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Dihybrid Crosses - looking at 2 traits

Dihybrid Crosses - looking at 2 traits. Mendel’s dihybrid crosses: Mendel also performed crosses involving two pairs of traits , e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green).

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Dihybrid Crosses - looking at 2 traits

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  1. Dihybrid Crosses - looking at 2 traits

  2. Mendel’s dihybrid crosses: • Mendel also performed crosses involving two pairs of traits, e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green). • If alleles sort independently, four possible phenotypes (2n) appear in the F2 generation in a 9:3:3:1 ratio. “Mendel’s Principle of Independent Assortment”: Alleles for different traits assort independently of one another. Modern formulation of independent assortment: Genes on different chromosomes behave independently in gamete production.

  3. The alignment of one pair of homologs is independent of any other. The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

  4. Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. RREE RE RE RE RE All gametes are RE

  5. Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. RREe Re RE RE Re You have 2 possible gametes: Re and RE

  6. Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. rrEe re rE rE re You have 2 possible gametes: rE and re

  7. How many gametes? To determine the number of different gametes a parent can have use the FOIL method. Number and kind of gametes TTYY = one TY TtYY = two TY and ty TtYy = four TY, Ty, tY, ty

  8. Possible Gametes (sperm/egg): If I cross: What are the different possible gametes for these parents?? RRTT x RrTt RT RT, Rt, rT, rt There are 4 total different gametes

  9. If I cross: What are the possible gametes? RRTT x RrTt RT Rt rT rt RRTT RRTt RrTT RrTt RT I have 4 possible offspring

  10. If I cross: What are the possible gametes and offspring? rrTt x RRTt RT Rt RrTT RrTt rT RrTt Rrtt rt I have 4 possible offspring

  11. If I cross: What are the possible gametes? RrTt x RrTt RT Rt rT rt RRTT RRTt RrTT RrTt RT RRTt RRtt RrTt Rrtt Rt I have 16 possible offspring 9:3:3:1 rT RrTT RrTt rrTT rrTt rt RrTt Rrtt rrTt rrtt

  12. Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another? Phenotype Ratio: 9 yellow/smooth 3 yellow/wrinkled 3 green/smooth 1 green/wrinkled Because it’s the only way to explain the pattern of inheritance!

  13. What Works for Peas Also Works for Humans Consider a cross between parents heterozygous for both deafness and albinism. This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

  14. January 10, 2012 • Get your homework out on your desk! Warm Up #1: A pea plant with the genotype TtWW is crossed with a pea plant with the genotype ttWw. How many different genotypes can be expressed in the offspring? (SHOW ALL WORK!) a. 1 b. 2 c. 3 d. 4

  15. Warm Up #2 • If a corn plant has a genotype of Ttyy, what are the possible genetic combinations that could be present in a single grain of pollen (sperm, male gamete) from this plant? a. Ty, ty b. TY, ty c. TY, Ty, ty d. Ty, ty, tY, TY

  16. Warm Up #3 3. The pea plant produces plants of two different sizes and seeds that are two different shapes. Tall pea plants (T) are dominant to dwarf pea plants (t) and round seeds (R) are dominant to wrinkled seeds, r. Two plants heterozygous for both traits are mated. What fraction of their offspring will be dwarf and have wrinkled seeds? (SHOW ALL WORK!)

  17. Great job on your quizzes! • Hand back quizzes and work – review most common mistakes. • Tonight: Talk to your parents about their blood type. Write down each parent and yours if you know yours. Turn this in tomorrow for extra credit. • What do you know about blood types?

  18. Multiple Alleles • More than two allelic forms of a gene that code for a trait • Phenotype is still determined by a pair of alleles! The gene pool simply contains more than two. • These alleles can be expressed in various ways: • Complete dominance • Co-dominance • Incomplete dominance

  19. Multiple Alleles: Human Blood Type Blood Type is Coded by Multiple Alleles A, B, and O. *A and B are co-dominant. *A and B are completely dominant over O. Genotypes AA AO BB BO AB OO Phenotypes Type A Type A Type B Type B Type AB Type O *The genes cause the production of a cell surface protein A or B. Type O cells have neither protein, so Type O individuals are Universal DONORS. Which individuals are Universal RECEPIENTS (have both proteins)?

  20. Type A Type B Type AB Type O No antibodies

  21. Multiple Alleles Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene. The AB phenotype (genotype IA IB) is an example of codominance!!

  22. Codominance The human ABO blood group illustrates another genetic phenomenon – codominance. Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote – both are expressed!

  23. Problem 1 1. In humans, the blood types A, B, AB and O are determined by three alleles. A man who has AB blood marries a woman with O blood. What are the genotypes and phenotypes of the offspring?

  24. Problem 2 2. A man who is heterozygous type A marries a woman who is heterozygous type B. What percentage of their offspring will be Type 0?

  25. Problem 3 3. If a man with blood type B, one of whose parents had blood type O, marries a woman with blood type AB. What percentage of their children should have blood type B?

  26. Problem 4 4. A man with type O blood and a woman with type AB blood get married. One of their children needs an operation. This child has type B blood. Can the child safely receive a blood transfusion from either parent? If not, why?

  27. Multiple Alleles: Whose Baby ? A major mix-up occurred. Have the babies been returned to the correct set of parents? 1 Baby A - Type O Blood Baby B - Type B Blood Smiths - Types O and AB Jones - Types A and B 2 Baby C - Type AB Blood Baby D - Type A Blood Squares -Types AB and AB Angles -Types AB and O

  28. 1/13/11: Warm Up Both Mrs. Smith and Mrs. Jones had babies the same day in the same hospital. Mrs. Smith took home a baby girl, whom she named Shirley. Mrs. Jones took home a baby girl, whom she named Jane. However, Mrs. Jones began to suspect that her child had been accidentally switched with another baby in the nursery. Mr. Smith – type A Mr. Jones – type A Mrs. Smith – type B Mrs. Jones – type A Shirley – type O Jane – type B **Did a mix-up occur? SHOW YOUR WORK!!!

  29. Sex – Linked Genes • Review: • What are the two sex chromosomes? • What is a woman’s genotype? • What is a man’s genotype? • Which do you think there are more of – x-linked genes or y-linked genes? • Who do you think inherits sex-linked disorders more – males or females?

  30. X-linked Genetics • Genes that are located on the • X chromosome but not the Y • MALES receive only ONE copy • FEMALES receive TWO copies

  31. Color Blind Test • Are you red-green colorblind? • Lets find out…

  32. Sex- Linked Inheritance • A male always receives a sex-linked condition from his mother, from whom he inherits an X chromosome. • A female must receive 2 alleles, one from each parent, for the trait to be expressed. X-linked disorders are often recessive.

  33. Sex-linked ( X-linked) disorders • Color –blindness • Hemophilia – protein for blood clotting is missing • Duchenne Muscular dystrophy- progressive wasting of muscles (There are about 205 X-linked recessive disorders)

  34. Color-blindness Father (normal vision) Male Female Son (normal vision) Daughter (normal vision) Mother (carrier) Daughter (carrier) Son (colorblind) More males than females are affected by this type of disorder. WHY?

  35. True or False? • The X chromosome carries alleles that are not on the Y chromosome. Therefore, a recessive allele on the X chromosome is expressed in males. • Certain traits that have nothing to do with the gender of the individual are controlled by genes on X chromosomes. • Males have 2 X chromosomes, and therefore, X-linked recessive alleles are expressed.

  36. Solving X-linked Problems • How do you know if x-linked? • Know color-blindness and hemophilia are! • Make Punnet Square using XX and XY • Attach alleles to X’s ONLY • To help determine percentages, circle the male offspring

  37. Let’s Practice 1. Both the mother and the father of a male hemophiliac appear to be normal. From whom did the son inherit the allele for hemophilia? What are the genotypes of the mother, the father, and the son?

  38. 2. A woman is color blind. What are the chances that her sons will be color blind? If she is married to a man with a normal vision, what are the chances that her daughters will be color blind? Will be carriers?

  39. Cont… 3. A husband and a wife give birth to a baby girl. Mom’s vision was normal, so she assumed the baby’s would be normal too. However, she gave birth to a color-blind daughter. What can you deduce about the girl’s parentage?

  40. Cont… 4. A man with normal color vision marries a woman who carries the recessive gene, although her vision is normal. What percentage of male children will be colorblind? What about females?

  41. Cont… 5. If a hybrid female for the colorblind trait marries a colorblind male, what percentages of offspring will be colorblind?

  42. Paternity Suit 6. One of the sons from question #5 is involved in a paternity suit. The child in question is a cb female. The judge rules that the child could not possibly belong to this man. Which sons, of problem #5, was involved in this case? Give his genotype and phenotype and tell how you know.

  43. Sex-Linked Problems • Finish for homework!!!!

  44. Article Anticipation Guide • Read the statements on your guide and mark your educated guesses in the boxes Skin Color and Genetics Article

  45. Polygenic Inheritance – when a Single Trait is influenced by many genes • Examples: • Height 2. Hair texture • 3. Skin color 4. eye color

  46. Types of Traits – Single Gene • Some traits are controlled by single gene • Expressed as “either/or” • Discontinuous Variation

  47. Types of Traits – Polygenic Traits • Polygenic Traits are controlled by the additive effects of 2 or more genes • Shows a range of phenotypes • Continuous Variation

  48. Types of Traits Example: A Biology Class Single Gene Trait Polygenic Trait Discontinuous Variation Continuous Variation

  49. Types of Traits – Polygenic Traits Human Population - Skin Color • Controlled by many genes • A given individual can have as many as six alleles (possibly more!) for pigmentation

  50. Polygenics What are other traits that are coded for by MANY genes????

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