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Non-Mendelian Genetics Hey! If you recall, Mendel was able to conclude from his experiments ……… 1. Alternate versions of genes account for variation. 2. For every trait an organism inherits 2 copies of every gene, one from each parent 3. One allele of each gene is dominant and the other is recessive 4. The two alleles for each trait separate during gamete formation – every gamete only getting one gene allele. These ideas make some big assumptions. 1. 2. 3. 4. One allele is always dominant That there are only ever two alleles for a single trait Each trait is only ever coded for by one gene All genes are separate from each other. We now know that while Mendel’s ideas do work with some traits, many other traits are inherited in more complicated ways that violate the assumptions that Mendel made. Geeeeesh! Let’s start with assumption #1 One allele is always dominant Darn! For instance, in snapdragons…….. When you cross a red parent With a white parent X The F1 is pink! Instead of one phenotype being expressed over the other, you get a blend of the two parent phenotypes in the F1. This is called incomplete dominance. With incomplete dominance, if an individual is heterozygous for the trait they have a phenotype that is a blend of the two alleles RR x rr If you cross 2 of the heterozygotes Rr x Rr Rr There is no strict convention for how to notate incomplete dominance. Sometimes you will see big and little letters used, sometimes you will see two different letters used. Either way, the heterozygote’s phenotype is always a blend of the parents. ¼ Red R r R RR Rr r Rr rr ½ Pink ¼ White With some traits, both alleles are expressed equally in a heterozygote. This is called co-dominance. Red blood cells have proteins on their surface that help your body know that they belong there. There are two types of these proteins A and B. Someone with the genotype AA will have only A proteins on their blood cells and are considered to have “type A” blood Someone with the genotype BB will have only B proteins on their blood cells and are considered to have “type B” blood Type A The difference between incomplete dominance and codominance is a subtle but important one. I suggest reading pgs. 319-320 in your text for a more complete treatment of the spectrum of dominance. Type B Someone with the genotype AB will have both A and B proteins on their blood cells and are considered to have “type AB” blood Type AB What about type O blood!!!! Be Patient……. Assumption #2 Ouch There are only two alleles for every trait. There are some traits that have more than two alleles. Strangely enough, the term for this is “multiple alleles”. Blood type works this way as well. There are actually three allele for the blood protein gene. A = A protein B = B protein O = no protein So if you are…… AA or AO: your phenotype is “type A” BB or BO: your phenotype is “type B” AB: your phenotype is “type AB” OO: your phenotype is “type O” Typically with multiple alleles a different letter is used for each allele. A woman with type A blood marries a man with type B blood and they have a child who is type O. Her mother and father were type A. What were: a. Her mother and father’s genotypes? b. Her genotype? Try this before c. Her husbands genotype? clicking for the d. Their child’s genotype? answer. e. The chance that their next child will be type O? The only way for a type A woman and a type B man could have a type O child is if they are AO and BO respectively. a. b. d. e. Her genotype = AO His genotype = BO Child’s genotype = OO Chances of having another OO child = 25% B O A AB AO O BO OO C. If she is AO and both her parent were type A, they had to be either AA x AO or AO x AO A A O O The woman A AA AO A AA AO A AA AO O AO OO Assumption #3 Sigh Every trait is controlled by only one gene. • We now know that sometimes a single trait can be controlled by more than one gene. This is called polygenic inheritance. Skin color is the classic example of a trait that is controlled by several genes. Skin color is coded for by three genes A B and C. The more dominant alleles you have the darker your skin, the more recessive the more light you are. This creates a huge range of skin pigments.