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Modern Genetics

Modern Genetics. Part 5. History. Gregor Mendel : “Father of Genetics” Austrian Monk (1822-1884) Published the results of scientific research on Garden Peas ( Pisium sativum ) in 1865. Why study peas?

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Modern Genetics

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  1. Modern Genetics Part 5

  2. History • Gregor Mendel: “Father of Genetics” Austrian Monk (1822-1884) • Published the results of scientific research on Garden Peas (Pisium sativum) in 1865. Why study peas? • Great natural variation- stem length, seed color, pod shape ,pod color, small, edible, easy to grow, many offspring, easy to cross fertilize

  3. Useful Terms Trait: characteristic of an organism. Gene: piece of DNA that codes for a protein. Allele: different forms of a gene. Genotype: The allele combination of an individual. Phenotype: The visible characteristics that result from a genotype.

  4. Chromosomes and Human Genetics • Human Chromosomes • Types • Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits) • X and Y do not look alike but behave as a homologous pair at meiosis • XX = female, XY = male • Autosomes – non-sex chromosomes (22 pairs) – genes are unrelated to sex determination

  5. PUNNETT SQUARE Method for determining possible allele combination for the offspring Gametes outside Offspring Inside

  6. Determining Sex… MOM X X X X X X X DAD X Y X Y Y

  7. One Example of Mendel’s Work Tall x Dwarf Phenotype P DD dd Genotype Homozygous Dominant Homozygous Recessive All Tall Clearly Tall is Inherited… What happened to Dwarf? F1 Dd Tall is dominant to Dwarf Use D/d rather than T/t for symbolic logic Heterozygous F1 x F1 = F2 possible gametes Punnett Square: D d 3/4 Tall 1/4 Dwarf F2 D Tall DD Tall Dd possible gametes d Tall Dd Dwarf dd Dwarf is not missing…just masked as “recessive” in a diploid state… there IS a female contribution.

  8. Sample Problem In hamsters, white fur color (W) is dominant to brown fur color (w). If you cross a heterozygous female with white fur color (Ww) with a male that has brown fur (ww), what genotypes and phenotypes would you see and in what ratios?

  9. After 1900 several scientists tried to replicate Mendel’s crosses using other species including snapdragon. Genetics After Mendel Red Yellow P x PRPR PYPY When these alleles go walking, they both do some talking (codominance)! OK, so we cannot use R/r nor Y/y so we pick a third letter…P for the petal color gene. Notice: we do NOT mix R/Y or r/y! All Orange F1 PRPY F1 x F1 = F2 possible gametes Punnett Square: PR PY F2 PR Red PRPR Orange PRPY This F2 will NOT have a 3:1 ratio of phenotypes. Instead it shows a 1:2:1 ratio! The exception here proves the rule. possible gametes PY Orange PRPY Yellow PYPY

  10. Sample Problem In horses, black and white coat colors are codominant. Heterozygous horses have gray coats. Black horse genotype: HBHB White horse genotype: HWHW Gray horse genotype: HBHW What would be the possible genotypes and phenotypes of the following crosses? Black x White Gray x Gray

  11. Blood Types Blood Type: • A • B • AB • O Genotype • IAIA , IAi • IBIB , IBi • IAIB • ii

  12. Try These • If a male is homozygous for blood type B and a female is heterozygous for blood type A, what are the possible blood types in the offspring?

  13. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

  14. Chromosomes and Human Genetics • Human Chromosomes • Types • Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits) • X and Y do not look alike but behave as a homologous pair at meiosis • XX = female, XY = male • Autosomes – non-sex chromosomes (22 pairs) – genes are unrelated to sex determination

  15. B. Determining Sex… MOM X X X X X X X DAD X Y X Y Y

  16. II. Gene Location • Linked – Linkage Groups – genes located so close together on a chromosome that the traits always seem to appear together Ex. Red hair and freckles Ex. Colorblindness and Hemophilia X X

  17. Sex-linked Traits – genes on the sex chromosomes • Expression of certain genes often appears more in one sex than the other • Males require only one copy of a gene since they only have one X chromosome • See Royal Families of Europe Pedigree • Ex. Eye color in fruit flies, hemophilia, color-blindness

  18. Colorblindness Tests

  19. X-Linked/Sex Linkage – do not write • Genes present on the X chromosome exhibit unique patterns of inheritance due to the presence of only one X chromosome in males. • X-linked disorders show up rarely in females • X linked disorders show up in males whose mothers were carriers (heterozygotes)

  20. Practice Sex-linked Problems…. • What will the result of mating between a normal (non-carrier) female and a hemophiliac male?

  21. A female carrier who is heterozygous for the recessive, sex-linked trait causing red-green colorblindness, marries a normal male. What proportions of their MALE progeny will have red-green colorblindness?

  22. Hemophilia is inherited as an X-linked recessive. A woman has a brother with this defect and a mother and father who are phenotypically normal. What is the probability that this woman will be a carrier if she herself is phenotypically normal?

  23. C. Gene Interactions • Polygenetic trait – many genes influence a single trait (ex. Height, intelligence) • Pleiotropic effect – one gene having many effects (ex. Gene to make testosterone)

  24. Pleiotropy ? • Expression of a single gene has multiple phenotypic effects • Marfan Syndrome – abnormal gene that makes fibrillin (important in connective tissues)

  25. III. Genetic Analysis • Karyotype – visualized chromosomes stained, squashed, and photographed at metaphase - They are characteristic of the species or individual

  26. B. Pedigree – chart showing family relationships (see worksheet)

  27. Pedigree Analysis • Method of tracking a trait through generations within a family. • Good method of tracking sex-linked traits as well as autosomal traits.

  28. Sex-Linked Pedigree • Shows gender bias with males exhibiting the trait more often than females

  29. Autosomal Dominant Pedigree • Autosomal dominant traits do not skip a generation • Autosomal dominant traits do not show gender bias

  30. Autosomal Recessive Pedigree • Autosomal recessive traits skip a generation • Autosomal recessive traits do not show gender bias

  31. IV. Non-Mendelian Genetics • Incomplete Dominance – blended inheritance • Neither form of the gene is able to mask the other • Ex. Snap dragon petal color • R1R1 – RED • R1R2 – PINK • R2R2 - WHITE

  32. Incomplete Dominance • Neither allele is dominant • Heterozygotes are a blend of homozygous phenotypes = no distinct expression of either allele

  33. Try these • In a plant species, if the B1allele (blue flowers) and the B2 allele (white flowers) are incompletely dominant (B1 B2is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?

  34. What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?

  35. 2. Codominance • No dominance and both alleles are completely expressed • Ex. Cat color • C1C1 – Tan • C1C2 – Tabby (black and tan spotted) • C2C2 - Black

  36. Try These • Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together. • Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

  37. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

  38. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). • What pattern of inheritance does this illustrate? • What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

  39. 3. Multiple Alleles • More than 2 alleles for one trait • Ex. Eye color, hair color, blood type, guinea pig fur color • ABO blood groups • Each individual is A, B, AB, or O phenotype • Phenotype controlled by marker on RBC • IA and IB alleles are dominant to the i allele • IA and IB alleles are codominant to each other

  40. Blood Type: A B AB O Genotype IAIA , IAi IBIB , IBi IAIB ii Blood Types

  41. Try These • If a male is homozygous for blood type B and a female is heterozygous for blood type A, what are the possible blood types in the offspring?

  42. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

  43. A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?

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