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Impacts, Issues: The Color of Skin

Impacts, Issues: The Color of Skin. Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins. Mendel’s Experimental Approach. Mendel was a monk with training in plant breeding and mathematics

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Impacts, Issues: The Color of Skin

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  1. Impacts, Issues:The Color of Skin • Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins

  2. Mendel’s Experimental Approach • Mendel was a monk with training in plant breeding and mathematics • He studied the garden pea (Pisum sativum), which breeds true for a number of traits

  3. Terms Used in Modern Genetics • Genes • Heritable units of info about traits • Parents transmit genes to offspring • Each gene has a specific locus on a chromosome • Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes

  4. Terms Used in Modern Genetics • A mutation is a permanent change in a gene • Allelesare different molecular forms of a gene • A hybrid has nonidentical alleles for a trait • Offspring of a cross between two individuals that breed true for different forms of a trait are hybrids

  5. Terms Used in Modern Genetics • Nonidenticalalleles of a gene is heterozygous • Identical alleles of a gene is homozygous

  6. Terms Used in Modern Genetics • An allele is dominant if its effect masks the effect of a recessive allele paired with it • Capital letters (A) signify dominant alleles; lowercase letters (a) signify recessive alleles • Homozygous dominant (AA) • Homozygous recessive (aa) • Heterozygous (Aa)

  7. Terms Used in Modern Genetics • Gene expression • Process by which info in a gene is converted to a structural or functional part • Expressed genes determine traits

  8. Terms Used in Modern Genetics • Genotype • The particular alleles an individual carries • Phenotype • An individual’s observable traits

  9. Terms Used in Modern Genetics • P stands for parents, F for filial (offspring) • F1: First generation offspring of parents • F2: Second generation offspring of parents

  10. 11.1 Key ConceptsWhere Modern Genetics Started • Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance • His meticulous work gave him clues that heritable traits are specified in units • The units, which are distributed into gametes in predictable patterns, were later identified as genes

  11. 11.2 Mendel’s Law of Segregation • Garden pea plants inherit two “units” of information for a trait, one from each parent

  12. homozygous dominant parent homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 11-5, p. 172

  13. Calculating Probabilities • Probability • A measure of the chance that a particular outcome will occur • Punnett square • A grid used to calculate the probability of genotypes and phenotypes in offspring

  14. Construction of a Punnett Square

  15. Phenotype Ratios in a Monohybrid Experiment

  16. Phenotype Ratios in a Monohybrid Experiment

  17. F1 offspring aa True-breeding homozygous recessive parent plant a a Aa Aa A Aa Aa AA A Aa Aa True-breeding homozygous dominant parent plant Aa Aa B A cross between two plants that breed true for different forms of a trait produces F1 offspring that are identically heterozygous. Fig. 11-7b, p. 173

  18. Mendel’s Law of Segregation • Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses • Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa) • This is the basis of Mendel’s law of segregation • Diploid cells have pairs of genes on pairs of homologous chromosomes • The two genes of each pair separate during meiosis, and end up in different gametes

  19. 11.3 Mendel’s Law of Independent Assortment • Mendel’s law of independent assortment • Many genes are sorted into gametes independently of other genes

  20. One of two possible alignments The only other possible alignment a Chromosome alignments at metaphase I: A A a a A A a a b B b B b b B B b The resulting alignments at metaphase II: a a A A A A a a b b b b B B B B a c Possible combinations of alleles in gametes: B A A B b a a b b A A b B a B AB ab Ab aB Fig. 11-8, p. 174

  21. Dihybrid Cross: In humans, there is a gene that controls formation (or lack thereof) of muscles in the tongue that allow people with those muscles to roll their tongues, while people who lack those muscles cannot roll their tongues. The ability to roll one’s tongue is dominant over non-rolling. The ability to taste certain substances is also genetically controlled. Let’s let R represent tongue-rolling, r represent a non-roller, T represent ability to taste PTC, and t represent non-tasting.

  22. Suppose a woman who is both a homozygous tongue-roller and a non-PTC-taster marries a man who is a heterozygous tongue-roller and is a PTC taster, and they have children Draw the Punnett square that predicts what their children will be. If the man is both RrandTt (How do we know that?), he would be RrTt and so could produce gametes with either R or r and either T or t (one allele for each gene). There are two choices for the first trait (R or r). No matter which of those go into a given sperm, there are still two choices for the second trait (T or t).

  23. Mendel’s Law of Independent Assortment • Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits • Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis

  24. Codominance in ABO Blood Types • Codominance • Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive • May occur in multiple allele systems • Multiple allele systems • Genes with three or more alleles in a population • Example: ABO blood types

  25. BB AA or or AB BO OO Genotypes: AO Phenotypes (Blood type): B AB O A Fig. 11-10, p. 176

  26. Incomplete Dominance • Incomplete dominance • One allele is not fully dominant over its partner • The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring • Example: Snapdragon color • RR is red • Rr is pink • rr is white

  27. homozygous parent (rr) homozygous parent (RR) heterozygous F1 offspring (Rr) x A Cross a red-flowered with a white-flowered plant, and all of the F1 offspring will be pink. Fig. 11-11a, p. 176

  28. R r R B Cross two F1 plants, and the three phenotypes of the F2 offspring will occur in a 1:2 :1 ratio: RR Rr r Rr rr Fig. 11-11b, p. 176

  29. Epistasis • Epistasis • Two or more gene products influence a trait • Typically, one gene product suppresses the effect of another • Example: Coat color in dogs • Alleles B and b designate colors (black or brown) • Two recessive alleles ee suppress color

  30. Epistasis in Coat Colors

  31. Pleiotropy • Pleiotropy • One gene product influences two or more traits • Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder

  32. The Distance Between Genes • The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes

  33. 11.6 Genes and the Environment • Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure • The result may be variation in traits

  34. Effects of Temperature on Gene Expression • Enzyme tyrosinase, works at low temperatures

  35. 11.7 Complex Variations in Traits • Individuals vary in some of their shared traits • Many traits (such as eye color) show a continuous range of variation

  36. Continuous Variation • Continuous variation • Traits with a range of small differences • The more factors that influence a trait, the more continuous the distribution of phenotype • Bell curve • When continuous phenotypes are divided into measurable categories and plotted as a bar chart, they form a bell-shaped curve

  37. Continuous Variation and the Bell Curve

  38. Regarding the Unexpected Phenotype • Phenotype results from complex interactions among gene products and the environment • Enzymes and other gene products control steps of most metabolic pathways • Mutations, interactions among genes, and environmental conditions may affect one or more steps

  39. 11.4-11.7 Key ConceptsVariations on Mendel’s Theme • Not all traits appear in Mendelian inheritance patterns • An allele may be partly dominant over a nonidentical partner, or codominant with it • Multiple genes may influence a trait; some genes influence many traits • The environments also influences gene expression

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