Mendelian Genetics

1. Mendelian Genetics • Chapter 12, part 1

2. Gregor Mendel • Born in 1822 in Moravia (now part of the Czech Republic. • Son of a tenant farmer; joined a monastery to get an education. • Deeply interested in science, particularly heredity.

3. At the monastery in Brno, Moravia, Mendel received the support of Abbot Napp. • From 1851-1855, studied at the University of Vienna, but did not receive a degree.

4. What was understood at the time: • Heredity appeared random and unpredictable. • Many traits seemed to blend in the offspring, suggesting a liquid factor controlled heredity. • Yet some traits, such as red hair, did not blend away.

5. With Abbot Napp’s encouragement, Mendel studied heredity in peas, carefully choosing traits that did not appear to blend. Collected data from 1856 - 1865. • Mendel’s creative contribution: he was the first to follow single traits from generation to generation instead of trying to document and follow every trait in the plants.

6. Mendel presented his findings to the Association of Natural Research in Brno in 1865. • Few people recognized the significance of Mendel’s research. His quantitative methods were uncommon at the time, and the “blending” theory was widely accepted.

7. In 1868, Mendel became abbot of his monastery. • His religious work left little time for research, which he set aside, though he was always convinced he had made a valuable contribution to science.

8. Mendel died in 1884. Sixteen years later, in 1900, his work was rediscovered by Hugo de Vries and others looking for clues into the puzzle of heredity. • Though criticized in some details, the main body of Mendel’s work still stands.

9. Mendel’s Laws • A scientific law is an evidence-based description of a natural phenomenon in a given set of circumstances. • Mendel’s three Laws of Heredity describe what Mendel observed in patterns of inherited traits.

10. Three Laws of Heredity • Law of Dominance • Law of Segregation • Law of Independent Assortment

11. Law of Dominance • Traits are controlled by two factors that can be called “dominant” or “recessive.” • A “dominant” trait shows if the offspring inherits at least one dominant factor from one parent. • A “recessive” trait shows only if the offspring inherits two recessive factors, one from each parent.

12. In this cross between two purple-flowered pea plants, one-quarter of the offspring have white flowers. Based just on this information, which is dominant: white or purple flowers? How do you know? X Hint: “Dominance” is not based on numbers of individuals with the trait. It is based on the number of copies of the allele that must be inherited to show the trait.

13. The offspring of a purple-flowered pea plant and a white-flowered pea plant all have purple flowers. The purple trait is dominant. Why? RR rr pollen Parental generation (P) pollen cross-fertilize true-breeding, purple-flowered plant true-breeding, white-flowered plant First-generation offspring (F1) Rr

14. Offspring of the F1 generation (the hybrids) may be purple-flowered if they inherit at least one factor for purple flowers, or may be white flowered if they inherit the white factor from both parents. First- generation offspring (F1) Rr Rr X RR Rr Rr rr Second- generation offspring (F2) 3/4 purple 1/4 white

15. The purple-flowered trait is dominant because each an individual who inherits at least one copy of the purple allele (R) shows the purple phenotype. The white-flowered trait is recessive because an individual must inherit two copies of the white allele (r) to show the white phenotype. Same letter, different case = same gene, different allele genotypes: RR or Rr rr phenotype purple white

16. Dexter freckles Darla freckles Derek no freckles Solving problems involving dominance Dexter has freckles. So does his wife, Darla. Their son, Derek has no freckles. Is having freckles a dominant or a recessive trait? Let’s start by making a family tree:

17. Law of Segregation • Each individual has a pair of factors controlling each trait, one inherited from each biological parent. • During the formation of gametes (sex cells) these two factors separate. Only one ends up in each sex cell.

18. In modern terms, the homozygous parents in the P generation can pass one one kind of allele to their offspring. homozygous parent gametes gene A A A A Homologous chromosomes

19. The heterozygous parents of the F1 generation have two alleles for the gene in question, and can pass one or the other, but not both, to their offspring. heterozygous parent gametes gene A a A a Homologous chromosomes

20. The genotypes can be represented with letters, which symbolize the alleles: capital for dominant alleles, small case for recessive. purple parent PP P + P all Psperm and eggs white parent pp p + p all p sperm and eggs

21. When the gametes join to produce the F1 generation, all offspring of homozygous dominant and homozygous recessive parents are heterozygous. F1 offspring gametes of parents sperm eggs P + p Pp or p P Pp +

22. gametes from F1 plants (Pp) F2 offspring The heterozygous F1 individuals can put either a dominant OR a recessive allele in each of their gametes. sperm eggs P + P PP P + p Pp p + P Pp pp p + p

23. A Punnet square is one way to predict the outcome of a cross by showing all the possible combinations of all the possible gametes. Pp self-fertilize 1/2 P eggs 1/2 p 1/2 P 1/4 Pp 1/4 PP sperm p 1/2 1/4 pp 1/4 pP

24. Solving single-gene (monohybrid) crosses with Mendelian (dominant-recessive) inheritance. Tomato fruit color can be red or yellow. a. A red tomato plant is crossed with a yellow tomato plant, and all the offspring have red tomatoes. Which trait is dominant? b. If two of the resulting hybrid red tomato plants are crossed, what will be the ratio of phenotypes in the offspring?

25. Solving single-gene (monohybrid) crosses with Mendelian (dominant-recessive) inheritance. Tomato fruit color can be red or yellow. a. A red tomato plant is crossed with a yellow tomato plant, and all the offspring have red tomatoes. Which trait is dominant? b. If two of the resulting hybrid red tomato plants are crossed, what will be the ratio of phenotypes in the offspring?

26. Law of Independent Assortment • When genetic factors segregate in the gametes, they segregate independently of one another. A dominant allele for one trait does not guarantee inheritance of a dominant allele for a different trait.

27. Trait Dominant form Recessive form Seed shape All organisms have multiple inheritable traits controlled by genes. Each trait is inherited independently of the others. A pea plant may, for example, have yellow seeds (dominant) but white flowers (recessive). smooth wrinkled Seed color yellow green Pod shape inflated constricted Pod color green yellow Flower color purple white Flower location at leaf junctions at tips of branches Plant size tall (1.8 to 2 meters) dwarf (0.2 to 0.4 meters)

28. S pairs of alleles on homologous chromosomes in diploid cells s Y y chromosomes replicate Traits carried on separate chromosomes sort independently of one another during gamete formation. replicated homologues pair during metaphase of meiosis I, y Y S S orienting like this or like this s s y Y meiosis I y S S Y Y s s y y S S s s y Y Y meiosis II S S S S s s s s y y y y Y Y Y Y independent assortment produces four equally likely allele combinations during meiosis Notice that each gamete receives ONE s-bearing and ONE y-bearing chromosome from the original cell.

29. Now consider this in terms of genotypes: S s Y y Genotype of this parent (for these two traits) is SsYy chromosomes replicate replicated homologues pair during metaphase of meiosis I, y Y S S orienting like this or like this s s y Y Meiosis puts ONE S-bearing and one Y-bearing chromosome in each gamete. meiosis I y S S Y Y s s y y S S s s y Y Y meiosis II S S S S s s s s y y y y Y Y Y Y Genotypes of the gametes that this parent can produce are: independent assortment produces four equally likely allele combinations during meiosis SY sy Sy sY

30. This Punnet square shows a cross between two pea plants which are heterozygous for two traits. Again, the Punnet square represents all possible combinations of the gametes that the plants can donate to their offspring. They must put one copy of a gene for each trait in their gametes. SsYy self-fertilize eggs SY Sy sY sy 1/4 1/4 1/4 1/4 1/4 SY SSYy SsYY SsYy 1/16 SSYY 1/16 1/16 1/16 1/4 Sy SSyY SSyy SsyY Ssyy 1/16 1/16 1/16 1/16 sperm 1/4 sY sSYY sSYy ssYY ssYy 1/16 1/16 1/16 1/16 1/4 sy sSyY sSyy ssyY ssyy 1/16 1/16 1/16 1/16 seed shape seed color phenotypic ratio (9:3:3:1) 3/4 smooth × 3/4 yellow = 9/16 smooth yellow 3/4 smooth × 1/4 green = 3/16 smooth green 1/4 wrinkled × 3/4 yellow = 3/16 wrinkled green 1/4 wrinkled × 1/4 green = 1/16 wrinkled yellow

31. Solving dihybrid crosses with Mendelian (dominant-recessive) inheritance. Pea plants can be tall (T) or short (t) and produce purple (R) or white (r) blossoms. a. A pure-breeding tall plant with purple flowers (TTRR) is crossed with a pure-breeding short plant with white flowers (ttrr). What will the offspring look like? b. If two of the hybrid (F1) plants are crossed, what offspring can they produce?

32. Solving dihybrid crosses with Mendelian (dominant-recessive) inheritance. Pea plants can be tall (T) or short (t) and produce purple (R) or white (r) blossoms. a. A pure-breeding tall plant with purple flowers (TTRR) is crossed with a pure-breeding short plant with white flowers (ttrr). What will the offspring look like? b. If two of the hybrid (F1) plants are crossed, what offspring can they produce?

33. Laws: “proven” forever? • Mendel’s Laws were good descriptions of what he observed in the peas and other plants he worked with. • New knowledge accumulated since Mendel’s time has refined his ideas. While his laws still hold true in some instances, there are many exceptions that we will explore in the next presentations.

34. Recap • Genes may have multiple alleles, such as dominant and recessive alleles. • Chromosomes, which carry genes, separate from one another during gamete formation. • Chromosomes sort independently of one another during gamete formation, but each gamete gets ONE of each kind of chromosome.