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01 mendelian genetics
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- 2. GENETICS Every trait (orcharacteristic) in your body comes from instructions from your mother and father Father Mother
- 3. GENETICS The instructionsare 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-color This 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 of each 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 of homologous chromosomes.
- 7. GENETICS Chromosomes and theirgenes are passed to the offspring (children) through sperm and egg cells (gametes) Egg cells 23 chromosomes Sperm cells Father Mother 23 chromosomes
- 8. GENETICS Chromosomes and theirgenes 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 eyes Brown 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 eyes Brown 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 eyes Brown 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 eyes 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.
- 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 that contains 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 that contains two alleles that are the same (ex. BB)
- 17. GENETICS But wait… Father Mother Is this possible?!
- 18. GENETICS Yes this is possible The 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. GENETICS The father’s parentscould 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 plants He 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 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
- 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 ovary Cross-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 a purple 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 flowers F1 stands for filius and filia, which in Latin means “son” or “daughter”
- 27. GENETICS Square Constructing a Simple Punnett Step 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 choice Let the allele for purple flower be represented by the letter B
- 29. GENETICS Square Constructing a Simple Punnett Step 3: Consider all possible gametes produced by the first parent. Write the alleles for these gametes across the 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 Punnett Step 4: Consider all possible gametes produced by the second parent. Write the alleles for these gametes down the side of the square Let the allele for purple flower be represented by the B b letter B b bb b
- 31. GENETICS Square Constructing a Simple Punnett Step 5: Complete the square by writing all possible allele combinations 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 Punnett Step 6: Determine the genotypic and phenotypic proportions 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. GENETICS A plant thatis 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 # 1 gametes Results: P P Therefore the results of the PP x pp cross Parent # 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. GENETICS Sheep ranchers preferwhite 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.
- 35. WHY WERE MENDEL’SFINDINGS 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”)
- 36. WHY WERE MENDEL’SFINDINGS IMPORTANT? -We can also mix traits that we like together from different species (in plants) -This process is called “hybridization”
- 37. WHY WERE MENDEL’SFINDINGS 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’SFINDINGS IMPORTANT? AMNIOCENTESIS: Looking at fetal cells from the amniotic fluid
- 39. WHY WERE MENDEL’SFINDINGS IMPORTANT? CHORIONIC VILLUS SAMPLING (CVS): Sampling tiny fingerlike projections on the placenta Can be performed earlier (10th to 12th week of pregnancy) than amniocentesis
- 40. THE STORY ISN’TAS SIMPLE… There are often more than 2 alleles per This is called gene… having multiple alleles for one …but each gene organism can ONLY have two different alleles for a trait at any one time We usually express these alleles like this: E1, E2, E3, E4
- 41. THE STORY ISN’TAS SIMPLE… Codominance: Both alleles are expressed at the same time
- 42. THE STORY ISN’TAS SIMPLE… Incomplete dominance: two alleles are equally dominant
- 43. THE STORY ISN’TAS SIMPLE…
- 44. SEX-LINKED TRAITS - Traitsthat are controlled by genes located on the sex chromosomes (usually the X chromosome) Ex: Duchenne muscular dystrophy, hemophilia, Charcot-Marie- Tooth disease and color blindness -Usually represented like this: XR Xr
- 45. SEX-LINKED TRAITS Females get2 X chromosomes: Protected by other X chromosome Males 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 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? Male RrYy x Female RrYy
- 48. 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? Male RrYy x Female RrYy