CHAPTER 9 Patterns of Inheritance

1. CHAPTER 9Patterns of Inheritance Modules 9.11 – 9.23

2. VARIATIONS ON MENDEL’S PRINCIPLES The relationship of genotype to phenotype is rarely simple • Mendel’s principles are valid for all sexually reproducing species • However, often the genotype does not dictate the phenotype in the simple way his principles describe

3. 1. Incomplete dominance results in intermediate phenotypes P GENERATION Whiterr Red RR • When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance Gametes R r PinkRr F1 GENERATION 1/2 R 1/2 r 1/2 R 1/2 R Eggs Sperm RedRR 1/2 r 1/2 r PinkRr PinkrR F2 GENERATION Whiterr Figure 9.12A

4. GENOTYPES: • Incomplete dominance in human hypercholesterolemia HH Homozygousfor ability to makeLDL receptors Hh Heterozygous hh Homozygousfor inability to makeLDL receptors PHENOTYPES: LDL LDLreceptor Cell Normal Mild disease Severe disease Figure 9.12B

5. 2. Many genes have more than two alleles in the population • In a population, multiple alleles often exist for a characteristic • The three alleles for ABO blood type in humans is an example • The alleles are iA…iB….and…i

6. 3. Codominance--The alleles for A and B blood types are codominant, and both are expressed in the phenotype BloodGroup(Phenotype) AntibodiesPresent in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Genotypes O A B AB Anti-A Anti-B O ii IA IA or IA i A Anti-B IB IB or IB i B Anti-A AB IA IB Figure 9.13

7. There are at least 29 different blood groups including the ABO blood group.

8. The Calico Cat-codominance Calico coloring is a mix of phaeomelanin based colors (red) and eumelanin based color (black, chocolate and cinnamon). Cats of this coloration are believed to bring good luck in the folklore of many cultures.[1] The spotting gene causes white patches to cover the colored fur

9. Co-Dominance ---Both Alleles express themselves

10. 4. A single gene may affect many phenotypic characteristics • A single gene may affect phenotype in many ways • This is called pleiotropy • The allele for sickle-cell disease is an example

11. Individual homozygousfor sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped Sickle cells Clumping of cells and clogging of small blood vessels Accumulation ofsickled cells in spleen Breakdown of red blood cells Physical weakness Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Anemia Pneumonia and other infections Impaired mental function Kidney failure Rheumatism Paralysis Figure 9.14

12. 5. A single characteristic may be influenced by many genes • This situation creates a continuum of phenotypes • Example: skin color

13. P GENERATION aabbcc(very light) AABBCC(very dark) F1 GENERATION AaBbCc AaBbCc Eggs Sperm Fraction of population Skin pigmentation F2 GENERATION Figure 9.16

14. 9.15 Connection: Genetic testing can detect disease-causing alleles • Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring Figure 9.15B • Dr. David Satcher, former U.S. surgeon general, pioneered screening for sickle-cell disease • Thallasemia, Tay Sachs Figure 9.15A

15. Canavan Disease -- This condition is most common in people of Ashkenazi Jewish ancestry, with a carrier incidence of 1 in 40. Canavan disease is a central nervous system disease that is usually fatal in childhood, with a few people surviving to adulthood. This disease is the result of a substance that destroys the central nervous system over time. There is presently no effective treatment for Canavan disease. • Fragile X Syndrome -- The Fragile X syndrome is not specific to a certain ethnic background. It is an inherited condition that can cause a range of intellectual and behavioral problems, from learning disabilities to mental retardation to autism. While Fragile X syndrome tends to be more severe in boys, it occurs in both males or females. It can be passed on to family members by individuals who have no signs of the syndrome. Review of your family history with a genetic counselor may help determine if Fragile X carrier testing is indicated. • Sickle Cell Disease -- This condition is most common in persons of African-American, African, Mediterranean, Hispanic and South American ancestry, with the carrier risk ranging from 1/10 to 1/40, depending on your ethnic background. Sickle cell disease is caused by a variant hemoglobin that changes the shape of the red blood cells. This causes anemia, severe pain, a tendency toward infection, and other serious health problems. Frequent blood transfusions and infection preventing antibiotics are available treatment. • Tay Sachs Disease -- People of both Ashkenazi Jewish and French Canadian ancestry have the greatest chance of being carriers of Tay Sachs disease, about 1/30 versus 1/250 in the general population. The disease results from a build up of certain substances in the brain, and is fatal in early childhood. There is presently no effective treatment for Tay Sachs disease. • Thalassemia -- Individuals of Mediterranean, Southeast Asian and African ancestry have the greatest chance - 1 in 3 and 1 in 30, respectively -- of being carriers for thalassemia. In general, this group of blood disorders affects a person's ability to produce hemoglobins, the protein in our blood that carries oxygen and nutrients to all parts of the body. In severe cases, children with thalassemia may not survive. Others have anemia, bone growth problems and liver and spleen involvement. Blood transfusions may be needed for treatment.

16. Prenatal diagnosis for couples testing positive after genetic testing • Chorionic villus sampling at 8 weeks of gestation. • Amniocentesis at 12-18 weeks of gestation. • Pre-implantation genetic testing and In vitro fertilization

17. THE CHROMOSOMAL BASIS OF INHERITANCE 9.17 Chromosome behavior accounts for Mendel’s principles • Genes are located on chromosomes • Their behavior during meiosis accounts for inheritance patterns

18. The chromosomal basis of Mendel’s principles Figure 9.17

19. Question? • What is the phenotypic ratio between a dihybrid cross involving two heterozygotes? • 9:3:3:1

20. 9.18 Genes on the same chromosome tend to be inherited together • Certain genes are linked • They tend to be inherited together because they reside close together on the same chromosome When would they not be inherited together?

21. If you cross PpLl x PpLl, what Phenotypic ratio do you expect? When the organism was selved or self pollinated, most of the progeny looked like the parent but the 9:3:3:1 ratio was not realized!! Linked genes were then suspected. Figure 9.18

22. 9.19 Crossing over produces new combinations of alleles • This produces gametes with recombinant chromosomes • The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

23. A B a b B A a b A b a B Tetrad Crossing over Gametes When does crossing over take place? Figure 9.19A, B

24. Question? • What is the phenotypic ratio between a dihybrid cross involving two heterozygotes? • 9:3:3:1

25. When you cross a di-hybrid heterozygote with a homozygous recessive, you expect a ¼ distribution of each potential phenotype. GgLl X ggll When this particular cross was performed, this was not the case. Most of the organisms resembled the parents. A few had the independently assorted phenotypes. What had occurred is that genes for body color and wing type were on the same chromosome (linked) and in some gametes, crossing over did occur, but in most gamestes, G and L were linked as were g and l. Figure 9.19C

26. Is crossing over more likely to occur between genes that are close together or farther apart? Does crossing over comply with Mendel’s Law of Independent Assortment? Law of Segregation? Crossing Over and Linkage

27. 9.20 Geneticists use crossover data to map genes • Crossing over is more likely to occur between genes that are farther apart • Recombination frequencies can be used to map the relative positions of genes on chromosomes. • Determine the location of g and c and l on the chromosome. Recombination frequency between g and c is 9%, between c and l is 9.5%, and between g and l is 17%. Chromosome g c l 17% 9% 9.5% Figure 9.20B

28. answer • Answer: 3.6% recombination • 3.6 map units • 3.6 centiMorgans

29. In corn C (colored) is dominant to c (colorless) and for the endosperm (part of seed where food is stored for embryo) Full (F) is dominant to shrunken (f). When an CcFf was test crossed, the results were as follows: • Colored, full 4032 • colored, shrunken 149 • Colorless, full 152 • Colorless, shrunken 4035 • What do you expect if the genes are not linked? CcFf x ccff? 1:1:1:1 • We got 27:1:1:27

30. The Chinese primrose-slate color (s) is recessive to blue (S), red stigma(r) is recessive to green stigma (R), and long style (l) is recessive to short style (L). All three genes are on the same chromosome. The F1 of a cross of true breeding strains, was test crossed and gave the following: • Slate, green, short 27 • Slate, red, short 85 • Blue, red short 402 • Slate, red, long 977 • Slate, green, long 427 • Blue, green, long 95 • Blue, green, short 960 • Blue, red ,long 27 • Total 3000

31. Answer the following questions: • What were the genotypes of the parents in the cross of the two true-breeding strains? • Make a map of the genes, showing gene order and distance between them. • Answer: hum??????????????????

32. slate (s) red (r) long (l) • Blue (S) Green (R) short (L) • SsRrLl x ssrrll Expect 1:1:1:1 If not expect crossing over. • To determine placement of genes on chromosomes: • Inspect for highest frequencies for parental phenotypes. (NO crossing over) • Inspect for other phenotypes to show single and double crossing over. (Lowest # is double crossing over) • Determine position of genes on chromosome. The gene that has changed position relative to the other two is the central (observe double cross overs only). • Designate regions I and II and calculate crossing over in those regions. Add all combinations in each region (both single cross over and double) to determine cross over frequency in that region.

33. Blue, Green, Short and slate, red, long are parental phenotypes • Blue, red, long and slate, Green, Sort are the double crossovers. • Flower color (Blue vs slate)has changed position relative to stigma and style. “S: is in the middle. Rsl and RSL • Region I ---Rsl and rSL + RsL and rSl = 427 + 402 + 27 + 27 = 883/3000 = 29.43% • Region II ---RSl and rsL + RsL and rSl = 95 + 85 + 27 + 27 = 234/3000 = 7.8% • Thus, R----------------S-----L

34. answer • Between r and s the distance is 29.4 map units • Between s and l is 7.8 map units.

35. Alfred H. Sturtevant, seen here at a party with T. H. Morgan and his students, used recombination data from Morgan’s fruit fly crosses to map genes Figure 9.20A

36. Mutant phenotypes Shortaristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Browneyes • A partial genetic map of a fruit fly chromosome Long aristae(appendageson head) Gray body (G) Red eyes (C) Normal wings (L) Redeyes Wild-type phenotypes Figure 9.20C

37. SEX CHROMOSOMES AND SEX-LINKED GENES 9.21 Chromosomes determine sex in many species • A human male has one X chromosome and one Y chromosome • A human female has two X chromosomes • Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

38. (male) (female) Parents’diploidcells X Y Male Sperm Egg Offspring(diploid) Figure 9.21A

39. The X-O system • Other systems of sex determination exist in other animals and plants • The Z-W system • Chromosome number Figure 9.21B-D

40. 9.22 Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes are said to be sex-linked • In many organisms, the X chromosome carries many genes unrelated to sex • Fruit fly eye color is a sex-linked characteristic Figure 9.22A

41. These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait • Their inheritance pattern reflects the fact that males have one X chromosome and females have two Female Male Female Male Female Male XRXR XrY XRXr XRY XRXr XrY XR Xr XR XR XR Xr Y XRXr XRXR XRXr Y Y Xr Xr XRY XrXR XRY XrXr XRY XrY XrY R = red-eye allele r = white-eye allele Figure 9.22B-D

42. 9.23 Connection: Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles • Examples: hemophilia, red-green color blindness • These are mostly seen in males • A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Figure 9.23A

43. A high incidence of hemophilia has plagued the royal families of Europe Albert QueenVictoria Alice Louis Alexandra CzarNicholas IIof Russia Alexis Figure 9.23B