CHAPTER 9 Patterns of Inheritance

1. CHAPTER 9Patterns of Inheritance Modules 9.11 – 9.23

2. VARIATIONS ON MENDEL’S PRINCIPLES 9.11 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. 9.12 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. 9.13 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

6. 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. 9.14 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

8. 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

9. 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 Figure 9.15A

10. 9.16 A single characteristic may be influenced by many genes • This situation creates a continuum of phenotypes • Example: skin color

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

12. 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

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

14. 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

15. Figure 9.18

16. 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

17. A B a b B A a b A b a B Tetrad Crossing over Gametes Figure 9.19A, B

18. Figure 9.19C

19. 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 Chromosome g c l 17% 9% 9.5% Figure 9.20B

20. 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

21. 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

22. 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

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

24. 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

25. 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

26. 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

27. 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

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