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Mendelian Genetics

Mendelian Genetics. Using a six-sided die, what is the probability of rolling either a 5 or a 6?   A) 1/6 x 1/6 = 1/36   B) 1/6 + 1/6 = 1/3   C) 1/6 + 1/6 = 2/3   D) 1/6 + 1/6 = 1/12   E) 1/6  . People knew that traits were inherited.

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Mendelian Genetics

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  1. Mendelian Genetics

  2. Using a six-sided die, what is the probability of rolling either a 5 or a 6?   A) 1/6 x 1/6 = 1/36   B) 1/6 + 1/6 = 1/3   C) 1/6 + 1/6 = 2/3   D) 1/6 + 1/6 = 1/12   E) 1/6  

  3. People knew that traits were inherited • Selective breeding is based on empirical knowledge that offspring resemble their parents • “Like begets like” • Mechansim unknown • Possible blending of traits, possible environmental factors

  4. Mendel studied the mechanism of inheritance • Particulate theory of inheritance • Mendel got low scores in biology

  5. Mendel’s model organism- the pea plant • Monoecious plant- both male + female reproductive parts on same flower • Pistil- contains unfertilized seeds • Stamen- hold pollen

  6. Mendel studied characters for which 2 traits existed • Purple vs. White flowers • Axial vs. Apical flowers • Yellow vs. green peas • Round vs. wrinkled seeds • Etc.

  7. Mendel crossed true-breeding pea plant strains • True-breeding plants were generated from plants by repeatedly inbreeding like traits • When purple flowers only begot purple flowers, and white only begot white, they were said to be true-breeding

  8. F1 generation showed dominance of one of the traits over the other • But when the F1 plants were allowed to self-pollinate, the lost trait returned • In the F2 generation, 25% of the plants had the recessive trait (3:1 ratio)

  9. Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • These concepts can be related to what we now know about genes and chromosomes

  10. There are different versions of a gene determining a trait (alleles) • Alleles are inherited in pairs- each parent donates one allele of a gene • When individuals inherit different alleles from each parent, some will be manifest (dominant) and others will hide (recessive) • Law of segregation- gamete formation involves placing a single allele for each trait into a gamete

  11. What Mendel did not know • The genes are on DNA • The DNA is on chromosomes • Alleles exist at specific loci on chromosomes • Chromosomes are segregated into gametes

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

  13. Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character • An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character • Unlike homozygotes, heterozygotes are not true-breeding • Physical apperance is called the phenotype • Genes which influence that appearance is called the genotype

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

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

  16. Gamete formation and fertilization are influenced by chance • For heterozygotes, half of gametes will have a given allele • 50% chance that a fertilizing gamete will contain a particular gene

  17. Making predictions about offspring with a Punnett square • If genotypes of parents are known, predictions can be made about offspring • The Punnett square is used to calculate odds • Notation: Capitals for dominant alleles, lower-case for recessive

  18. Like algebra, doing Punnett squares correctly depends on setting up the problem correctly • Setting up the problem correctly depends on generating gametes correctly • Recall, gametes are generated by meiosis

  19. Generating gametes are generated only for the genes being considered • Living things have thousands of genes • Punnett squares only look at one or two (maybe three) • Aa • AA • A, a • ___?

  20. Aa • AA • AaBB • A, a • A • ____?

  21. Aa • AA • AaBB • AaBb • A, a • A • AB, aB • _____?

  22. Aa • AA • AaBB • AaBb • A, a • A • AB, aB • AB, Ab, aB, ab

  23. Aa • AA • AaBB • AaBb • A, a • A • AB, aB • AB, Ab, aB, ab • ≠ AA, BB, aa, bb • Why are these gametes not formed?

  24. Gamete formation and fertilization are influenced by chance • For heterozygotes, half of gametes will have a given allele • 50% chance that a fertilizing gamete will contain a particular gene

  25. The Test Cross • For determining Genotype of a dominant phenotype • Crossed with a recessive, a homozygote yields all domiant phenotype, with a heterozygote, a 1:1 ratio

  26. The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding themystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

  27. Mendel studied two traits at a time • Punnett squares can be used in crosses of two genes • Mendel found that the two genes were inherited independently of each other • The law of independent assortment

  28. P Generation YYRR yyrr Gametes yr YR YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment LE 14-8 Sperm YR Yr yR yr 1 1 1 1 4 4 4 4 Sperm 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

  29. Meiosis accounts for Independent Assortment of genes • The probablility of one chromosome being passed to a gamete is 50% • Different genes are on different chromosomes

  30. Mendel’s data was extremely good • Improbably good, in fact • R.A. Fisher analyzed the odds of his data- extremely remote! • Did Mendel fudge his data?

  31. Mendel’s data was extremely good • Improbably good, in fact • R.A. Fisher analyzed the odds of his data- extremely remote! • Did Mendel fudge his data? • Did Mendel suffer from confirmation bias?

  32. Other patterns of inheritance • Incomplete dominance • Codominance • Multiple alleles • Pleiotropy • Polygenic inheritance • Epistasis

  33. Incomplete dominance • Neither gene is dominant of the other • Result is a blending of phenotypes • Dominance: Genotype ≠ Phenotype • Incomplete dominance: Genotype = phenotype

  34. Codominance • Both genes expressed simultaneously

  35. Codominance in rhododendrons • Red and white genes expressed simultaneously

  36. Many human diseases exhibit Mendelian (and non-Mendelian) inheritance

  37. Mendelian Ineritance in Man • Inheritance of certain alleles can be analyzed using pedigrees • Many genetic diseases exhibit Mendelian inheritance

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

  39. Sickle cell anemia • Round cells- normal • Crescent-shaped cells- sickle cell • Caused by a single mutation in the human gene for hemoglobin

  40. Cystic Fibrosis and Sickle Cell Anemia exhibit Mendelian Inheritance • Both are recessive disorders • Both exhibit multiple phenotypic traits for a single gene defect (pleiotropy)

  41. Pleiotropy

  42. Individuals heterozygous for sickle cell exhibit resistance to malaria • Incidence of sickle cell correlates to the prevalence of malria-carrying mosquitoes in the environment • What is the inheritance pattern of sickle cell- dominance?

  43. INHERITANCE OF HD Completely Genetic Autosomal Dominant 100% Lethal Why has Huntington’s not been totally eradicated from the population?

  44. Genetic testing raises many ethical questions • Alro Guthrie’s dad, Woody, had HD • What were the chances Arlo would have HD? • What if he could have been tested? • What about insurance? • Should he be allowed to have children?

  45. Polygenic inheritance • One trait is affected by many genes • Human height is an example

  46. Many traits have multiple alleles

  47. Sex Linkage • Genes on sex chromosomes are not necessarily involved in sex determination • Genes for color blindness and hemophilia are found on human X-chromosomes • Sex-linkage (or X-linkage) shows different patterns of inheritance • In males, Recessive traits cannot be masked by domiannt ones • Sex is not always determined by presence of a Y chromosome

  48. Parents Ova Sperm Zygotes (offspring) LE 15-9 The X-Y system The X-0 system The Z-W system The haplo-diploid system

  49. Epistasis • One gene nullifies the effects of another • Bombay O blood type is an example of recessive epistasis

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