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Genetics: Beyond Mendel
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Incomplete Dominance:
Neither allele is dominant, “blending” of both alleles to create a different phenotype. Example: If snapdragons have a red flower allele and a white allele, flowers are pink.
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Representing Incomplete Dominance
Since neither allele is dominant, alleles with incomplete dominance are represented as a superscript above a letter representing the gene. Example: Red color – CR White color - CW
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Codominance Both alleles are expressed in a heterozygote. Example: Roan cattle with mixed red and white hairs are heterozygotes for red and white hair color alleles.
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Representing Codominance
As with incomplete dominance, codominance is represented with a superscript. Example: red CR, white CW The difference between incomplete dominance and codominance is subtle, and they can be treated the same.
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Applying the Concepts Which produces the greatest number of roan (CRCW) offspring? Roan x Roan (CRCW x CRCW) OR Red x White (CR CR x CWCW) CR CW CR CW CRCR CRCW CRCW CWCW CR CR CW CRCw CRCW CRCW CRCW ALL ROAN
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For incomplete dominance, the phenotype ratio is the same as the genotype ratio. A heterozygote has a different phenotype than either homozygote in incomplete dominance, so there are three possible phenotypes.
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Multiple Alleles For some traits, more than two possible alleles exist. Example: 3 Blood Type Alleles A ( IA) B (IB) O (i) A and B are codominant O is recessive
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Blood type AB has both marker proteins (A and B) on red blood cell surface.
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Blood Type (Phenotype) Possible Genotype(s) A B AB O IAIA or IAi
IBIB or IBi IAIB only ii only
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Paternity Tests A mother has blood type A, and her child has blood type O. Can a man with blood type B be the father? Yes, the mother can be IAi and the father can be IBi. A mother has blood type B and her child has blood type O. Can a man with blood type AB be the father? No, the child must have received an i allele from each parent.
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Gender Determination In humans, the 23rd pair of chromosome (X/Y) are the sex chromosomes which determine the gender. Men are XY (X and Y are the only nonhomologous pair- which means that the pair does not contain the same type of gene.) Woman are XX The other 22 pairs of chromosomes are called autosomal chromosomes.
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FEMALE MALE
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Autosomal Chromosomes
Sex Chromosomes
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Sex Linked Traits Traits for which the gene is found on the X chromosome. Since men are XY, they have only one copy of sex-linked genes. Examples: Red-Green Colorblindness Hemophilia
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Colorblindness Test
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Sex-Linked Traits Video Click once on image to start video
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Applying the Concepts Colorblindness is caused by a recessive allele on the X chromosome (Xb). If a colorblind woman(XbXb) marries a man with normal vision (XBY), what is the probability of a daughter being colorblind? A son being colorblind? Xb Xb XbXb x XBY XBXb XB XBXb Daughter being CB: O% Son being CB: 100% Y XbY XbY
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Wording of Probability in Sex-Linked Trait Questions
If the question asks: “What is the probability of the parents having a daughter that has normal color vision”- it is not a given that the child is a daughter, so divide by all 4 possibilities = 2 out of 4 = 50% If the question asks: “What is the probability of a daughter having normal color vision”, it is a given that the child is a daughter. Only look at the 2 daughter possibilities = 2 out of 2 = 100%
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Inheritance of Sex-Linked Traits
A male can inherit colorblindness if his mother is a carrier of colorblindness. (Doesn’t matter father’s color vision, because males inherits Y from dad). A female must inherit the colorblindness allele from both parents. Mother must be at least a carrier. Father must be colorblind.
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Polygenic Traits Traits that are controlled by multiple genes.
Examples: Human Height Skin Color Traits exhibit a greater range of variation
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