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Genetics

Genetics. Gregor Mendel. Father of modern genetics Studied pea plants variety of distinct heritable features, or characters character variations are called traits Mating of plants can be controlled used varieties that were “true-breeding”

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Genetics

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  1. Genetics

  2. Gregor Mendel • Father of modern genetics • Studied pea plants • variety of distinct heritable features, or characters • character variations are called traits • Mating of plants can be controlled • used varieties that were “true-breeding” • plants that produce offspring of the same variety when they self-pollinate • When cross pollination occurs offspring showed a mix of characteristics of both parents

  3. Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Stamens Carpel LE 14-2 Pollinated carpel matured into pod Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F1)

  4. Terms • Trait • specific characteristic that changes from one individual to another • Gene • chemical factors that determine traits • Found in DNA • Genes for various traits are found on chromosomes • Allele • different forms of a gene • Example • Trait = plant height • Alleles (different forms of gene) • Short • Tall

  5. Dominance • Principle of Dominance • some alleles are dominant and others are recessive • Dominant alleles • are generally expressed using a capital letter • Recessive alleles • are usually expressed using a lower case letter • Ex. Plant height • Dominant = tall (T) • Recessive = short (t) • Each gene is coded for using 2 alleles • One allele from each parent • Dominant alleles control the appearance of the trait • If two recessive alleles are present the recessive condition will be expressed

  6. Alleles • Homozygous • individual with 2 of the same allele for a given trait • Ex. TT, RR = homozygous dominant • Ex. tt,, rr = homozygous recessive • Can “true breed” = create offspring like self in self polination • Heterozygous • individual with 2 different alleles for a given trait • Ex. Tt, Rr, Ww • Are not “true breeders”

  7. Genotype • The combination of alleles for a given trait • Ex. TT, Tt, tt • Phenotype • The physical appearance caused by the interaction of alleles • In general, if a dominant allele is present then the dominant condition will be shown • Ex. Tall, Short

  8. Parent • First generation studied • F1 (first filial) • Offspring of 2 individuals from the parent generation • F2 (second filial) • Offspring of 2 individuals from the first filial generation

  9. The Law of Segregation • Each individual contains 2 alleles for each gene (one from each parent) • During meiosis, when the number of chromosomes are reduced (diploid to haploid), gametes are produced that contain one allele for each trait • Segregation = the separation of alleles for a certain trait • Each gamete only carries a single allele for each gene • During fertilization, two gametes fuse forming a diploid cell. • This cell contains 2 alleles for each gene

  10. P Generation (true-breeding parents) Purple flowers White flowers F1 Generation LE 14-3 (hybrids) All plants had purple flowers F2 Generation

  11. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene LE 14-4 Allele for white flowers

  12. P Generation Purple flowers PP White flowers pp Appearance: Genetic makeup: p P Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp LE 14-5_2 Gametes: 1 1 p P 2 2 F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

  13. Genotype Phenotype PP (homozygous Purple 1 Pp (heterozygous 3 Purple LE 14-6 2 Pp (heterozygous Purple pp (homozygous White 1 1 Ratio 1:2:1 Ratio 3:1

  14. Probability • The likelihood that a particular event will occur. • Ex. If you flip a coin you have a probability of ½ that you will end up with heads. • What is the probability of flipping 3 heads in a row? • ½ x ½ x ½ = 1/8 • The principles of probability can be used to predict outcomes of genetic crosses. • Punnet Squares

  15. Tt X Tt Cross Section 11-2 Go to Section:

  16. Tt X Tt Cross Section 11-2 Go to Section:

  17. The Testcross • breeding themystery individual with a homozygous recessive individual

  18. Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If Pp, then 1 2 offspring purple and 1 2 offspring white: If PP, then all offspring purple: LE 14-7 p p p p P P Pp Pp Pp Pp P P pp pp Pp Pp

  19. The Law of Independent Assortment • Genes for different traits can segregate independently during the formation of gametes. • Genes for different traits are not necessarily linked.

  20. P Generation YYRR yyrr Gametes yr YR YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment Sperm YR Yr yR yr 1 1 1 1 4 4 4 4 Sperm LE 14-8 Eggs YR yr 1 1 2 2 YR 1 4 Eggs YYRR YYRr YyRR YyRr YR 1 2 F2 Generation (predicted offspring) YYRR YyRr Yr 1 4 YYRr YYrr YyRr Yyrr yr 1 2 YyRr yyrr yR 1 4 YyRR YyRr yyRR yyRr 3 1 4 4 yr 1 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 3 3 3 16 16 16 16 Phenotypic ratio 9:3:3:1

  21. The Spectrum of Dominance • Complete dominance • phenotypes of the heterozygote and dominant homozygote are identical • Codominance • two dominant alleles affect the phenotype in separate, distinguishable ways • Incomplete Dominance • phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

  22. P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation LE 14-10 1 1 Gametes CR CW 2 2 Sperm 1 1 CR CW 2 2 Eggs F2 Generation 1 CR 2 CRCR CRCW 1 CW 2 CRCW CWCW

  23. Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • Polydactyl individuals • 1/400 in the U.S.

  24. Multiple Alleles • more than two allelic forms • Example – blood type • four phenotypes: A, B, AB, O • Three alleles: IA, IB, and i.

  25. Pleiotropy • multiple phenotypic effects • Ex. multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

  26. Polygenic Traits • Traits controlled by 2 or more genes • Ex. Eye color, hair color, skin color

  27. AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20/64 Skin color 15/64 Fraction of progeny 6/64 1/64

  28. Epistasis • a gene at one locus alters the phenotypic expression of a gene at a second locus • Ex. Coat color of mice • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for pigment color and c for no pigment color ) determines whether the pigment will be deposited in the hair

  29. BbCc BbCc Sperm bC Bc 1 1 1 1 BC bc 4 4 4 4 1 BC BBCC BbCC BBCc BbCc 4 LE 14-11 1 bC BbCC bbCC BbCc bbCc 4 BBcc Bbcc BBCc BbCc 1 Bc 4 bbcc Bbcc 1 bbCc bc BbCc 4 9 3 4 16 16 16

  30. Nature and Nurture: The Environmental Impact on Phenotype • phenotype for a character depends on environment as well as genotype • Ex. hydrangea flowers • same genotype range from blue-violet to pink • depends on soil acidity

  31. Human Inheritance • Humans are not good subjects for genetic research • generation time is too long • parents produce relatively few offspring • breeding experiments are unacceptable Personal Pedigree

  32. Pedigree Analysis • Pedigree • family tree that describes the interrelationships of parents and children across generations • Can be used to make predictions about future offspring

  33. First generation (grandparents) Ww ww ww Ww Second generation (parents plus aunts and uncles) Ww ww ww Ww Ww ww Third generation (two sisters) LE 14-14a WW ww or Ww Widow’s peak No widow’s peak Dominant trait (widow’s peak)

  34. First generation (grandparents) Ff Ff ff Ff Second generation (parents plus aunts and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) LE 14-14b ff FF or Ff Free earlobe Attached earlobe Recessive trait (attached earlobe)

  35. Down’s Syndrome • Occurs in 1/800-1,000 births • Caused by nondisjunction • Trisomy 21 = three copies of chromosome 21

  36. Frequency of Down SyndromePer Maternal Age Age (years) Frequency of Fetuses with DownSyndrome to Normal Fetusesat 16 weeks of pregnancy Frequency of Live Births ofBabies with Down Syndrometo Normal Births 15 - 19 ---- 1 / 1250 20 - 24 ---- 1 / 1400 25 - 29 ---- 1 / 1100 30 - 31 ---- 1 / 900 32 ---- 1 / 750 33 1 / 420 1 / 625 34 1 / 325 1 / 500 35 1 / 250 1 / 350 36 1 / 200 1 / 275 37 1 / 150 1 / 225 38 1 / 120 1 / 175 39 1 / 100 1 / 140 40 1 / 75 1 / 100 41 1 / 60 1 / 85 42 1 / 45 1 / 65 43 1 / 35 1 / 50 44 1 / 30 1 / 40 45 and older 1 / 20 1 / 25 Return to Prenatal Testing for Down SyndromeReturn to Down Syndrome: Health Issues Homepage

  37. Down’s Syndrome • The image shows a karyotype of a person with Down’s Syndrome, Trisomy 21

  38. Sex Chromosome Disorders • Turner’s Syndrome (XO) • underdeveloped ovaries, short stature, webbed neck, and broad chest. Individuals are sterile, and lack expected secondary sexual characteristics. Mental retardation typically not evident.

  39. Sex Chromosome Disorders • Klinefelter’s Syndrome (XXY) • Nondisjunction in males • Some development of breast tissue, little body hair is present; typically tall, with or without evidence of mental retardation. Males with XXXY, XXXXY, and XXXXXY karyotypes have a more severe presentation, and mental retardation is expected.

  40. Recessively Inherited Disorders • Only expressed in individuals that are homozygous recessive • Carriers • heterozygous individuals • carry the recessive allele • phenotypicallynormal

  41. Albinism

  42. Cystic Fibrosis • most common lethal genetic disease in the • 1/2,500 people of European descent • results in defective or absent chloride transport channels in plasma membranes • Symptoms: • mucus buildup in some internal organs • abnormal absorption of nutrients in the small intestine

  43. Sickle-Cell Disease • 1/400 African-Americans • Incompletely recessive • caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • Symptoms: • physical weakness • Pain • organ damage • even paralysis

  44. Sickle Cell Anemia

  45. Dominantly Inherited Disorders • Achondroplasia • form of dwarfism • lethal when homozygous for the dominant allele

  46. Achondroplasia

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