Mendel & the gene idea

1. Mendel & the gene idea Chapter 14

2. Key Vocabulary • Genetics:The scientific study of heredity • Heredity: the passing of traits from parents to offspring • Inheritance: You get your genes from your parents - in meiosis, half of the chromosomes in a pair come from the Dad, half come from the Mom

3. Key terms to know • Allele – each form of a gene for a certain trait (R or r) • Gene– sequence of DNA that codes for a protein a thus determines a trait • Genotype – combination of alleles for a given trait (RR or Rr or rr) • Phenotype – Appearance of trait ( round seeds or wrinkled seeds • Homozygous - when you have 2 or the same alleles for a given trait (RR or rr) • Heterozygous– when you have 2 different alleles for a trait (Rr)

4. Characters and Traits • Character – heritable feature that varies among individuals • ex. Flower color • Trait – each variant for a character • ex. Purple vs. white flowers • Originally believed that traits of parents blended together to give offspring results!!!

5. Gregor Mendel's Discoveries • Gregor Mendel – studied pea plants in monastery garden – COUNTED the plants and compiled data (QUANTITATIVE APPROACH to science). • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.

6. Figure 14.1 A genetic cross For his experiments, Mendel chose to CROSS POLLINATE (mate different plants to each other) plants that were TRUE BREEDING (meaning if the plants were allowed to self-pollinate, all their offspring would be of the same variety). P generation – parentals; true-breeding parents that were cross-pollinated F1 generation – (first filial) - hybrid offspring of parentals that were allowed to self-pollinate F2 generation – (second filial) - offspring of F1’s

7. Figure 14.2 Mendel tracked heritable characters for three generations If the blending model of inheritance were correct, the F1 hybrids from a cross between a purple-flowered and white-flowered pea plants would have pale purple flowers (an intermediate between the two traits of the parents…BUT: When F1 hybrids were allowed to self-pollinate, or when they were cross-pollinated with other F1 hybrids, a 3:1 ratio of the two varieties occurred in the F2 generation. So what happened to the white flowers in the F1 generation?

8. Mendel’s 4 ideas… • Alternative versions (different alleles)of genes account for variations in inherited characters. • For each character, an organism inherits two alleles, one from each parent. • If the two alleles differ, the dominant allele is expressed in the organism’s appearance, and the other, arecessiveallele is masked. • (Law of Dominance) • Allele pairs separate during gamete formation. This separation correspondes to the distribution of homologous chromosomes to different games in meiosis. • (Law of Segregation)

9. Figure 14.3 Alleles, alternative versions of a gene The gene for a particular inherited character, such as color, resides at a specific locus (position) on a certain chromosome. Alleles are variants of that gene. In the case of peas, the flower-color gene exists in two versions: the allele for purple flowers and the allele for white flowers. This homologous pair of chromosomes represents an F1 hybrid, which inherited the allele for purple color from one parent and the allele for white flowers from the other parent.

10. Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants MENDEL’S TEST CROSSES ON PEA PLANTS Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall *Flower color – purple (P) vs. white (p) Seed coat color and flower color are often put in for one another – thus, the EIGHT traits!!!

11. Figure 14.4 Mendel’s law of segregation (Layer 1) Each true-breeding plant of the parental generation has matching alleles, PP or pp. Gametes (circles) each contain only on allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F1 hybrids having a Pp combination (because the purple allele is dominant, all these hybrids have purple flowers.) When the hybrid plants produce gametes, the two alleles segregate (separate), half the gametes receiving the P allele and the other half the p allele. This Punnett square shows all possible combinations of alleles in offspring. Each square represents an equally probable product of fertilization. Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. The LAW OF SEGREGATION states that allele pairs separate during gamete formation, and then randomly re-form as pairs during the fusion of gametes at fertilization.

12. Figure 14.4 Mendel’s law of segregation (Layer 2) The LAW OF SEGREGATION states that during the formation of gametes, the two traits carried by each parent separate. Parent cell with full gene and Tt alleles. Traits have separated during gamete formation from meiosis.

13. Figure 14.5 Genotype versus phenotype Grouping F2 offspring from a cross for flower color according to phenotype results in the typical 3:1 ratio. In terms of genotype, there are actually two categories of purple-flowered plants (PP and Pp).

14. Law of Independent Assortment • States that each allele pairs of different genes segregates independently during gamete formation; • applies when genes for two characteristics are located on different pairs of homologous chromosomes. • See figure 14.7 (page 253) • http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

15. Punnett Square • Device for predicting the results of a genetic cross between individuals of a known phenotype. • Developed by R.C. Punnett • Rules: • must predict possible gametes first • male gametes are written across top, female gametes on left side • when reading a Punnett, start in upper left corner and read as if a book – WRITE OUT GENOTYPES IN ORDER!

16. Board examples • Character – flower color • Alleles – Purple (P) and white (p) Genotypic Combos possible – two dominants: PP (homozygous dominant) two recessives: pp (homozygous recessive) One of each: Pp (heterozygous) Phenotypes possible – • PP – looks purple, so phenotype is purple • pp – looks white • Pp – looks purple (white is masked, but still part of genotype)

17. Testcross • Designed to reveal the genotype of an organism that exhibits a dominant trait • it is homozygous dominant or heterozygous? • Involves the breeding of a recessive homozygote with an organism of dominant phenotype by unknown genotype

18. Figure 14.6 A testcross Is the dominant phenotype homozygous or heterozygous? A testcross will tell us!

19. Monohybrid crosses – only one character considered Steps to do: • Write out genotypes of parents • Write out possible gametes produced • Draw 4 box Punnett square • Put male gametes on top, female on left side • Fill in boxes • Determine genotypes by reading Punnett starting from top left • Determine phenotypes by reading from genotype list Ex. • White flowered plant X Purple flowered plant • Yellow peas X Green peas • Tall plant X short plant

20. Dihibrid cross • Developed following TWO characters at the same time… Dihybrid cross Ex. Homozygous dominant for seed color, homozygous dominant for seed shape X homozygous recessive for seed color, homozygous recessive for seed shape

21. Steps to do… • Write out genotypes of parents • Write out possible gametes produced – “hopscotch method” • Draw 16 box Punnett square • Put male gametes on top, female on left side • Fill in boxes • Determine genotypes by reading Punnett starting from top left • Determine phenotypes by reading from genotype list

22. Dihybrid practice problems… 1. heterozygous for shape, heterozygous for color X heterozygous for shape, heterozygous for color 2. heterozygous for shape, homozygous recessive for color X homozygous dominant for shape, homozygous recessive for color

23. Beyond Mendel • Mendel’s two laws, segregation and independent assortment, explain heritable variations in terms of alternative forms of genes (hereditary “particles”) that are passed along, generation after generation, according to simple rules of probability. • Figure 14.4 in text (be able to explain) • Figure 14.7 B in text (be able to explain) • Now let’s go beyond basic Mendelian genetics….

24. Other Genetic Landmarks • 1879 Walther Flemming – German biologist who stained cells with dye and saw tiny, threadlike structures in the nucleus CHROMOSOMES! • also observed and describedMITOSIS and noted that a full set of chromosomes was being passed on to each daughter cell. • Sixteen years after Mendel’s death, his paper is rediscovered and scientists realize that the chromosomes are the carriers of heredity – Mendel’s FACTORSare ensuring the passing of traits from parents to offspring. • 1902 Walter Sutton – American biologist who supports idea that “factors” are located on chromosomes

25. Other Genetic Landmarks • 1905 E.B. Wilson and Nettie Stevens – Americans studying insect chromosomes • Saw that male insects always showed a chromosome that did not seem to have a match (females always had a perfect matching set of chromosomes.) Thus, they referred to the non-matching chromosomes as Sex Chromosomes. • In females the sex chromosomes do match XX • In males, one of the chromosomes looked as if it were missing a part, so called it a Y XY

26. Other Genetic Landmarks • 1909 Wilhelm Johannsen – Danish biologist who coined the term “gene” to define the physical units of heredity. • GENE: segment of DNA molecules that carries the instructions for producing a specific trait.

27. Other Genetic Landmarks • 1912 Thomas Hunt Morgan – Showed evidence that the presence of white eye color in fruit flies was associated with a particular gene on a particular chromosome. • Drosophila melanogaster -- scientific name for fruit fly .

28. Why Study Fruit Flies? • Produces about 100 offspring per egg lay – good statistics! • Matures in only 15-20 days! • Only have 8 chromosomes (4 pair) so less to look at! • Easy/inexpensive to raise! • Chromosomes are VERY large and easy to see and locate! • Sexes are easily distinguished • female is larger • shapes of abdomen identify sexes at a glance

29. Drosophila Crosses • Normally, fruit flies always have RED eyes, but Morgan saw a white eyed one show up, and it was MALE!! Thought that this was strange, so he conducted an experiment: P white eyed X red eyed F1 all red eyed offspring (thus concluded that red is dominant over white for color) F1 red eyed X red eyed F2 ¾ red eyed & ¼ white eyed (AND ALL OF THE WHITE EYED ONES WERE MALE!!!) • Determined that this was a sex-linked trait – the trait for eye color in fruit flies is carried on the sex chromosome. • Examples of other sex-linked traits: hemophilia & color blindness • C = normal vision, c = colorblindness • Xc Y crossed with XCXc….work this problem out!

30. Dominance, Multiple Alleles, and Pleiotrophy • Involve effects of alleles for SINGLE GENES

31. DOMINANT Alleles • See pages 256 and 257 • Definition is NOT clear cut… • Three points: • They range from complete dominance, through various degrees of incomplete dominance, to codominance. • They reflect the mechanisms by which specific alleles are expressed in phenotype and do not involve the ability of one allele to subdue another at the level of the DNA. • Dominant alleles are not necessarily more common.

32. Incomplete Dominance • Incomplete Dominance: when BOTH alleles in an individual affect the appearance of a trait and you get a brand new color that was not found in the original parents. Both traits are written in capitals and have different letters because BOTH control the appearance. • Example: flower color in snapdragons • Pure red (RR) X Pure white (WW) • Offspring will be pink (RW)

33. Incomplete Dominance

34. Codominance • Codominance: when 2 alleles work together and BOTH are expressed without one masking the other (NO intermediate phenotype) • TWO ALLELES AFFECT THE PHENOTYPE IN SEPARATE, DISTINGUISHABLE WAYS!

35. Multiple Alleles • Multiple Alleles:when more than two possibilities for a trait are present. • Example: Blood type – see pages 257 and 258 • There are 3 alleles for blood type -- A, B, O • Here, A and B are dominant over O, but if A and B are present together, neither dominates!!! This is codominance – they share the power of expression.

36. More on Blood Types • The letters A, B, and O refer to 2 carbohydrates found on the surfaced of RED BLOOD CELLS. • Will often see the A,B designation as superscripts with a base of I; • O (since is recessive to A and B) is shown as i. • Matching compatible blood groups is critical – proteins called antibodies are produced against foreign blood factors. • Antibodies bind to foreign molecules and cause donated blood cells to clump together(agglutination).

37. Figure 14.10 Multiple alleles for the ABO blood groups

38. Pleiotropy • Most genes have MULTIPLE phenotypic effects • Ability of a gene to affect an organism in many ways is called PLEIOTROPHY • This is due to molecular and cellular interactions that are responsible for an organism’s development • Ex. Sickle-cell disease (page 262)

39. Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote Sickle cell is a disease caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells. When oxygen concentration of affected individual is low, the hemoglobin crystallizes into long rods. Heterozygotes for sickle cell have increased resistance to malaria because the rod shape of blood interrupts the parasites life cycle. So, sickle cell is prevalent among African Americans.

40. Epistasis • Involves MORE THAN ONE GENE • Defined as when a gene at one locus alters the phenotypic expression of a gene at a second locus • Mouse coat color – page 258 • coat color – B = black, b = brown • second gene determines whether pigment will be deposited in the hair: C = color, c = albino

41. Figure 14.11 An example of Epistasis One gene determines whether the coat will be black (B) or brown (b). The second gene controls whether or not pigment of any color will be deposited in the hair, with the allele for the presence of color (C) dominant to the allele for the absence of color (c).

42. Polygenic Inheritance Additive effect of two or more genes on a single phenotypic character Ex. Skin color in humans – page 259

43. Nature vs. Nurture • Phenotype depends on nature AND genes… • See NORM OF REACTION: phenotypic range of possibilities due to environmental influences on genotype…READ TEXT PAGE 259! • Ex. Blood count of RBC’s and WBC’s depends on altitude, physical activity, presence of infection • Ex. Color of hydrangea blooms depends on soil acidity

44. Figure 14.13 The effect of environment of phenotype

45. Human Genetics • Humans are difficult to study…but we have developed ways to approach these difficulties. • Pedigree analysis – family history for a particular trait • Study of Genetic diseases • Twin studies – Nature vs. nurture • Population Sampling • Genetic Technology

46. Figure 14.14 Pedigree analysis • Males are shown as squares, Females are shown as circles • Horizontal lines – “marriage” or mating lines • Vertical lines – offspring lines • Shaded symbols represent individuals with the trait being studied • CARRIERS of the trait are those individuals that are heterozygous (Ww OR Ff) because they may transmit the recessive allele to their offspring even though they do not express the trait. • See text page 261 – PEDIGREE ANALYSIS

47. Errors in Chromosomes • Mistakes in numbers of chromosomes: • nondisjunction -- members of a pair of homologous chromosomes do not move apart properly…result in offspring that have: • Aneuploidy – abnormal chromosome number: • Can be…Trisomy or Monosomy or Polyploidy

48. Chromosomal Mistakes 2. Mistakes in shape of chromosomes: • deletion – part of chromosome is broken off and lost completely • duplication – broken fragment of chromosome attaches to sister chromatid so section is repeated on that chromatid • inversion – when fragment reattaches to original chromosome but in reverse order • translocation – broken fragment attaches to a nonhomologous chromosome • (can exist as reciprocal or nonreciprocal)

49. Figure 15.13 Alterations of chromosome structure

50. Technology is Providing New Tools for Genetic Testing and Counseling • Carrier recognition with genetic screening and Fetal testing: -ultrasound and sonograms -amniocentesis -chorionic villi sampling -fetoscopy -blood/urine tests of newborns