Mendel and His Peas

1. Mendel and His Peas MUPGRET Workshop Feb. 7, 2004

2. Genetic variation • In the beginning geneticists studied differences they could see in plants. • These differences are called morphological differences. • Individual variants are referred to as phenotypes, ex. tall vs. short plants or red vs. white flowers.

3. Trait • A broad term encompassing a distribution of phenotypic variation. • Example: • Trait: Disease resistance • Phenotype: resistant vs. susceptible • Morphological differences associated with the trait might include fungal infection, fungal growth, sporulation, etc.

4. Mendel • Monk at the St. Thomas monastery in the Czech Republic. • Performed several experiments between 1856 and 1863 that were the basis for what we know about heredity today. • Used garden peas for his research. • Published his work in 1866.

5. Mendel • Results are remarkably accurate and some have said they were too good to be unbiased. • His papers were largely ignored for more than 30 years until other researchers appreciated its significance.

6. Garden Pea • Pisum sativum • Diploid • Differed in seed shape, seed color, flower color, pod shape, plant height, etc. • Each phenotype Mendel studied was controlled by a single gene.

7. Terms • Wild-type is the phenotype that would normally be expected. • Mutant is the phenotype that deviates from the norm, is unexpected but heritable. • Notice that this definition does not imply that all mutants are bad in fact many beneficial mutations have been selected by plant breeders.

8. Advantages of plants • Can make controlled hybrids. • Less costly and time consuming to maintain than animals. • Can store their seed for long periods of time. • One plant can produce tens to hundreds of progeny.

9. Advantages of plants • Can make inbreds in many plant species without severe effects that are typically seen in animals. • Generation time is often much less than for animals. • Fast plants (Brassica sp.) • Arabidopsis

10. Principle of Segregation X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850

11. Mendel’s Results

12. Important Observations • F1 progeny are heterozygous but express only one phenotype, the dominant one. • In the F2 generation plants with both phenotypes are observedsome plants have recovered the recessive phenotype. • In the F2 generation there are approximately three times as many of one phenotype as the other.

13. 3 : 1 Ratio • The 3 : 1 ratio is the key to interpreting Mendel’s data and the foundation for the the principle of segregation.

14. The Principle of Segregation • Genes come in pairs and each cell has two copies. • Each pair of genes can be identical (homozygous) or different (heterozygous). • Each reproductive cell (gamete) contains only one copy of the gene.

15. Principle of Segregation • Either copy of the gene is equally likely to be included in a gamete. • One male and one female gamete combine to generate a new individual with two copies of the gene.

16. Allele • One of two to many alternative forms of the same gene (eg., round allele vs. wrinkled allele). • Alleles have different DNA sequences that cause the different appearances we see.

17. Mendel’s Principle of Segregation • In the formation of gametes, the paired hereditary determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair.

18. Principle of Segregation X Parental Lines Round (WW) Wrinkled (ww) All round F1 progeny (Ww) Self-pollinate 3 Round : 1 Wrinkled Round (WW + Ww) 5474 Wrinkled (ww) 1850

19. Punnett Square Male Female ¼ AA :½ Aa : ¼ aa

20. Round vs. wrinkled • The SBEI causes the round vs. wrinkled phenotype. • SBEI = starch-branching enzyme • Wrinkled peas result from absence of the branched form of starch called amylopectin. • When dried round peas shrink uniformly and wrinkled do not.

21. Round vs. wrinkled • The non-mutant or wild-type round allele is designated W. • The mutant, wrinkled allele is designated w. • Seeds that are Ww have half the SBEI of wild-type WW seeds but this is enough to make the seeds shrink uniformly. • W is dominant over w.

22. Round vs. wrinkled • An extra DNA sequence is present in the wrinkled allele that produces a non-functional SBEI and blocks the starch synthesis pathway at this step resulting in a lack of amylopectin.

23. A Molecular View Parents F1 F2 Progeny WW ww Ww ¼WW ¼Ww ¼wW ¼ww 1: 2 : 1 Genotype = 3: 1 Phenotype

24. Chi-Squared Analysis • Tests if your observations are statistically different from your expectation. • For example does the Mendel data fit the 3:1 hypothesis? • Chi-squared = [(observed-expected)2/expected2]