with white snapdragons (rr), the F₁ offspring (Rr) are pink.
      2. This looks like "blending inheritance" -- but if you then cross two pink snapdragons (Rr x Rr) you get one red (RR) for every two pink (Rr) and one white (rr).
   2. In other cases, there may be codominant alleles
      1. If two different codominant alleles are present, the organism just shows both traits, and neither one masks the other.
      2. EXAMPLE: Human ABO blood types are governed by a gene with three alleles. Technically they're written I^A, I^B, and i, but you can call them A, B and O. . .
      3. A and B are codominant and O is recessive. If you have an A and a B allele, you have blood type AB.
      4. These alleles determine which molecular "dogtags" your blood cells carry: type A, type B, both (AB), or neither (O). Your immune system attacks whichever one you don't make: if you have type A blood, you will attack any blood cells that are carrying the B tag (i.e., type B or type AB blood).
      5. People with type AB blood attack neither A nor B blood -- they are universal recipients because they can accept blood from anybody
      6. People with type O blood have no "dogtags" -- they are universal donors because anybody can accept blood from them.
   3. Multiple genes may influence one trait. This is called epistasis.
      1. EXAMPLE: Human height and skin color are inherited, but in each case something like six different genes influence the actual outcome.
      2. EXAMPLE: The color and pattern of a cat's fur is affected by about ten genes, each of which exists in several alleles. (I will go into this in some detail in lecture. . .)
   4. A single gene may have multiple effects -- a phenomenon called pleiotropy.
      1. EXAMPLE: The Devon Rex breed of cat has a single allele that has multiple effects -- it gives it wavy or curly hair, and also a "pixie"-shaped face with big eyes, wide cheekbones, and big ears. Devon Rex cats look like Yoda! (Check 'em out. . .)

         
         A Devon Rex cat.

      2. EXAMPLE: In humans, the genetic disorder Marfan's syndrome is caused by a single dominant allele -- its effects include: tallness; long arms and legs; long fingers; tendency to nearsightedness or other vision problems; tendency to develop weak spots in the aorta.
   5. Some genes may be lethal when they are homozygous -- this can mess up the Mendelian ratios.
      1. EXAMPLE: Manx cats are cats with no tails; the tailless allele (T) is dominant to the normal, tailed allele (t).
      2. When you mate two Manx cats, the breed doesn't "breed true"; you get a mix of tailed and tailless cats.
      3. What's more, you get only two Manx kittens for every tailed kitten. This is not what Mendel would have predicted. . .
      4. The reason is that having two copies of the tailless allele causes a kitten to die very early in development, and never be born. So crossing two Manx cats (Tt x Tt) yields 1 TT (lethal) : 2 Tt (Manx) : 1 tt (normal). (Check 'em out. . .)
         
         A Manx cat.