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01 genetics version 2
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01 genetics version 2

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  • 1. GENETICS
  • 2. Every trait (or characteristic) in your body comes from instructions from your mother and father GENETICS Father Mother
  • 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. 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. 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. GENETICS Chromosomes and their genes are passed to the offspring (children) through sperm and egg cells ( gametes ) Father Mother
  • 7. GENETICS 9 months later… The offspring is born Father Mother
  • 8. GENETICS A closer look… Half of the offspring’s chromosomes are from mommy, and half are from daddy Father Mother
  • 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. 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. 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. 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. 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. 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. 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. GENETICS But wait… Is this possible?! Father Mother
  • 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. GENETICS Father B b The father’s parents could have passed the blue eye allele to him bb BB Grandpa Grandma
  • 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. GENETICS Mendel’s pea plants He observed 2 traits for each part of the plant
  • 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. 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. 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. 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. 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. GENETICS Constructing a Simple Punnett Square Step 1: Draw a square with a 2 by 2 grid
  • 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. 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. 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. 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. 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. 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. 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. 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. WHY WERE MENDEL’S FINDINGS IMPORTANT?
    • We can also mix traits that we like together from different species (in plants)
    • This process is called “ hybridization ”
  • 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. WHY WERE MENDEL’S FINDINGS IMPORTANT? AMNIOCENTESIS: Looking at fetal cells from the amniotic fluid
  • 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. 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. THE STORY ISN’T AS SIMPLE… Codominance: Both alleles are expressed at the same time
  • 41. THE STORY ISN’T AS SIMPLE… Incomplete dominance: two alleles are equally dominant
  • 42. THE STORY ISN’T AS SIMPLE…
  • 43. SEX-LINKED TRAITS - Traits that are controlled by genes located on the sex chromosomes (usually the X chromosome)
    • Usually represented like this:
    • X R X r
    Ex: Duchenne muscular dystrophy, hemophilia, Charcot-Marie-Tooth disease and color blindness
  • 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. SEX-LINKED TRAITS X X Females get 2 X chromosomes:
    • 1 gets turned off (called a Barr Body)
    • some cells have one X chromosome inactive, while other cells have the other inactive
  • 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. 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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