The Rest of the Story. Fine Points of Mendelian Genetics. Alleles don t necessarily come in two forms only! The Rest of the Story 3/9/11

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1 Fine Points of Mendelian Genetics illustrated copiously, primarily with examples of Felis domesticus plagiarized from web pages too numerous to count The Rest of the Story Mendel announced his findings in 1865, and published them in 1866 You can read his original paper at if you're interested. His work was mostly ignored for 35 years. Mendel tried to continue his experiments, but was elected Abbot (head of the monastery) in 1868, and had to spend the rest of his life administering the monastery's affairs. He died in The Rest of the Story But in 1900, Mendel's work was rediscovered by several scientists, independently of each other. Experiments with other plants, and with animals, soon revealed that Mendel's basic laws of heredity applied to virtually all eukaryotes (including humans). Of course, there were bound to be a few new wrinkles... Don't memorize the details of the case studies that follow. Learn the principles that they illustrate. Alleles don t necessarily come in two forms only! There are actually three alleles of the gene for dark base color in cats: B, b, and b' (pronounced bprime ). Black (B) is dominant to medium brown (b) and light brown (b') Technically, there s no such thing as a black cat what we call black is actually an extremely dark brown Medium brown is dominant only to light brown (b') Cat breeders call medium brown chocolate and light brown cinnamon 1

2 3/9/11 Compare the black cat, chocolate brown cat, and cinnamon brown cat... In humans, the ABO blood group discovered in 1900 by Karl Landsteiner is controlled by a single gene with three alleles. The A and B alleles are codominant to the O allele. A person with type A blood could have the AA or the AO genotype, and so on. Note that a black cat could have any one of three genotypes; a dark brown cat could have one of two, and a light brown cat can only have one. And then there is the phenomenon of incomplete dominance, as shown in these snapdragons (Antirrhinum majus)... 2

3 Cross a red snapdragon with a white snapdragon, and the offspring are all... pink. Pink? X But if you cross two pink snapdragons, you get a ration of one red and one white for every two pinks! X The reason is that the red allele (R) is incompletely dominant to the white (r) allele. The red allele doesn't mask the white allele completely, so heterozygotes are pink! Rr X Rr Incomplete Dominance in Cats The allele for white spotting (S) is incompletely dominant to the allele for solid color (s). Left: Black cat with Ss genotype. Right: Black cat with SS genotype. RR Rr Rr rr 3

4 A non-cat example of incomplete dominance: Cross a white horse and a chestnut brown horse and the result is in-between: a golden palomino. Epistasis, simple example The dilute allele (d) is responsible for taking whatever coat color a cat has and lightening it. It s recessive to the non-dilute or dense allele (D). This is epistasis several genes affecting the same trait. Some genes set the basic color, and others lighten or darken it. These cats are, genetically, black (top) and chocolate brown (bottom) but both have the dd genotype and thus the dilute phenotype. Dilution produces the gray and tan coat colors (which cat breeders call blue and lilac). Epistasis: A more complex example Agouti is a fur pattern in which whatever color the cat has is only produced at the tips and bases of the hairs. (This is also called ticking ). Agouti (A) is dominant to nonagouti (a). 4

5 Tabby cats have alternating stripes of agouti hairs (which look light) and non-agouti hairs (which look dark). Mackerel tabbies have narrow straight stripes, while classic tabbies have a blotched or swirled pattern... How s it work? T ("mackerel tabby") is dominant to classic tabby or blotched tabby (t) A cat with, say, the Tt genotype would be mackerel tabby, of course... There are a few more alleles of the tabby gene, but for the sake of simplicity, let's leave it at that. (Look up "ticked tabby" or "Abyssinian" sometime, if you're curious.) How s it work? BUT... for the tabby patterns to be seen, the agouti gene must be present. A non-agouti cat (aa) will have tabby genes all cats do but never show them. This is epistasis one gene affecting how others are expressed in the phenotype. You could think of it like this: there must be a dominant agouti allele to switch on the alleles for the tabby pattern. 5

6 Both the agouti and the tabby genes affect the expression of the base color genes (black, brown, etc.) So this cat must have genes for black fur color... and genes for agouti hairs, which modify where the black color goes on the hairs... and genes for the classic tabby pattern, which determine the pattern of agouti and non-agouti hairs! And it gets worse! Still other genes may modify a tabby phenotype for instance, by breaking up the stripes of a mackerel tabby to produce a spotted cat. (Geneticists still aren t quite sure how this works, though.) Pleiotropy The r e r e genotype produces wavy hair, big low-set ears, and a wide face. Devon rex cats look like Yoda! Pleiotropy refers to cases in which a single gene has several effects on the phenotype. Example: In domestic cats, the Devon Rex allele (r e ) is recessive to the non- Devon allele (R e )... 6

7 A somewhat more poignant example of pleiotropy: Cats that are homozygous for one type of white fur allele also have blue eyes and are also usually deaf. Another deviation from Mendel s laws can be seen in Manx cats, a breed born without tails. The tailless allele (T) is dominant to the tailed allele (t)... Non-Mendelian Manx? If you cross two Manx cats, you get two tailless kittens for every one tailed kitten... The reason is that the TT genotype is embryonic lethal it causes the embryo to fail to develop. TT embryos are never born. Manx cats must therefore have the Tt genotype. The Tt x Tt cross gives you one TT (lethal; never born) to two Tt (Manx) to one tt (tailed). Environmental Effects The color allele (C) in cats allows whatever color and pattern is present to be expressed. It s dominant to the albino allele, c. (This is yet another example of epistasis). However, another allele of this gene, c s, causes whatever color is present to be expressed but only on the parts of the cat that are cooler than the rest. This is an example of how the environment can affect gene expression! 7

8 Here s a c s c s cat with black color ( seal point Siamese ) and a c s c s cat with light brown color ( cinnamon point Siamese ). Pigment only appears on the cooler parts of the cats bodies. You can even have a c s c s cat with a base tabby pattern ("lynx point Siamese"). Stripes and pigment again only appear on the cooler parts of the cat s body. 8