Purebreds and Mutts — A Difference of Heredity

1. Purebreds and Mutts — A Difference of Heredity • Genetics is the science of heredity • These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups • Purebreds often suffer from serious genetic defects

2. Their behavior and appearance is more varied as a result of their diverse genetic inheritance • The parents of these puppies were a mixture of different breeds

3. Experimental genetics began in an abbey garden • Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants Stamen Carpel Figure 9.2A, B

4. White 1 Removed stamensfrom purple flower • Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower PARENTS(P) Purple 3 Pollinated carpel matured into pod • This illustration shows his technique for cross-fertilization 4 Planted seeds from pod OFF-SPRING(F1) Figure 9.2C

5. FLOWER COLOR Purple White FLOWER POSITION • Mendel studied seven pea characteristics Axial Terminal • He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity SEED COLOR Yellow Green SEED SHAPE Round Wrinkled POD SHAPE Inflated Constricted POD COLOR Green Yellow STEM LENGTH Figure 9.2D Tall Dwarf

6. Mendel’s principle of segregation describes the inheritance of a single characteristic P GENERATION(true-breedingparents) • From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic • One characteristic comes from each parent Purple flowers White flowers All plants have purple flowers F1generation Fertilization among F1 plants(F1 x F1) F2generation 3/4of plantshave purple flowers 1/4of plantshave white flowers Figure 9.3A

7. GENETIC MAKEUP (ALLELES) P PLANTS PP pp Gametes All P All p • The pairs of alleles separate when gametes form • This process describes Mendel’s law of segregation • Alleles can be dominant or recessive • A sperm or egg carries only one allele of each pair F1 PLANTS(hybrids) All Pp Gametes 1/2P 1/2p P P Eggs Sperm PP F2 PLANTS p p Pp Pp Phenotypic ratio3 purple : 1 white pp Genotypic ratio1 PP : 2 Pp : 1 pp Figure 9.3B

8. Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes GENE LOCI DOMINANT allele P a B P a b RECESSIVE allele GENOTYPE: PP aa Bb HOMOZYGOUSfor thedominant allele HOMOZYGOUSfor therecessive allele HETEROZYGOUS Figure 9.4

9. Mendel’s principles reflect the rules of probability F1 GENOTYPES • Inheritance follows the rules of probability • The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring Bb female Bb male Formation of eggs Formation of sperm 1/2 B B 1/2 B B 1/2 b b 1/2 1/4 B b b B 1/4 1/4 b b F2 GENOTYPES 1/4 Figure 9.7

10. Connection: Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability Figure 9.8A

11. 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

12. 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

13. ABO blood types Figure 9.13x

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

15. Normal and sickle red blood cells Figure 9.14x1

16. 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

17. 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

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

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

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

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

22. 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

23. Figure 9.18

24. 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

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

26. Figure 9.19C

27. 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

28. 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

29. 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

30. SEX CHROMOSOMES AND SEX-LINKED GENES 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

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

32. 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

33. 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

34. 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

35. 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

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