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OBJECTIVE 6 NOTES: POLYGENIC INHERITANCE

OBJECTIVE 6 NOTES: POLYGENIC INHERITANCE. Objective 6 - Describe a simple model for polygenic inheritance and explain why most polygenic characters are described in quantitative terms. Polygenic traits – more than two alleles and multiple genes control the expression of a trait.

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OBJECTIVE 6 NOTES: POLYGENIC INHERITANCE

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  1. OBJECTIVE 6 NOTES: POLYGENIC INHERITANCE

  2. Objective 6 - Describe a simple model for polygenic inheritance and explain why most polygenic characters are described in quantitative terms. Polygenic traits – more than two alleles and multiple genes control the expression of a trait

  3. TWO TYPES OF TRAITS Discrete or discontinuous traits: traits occur in distinct categories: Trait is there or it is not (examples: cystic fibrosis, Huntington’s disease) Mendelian inheritance, single genes, complete dominance) Continuous traits: Distribution of phenotypes in the population varies along a continuum. Individuals differ by small degrees. (examples include height, blood pressure, reaction time, learning ability) Polygenic quantitative or multifactorial inheritance. Genes act additively.

  4. Examples of Discontinous Polygenic Traits   Cardiac defects (most types)  Cleft lip (alone)  Cleft lip, with or without cleft palate  Diaphragmatic hernia  Hip dislocation  Limb reduction defect (most forms)  Neural tube defects (anencephaly, spina bifida)

  5. Height is a continuous trait Crow, 1997 Genetics 147:1

  6. INHERITANCE OF CONTINUOUS TRAITS A simple case with two loci, two alleles each A A1 B B1 An allele with a prime 1, adds one unit of measure to the phenotype

  7. If a trait, say height, is controlled by two loci, A and B, and each locus has two alleles, one regular and one prime allele, what are the possible genotypes and phenotypes? AABB 0----------short AA1BB 1----------semi-short AABB1 1----------semi-short AA1BB1 2----------average A1A1BB 2----------average AAB1B1 2----------average A1A1B1B 3----------semi-tall A1AB1B1 3----------semi-tall A1A1B1B1 4----------tall

  8. semi- short semi- tall average short tall AABB AA1BB AA1BB1 A1A1B1B A1A1B1B1 AABB1 A1A1BB A1AB1B1 AAB1B1 0 1 2 3 4

  9. Two loci, Two alleles Three loci, Two alleles

  10. Characteristics of polygenic inheritance: A substitution at one locus usually produces the same effect on the phenotype as a substitution at another. Many loci with small, additive effects. Tall parents can produce a short child, tall child or in between, with limits. 4. Average parents can produce a tall, average or short child

  11. Why are the traits continuous? Because of the environment. In height, for example, nutritional differences can play a major role in variation For skin color, exposure to sun can modify the phenotype How do we know how much of the variability we see among people is due to genetic differences between them as opposed to environmental differences?

  12. Fig 14.12 Polygenic inheritance of skin tone 3 loci: each has two possible alleles: A,a B,b C,c, each capital allele adds one unit of darkness each lower case allele adds nothing Parents with intermediate tone AABBCC aabbcc Offspring can have tone darker or lighter than either parent. Can two light toned people have a darker child?

  13. Fig 14.12 0 1 2 3 4 5 6 Number of ‘darker’ alleles

  14. Another example of a polygenic trait: • Hair Color • Hair color is controlled by alleles on chromosomes 3, 6, 10, and 18. • The more dominant alleles that appear in the genotype, the darker the hair!

  15. Hypothetical mechanism for determination of eye color in Humans Gene for melanin production B (produce) dominant to b (none) 2 Modifier loci affect amount of pigment deposited CC’ and DD’ each non prime allele contributes one unit of deposition G’type at B Modifier loci Phenotype B_ CCDD Dk brown (+4) B_ CCDD’ Med. Brown (+3) B_ CC’DD’ Lt Brown (+2) B_ CC’D’D’ Hazel (+1) BB C’C’D’D’ Green Bb C’C’D’D’ Greenish blue bb any g’type Blue

  16. TO CONFUSE EYE COLOR FURTHER: University of Queensland geneticist Rick Strum suggests that the genetics are not so clear. “There is no single gene for eye color,” he says, “but the biggest effect is the OCA2 gene.” (THE ONE CALLED B IN THE PREVIOUS SLIDE) This gene Accounts for about 74 percent of the total variation in people’s eye color. Sturm found that how OCA2 is expressed—and how much pigment a person has—is strongly linked to three single nucleotide polymorphisms (SNPs), or single letter variations, in a DNA sequence near the OCA2 gene.

  17. For example, among individuals carrying the SNP sequence “TGT” at all three locations on both copies of the gene, 62 percent were blue-eyed,” By contrast, only 21 percent of individuals carrying only one TGT copy at each location and 7.5 percent of those lacking the TGT entirely had blue eyes. Depending on the particular combination of SNPs inherited, a person can have a range of OCA2 activity that lands them on the spectrum between blue and brown eyes. What about green eyes? “Green eyes probably represent the interaction of multiple variants within the OCA2 and in other genes, including perhaps the red-hair gene,” Duffy says.

  18. Hypothetical mechanism for determination of eye color in Humans A possible cross: P bbCCDD X BbC’C’D’D” (Blue) (Gr-blue) Gametes bCD BC’D’, bC’D’ F1 BbCC’DD” bbCC’DD’ (Lt. Brown) (Blue)

  19. James Bond Height Chart

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