Mendel’s particulate theory of heredity

1. Mendel’s particulate theory of heredity Parents transmit genes to their offspring. Genes remain as separate factors from one generation to the next—they’re NOT blended. Gregor Mendel studied pea plants.

2. WHY PEAS? • Available in lots of varieties • It was easy to control mating so that he knew which peas belonged to which parents. Vocabulary: character(istic)—inheritable feature of an organism that can be seen Ex. flower color trait—variant of a character Ex. purple or white true-breeding—always producing offspring with the same traits as the parents when the parent plants are self-fertilized

3. How to cross pea plants (in case you ever want to try…)

4. Mendel chose 7 characters of peas—each occurred in 2 different forms. • flower color • flower position • seed color • seed shape • pod shape • pod color • stem length

5. Mendel started all of his experiments with true-breeding parents. Colors didn’t blend, and the gene for white wasn’t lost! The gene for purple flowers masks the gene for white flowers.

6. More Vocabulary: • Alleles—alternative versions of genes • Dominant allele—one that is fully expressed • (ex. P = purple) • Recessive allele—one that is masked by the dominant allele (ex. p = white) • Locus (plural loci)—a specific place on a chromosome

7. Homozygous—having 2 of the same allele • ex. PP, pp homozygous recessive homozygous dominant • Heterozygous—having 2 different alleles • ex. Pp • Genotype—organism’s genetic makeup • ex. PP, Pp, pp • Phenotype—organism’s expressed traits • ex. purple flowers, white flowers

8. Alleles separate in meiosis since homologous chromosomes separate = Mendel’s Law of Segregation The paired condition is restored by the random fusion of gametes at fertilization. A Punnett Square can be used to predict probabilities of offspring genotype and phenotype.

9. P p PP Pp P purple purple Pp pp p white purple Genotype ratio— 1 PP : 2 Pp : 1 pp (¼ : ½ : ¼) Phenotype ratio– 3 purple: 1 white (¾ : ¼)

10. Mendel found the same thing for all seven characters! One parental trait disappeared in the F1 generation… but it reappeared in ¼ of the F2 generation! If we have a plant with purple flowers, how can we tell if it’s homozygous dominant (PP) or heterozygous (Pp)? We can do a TESTCROSS, which means that we cross it to a white flowered plant. We know the genotype of the white flowered plant, so we should be able to predict what would happen for the 2 options.

11. P P P p p p p p Heterozygous X Homozygous recessive Homozygous dominant X Homozygous recessive ___ purple offspring ___ white offspring ___ purple offspring ___ white offspring

12. Monohybrid cross = a cross between 2 organisms that are heterozygous for a single trait. • Dihybridcross = a cross between 2 organisms that are heterozygous for 2traits. Mendel wanted to see if traits segregated together or independently, so he looked at seed color and seed shape. Yellow is dominant to green. Round is dominant to wrinkled. If we cross a yellow, round plant (YYRR) to a green, wrinkled plant (yyrr), we’ll get a yellow, round plant (YyRr). If we cross 2 YyRrplants, will we ever see a yellow, wrinkled or green, round plant?

13. The parents are both YyRr. If the characters segregate dependently, then Y and R will always stay together, and y and r will always stay together. If they segregate independently, we will see Yr gametes and yR gametes—that means we’ll need a 4 x 4 Punnett Square!

15. Mendel’s Law of Independent Assortment: Each pair of alleles segregates independently during gamete formation. Instead of drawing a big Punnett Square, we can use probability when multiple genes are involved.

16. Random events are independent of each other. A couple has 10 children. Each time, there is a 50% chance that the child will be a boy and a 50% chance that it will be a girl. Even if they have 9 girls, the chance that their 10th child will be a girl is still 50%!

17. Rule of Multiplication: Probability of independent events occurring simultaneously (ie. AND) Premise: Dodger fan (D) is dominant to Giants fan (d) The parents are each heterozygous for the baseball fan gene (Dd). What’s the probability that they will have a daughter who’s a Dodger fan?

18. ½ chance that the child will be a girl ¾ chance that the child will be a Dodger fan ½ x ¾ = 3/8 chance that they will have a girl who’s a Dodger fan.

19. Rule of Addition: Probability of independent events occurring (ie. OR) What’s the probability that the same 2 parents from the last problem will have a child who’s EITHER a girl or a Dodger fan?

20. ½ chance that the child will be a girl ¾ chance that the child will be a Dodger fan Add the probability of EACH event occurring on its own and subtract the possibility of BOTH of them occurring together. So… ½ + ¾ - (½ x ¾) = 1¼ - 3/8 = 7/8 chance that the child will be EITHER a Dodger fan or a girl.

21. Since each gene assorts independently, we can use the rules of probability to figure out all sorts of seemingly complicated problems. Question: What is the probability that a trihybridcross between 2 organisms with the genotypes AaBbCc and AaBbCcwill produce an offspring with the genotype aabbcc?

22. Consider a trihybrid cross of garden peas Flower color: Purple (P) is dominant to white Seed color: Yellow (Y) is dominant to green Seed shape: Round (R) is dominant to wrinkled Parents: PpYyRr x Ppyyrr What are the chances that any given offspring is homozygous recessive for at least 2 of the traits?

23. Flower position: axial (A) is dominant to terminal • Stem length: tall (T) is dominant to short • Seed shape: round (R) is dominant to wrinkled • If a plant that is heterozygous for all three • characters were allowed to self-fertilize, what • proportion of the offspring would be expected • to be: • homozygous for the 3 dominant traits? • homozygous for the 3 recessive traits? • heterozygous for all 3 characters? • homozygous for axial and tall, heterozygous • for seed shape?

24. In sesame plants, the one-pod condition (P) is • dominant to the 3-pod condition. Normal leaf (L) • is dominant to wrinkled leaf. Determine the • genotypes for the 2 parents for all possible • matings producing the following offspring. • 318 one-pod normal, 98 one-pod wrinkled • 323 3-pod normal, 106 3-pod wrinkled • 401 one-pod normal • 150 one-pod normal, 147 one-pod wrinkled • 51 3-pod normal, 48 3-pod wrinkled • 5) 223 one-pod normal, 72 one-pod wrinkled, • 76 3-pod normal, 27 3-pod wrinkled

25. Incomplete dominance: the dominant phenotype is not fully expressed in the heterozygote. The heterozygous phenotype is between the phenotypes of the heterozygotes. Ex: flower color in carnations

26. Codominance—both alleles are fully expressed in the heterozygote. Ex. production of glycoproteins on red blood cells (MN blood groups)

27. Multiple Alleles—often there are more than 2 alleles for a certain characteristic. (Ex. ABO blood types)

29. Epistasis—interaction between 2 genes where one modifies the phenotype expressed by the other Ex. Genes affecting coat color in mice C—pigment can be deposited in fur B—black fur If a mouse is cc, B cannot be expressed

30. So we don’t get the normal 9:3:3:1 ratio of phenotypes

31. Polygenic inheritance—2 or more genes determine a single phenotypic character Ex. skin pigmentation in humans varies on a continuum rather than as an either/or trait like Mendel’s peas.

32. We can’t testcross humans, so we have to use pedigrees to trace inheritance patterns Pedigree—a family tree that shows the inheritance pattern of a particular phenotype. From pedigrees, we can attempt to determine genotypes. Males— Females— Shaded—has the trait Vertical lines connect generations

33. This pedigree shows Widow’s peak. Is Widow’s Peak a dominant or recessive trait?

34. This pedigree shows attached earlobes. Are attached earlobes dominant or recessive?