11.1 Mendel and the Black Box

2. 11.1 Mendel and the Black Box

3. Mendel and the Black Box • Gregor Mendel was the first person to comprehend some of the most basic principles of genetics.

4. Gregor Mendel Figure 11.1

5. Gregor Mendel • Mendel reached these understandings in the mid-nineteenth century working in what is now the Czech Republic and using as his experimental subjects, a species of garden pea, Pisum sativum.

6. 11.2 The Experimental Subjects: Pisum sativum

7. The Experimental Subjects • Mendel looked at seven characters in his plants—attributes such as seed color and texture. • He observed which of those traits showed up in succeeding generations. • In his plants, each of these characters came in two varieties or traits, one of them dominant, the other recessive.

8. Cross Pollination 1. Before fertilization occurs, peel back the closed petals of a pea plant (in this case, one that came from a line that yielded yellow peas). Then pull out the pollen-bearing stamens with tweezers so that self-fertilization is no longer possible. flower grown from a yellow seed 2. Next, gather pollen from a green-seedplant by dabbing its anthers with a paintbrush. flower grown from a green seed cross- pollination 3. Finally, rub these pollen grains onto the stigma of the first plant. The results of the cross-pollination can be observed when the fertilized eggs mature into seeds in the ovary, meaning peas in a pod. The resulting seeds are yellow in this case because yellow is dominant over green. offspring (yellow seeds) Figure 11.3

9. Phenotypes and Genotypes • A phenotype is any physiological feature, bodily characteristic, or behavior of an organism. • In Mendel’s plants, purple flowers were one phenotype, and white flowers were another.

10. Phenotypes and Genotypes • Phenotypes in any organism are in significant part determined by that organism’s genotype, meaning its genetic makeup.

11. Table 11.1 Pea-Plant Characters Studied by Mendel Dominant Trait Recessive Trait Character Studied smooth wrinkled Seed shape yellow green Seed color wrinkled Pod shape inflated yellow Pod color green Flower color purple white Flower position at tip on stem Stem length dwarf tall Table 11.1

12. Three Genotypes Yield Two Phenotypes Y y Y y y y Y Y Three genotypes yield . . . two phenotypes. green yellow Figure 11.7

13. 11.3 Starting the Experiments: Yellow and Green Peas

14. Starting the Experiments • Mendel realized that it was possible for organisms to have identical phenotypes—for all his pea plants to have yellow seeds, for example—and yet to have differing underlying genotypes.

15. Starting the Experiments One of Mendel’s central insights was that the basic units of genetics are material elements that, in his pea plants, came in pairs. These elements, today called genes, come in alternative forms called alleles.

16. Starting the Experiments One allele for a gene resides on one chromosome. The other allele for the same gene resides on a second chromosome that is homologous to the first.

17. possible pairing of homologous chromosomes dominant allele recessive allele location of gene for seed color maternal paternal maternal maternal paternal paternal heterozygous homozygous recessive homozygous dominant yellow seeds yellow seeds green seeds Figure 11.8

18. Genes Retain Their Character Another of Mendel’s insights was that genes retain their character through many generations rather than being “blended” together.

19. (a) P generation crosses 1. Female gametes are being provided by a plant that has the dominant, yellow alleles (YY); male gametes are being provided by a plant that has the recessive, green alleles (yy). female male 2. The cells of the pea plants that give rise to gametes start to go through meiosis. P generation 3. The two alleles for pea color, which lie on separate homologous chromosomes, separate in meiosis, yielding gametes that each bear a single allele for seed color. In the female, each gamete bears a Y allele; in the male, each bears a y allele. male gametes female gametes 4. The Punnett square shows the possible combinations that can result when the male and female gametes come together in the moment of fertilization. (If you have trouble reading the Punnett square, see Figure 11.5b). The single possible outcome in this fertilization is a mixed genotype, Yy. possible outcomes in fertilization 5. Because Y (yellow) is dominant over y (green), the result is that all the offspring in the F1 generation are yellow because they all contain a Y allele. F1 generation (b) How to read a Punnett square female gametes female gametes male gametes male gametes 1. A p gamete from the male combines with a p gamete from the female to produce an offspring of pp genotype (and white color). 2. A p gamete from the male combines with a P gamete from the female to produce an offspring of Pp genotype (and purple color). Figure 11.5

21. 11.4 Another Generation

22. Law of Segregation • A third insight of Mendel’s was that alleles separate prior to the formation of gametes. • The alleles Mendel was observing resided on homologous chromosomes, which always separate in meiosis. • This concept is known as the law of segregation.

23. Law of Segregation • An organism that has two identical alleles of a gene for a given character is said to be homozygous for that character.

24. Law of Segregation • An organism that has differing alleles for a character is said to be heterozygous for that character.

25. Law of Segregation • Dominant: expressed in the heterozygous condition. • Example: the yellow color of peas present in the heterozygous Yy condition.

26. Law of Segregation • Recessive: not expressed in the heterozygous condition. • Example: the green color of peas absent in the Yy condition.

27. Law of Segregation Y y Y y y y Y Y Three genotypes yield . . . two phenotypes. green yellow Figure 11.7

28. 11.5 Crosses Involving Two Characters

29. Crosses Involving Two Characters • Mendel observed that the genes for the different characters he studied were passed on independently of one another. • This was because the genes for these characters resided on separate, non-homologous chromosomes.

30. Crosses Involving Two Characters The physical basis for what he found is the independent assortment of chromosome pairs during meiosis.

32. 11.6 Reception of Mendel’s Ideas

33. Reception of Mendel’s Ideas • Gregor Mendel published his work, but the significance of it was never recognized in his lifetime. • It was only rediscovered 16 years after his death, in 1900.

34. 11.7 Incomplete Dominance and Codominance

35. Incomplete Dominance • Incomplete dominance operates when neither allele for a given gene is completely dominant, with the result that heterozygous genotypes can yield an intermediate phenotype (such as pink snapdragons).

36. P generation 1. The starting plants are a snapdragon homozygous for red color (RR) and snapdragon homozygous for white color (rr). rr white RR red F1 generation 2. When these plants are crossed, the resulting Rr genotype yields only enough pigment to produce a flower that is pink—the only phenotype in the F1 generation. Rr 100% pink sperm R r F2 generation 3. In the F2 generation, alleles combine to produce red, pink, and white phenotypes. R Rr RR egg r Rr rr 1 red 2 pink 1 white : : Figure 11.10

37. Variations on Mendel Animation 11.2: Variations on Mendel

38. Codominance • In some instances, differing alleles of the same gene will have independent effects in a single organism. • Such is the case with the gene that codes for the type A and B glycolipids that extend from the surface of human red blood cells.

39. Codominance • An individual who has one A and one B allele will have type AB blood. • In such a situation, neither allele is dominant; rather, each is having a separate phenotypic effect.

40. Table 11.3 Human Blood Types This blood type (phenotype) . . . . . . and is produced by these genotypes . . . has these surface glycolipids . . . AA or AO A B BB or BO AB AB O OO (no surface glycolipids) The familiar ABO human blood-typing system refers to glycolipid molecules that extend from the surface of red blood cells. People whose blood is “type A” have A extensions on their blood cells. It is also possible to have only B extensions (and be type B); to have both A and B extensions (and be type AB); or to have none of these extensions (and be type O). Note that a person whose genotype is AO is phenotypically type A; likewise, a person whose genotype is BO is phenotypically type B. Table 11.3

41. Codominance When differing alleles of a single gene have independent effects on the phenotype of an individual, the alleles are said to be codominant.

42. 11.8 Multiple Alleles and Polygenic Inheritance

43. Polygenic Inheritance • Human beings and many other species can have no more than two alleles for a given gene, each allele residing on a separate, homologous chromosome. • However, many allelic variants of a gene can exist in a population.

44. Polygenic Inheritance • Most traits in living things are governed by many genes. • These genes often have several allelic variants.

45. Polygenic Inheritance • Polygenic inheritance means the inheritance of a genetic character is determined by the interaction of multiple genes, with each having a small additive effect on the character.

46. Polygenic Inheritance • Polygenic inheritance tends to produce continuous variation in phenotypes, in which there are no fixed increments of difference between individuals. • Human skin, for example, comes in a range of colors in which one color shades imperceptibly into the next.

47. Polygenic Inheritance • The traits produced in polygenic inheritance tend to manifest in bell-curve distributions, in which most individuals display near average trait values rather than extreme trait values.

48. Continuous Variation and the Bell Curve Figure 11.12

49. Polygenic Inheritance • Gene interactions and gene–environment interactions are so complex in polygenic inheritance that predictions about phenotypes are a matter of probability, not certainty.

50. 11.9 Genes and Environment