01 genetics version 2

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01 genetics version 2

  1. 1. GENETICS
  2. 2. Every trait (or characteristic) in your body comes from instructions from your mother and father GENETICS Father Mother
  3. 3. GENETICS Segments of DNA that code for specific traits The instructions are coded in the DNA as genes . Genes are located in chromosomes . For example… This is not an accurate example. It’s just used to illustrate a point. Genes Gene for height Gene for eye-color
  4. 4. GENETICS Humans have 46 chromosomes . Father’s side Mother’s side Each chromosome has 2 copies: one from mommy one from daddy 2 sex chromosomes 46
  5. 5. GENETICS Chromosomes and their genes are passed to the offspring (children) through sperm and egg cells ( gametes ) Father Mother Sperm cells 23 chromosomes Egg cells 23 chromosomes
  6. 6. GENETICS Chromosomes and their genes are passed to the offspring (children) through sperm and egg cells ( gametes ) Father Mother
  7. 7. GENETICS 9 months later… The offspring is born Father Mother
  8. 8. GENETICS A closer look… Half of the offspring’s chromosomes are from mommy, and half are from daddy Father Mother
  9. 9. GENETICS Blue eyes Brown eyes Each gene has alternate forms, called alleles . For instance, the gene for eye colour* may have 2 alleles: brown vs. blue Brown eyes Blue eyes Father Mother
  10. 10. GENETICS Some alleles can “ mask ” the effects of the other allele. Although the mother has blue eyes, the child has brown eyes. In this case, brown eyes are “ dominant ” As a result, blue eyes are “ recessive ” Brown eyes Blue eyes Father Mother
  11. 11. GENETICS Some alleles can “ mask ” the effects of the other allele. Dominant – traits that are expressed more often. Alleles that are dominant are usually represented by a capitalized letter symbolizing that allele (i.e. B ) Recessive – traits that are expressed less frequently. Alleles that are recessive are usually represented by a lower-case letter symbolizing that allele (i.e. b ) Brown eyes Blue eyes
  12. 12. GENETICS Dominant and Recessive Alleles: How can the daughter’s two alleles ( genotype ) be written? Bb Let the allele for brown eyes be B , and the allele for blue eyes be b Brown eyes Blue eyes Daughter’s genetic makeup: Brown eyes Blue eyes
  13. 13. GENETICS Dominant and Recessive Alleles: Bb Notice how the daughter carries the allele for blue eyes, but she does not have blue eyes. Thus her phenotype (observable trait) is brown eyes. Brown eyes Blue eyes
  14. 14. GENETICS Since she carries two different alleles for eye colour, we can say that she is heterozygous for eye color. Heterozygous – describes the genotype of an organism that contains two different alleles (ex. Bb ) Homozygous vs. heterozygous Bb Brown eyes Blue eyes
  15. 15. GENETICS If she had blue eyes, we can say that she is homozygous for eye color. Homozygous – describes the genotype of an organism that contains two alleles that are the same (ex. BB ) Homozygous vs. heterozygous bb blue eyes Blue eyes
  16. 16. GENETICS But wait… Is this possible?! Father Mother
  17. 17. GENETICS Yes this is possible Bb bb The father could have carried the recessive allele for blue eyes as well… … although you can’t tell because he has the dominant brown eye allele (which “masks” blue eyes) b b Father Mother
  18. 18. GENETICS Father B b The father’s parents could have passed the blue eye allele to him bb BB Grandpa Grandma
  19. 19. GENETICS All too complicated? Let’s take a look at how it all started… Gregor Mendel (1822-1884) - Known as the father of genetics - Worked with pea plants
  20. 20. GENETICS Mendel’s pea plants He observed 2 traits for each part of the plant
  21. 21. GENETICS Mendel’s pea plants Mendel came up with the concept of alleles . He noticed that alleles are hereditary , and that you can predict the probability of the offspring having certain alleles.
  22. 22. GENETICS Mendel’s pea plants He also noticed that some traits dominated over others For instance, if you “crossed” a yellow-pea plant with a green-pea plant, you generally get a yellow-pea plant Mendel Video
  23. 23. GENETICS Mendel’s pea plants What does “crossing” the pea plants mean? Garden peas are both self-fertilizing and cross-fertilizing . Self-fertilizing – a plant’s pollen grains fertilize it’s own egg cells in the ovary Cross-fertilizing – a plant’s pollen grains fertilize another plant’s egg cells in the ovary It means to mate a plant with another plant by pollination.
  24. 24. GENETICS Mendel’s pea plants This allowed Mendel to mate pea plants with each other as well as with itself. For example, you can mate a purple flower pea plant with itself. MATE! This is called a Punnett Square
  25. 25. GENETICS Punnett Square This means that mating a pea plant that is heterozygous for flower colour (Bb) with itself will produce… F1 GENOTYPE: 25% BB 50% Bb 25% bb A 1:2:1 ratio F1 PHENOTYPE: 75% purple flowers 25% white flowers F1 stands for filius and filia , which in Latin means “son” or “daughter”
  26. 26. GENETICS Constructing a Simple Punnett Square Step 1: Draw a square with a 2 by 2 grid
  27. 27. GENETICS Constructing a Simple Punnett Square Step 2: Choose a letter for your allele and record this choice Let the allele for purple flower be represented by the letter B
  28. 28. GENETICS Constructing a Simple Punnett Square Step 3: Consider all possible gametes produced by the first parent. Write the alleles for these gametes across the top of the square Bb B b Let the allele for purple flower be represented by the letter B
  29. 29. GENETICS Constructing a Simple Punnett Square Step 4: Consider all possible gametes produced by the second parent. Write the alleles for these gametes down the side of the square B b bb b b Let the allele for purple flower be represented by the letter B
  30. 30. GENETICS Constructing a Simple Punnett Square Step 5: Complete the square by writing all possible allele combinations from the cross B b b b B b b b b B b b Let the allele for purple flower be represented by the letter B
  31. 31. GENETICS Constructing a Simple Punnett Square Step 6: Determine the genotypic and phenotypic proportions of the offspring B b b b B b b b b B b b b Let the allele for purple flower be represented by the letter B F1 Genotypes: 50% Bb 50% bb F1 Phenotypes: 50% of the plants have purple flowers 50% of the plants have white flowers
  32. 32. GENETICS A plant that is homozygous for purple flowers is crossed with a plant that has white flowers. If the purple condition is dominant over the white condition, what are the genotypes and phenotypes of the F1 generation? GIVEN: Let the allele for flower color be presented by the letter P Parent genotypes: PP X pp Parent gametes: P or P X p or p Parent # 2 gametes Parent # 1 gametes Results: Therefore the results of the PP x pp cross indicate that: F1 genotypes: 100% are Pp (or 4 out of 4 are Pp) F1 phenotypes: all plants have purple flowers Pp 25% Pp 25% p Pp 25% Pp 25% p P P
  33. 33. GENETICS Sheep ranchers prefer white sheep over black sheep, because black fur is hard to die and is brittle. The allele for black fur is recessive . As a result, if a sheep rancher wishes to purchase a white fur sheep for breeding, how does she/he know if it will make black fur babies (in other words, how does the rancher know if her/his sheep is homozygous or heterozygous ?)? A test cross can be performed to determine the genotype of a dominant phenotype, which involves breeding the unknown genotype with a homozygous recessive genotype. In this case the white sheep with an unknown genotype will be bred with a homozygous recessive black fur sheep.
  34. 34. WHY WERE MENDEL’S FINDINGS IMPORTANT? Once we find traits that we like in an organism (for example, a dog), we can maintain these traits by mating closely related individuals for the purpose of maintaining or perpetuating these characteristics (this is called “ inbreeding ”)
  35. 35. WHY WERE MENDEL’S FINDINGS IMPORTANT? <ul><li>We can also mix traits that we like together from different species (in plants) </li></ul><ul><li>This process is called “ hybridization ” </li></ul>
  36. 36. WHY WERE MENDEL’S FINDINGS IMPORTANT? Genetic Screening: -we can tell if an individual carries an allele (or two alleles) for genetic disorders - Amniocentesis and Chorionic Villi Sampling (CVS)
  37. 37. WHY WERE MENDEL’S FINDINGS IMPORTANT? AMNIOCENTESIS: Looking at fetal cells from the amniotic fluid
  38. 38. WHY WERE MENDEL’S FINDINGS IMPORTANT? CHORIONIC VILLI SAMPLING (CVS): Sampling tiny fingerlike projections on the placenta Can be performed earlier (10 th to 12 th week of pregnancy) than amniocentesis
  39. 39. THE STORY ISN’T AS SIMPLE… There are often more than 2 alleles per gene… … but each organism can ONLY have two different alleles for a trait at any one time We usually express these alleles like this: E 1 , E 2 , E 3 , E 4 This is called having multiple alleles for one gene
  40. 40. THE STORY ISN’T AS SIMPLE… Codominance: Both alleles are expressed at the same time
  41. 41. THE STORY ISN’T AS SIMPLE… Incomplete dominance: two alleles are equally dominant
  42. 42. THE STORY ISN’T AS SIMPLE…
  43. 43. SEX-LINKED TRAITS - Traits that are controlled by genes located on the sex chromosomes (usually the X chromosome) <ul><li>Usually represented like this: </li></ul><ul><li>X R X r </li></ul>Ex: Duchenne muscular dystrophy, hemophilia, Charcot-Marie-Tooth disease and color blindness
  44. 44. SEX-LINKED TRAITS Females get 2 X chromosomes: Males get ONE X chromosome: Disease!!! Protected by other X chromosome X X X X Y X Y X
  45. 45. SEX-LINKED TRAITS X X Females get 2 X chromosomes: <ul><li>1 gets turned off (called a Barr Body) </li></ul><ul><li>some cells have one X chromosome inactive, while other cells have the other inactive </li></ul>
  46. 46. GENETICS Dihybrid cross So far what we have done is a monohybrid cross, which only involves one trait. What if you wanted to see how two different traits are passed on to the next generation?
  47. 47. GENETICS Dihybrid cross So far what we have done is a monohybrid cross, which only involves one trait. What if you wanted to see how two different traits are passed on to the next generation?

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