Presentation on theme: "Beyond Mendel: Exceptions/Additions to Mendel’s Laws"— Presentation transcript:
1Beyond Mendel: Exceptions/Additions to Mendel’s Laws AP Biology
2Exceptions to Mendelian Genetics Mendel chose traits in peas that showed 2 distinct forms.Not all genes exhibit such simple inheritance.Alleles interactGenes interactSegregation of genes on same chromosomeMitochondrial DNA
3Multiple Alleles A population can have more than 2 alleles for a gene. Ex. Labrador Retriever coat colorDetermined by 1 gene with 4 alleles.Even if more than 2 alleles exist in a population, any given individual can only have 2 of them(1 from mother, 1 from father)
4Multiple Alleles Black is dominant to chocolate B: blackb: chocolateYellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles)Yellow: E or e* must be ee to produce a yellow lab
6Problem #1How do the multiple alleles act together to determine the coat color of a mouse?There are 3 varieties of coat color: black, brown, and whiteThey are controlled by 4 different alleles (similar to the Labradors)BbCc x BbCcB=black, bb = browncc is epistatic (white)
8Problem #2Determine the number of chocolate labs produced from a black female and a yellow maleBbEe x bbee
9Codominance Both alleles can be expressed For example, red cows crossed with white will generate roan cows.Roan cows have red coats with white blotches.
10Incomplete DominanceIn some cases, an intermediate phenotype is shown.Neither allele is dominant.Ex. Snapdragons – flower color3 phenotypes: red, white, pinkHeterozygous condition results in pink flowers (the intermediate trait).
11Incomplete Dominance - Snapdragons A white (rr) snapdragon crossed with a red (RR) snapdragon produces all pink (Rr) offspring.Two pinks crossed together (Rr x Rr) produce 1/4 white, 2/4 pink, 1/4 red
13Sex-Linked Genes Genes that are located on the X chromosome. Females receive 2 alleles; males receive one.Ex. Color blindness, hemophiliaWomen can be carriers when they carry one gene for the disorder and one normal gene.Carriers can have sons with the disorder.
15Problem #3Determine the probability of a woman with hemophilia having children with hemophilia assuming she marries a normal man.
16PleiotropySome single alleles have more than one distinguishable phenotypic effect.This is called pleiotropy.Ex. Coloration pattern and crossed eyes of Siamese catsBoth caused by the same allele.Unrelated characters caused by the same protein produced by the same allele.
17Siamese CatSiamese cats have a gene that codes for darker pigments - this gene is more active at low temperatures.Parts of the body that are colder will develop the darker pigmentation - ears, feet tail of the siamese cats
18PleiotropyAnother example is the gene that causes pigment color in rats.White rats also have very sensitive eyes and often become blind.
19Pleiotropy Another example is Marfan Syndrome. Marfan Syndrome is a disease of the connective tissue.Symptoms: tall & thin, long extremities, deficiencies in eyes and skeletal system, enlarged heart
20Polygenic TraitsIndividual heritable characters are often controlled by groups of several genes.These genes are called polygenes.Each allele intensifies or diminishes the phenotype.Variation is continuous or quantitative (adding up)Also called quantitative inheritance
21Polygenic Traits Examples: Human hair, eye, and skin color Height WeightIntelligence
22Lethal Genes Some genes are lethal when both alleles are present. Lethality (death) can occur before or after birth.
23Lethal GenesEx. The “creeper” allele in chickens, which causes the legs to be short and stunted.Creeper is a dominant gene.Heterozygous chickens display the creeper phenotype.If 2 creeper chickens are crossed, one would expect to have ¾ creeper and ¼ normalInstead, the ratio is 2/3 creeper and 1/3 normal.
25Lethal GenesMexican hairless dogs result from a mutation in a gene that shows lethality.hh hairy normal traitHh hairless one mutation presentHH lethal two mutations = lethal
26Manx Cats Cats possess a gene for producing a tail. The tailless Manx phenotype in cats is produced by an allele that is lethal in its homozygous state.The allele interferes with normal spinal development, in heterozygous cats this results in lack of a tail.
27Blood Types – Multiple Alleles and Codominance 4 blood types (humans)A, B, AB, and OBlood type is controlled by 3 allelesA, B, OO is recessive (must have two O alleles to have blood type O)A and B are codominant (if inherit an A and B, blood type is AB)Crosses involving blood type often use an I to represent the alleles
28Blood TypesThe blood type determines what antibodies are located within the blood.Type A blood has type B antibodiesIf Type B blood is put into their body, their immune system reacts and antibodies clump the blood – can cause deathType AB blood has no antibodies, any blood can be donated to them; they are “universal acceptors”Type O blood has no antigens, antibodies in the blood do not react to type O blood, they are “universal donors”