1 / 78

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.

maree
Télécharger la présentation

Mendelian Genetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related