2What happens when inheritance doesn’t follow the patterns observed by Mendel? There are many reasons why traits might deviate from expected.
3Alleles Alter Phenotypes in Different Ways Main ConceptsAlleles Alter Phenotypes in Different WaysIncomplete DominanceCodominanceMultiple AllelesLethal AllelesPhenotypes Often Affected by More Than One Gene (Polygenic)A Single Gene May Have Multiple Effects (Pleiotropy)Sex Linked TraitsSex-Limited and Sex-Influenced Inheritance
4Different types of mutations can alter alleles (or produce new ones) How are alleles formed?MUTATIONWild-type allele: occurs most frequently in nature and is usually dominantDifferent types of mutations can alter alleles (or produce new ones)
5MutationsLoss of function mutation: a mutation that causes the reduction/loss of the specific wild-type functionIf the loss is complete, the mutation has resulted in what is called a null alleleGain of function mutation: a mutation that enhances the function of the wild-type allele
6Incomplete DominanceIncomplete dominance – heterozygotes show a distinct intermediate phenotype different from homozygous genotypesNeither trait is dominant
7SymbolsAlleles that are incompletely dominant are written using a superscript letter.Flower color:CWCW (white) plant X CRCR (red) plant will produce all CRCW offspring
11Are these traits blended? NO … the alleles are particulate (they remain separate and do not influence each other).How do we know?A cross of two pink individuals will produce red, white and pink individuals.The phenotypic ratio will be 1:2:1, like the genotypic ratio
14What is really going on?White allele is most likely a “loss of function” mutationWild type (red) “codes” for synthesis of red pigmentMutant allele (white) cannot synthesize the pigmentTherefore, the heterozygote only produces ½ the amount of pigment (pink)
15True incomplete dominance is rare Individuals may appear completely dominant until viewed at the molecular levelTay-Sachs diseaseHomozygous recessive = severely affected, death before age 3Heterozygotes appear normal, but only produce about 50% of normal enzymesThreshold effect
16The threshold effect - normal phenotypic expression occurs whenever a certain level (≤ 50%) of gene product is attained.
17CodominanceCodominance – 2 alleles affect the phenotype in separate, distinguishable waysAlleles for curly hair and straight hair are codominantCurly hair = homozygous for curly hair allelesStraight hair = homozygous for straight hair allelesHeterozygous individuals have wavy hair
21Incomplete Dominance vs. Codominance With incomplete dominance we get a reduced function of the dominant trait (due to recessive), so that the third phenotype is something in the middle (red x white = pink).In codominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms – appears to be a “blend”
22Dominance = common?Polydactyly – an allele that is dominant to the recessive allele for 5 digitsRecessive allele more common – 99% have 5 digits
23Multiple Alleles Can only be studied in populations WHY? Because individuals can only have 2 alleles for a gene!Example of trait covered by multiple alleles?Human Blood TypeABO antigens
24I stands for “isoagglutinogen”, which is another word for antigen. ABO blood groups in humans are determined by three alleles, IA, IB, and IO (also referred to as i)Both the IA and IB alleles are dominant to the IO alleleThe IA and IB alleles are codominant to each otherI stands for “isoagglutinogen”, which is another word for antigen.
25How many blood types are possible? Because each individual carries two alleles, there are six possible genotypes and four possible blood typesIA IA or IAIO- type AIB IB or IBIO- type BIA IB - type ABIOIO - type O
33Lethal allelesIn some cases, the lethal allele is DOMINANT, so even heterozygotes will die.Why does this allele persist in the population?Late acting (Huntington’s)Individuals reproduce before allele takes affect
34Polygenic TraitsMany traits with a distinct phenotype are affected by more than one geneThe cellular function of numerous gene products contributes to the development of a common phenotypeEx - skin color in humans is controlled by at least 3 different genes
35AABBCC (dark) and aabbcc (light) Imagine - each gene has 2 alleles, (light/dark), demonstrate incomplete dominanceAABBCC (dark) and aabbcc (light)Cross between 2 AaBbCc (intermediate) produces wide range of shades
37EpistasisEpistasis - a gene at one locus alters the phenotypic expression of a gene at a second locusOne gene can mask the effect of the other geneTwo gene pairs can complement each other, such that one dominant allele is required at each locus to express a certain phenotype
38Epistasis exampleMice (and many other mammals) - coat color depends on two genesOne (epistatic gene), determines whether pigment will be deposited in hairPresence (C) is dominant to absence (c)Second determines whether pigment deposited is black (B) or brown (b)The black allele is dominant to the brown alleleIndividual with cc has a white (albino) coat regardless of the genotype of the 2nd gene
41Epistasis – more complex In cats (and other mammals), a pattern of hair termed “agouti” is an example of epistasisSome cats have hairs in which there is more than one color distributed along the hair shaft (banded – agouti)Agouti fur color is typical of many wild animals such as mice squirrels and rabbits – good for camouflage!
42Agouti is determined by the dominant agouti allele, A Hairs on non-agouti cats are unbanded, producing a solidly colored coatSuch a cat is homozygous for the non-agouti allele (aa) at the agouti locusAgain – regardless of color. SO … you could have an agouti brown, agouti black, etc.
44Epistasis RatiosWhen studying a single characteristic, a ratio expressed in 16 parts (e.g., 3:6:3:4) suggests that epistasis is occurring.
45PleiotropyPleiotropy occurs when expression of a single gene has multiple phenotypic effects, and it is quite commonFor example, the wide-ranging symptoms of sickle-cell disease are due to a single gene
54Environmental Mutations Conditional or temperature-sensitive mutations - mutations affected by temperatureuseful in studying mutations that affect essential processesNutritional mutations – mutations affected by dietmay prevent phenotype from reflecting genotype. Ex -mutations in a biosynthetic pathway
55X-linked or Sex linked traits Genes are located on the X chromosomePresent a unique pattern of inheritance due to the presence of only one X chromosome in malesFemales (XX) can be heterozygous (carriers), while their sons (XY) can express the disease/trait.
56Drosophila (fruit fly) eye color was one of the first examples of X-linkage described Drosophila was a favorite model organism for Thomas Hunt MorganMorgan studied eye color in fruit flies – the trait did not have normal Mendelian ratios
57Crosses between F1 produced classic 3:1 ratio Crosses of white-eyed male with a red-eyed female - all F1 offspring had red eyesThe red allele appeared dominantCrosses between F1 produced classic 3:1 ratioSurprisingly, the white-eyed trait appeared only in malesAll the females and half the males had red eyesMorgan concluded that a fly’s eye color was linked to its sex