BIO1107 Test 5 Study Guide 14 3 Inheritance Patterns are more complex than predicted by Mendel In some scenarios patterns are more complex due to several reasons italicized below A Extending Mendelian Genetics for a Single Gene Alleles are not completely dominant or recessive Degrees of Dominance alleles can show different degrees of dominance and recessiveness to each other In Mendel s rst experiment complete dominance was displayed as one allele showed up over another e g purple vs white owers Incomplete dominance occurs when neither allele is completely dominant and the F1 hybrids have a phenotype somewhere in the middle of the two parental alleles e g cross of red vs white snapdragons yields pink F1 generation This is not considered blending because the original red and white phenotypes can be seen again in the F2 generation F2 offspring produced have same genotypic and phenotypic ratios Codominance two alleles both affect the phenotype in a separate of 1 2 1 red pink white distinguishable way e g human MN blood group is determined by codominant alleles for two speci c molecules located on the surface of R B Cs the M and N molecules A single gene locus which has 2 possible allelic variations determines the phenotype of this blood group homozygous MM have only M molecules on RBCs homozygous NN have only N molecules But both M and N are on RBCs of heterozygous MN individuals MN not an intermediate like in codominance distinguishes codominance from incomplete dominance When a particular gene has more than two alleles ABO blood groups for example are determined by 3 alleles of a single gene IA IB and i BIO1107 Test 5 Study Guide A person s blood may be 4 phenotypes A B O or AB these letters refer to different carbohydrates that may be found on surface of RBCs O being neither When a single gene produces multiple phenotypes most genes have multiple phenotypic effects even though we have been studying genes mostly with one phenotypic effect Pleiotropy Pleiotropic alleles in humans are responsible for the multiple symptoms associated with certain hereditary diseases e g cystic brosis or sickle cell anemia e g In the garden pea the gene that determined ower color also affected the color of the coating of the outside of the seed Frequency of Dominant Alleles Polydactyly is condition when born with extra ngers or toes caused by Dominant allele this shows that there is a higher frequency of recessive alleles than dominant in the human population The Relationship between Dominance and Phenotype Tay Sachs Disease crucial enzymes can not metabolize lipids in brain Organismal level the Tay Sachs allele quali es as recessive because only children who inherit two copies of the Tay Sachs allele homozygotes have the disease Biochemical Level the intermediate phenotype is characteristic of incomplete dominance of either allele BIO1107 Test 5 Study Guide enzyme activity in heterozygotes is intermediate between homozygous for Tay Sachs or homozygous for allele not causing Molecular Level the normal allele and the Tay Sachs allele are Tay Sachs codominant Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules B Extending Mendelian Genetics for Two or More Genes Situations where 2 or more genes are responsible for determining phenotype Epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus e g In mice black coat is dominant to brown B and b acting as the two alleles For brown fur mouse must have genotype bb BUT a second gene determines whether or not the pigment will show up in the hair C for color cc giving albino so the C c gene for deposition is epistatic to the B b gene coding black or brown pigment BIO1107 Test 5 Study Guide Polygenic Inheritance For certain characteristics such as skin either or classi cation is impossible because the population is set up as a gradient These are called quantitative characters Quantitative variation usually indicates polygenic inheritance an additive effect of two or more genes on a single phenotypic character opposite of pleiotropy where one gene affects several different evidence that human skin is controlled by at least 3 different phenotypic characters genes Nature vs Nurture The Environmental Impact of Phenotype Heredity gets more complicated and steers away some from Mendelian Genetics when the phenotype for a character depends on environment as well as genotype nutrition working out sun exposure to skin all alter already set genotypes in humans Genotypes are not associated with rigidly de ned phenotypes but rather a range of phenotypes that are sometimes in uenced by nature this range is called the norm of reaction for a genotype Generally these norms of reaction are broader for polygenic characters Multifactorial characters regarding phenotype are in uenced by many factors both genetic and environmental 14 4 Many human traits follow Mendelian patterns of inheritance Pedigree Analysis are used to trace back heredity in human families since we are not able to predict human mating patterns Recessively Inherited Disorders shows up only in homozygous recessive people aa One from each parent Aa phenotype appears normal as there is enough of the speci c protein produced Carriers are heterozygotes who transfer the recessive allele to their offspring BIO1107 Test 5 Study Guide Most people with recessive disorders have parents who are carriers also who do not show symptoms A mating between 2 carriers corresponds to Mendelian monohybrid Cross so the predicted genotypic ratio of the offspring is 1AA 2Aa 1aa Thus each child has a 1 4 chance of inheriting a double dose of the recessive allele 2 out of 3 normal phenotypes are predicted to be heterozygous carriers 2 3 chance When a disease causing recessive allele is rare it is unlikely that two people with the same allele will mate same family e g incest chances of same harmful allele in both partners increase when from the Consanguineous marriages are indicated by double lines on pedigrees Cystic Fibrosis ex of pleiotropy most common lethal genetic disease in U S 1 out of 2500 European descent 4 of European American population are carriers Sickle Cell Disease ex of pleiotropy most common inherited disorder among african american people 1 out of 400 African Americans affected caused by 1 substitution in amino acids in the hemoglobin protein val glut 6th position When oxygen levels are low cells form sickle shape blocking veins and arteries