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Theoretical Genetics

Theoretical Genetics. 4.3, 10.2, and 10.3. Definitions. 1. Define Genotype, Phenotype, Dominant Allele, Recessive Allele, Codominant Allele, Locus, Homozygous, Heterozygous, Carrier, and Test Cross. Definitions. Locus- the particular position on homologous chromosomes of a gene

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Theoretical Genetics

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  1. Theoretical Genetics 4.3, 10.2, and 10.3

  2. Definitions 1. Define Genotype, Phenotype, Dominant Allele, Recessive Allele, Codominant Allele, Locus, Homozygous, Heterozygous, Carrier, and Test Cross.

  3. Definitions • Locus- the particular position on homologous chromosomes of a gene • Homozygous-having 2 identical alleles of a gene • Heterozygous- having 2 different alleles of a gene • Genotype- the alleles (genes) of an organism • Phenotype- The physical characteristics of an organism

  4. Definitions • Dominant Allele- an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state • Recessive Allele- an allele that only has an effect on the phenotype when present in the homozygous state • Codominant Allele- pairs of alleles that both affect the phenotype when present in a heterozygote • Carrier- an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele • Test Cross- testing a suspect heterozygote by crossing it with a known homozygous recessive

  5. Definitions • P Generation- Parent Generation • F1 Generation- Offspring of the Parent Generation (Filial) • F2 Generation- Offspring from when the F1 generation are allowed to self fertilize

  6. Monohybrid Crosses 2. Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett Grid. Flower color gene- R= Red r= white Cross a Homozygous Red flower with a White flower Red flower Genotype= RR Gametes= R or R White flower Genotype= rr Gametes= r or r Phenotypic Ratio: All (4) Red Genotypic Ratio: All (4) Rr

  7. Monohybrid Crosses Flower color gene- R= Red r= white Cross a Heterozygous Red flower with a White flower Red flower Genotype= Rr Gametes= R or r White flower Genotype= rr Gametes= r or r Phenotypic Ratio: 2 Red : 2 White Genotypic Ratio: 2 Rr : 2 rr

  8. Monohybrid Crosses Flower color gene- R= Red r= white Cross 2 Heterozygous Red flowers Red flower Genotype= Rr Gametes= R or r Phenotypic Ratio: 3 Red : 1 White Genotypic Ratio: 1 RR: 2 Rr : 1 rr

  9. Multiple Alleles 3. State that some genes have more than two alleles (multiple alleles). • Multiple alleles means a gene has 3 or more alleles • Example: ABO Blood type- 3 alleles

  10. Multiple Alleles 4. Describe ABO blood groups as an example of codominance and multiple alleles.

  11. Multiple Alleles • Any two of these alleles are present in an individual • A and B are codominant • O is recessive • Homozygotes- IAIA, IBIB, or ii • Heterozygotes- IAi, IBi, or IAIB

  12. Monohybrid Crosses Cross a Homozygous A blood type with an AB blood type A Genotype= IAIA Gametes= IA AB Genotype= IAIB Gametes= IA or IB Phenotypic Ratio: 2 A blood type and 2 AB Genotypic Ratio: 2 IAIA : 2 IAIB

  13. Monohybrid Crosses Cross a Heterozygous A blood type with a Heterozygous B blood type A Genotype= IAi Gametes= IA or i B Genotype= IBi Gametes= IB or i Phenotypic Ratio: 1 A : 1 B: 1 AB : 1 O Genotypic Ratio: 1 IAi: 1 IBi : 1 IAIB : 1 ii

  14. Monohybrid Crosses Cross 2 individuals with O blood O Genotype= ii Gametes= i Phenotypic Ratio: All (4) O blood type Genotypic Ratio: All (4) ii

  15. Sex Chromosomes 5. Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans. • Two sex chromosomes determine the gender of a child. • The X chromosome is relatively large and carries many genes. • The Y chromosome is much smaller and carries only a few genes.

  16. Sex Chromosomes • XX  Female • XY  Male • Females only pass on X in their gametes • Males pass on X or Y in their gametes • Therefore gender is determined by the Male

  17. Sex Chromosomes 6. State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

  18. Sex Chromosomes 7. Define Sex Linkage. • Sex linkage is the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome. • Sex linked genes are almost always located on the X chromosome. • Females have 2 X chromosomes and therefore have 2 copies of sex linked genes. • Males only have 1 X chromosome and therefore only have one copy of sex linked genes. • In humans, hemophilia and red-green color blindness are examples of sex-linked characteristics.

  19. Sex Chromosomes • Describe the inheritance of color blindness and hemophilia as examples of sex linkage. • Both color blindness Xb and hemophilia Xh are recessive sex linked alleles on the X chromosome. • The normal alleles are represented by XB and XH

  20. Sex Chromosomes • State that a human female can be homozygous or heterozygous with respect to sex-linked genes. 10. Explain that female carriers are heterozygous for X-linked recessive alleles. • Normal females are homozygotes- XBXB • Carrier female carriers are heterozygotes- XBXb the are heterozygotes, but since CB is recessive they see color. • CB females are homozygotes- XbXb and do not see color normally

  21. Sex Chromosomes 11. Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

  22. Monohybrid Crosses Cross a colorblind male and a normal female Male Genotype= XbY Gametes= Xb or Y Female Genotype= XBXB Gametes= XB Phenotypic Ratio: 2 carrier females and 2 normal males Genotypic Ratio: 2 XBXb and 2 XBY

  23. Monohybrid Crosses Cross a colorblind male and a carrier female Male Genotype= XbY Gametes= Xb or Y Female Genotype= XBXb Gametes= XB or Xb Phenotypic Ratio: 1 carrier ♀, 1 CB ♀ & 1 normal ♂, 1 CB ♂ Genotypic Ratio: 1 XBXb , 1 XbXb , 1 XBY, and 1 XbY

  24. Pedigree • Deduce the genotypes and phenotypes of individuals in pedigree charts. • For dominant and recessive alleles, upper-case and lower-case letters, respectively, should be used. • Letters representing alleles should be chosen with care to avoid confusion between upper and lower case. • For codominance, the main letter should relate to the gene and the suffix to the allele, both upper case. • For example, red and white codominant flower colors should be represented as CR and CW. • Sickle cell anemia- HbA is normal and HbS is sickle cell.

  25. Pedigree Sex linked- • One gender will be affected more than the other • If a female has the condition- all of her sons must have it. Dominant or Recessive- • If both individuals have the condition and they give rise to a child without the condition it is Dominant

  26. Pedigree • Possible genotypes of individuals I, II, and III • I- Rh+ Rh+ or Rh+ Rh- • II- Rh+ Rh+ or Rh+ Rh- • III- Rh+ Rh- • However, with respect to I and II, one is Rh+ Rh+ and one is Rh+ Rh-

  27. Pedigree

  28. Deduce, with a reason, whether the allele producing the condition is dominant or recessive. • Recessive- in 2nd generation individuals 2 and 3 do not have the condition but they give rise to children that do.

  29. Determine all the possible genotypes of the individual (2nd generation-1) using appropriate symbols. • XaY (where a=condition)

  30. Determine all the possible genotypes of the individual (3rd generation-4) using appropriate symbols. • XAXa or XAXA where A=normal, a=condition

  31. Theoretical Genetics 10.2

  32. Dihybrid Crosses 1. Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

  33. Dihybrid Crosses

  34. Dihybrid Crosses

  35. Dihybrid Crosses Flower color gene- R= Red r= white Flower size gene- B= Big b= small Cross a Heterozygous Red Big flower with a white small flower Red Big Genotype= RrBb Gametes= RB, Rb, rB or rb White Small Genotype= rrbb Gametes= rb Phenotypic Ratio: 4 Red Big : 4 Red Small : 4 White Big : 4 White Small Genotypic Ratio: 4 RrBb : 4 Rrbb : 4 rrBb : 4 rrbb

  36. Dihybrid Crosses Cross a Homozygous Red Heterozygous Big flower with a white Homozygous Big flower Big Red Genotype= RRBb Gametes= RB or Rb Big White Genotype= rrBB Gametes= rB Phenotypic Ratio: 16 Red Big Genotypic Ratio: 8 RrBB : 8 RrBb

  37. Dihybrid Crosses Cross two individuals that are heterozygotes for both traits Big Red Genotype= RrBb Gametes= RB, Rb, rB, or rb Phenotypic Ratio: 9 Red Big:3 Red small: 3 white big:1 white small Genotypic Ratio: 1 RRBB 2 RRBb 1 RRbb 2 RrBB 4 RrBb 2 Rrbb 1 rrBB 2 rrBb 1 rrbb

  38. Sex Chromosomes vs. Autosomes 2. Distinguish between autosomes and sex chromosomes

  39. Sex Chromosomes vs. Autosomes

  40. Crossing Over 3. Explain how crossing over between non-sister chromatids of a homologous pair in Prophase I can result in an exchange of alleles.

  41. Crossing Over • Some genes do not follow the law of independent assortment • They will deviate from the 9:3:3:1 ratio when 2 heterozygotes are crossed. • Combinations of genes tend to be inherited together because of their loci being close together on the same chromosome. • This is called Gene Linkage.

  42. Crossing Over • New combinations of alleles can only be produced if crossing over occurs which results in recombination. • The chromosomes pair up and form a synapsis. • The DNA of one chromosome is cut and a second cut is made at the exact same point on the other chromosome. • Crossing over creates a chiasmata which holds the two homologous chromosomes together and the chromosomes switch information (alleles). • Resulting in recombination and increased genetic diversity. • The new combinations are referred to as recombinants- Ab and aB

  43. Recombination • Recombination- the reassortment of genes or characteristics into different combinations from those of the parents. • Recombination occurs for linked genes by crossing over and for unlinked genes by chromosome assortment.

  44. Linkage Group • Define Linkage Group • All of the genes that have their loci on the same chromosome from a Linkage Group. • Crossing over allows recombination of linked genes

  45. Linked Genes 5. Explain an example of a cross between two linked genes. • Alleles are shown in vertical pairs when they represent linkage groups. TB tb

  46. Linked Genes P genotypes- PPLL ppll Phenotypes- purple flower red flower long pollen round pollen Gametes- PL pl F1 Genotype- PpLl F1 Phenotype purple flower long pollen Allow the F1 to self fertilize and product a F2 generation

  47. Linked Genes • Chi Squared Test is used to see if the observed ratios and expected ratios are significantly different.

  48. Linked Genes 6. Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

  49. Linked Genes • Two genes A and B are linked together as shown below A b a B • If the genes are far enough apart such that crossing over between the alleles occurs occasionally, which statement is true of the gametes? • All of the gametes will be Ab and aB • There will be 25% Ab, 25% aB, 25% ab, & 25% AB • There will be approximately equal numbers of Ab and ab gametes. • The number of Ab gametes will be greater than the number of ab gametes.

  50. Recombination in non-linked Genes Cross tall, white (Ttrr) with short, red (ttRr). tall, white Genotype= Ttrr Gametes= Tr or tr short, red Genotype= ttRr Gametes= tR or tr Phenotypic Ratio: 4 Tall red: 4 Tall white: 4 short red: 4 short white Genotypic Ratio: 4 TtRr: 4Ttrr: 4ttRr: 4 ttrr Which are the recombinants? Tall red- TtRr Short White- ttrr

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