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Mendelian Genetics

Mendelian Genetics. Genetics: The Basics. Allele- An alternative form of a gene Diploid organisms have one copy on each homologous chromosome Represented by letters: Capital letter = dominant form Lower case letter = recessive form Example= Eye Color Controlled by 2 alleles

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Mendelian Genetics

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  1. Mendelian Genetics

  2. Genetics: The Basics • Allele- An alternative form of a gene • Diploid organisms have one copy on each homologous chromosome • Represented by letters: • Capital letter = dominant form • Lower case letter = recessive form • Example= Eye Color • Controlled by 2 alleles • Blue Eyes = bb • Brown eyes= Bb or BB

  3. Dominant allele-fully expressed in the organism's appearance Recessive allele-no noticeable effect on the organism's appearance

  4. Genetics: The Basics • Heterozygous: Have 2 different forms of the allele • Example: • Brown Eyes = Bb = heterozygous • Homozygous: Have 2 of the same forms of the allele • Example: • Blue Eyes = bb = homozygous recessive • Brown Eyes = BB = homozygous dominant

  5. Genetics: The Basics • Genotype = the genetic makeup of an organism • Example = BB, Bb, bb • Phenotype = the physical expression of genes • Example = • Brown Eyes = phenotype of either the BB or Bb genotype • Blue Eyes= phenotype of the bb genotype Remember that phenotype is not necessarily an appearance- It can be things like enzyme production, behavior, etc!! It is ANY expression of a gene!!

  6. Gregor Mendel *1843 entered monastery *1851-53 studied at Univ. of Vienna *1857 started breeding garden peas * 1860 started forging data!!

  7. MENDEL'S MAIN QUESTION Do units of inheritance retain integrity (preserved) or blend???? Sample Question: If you cross a purple flower with a white flower are these flower colors retained in future crosses or are they blended to form an intermediate color?

  8. Law of Segregation-two alleles for a character are packaged into separate gametes 2 plants crossed Self-fertilized

  9. Mendel's findings 1. Alternative version of genes (alleles) account for variations in inherited characters Homologous chromosomes Purple flowers White flowers

  10. Mendel's findings 2. For each character, an organism inherits two alleles, one from each parent. maternal Homologous chromosomes paternal Purple flowers White flowers

  11. Mendel's findings 3. If two alleles differ, the dominant allele is fully expressed in the organism's appearance. recessive dominant Purple flowers White flowers

  12. Mendel's findings 4. The two alleles for each character segregate during gamete production. dominant recessive Seed shape PP pp Gametes P p

  13. Punnett Square Predicts the results of a genetic cross between individuals with known genotypes

  14. Rules for Genetic Problems 1. Identify traits (alleles) and assign letters to represent the various traits: capital letters for dominant traits; lower case letters for recessive traits. 2. Set up parental cross. 3. Draw individual gametes with corresponding letter for trait. 4. Identify F1 offspring phenotype and genotype. 5. Setup F1 cross. 6. Draw individual gametes with corresponding letter for trait. 7. Set up Punnett square to identify individual genotypes and phenotypes for F2 offspring.

  15. EXAMPLE: SEED COLOR C c c C c dominant recessive CC cc c C C CC 3 Cc Cc cc 1

  16. EXAMPLE: POD SHAPE S s s S s dominant recessive SS ss s S S SS 3 Ss Ss ss 1

  17. Monohybrid Cross Follows single trait

  18. Test Cross Breeding a homozygous recessive with a dominant phenotype (unknown genotype) can determine an unknown allele.

  19. In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? F1 generation, test cross: Ss Ss What is the genotypic ratio? What is the phenotypic ratio?

  20. The test cross To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______. A. y B. Y C. yy D. YY E. Yy

  21. Predicting the results of a test cross A test cross is used to determine if the genotype of a plant with the dominant phenotype is homozygous or heterozygous. If the unknown is homozygous, all of the offspring of the test cross have the __________ phenotype. If the unknown is heterozygous, half of the offspring will have the __________ phenotype. A. dominant, recessive B. recessive, dominant

  22. Question: How are two traits inherited? • DIHYBRID CROSS • Experimental Approach: A cross involving two true-breeding traits. • System: Pea Plants; seed color (Y/y) • and seed shape (S/s).

  23. F1 Generation

  24. F1 Generation

  25. F1 Generation • Each of the male gametes types (SY, Sy, sY, sy) can fuse with each of the female gametes types (SY, Sy, sY, sy). • 16 possible combinations of gametes are possible. • We will see that there are 9 possible genotypes and 4 possible phenotypes. • 4. The two parental phenotypes, and • two new phenotypes were obtained.

  26. Dihybrid Cross Follows two traits 9:3:3:1 RATIO

  27. The phenotypes of two independent traits show a 9:3:3:1 ratio in the F2generation. coat color is indicated by B (brown, dominant) or b (white) tail length is indicated by S (short, dominant) or s (long). If the children mate with each other, in the F2 generation all combinations of coat color and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box).

  28. Dihybrid Cross In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d)..  If a squash plant true-breeding for white, disk-shaped fruit  is crossed with a plant true-breeding for yellow, sphere-shaped fruit,  what will the phenotypic and genotypic ratios be for: a. the F1 generation?     b. the F2 generation?

  29. Genotypic ratios: 1/16 will be homozygous dominant for both traits (WWDD)2/16 will be homozygous dominant for color and heterozygous for shape (WWDd)2/16 will be heterozygous for color and homozygous dominant for shape (WwDD)1/16 will be homozygous dominant for color and homozygous recessive for shape (WWdd)4/16 will be heterozygous for both traits  (WwDd)2/16 will be heteozygous for color and homozygous recessive for shape (Wwdd)1/16 will be homozygous recessive for color and homozygous dominant for shape (wwDD)2/16 will be homozygous recessive for color and heterozygous for shape (wwDd)1/16 will be homozygous recessive for both traits (wwdd) This is a 1:2:2:1:4:2:1:2:1 genotypic ratio Phenotypic ratios:9/16 will have white, disk-shaped fruit3/16 will have white, sphere-shaped fruit3/16 will have yellow, disk-shaped fruit1/16 will have yellow, sphere-shaped fruit This is a 9:3:3:1 phenotypic ratio

  30. Law of Segregation- Every individual possesses a pair of alleles for any particular trait and that each parent passes a randomly selected copy (allele) of only one of these to its offspring. 

  31. Law of Independent Assortment- Separate genes for separate traits are passed independently of one another from parents to offspring. These allele pairs are then randomly united at fertilization. 

  32. Inheritance that diverges from Mendel's inheritance GENE INTERACTIONS The relationship between the genotype and phenotype is rarely simple. * Each character is rarely controlled by one gene *Each gene usually has more than two alleles, with one not always being dominant over the other

  33. Incomplete Dominance Heterozygotes show a distinct intermediate phenotype, not seen in homozygotes Traits are separable in further crosses Not BLENDED

  34. In CODOMINANCE, both alleles are expressed and functional, though they may be different. Most genes have more than two alleles in a population. (IA, IB, I)

  35. Pleiotrophic Most genes affect more than one phenotypic character.

  36. Pleiotropy:Albinism A single defect in one of the enzymes catalyzing tyrosine to melanin can affect multiple phenotypic characters, from eye color to skin color to hair color.

  37. Epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus. bb with dominant C allele results in brown mouse

  38. Additive effect of two or more genes on a single phenotypic character. Polygenic Inheritance SKIN COLOR Controlled by at least 4 different genes

  39. Sex-linked traits • In humans, 2 of our 46 chromosomes are • classified as sex chromosomes • Females = XX • Carried on ova • Males = XY • Carried on sperm • In females, only 1 X chromosome is active • Sex linked traits usually aren’t expressed- • In males, their only X chromosome is active • No other X chromosome to block sex linked trait

  40. Sex-linked traits In humans, the genes for colorblindness are both located on the X chromosome with no corresponding gene on the Y. Strawberries as they would appear to someone who is red/green colorblind.

  41. Sex Linked Traits • Alleles are expressed on each of the sex chromosomes • Female: XAXA or XAXa or XaXa • Male: XAYor XaY Setting up a punnet square for sex-linked traits: Mom= XAXa Dad = XAY XA Xa XA Y

  42. Mom is carrier, dad does not have x-linked recessive disorder Mom isn’t carrier, dad has x-linked recessive disorder

  43. Sex Linked Traits • Can a female end up with an X-linked trait???? • Example = Sex-linked baldness • assume that baldness (b) is recessive • Full head-o-hair (B) is dominant

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