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Observable Patterns of Inheritance

Observable Patterns of Inheritance. Chapter 20. Earlobe Variations. If you have attached earlobes, you inherited two copies of the recessive allele If you have detached earlobes, you may have either one or two copies of the dominant allele. Gregor Mendel.

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Observable Patterns of Inheritance

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  1. Observable Patterns of Inheritance Chapter 20

  2. Earlobe Variations • If you have attached earlobes, you inherited two copies of the recessive allele • If you have detached earlobes, you may have either one or two copies of the dominant allele

  3. Gregor Mendel • Strong background in plant breeding and mathematics • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

  4. Alleles • Different molecular forms of a gene • Arise by mutation • Dominant allele masks a recessive allele that is paired with it

  5. Allele Combinations • Homozygous • having two identical alleles at a locus • AA or aa • Heterozygous • having two different alleles at a locus • Aa

  6. Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes

  7. Genotype & Phenotype • Genotype refers to particular genes an individual carries • Phenotype refers to an individual’s observable traits • Cannot always determine genotype by observing phenotype

  8. Tracking Generations • Parental generation P mates to produce • First-generation offspring F1 mate to produce • Second-generation offspring F2

  9. Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other

  10. Segregation Parents: CC cc (meiosis) (meiosis) C C c c Gametes: AA parent produces only A gametes; aa parent produces only a gametes

  11. Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

  12. Female gametes C c C CC Cc Male gametes c Cc cc Punnett Square of a Monohybrid Cross Dominant phenotype can arise three ways, recessive only one

  13. Test Cross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring enables you to determine the genotype of the dominant individual

  14. c c c c C C Cc Cc Cc Cc C c cc Cc cc Cc Punnett Squares of Test Crosses Two phenotypes All dominant phenotype

  15. Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

  16. Independent Assortment Metaphase I: OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

  17. Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits

  18. Dihybrid Cross - F1 Results ccdd (smooth chin, no dimples) CCDD (chin fissure, dimples) Parents: Gametes: cd cd CD CD F1 offspring: CcDd

  19. Allele Combinations in F2 CcDd 1/4 CD 1/4 Cd 1/4 cD 1/4 cd 1/4 CD 1/16 CCDD 1/16 CCDd 1/16 CcDD 1/16 CcDd 1/4 Cd 1/16 CCDd 1/16 CCdd 1/16 CcDd 1/16 Ccdd CcDd 1/4 cD 1/16 CcDD 1/16 CcDd 1/16 ccDD 1/16 ccDd 1/4 cd 1/16 CcDd 1/16 Ccdd 1/16 ccDd 1/16 ccdd

  20. Pleiotropy • Alleles at a single locus may have effects on two or more traits • Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

  21. Genetics of Sickle-Cell Anemia • Two alleles 1) HbA Encodes normal beta-hemoglobin chain 2) HbS Mutant allele encodes defective chain • HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

  22. Sickle-Cell Anemia • At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together • This impedes oxygen delivery and blood flow • Over time, it causes damage throughout the body

  23. Campodactyly: Unexpected Phenotypes • Effect of allele varies: • Bent fingers on both hands • Bent fingers on one hand • No effect • Many factors affect gene expression

  24. Polygenic Traits • Result from the combined expression of several genes • Skin color, eye color • Population may show continuous variation

  25. Continuous Variation • A more or less continuous range of small differences in a given trait among individuals • The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait

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