1. Mendelian Genetics G.Burgess 2011.

2. Genetics n Genetics = the science of heredity that involves the structure and function of genes and the way genes are passed from one generation to the next. n Hereditary trait = a characteristic under control of the genes that is transmitted from one generation to another.

3. Gregor Mendel n Mendel was the first person to gain some understanding of the principles of heredity. n 1866 – he did research involving pea plants to discover the mechanisms of genetics and applied what he learned about heredity to people and animals. n Used statistical analysis to help prove his hypothesis

4. Homologous Pairs Mendel understood that we get our traites from both our mother and father. Further research indicated that the traites are located on chromosomes at specific locations. Our chromosomes are in pairs. Each chromosome in the pair has genes in the same location. This is a homologous pair.

5. Why did Mendel use pea plants? n Easy to grow n Grow quickly n Easy to crossbreed

6. Mendel’s Experiment n Mendel obtained 34 strains of pea plants that differed in a number of traits ( ie. seed shape, seed color, height, flower color etc. ) n Allowed strains to self-fertilize for many generations to ensure pure-breeding strains (ensures that the offspring will have the same traits (genes) as the parents)

7. Mendel’s Experiment continued n Mendel specific traits (or physical characteristics: phenotype) for study –Flower color (white & purple) –Seed color ( yellow& green) –Seed shape ( smooth & wrinkled) –Pod color ( green & yellow) –Stem height ( tall & short)

8. Terminology n Before discussing Mendel’s experiments, you must understand some basic terminology –P generation = parental generation –F1 generation = first set of offspring –F2 generation = produced by breeding the F1 offspring –Allele = a form of a gene (dominant or recessive) –Dominant = the allele that is expressed over another –Recessive = the allele that is ‘masked’ –X means crossed with…

9. Mendel’s Experiment n Crossed 2 true-breeding strains of peas that differed in a single trait (monohybrid cross) –i.e P =smooth seed plant X wrinkled seed plant –F 1 = all smooth seeds

10. Mendel’s Experiment n Mendel’s second step = he allowed the F 1 generation to self-fertilize to produce the F 2 – F 1 X F 1 ( smooth seeds) –= F 2 (3 smooth: 1 wrinkled) –How can a trait be present in the P generation disappear in the F 1 and reappear in the F 2 ?

11. Dominant vs. Recessive Traits n In the case of the pea plants, there are 2 alternative forms (or alleles) for each characteristic n FOR EXAMPLE the gene for determining the shape of the seed has two forms (alleles); wrinkled and smooth.

12. Dominant & Recessive cont. n Recall the example; –P = smooth X wrinkled = F 1 (All smooth) –*the allele for smooth is dominant because it is “masking” the wrinkled gene –*the allele for wrinkled is recessive because in it is no shown

13. Dominant and Recessive n Dominant –The expressed allele (the one you can see) –Represented by a capital letter –ie S = smooth n Recessive –The allele that is not expressed ( or seen) –Represented by lower case letter –ie s = wrinkled

14. Example of a Smooth vs Wrinkled Cross n Remember S = smooth s = wrinkled n (these are 2 alleles for the gene that determines shape of the seed) n Each plant has 2 alleles for each gene (one from the mother, one from the father: homologous pair)

15. Example of a Smooth vs Wrinkled Cross (continued) n Therefore, plants can be SS or Ss or ss n Homozygous = 2 copies of the same allele (ie SS or ss) both are recessive or dominant n Heterozygous = 2 different alleles (ie Ss) 1 dominant and 1 recessive (the dominant trait is shown in the phenotype

16. What would these plants look like? n Wrinkled or smooth? n (**remember S = smooth, s = wrinkled) n SS = ????? n ss = ????? n Ss = ????? Smooth Wrinkled Smooth

17. It’s a good time to talk about genotype and phenotype... n Phenotype = the observable characteristics of an organism (is smooth or wrinkled appearance) n Genotype = the genetic make-up of an organism (ie SS or Ss or ss)

18. IMPORTANT n *Note: A plant with different GENOTYPE (SS and Ss) will have the SAME PHENOTYPE (they will both appear wrinkled because the smooth allele is dominant over the wrinkled)

19. Punnett Square http://www.blc.arizona.edu/courses/1 81gh/rick/genetics1/punnet.html

20. PUNNETT SQUARE n Punnett square = a matrix that describes all the possible gametic fusion that will give rise to the zygotes that will produce the next generation

21. Punnett Square F1 Gametes Yy X Yy ♂ Male YY ♀ Female YYYYyYy yYyYyyy

22. Mendel’s Laws n Law of Segregation - during gamete formation the pair of genes responsible for each trait separates so that each gamete receives only one gene for each trait n Law of Independent Assortment - genes for different traits segregate independently during gamete formation.

23. Two-Trait Crosses n Mendel also studied the inheritance of two traits at once n (ie round yellow plants (RRYY) X wrinkled green (wwyy)

24. Two-trait continued n Remember, the law of segregation stated that each gamete must contain ONE allele from each trait n therefore, the gametes of a round, yellow parent are R, Y and the gametes of a wrinkled, green parent are r, y.

25. Two-trait Cross example

26. Incomplete Dominance n In all the examples we have seen this far, one trait has been completely dominant over the other (ie tall (T) is dominant over short(t)) n Dominant and recessive traits are not always clearly defined, incomplete dominance also exists n Occurs when one allele only partially masks another.

27. Incomplete Dominance n For example - four o’clock plants n they have red and white flowers (neither are dominant) n red = RR n white = rr n When crosses, the Rr plants are pink

28. Incomplete Dominance

29. Multiple Alleles n Multiple Alleles = a set of three or more different alleles controlling a trait. n ie human blood type is determined by three alleles A, B & i n A and B are dominant over i n BUT A and B are codominant

30. Multiple Alleles (blood types) GenotypePhenotype IAIAIAiIAIAIAi A IBIBIBiIBIBIBi B IAIBIAIB AB iiO A is incompletely dominant over B. An individual with blood type AB has characteristics of both blood types. A person who has blood type O does not have either antigen. Blood type AB is also an example of codiminance Blood typing is done by examining the surface proteins found on cells. Type I A blood has antigen A. Blood type I B has antigen B. Type I blood does not have either antigen.

31. Codominance n Occurs when two alleles for a genetic trait are neither dominant nor recessive