GEN 3000 Notes Set 7 01 23 2016 Dr Tsai Clemson University Chapter 4 Continued from video lecture Modification of Mendelian Ratios Recessive Lethality o Cuenot In 1900s Cuenot was attempting to replicate and build off of the work of Mendel He began to do crosses in mice for color traits o He set up his experiment the same way that Mendel did and found that there was the expected 3 1 ratio between gray and white mice Then when he tried a different phenotype Yellow crossed with gray he originally believed he was again getting the 3 1 ratio in F1 of the cross As he continued to do more generations he found that he was getting no true breeding yellow mice YY and was getting a 2 1 ratio of yellow to gray instead of 3 1 o It turns out that this yellow allele in mice is a pleiotropic gene a gene that can impact several aspects of the overall phenotype When looking at the phenotype the Yellow gene does appear dominant However if two copies of Y were present the mice were still born Therefore the homozygous dominant gene is recessively lethal 2 3 of live progeny are Yy yellow 1 3 of progeny are yy nonyellow Note even though lethality is related to a dominant gene the lethality itself is recessive because it is hidden in heterozygous mice Yy o Another example of recessive lethality The manx phenotype in cats includes the ML allele the lethal allele that stops the spine from developing properly The M allele is the normal one If only one copy of the ML gene is in the animal it is non lethal and only the tail is missing However if two copies of the ML are present it is embryonic lethal making it recessively lethal for the dominant allele o There are also examples of dominant lethal alleles which can only happen and surivive in a population if lethality is late onset after reproductive capabilities An example of this is Huntington s disease in humans It only takes one copy of the dominant allele to cause lethality Dominant Lethality Multiple Alleles Genes can have more than just two alleles Within any giving individual there will be two alleles but within the total population there can be numerous alleles o Example The ABO blood group has three alleles the A allele IA for the A antigen the B allele IB for the B antigen and the O allele i that simply makes no antigen The O allele i is recessive to both IA and IB while IA and IB are Codominant This results in various combination possibilities O blood type has to be ii and AB blood type has to be IAIB However when only one antigen is expressed the genotype may be IAi IAIA IBi or IBIB O ii is the universal donor due to lacking antigens AB IAIB is the universal recipient because it is used to both antigens but can also receive the O type that lacks antigens o Some genes have a great many allele possibilities In the Drosophila the fruit fly there are at least 100 allele combinations for the eye color They are all designated with wn where n is the allele designation o Probability Within a single individual we will still only have two alleles However our possible genotypes and phenotypes are going to change We can still use probability we just have to modify it Example Cross two parents and look for possible blood types and chances of albinism aa or pigmentation A of skin Cross is between two pigmented parents one with type A blood and one with type B blood AaIBi x AaIAi AaIBi x AaIAi 3 4 A pigmented 1 4 aa albino 1 4 IAIB AB blood type 1 4 IAi A blood type 1 4 IBi B blood type 1 4 ii O blood type 1 4 IAIB AB blood type 1 4 IAi A blood type 1 4 IBi B blood type 1 4 ii O blood type 3 16 chance of pigmented with type AB blood 3 16 chance of pigmented with type A blood 3 16 chance of pigmented with type B blood 3 16 chance of pigmented with type O blood 1 16 chance of albino with type AB blood 1 16 chance of albino with type A blood 1 16 chance of albino with type B blood 1 16 chance of albino with type O blood Gene Interactions o More than one gene contributes to the same phenotype Both genes work together to create an overall appearance o For example In peppers there are two alleles that contribute to coloration R r and C c R is dominant and produces a red color An r is recessive and produces no color C is dominant and breaks down chlorophyll to get rid of the green color while the c is the recessive allele that allows the chlorophyll to remain intact producing the green color So if the RRCC red pepper is crossed to an rrcc green the F1 generation appears to be simple recessive However in the F2 generation four phenotypes make an appearance You get 9 16 Red pigments R C 3 16 of a brownish color R cc 3 16 yellow peppers rrC and 1 16 green rrcc Recessive Epistasis Sometimes one gene can completely hide what s going on at another gene if two recessive alleles are present o Labrador retriever coloration Labs can be black chocolate or yellow The first gene B or b is the actual pigmentation of Black B or brown b where B is dominant to b The second gene is responsible for attaching pigmentation to hair color but is only functional at dominant E and nonfunctional at recessive e o Black genotypes are BBEE BbEE BBEe or BbEe Chocolate are bbEE or bbEe and Yellow are BBee Bbee bbee As long as there is an E present the pigment will be attached to the hair If it is homozygous for e ee then the allele is non functional and produces a yellow color This is our epistatic gene which will mask the effect of the hypostatic or hidden gene In this case it is the B b gene o How does this affect our ratios We start with a black lab and yellow lab true bred both BBEE x bbee The F1 generation will produce all black puppies BbEe F2 generation BbEe x BbEe will give us a 9 3 4 ratio of black brown yellow We can break it down since we know every yellow lab has to be ee every black lab has B E and every chocolate lab has bbE As a side note you can distinguish between a BBee Bbee and bbee the BBee Bbee will be yellow with a black nose and the bbee will be yellow with a brown nose giving us the 9 3 3 1 ratio o Bombay Phenotype Recessive epistasis example in humans A woman s blood showed no antigens so they gave her some O blood and she had a reaction The doctors couldn t figure …