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01 mendelian genetics

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  • 1. GENETICS
  • 2. GENETICSEvery trait (or characteristic) in your body comes from instructions from your mother and father Father Mother
  • 3. GENETICS The instructions are coded in the DNA as genes. Genes are located in chromosomes. Genes Segments of DNA that code for specific traits For example… Gene for height Gene for eye-colorThis is not an accurate example. It’s just used to illustrate a point.
  • 4. GENETICS A closer look…Half of the offspring’s chromosomes are from mommy, and half are from daddy.Father Mother
  • 5. GENETICS A closer look…Humans have 23 different chromosomes. We get 1 ofeach from our parents, for a total of 46 in somatic cells.
  • 6. GENETICS A closer look… Homologues Homologues Not homologues A pair of the same types of chromosomes are called homologous chromosomes, or just homologues.This picture has 22 pairs ofhomologous chromosomes.
  • 7. GENETICSChromosomes and their genes are passed to the offspring (children) through sperm and egg cells (gametes) Egg cells 23 chromosomes Sperm cells Father Mother 23 chromosomes
  • 8. GENETICSChromosomes and their genes are passed to the offspring (children) through sperm and egg cells (gametes) Father Mother
  • 9. GENETICS 9 months later…Father Mother The offspring is born
  • 10. GENETICS Each gene has alternate forms, called alleles. For instance, the gene for eye colour may have 2 alleles: brown or blue Brown eyes Blue eyes Father passed-on: Mother passed-on: Blue eyesBrown eyes
  • 11. GENETICS Some alleles can “mask” the effects of the other allele. Although the mother has blue eyes, the child has brown eyes. Father Mother In this case, brown eyes are “dominant” Blue eyesBrown eyes As a result, blue eyes are “recessive”
  • 12. 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. Blue eyesBrown eyes Alleles that are recessive are usually represented by a lower-case letter symbolizing that allele (i.e. b)
  • 13. GENETICS Dominant and Recessive Alleles: Daughter’s genetic makeup: Brown eyes Blue eyes How can the daughter’s two alleles (genotype) be written?Let the allele for brown eyes be B, and the allele for blue eyes be b Bb Brown eyes Blue eyes
  • 14. GENETICS Dominant and Recessive Alleles: Bb Brown eyes Blue eyesNotice how the daughter carries the allele for blue eyes, but she does not have blue eyes. Thus her phenotype (observable trait) is brown eyes.
  • 15. GENETICS Homozygous vs. heterozygous Bb Brown eyes Blue eyes 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 thatcontains two different alleles (ex. Bb)
  • 16. GENETICS Homozygous vs. heterozygous bb blue eyes Blue eyes If she had blue eyes, we can say that she is homozygous for eye color.Homozygous – describes the genotype of an organism thatcontains two alleles that are the same (ex. BB)
  • 17. GENETICS But wait…Father Mother Is this possible?!
  • 18. GENETICS Yes this is possibleThe father could have carried the recessive allele for blue eyes as well… Father bb Mother Bb bb …although you can’t tell because he has the dominant brown eye allele (which “masks” blue eyes)
  • 19. GENETICSThe father’s parents could have passed the blue eye allele to him Grandpa Grandma bb BB Father Bb
  • 20. 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
  • 21. GENETICS Mendel’s pea plantsHe observed 2 traits for each part of the plant
  • 22. 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.
  • 23. GENETICS Mendel’s pea plantsHe also noticed that some traits dominated over othersFor instance, if you “crossed” a yellow-pea plant with a green-pea plant, you generally get a yellow-pea plant Mendel Video
  • 24. GENETICS Mendel’s pea plants What does “crossing” the pea plants mean? It means to mate a plant with another plant by pollination. Garden peas are both self-fertilizing and cross-fertilizing. Self-fertilizing – a plant’s pollen grains fertilize it’s own egg cells in the ovaryCross-fertilizing – a plant’s pollen grains fertilize another plant’s egg cells in the ovary
  • 25. 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 apurple flower pea plant with itself. This is called a Punnett Square MATE!
  • 26. 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 flowersF1 stands for filius and filia, which in Latin means “son” or “daughter”
  • 27. GENETICS Square Constructing a Simple PunnettStep 1: Draw a square with a 2 by 2 grid
  • 28. GENETICS Square Constructing a Simple Punnett Step 2: Choose a letter for your allele and record this choiceLet the allele for purple flower be represented by the letter B
  • 29. GENETICS Square Constructing a Simple PunnettStep 3: Consider all possible gametes produced by thefirst parent. Write the alleles for these gametes acrossthe top of the square Bb Let the allele for purple flower be represented by the B b letter B
  • 30. GENETICS Square Constructing a Simple PunnettStep 4: Consider all possible gametes produced by thesecond parent. Write the alleles for these gametes downthe side of the square Let the allele for purple flower be represented by the B b letter B bbb b
  • 31. GENETICS Square Constructing a Simple PunnettStep 5: Complete the square by writing all possible allelecombinations from the cross Let the allele for purple flower be represented by the B b letter B b b
  • 32. GENETICS Square Constructing a Simple PunnettStep 6: Determine the genotypic and phenotypicproportions of the offspring Let the allele for purple flower be represented by the letter B B b F1 Genotypes: 50% Bb b Bb bb 50% bb F1 Phenotypes: 50% of the plants b Bb bb have purple flowers 50% of the plants have white flowers
  • 33. GENETICSA plant that is homozygous for purple flowers is crossed with a plantthat has white flowers. If the purple condition is dominant over thewhite condition, what are the genotypes and phenotypes of the F1generation? 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 # 1 gametes Results: P P Therefore the results of the PP x pp crossParent # 2 Pp Pp indicate that:gametes p 25% 25% F1 genotypes: 100% are Pp (or 4 out of 4 are Pp) Pp Pp p F1 phenotypes: all plants 25% 25% have purple flowers
  • 34. GENETICSSheep 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 awhite 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 unknowngenotype 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.
  • 35. WHY WERE MENDEL’S FINDINGS IMPORTANT?Once we find traits that we like in an organism (forexample, a dog), we can maintain these traits bymating closely related individuals for the purpose ofmaintaining or perpetuating these characteristics (thisis called “inbreeding”)
  • 36. WHY WERE MENDEL’S FINDINGS IMPORTANT?-We can also mix traits that we like together fromdifferent species (in plants)-This process is called “hybridization”
  • 37. 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 Villus Sampling (CVS)
  • 38. WHY WERE MENDEL’S FINDINGS IMPORTANT?AMNIOCENTESIS:Looking at fetalcells from theamniotic fluid
  • 39. WHY WERE MENDEL’S FINDINGS IMPORTANT?CHORIONIC VILLUSSAMPLING (CVS):Sampling tinyfingerlike projectionson the placentaCan be performedearlier (10th to 12thweek of pregnancy)than amniocentesis
  • 40. THE STORY ISN’T AS SIMPLE…There are oftenmore than 2alleles per This is calledgene… having multiple alleles for one…but each geneorganism canONLY have twodifferent allelesfor a trait at anyone time We usually express these alleles like this: E1, E2, E3, E4
  • 41. THE STORY ISN’T AS SIMPLE…Codominance:Both alleles areexpressed at thesame time
  • 42. THE STORY ISN’T AS SIMPLE…Incompletedominance: twoalleles are equallydominant
  • 43. THE STORY ISN’T AS SIMPLE…
  • 44. SEX-LINKED TRAITS- Traits that arecontrolled by geneslocated on the sexchromosomes(usually the Xchromosome)Ex: Duchennemusculardystrophy,hemophilia,Charcot-Marie-Tooth disease andcolor blindness-Usuallyrepresented likethis:XR Xr
  • 45. SEX-LINKED TRAITSFemales get 2 X chromosomes: Protected by other X chromosomeMales get ONE X chromosome: Disease!!!
  • 46. SEX-LINKED TRAITS 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
  • 47. GENETICS Dihybrid crossSo far what we have doneis a monohybrid cross,which only involves onetrait. What if you wantedto see how two differenttraits are passed on to thenext generation?Male RrYy x Female RrYy
  • 48. GENETICS Dihybrid crossSo far what we have doneis a monohybrid cross,which only involves onetrait. What if you wantedto see how two differenttraits are passed on to thenext generation?Male RrYy x Female RrYy