GEN 3000 Notes Set 6 01 20 2016 Dr Tsai Clemson University Chapter 3 Continued Beginning of Chapter 4 Chapter 3 Cont Pedigrees Pedigree charts can be used to decide whether or not the trait we re looking at is being inherited in a recessive fashion or a dominant fashion We ll start by looking at autosomal chromosomes in other words no sex chromosome involvement Genetic diseases within the human population particularly recessive traits or diseases are relatively rare Recessive Diseases Traits o Recessive traits or diseases A disease for which an individual must have two copies of the recessive allele in order to display the phenotype A recessive disease often skips generations If there is an affected offspring both parents HAVE to be carriers or also affected by the disease o Recessive diseases tend to be rare within a family If there is a huge generational skip but then a generation displays a great deal of a recessive disease there is most likely some level of consanguinity mating between related individuals If it is rare in the population but a family is consanguineous in any form you re more likely to see recessive alleles Autosomal Recessive disorders are more commonly seen in animal type pedigrees due to the higher instances of consanguinity o Genetic diseases can also be caused by dominant alleles so you only need one copy of the allele to get an affected offspring Every generation is going to have an affected individual If there is an unaffected individual born in a family line meaning they have two recessive alleles they will NOT be able to pass on the trait unless someone new brings it into the genetic line Generally affected individuals for a dominant disease or trait are going to heterozygous Hh instead of homozygous for the allele due to higher ratios of heterozygous genotypes o Example Waardenburg syndrome is an autosomal dominant trait that causes deafness fair skin white forelock and visual problems Dominant Disease Trait Note Not everyone draws their pedigrees in that standard format especially in animals like canines that are bred to many different individuals Designed pedigree Within a designed pedigree you can often see a lot of backcrossing used lines down with gaps to show how they follow each other However any questions asked in this class will follow standard generation and numbering format Deciphering genotypes in pedigrees Example A male individual with a family history of a recessive disease but who is not affected himself is to have an offspring with a female individual They want to know what the likelihood of their offspring having the disease will be o The disease is autosomal recessive meaning two recessive alleles must be present in order to be expressed Since the male is unaffected he either has Dd dD or DD and no possible chance of being a dd therefore we can eliminate this from possibility Therefore dad has a 2 3 chance of carrying the affected gene o Mom s history is very important in this case She is unaffected so if there is no history of the disease in her lineage we can assume that she has a DD genotype and the offspring will have no chance of being affected due to the impossibility of a dd phenotype However if mom does have a family history of the trait we must go back through her lineage to determine the likelihood of her being a carrier So let s say her parents are unaffected but she does have a sibling that is affected This means both of her parents carry the d gene meaning she has a 2 3 chance just like dad of being a carrier If the couple has a child and they are both carriers that offspring will have a chance of getting two d alleles All three of these probabilities would have to happen meaning we must take 2 3 x 2 3 x 4 36 1 9 chance of the child being affected Chapter 4 Modification of Mendelian Ratios Vocab o Allele Different forms of the same gene If the allele loses its ability to work properly it has a loss of function mutation o Loss of function mutation A partial loss of function black pigmentation loses some capability to create a grey color or loses color partially o Null allele Allele is not working at all black pigmentation no longer causes black coloration o Gain of function mutation The allele is working so much that it ends up gaining more work for the protein or doing it at the wrong time black pigmentation is showing up in the wrong spot or is just a very dark color o Change of function neutral mutation alleles can change function as well This is a neutral mutation that doesn t affect what happens but it does something else o Gene Interaction Some genes interact together to create different possibilities Phenotypes vary o X linkage Y linkage Sex chromosome linkage Other allele notation So far when differentiating alleles we ve been using capital and lowercase However this nomenclature isn t always applicable or preferred o Wild Type or Mutant Allele The wild type allele is considered the most common allele appearance in a population Anything that alters the wild type is mutant Red eyes in the fruit fly are considered a wild type allele So instead of denoting an allele as dominant or recessive we consider it or where is the wild type and the is for the mutant o Superscripts You can also distinguish between different alleles with numbers or letters as superscripts allele 1 and 2 for example R1 or R2 two forms of the allele Incomplete Dominance A blending of dominant and recessive characteristics to create a new phenotype When Mendel set up his experiments the offspring of two parents looked exactly like one parent or the other However it doesn t always work that way o If we had homozygous of each parent of a certain colored plant and the crossed PP x pp all Pp the offspring would have characteristics of both parents creating an intermediate color Then in the second generation Pp x Pp 1PP 2Pp 1pp you get three different genotypes and three different phenotypes In the case of eggplants the parent generation would be a dark purple crossed with a white plant the F1 generation would be all lighter purple violet plants and the F2 generation would have a 1 2 1 ratio of the three colors Codominance Different than incomplete dominance Instead of a blend you clearly see the effect of both alleles at once The example of that is a blood antigen group There are two different alleles LM and LN Homozygous LMLM LNLN are only going to produce one antigen while a heterozygous allele combo LMLN produces BOTH M and N antigens Sickle Cell