1.  Linked Genes Learning Objective DOT Point: predict the difference in inheritance patterns if two genes are linked Sunday, June 05, 2016 1

2.  Linked Genes vs. Unlinked Genes Sunday, June 05, 20162

3. 1.One allele completely dominates the other 2.All genes have 2 allelic forms 3.All traits are monogenic (affected by only one locus) 4.All chromosomes occur in homologous pairs 5.All genes assort independently Application of Mendel’s Rules assumes:

4. Mendel's Law of Independent Assortment  Allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another. Sunday, June 05, 20164

5. Dihybrid cross - phenotypes

6. Independent assortment An Interpretation from the Dihybrid cross  During gamete formation, segregating pairs of traits will sort independently.  In other words, segregation of 2 alleles at one genetic locus has no effect on the segregation of 2 alleles at another locus.  For example, the assortment of yellow and green alleles has no effect on the assortment of round and wrinkled alleles, and vice versa.

7. Discovery of Linkage  William Bateson and R.C. Punnett were working with several traits in snapdragons, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).

8. Flower Colour:P = purplep = red Pollen seed shape:L = longl = round True Breeding lines:LLPP xllpp PpLlF1 PhenotypeNumber Exp RatioExp Number Purple long2849215 Purple round213 71 Red long213 71 Red round551 24 What is going on???? Crosses produced a deviation from the predicted Mendelian independent assortment ratios. Bateson and Punnett’s snapdragons:

9.  The parental phenotypes reappeared more frequently than expected  Bateson and Punnett hypothesized that there was a coupling, or linkage, between the parental alleles for flower colour and pollen grain shape  This coupling resulted in the observed deviation from independent assortment. Linkage

10. Linked genes  Genes are said to be “linked” when their loci are found on the same chromosome.  It means that those alleles are usually inherited together… but not always!! The offspring of this couple gets one of each parental chromosome. The genes are intact, so the alleles that are inherited are of the “parental type”

11. Linkage  Linkage is the tendency for a group of genes, on the same chromosome, to be inherited together via crossing over Sunday, June 05, 201611

12. Linkage II  However, for linkage to occur, the genes, located on the same chromosome, must be close enough to each other.  If the genes are close enough to each other, and have a high probability of crossing over together, then they are considered to be linked genes. Sunday, June 05, 201612

13. Linkage III  If genes are linked together, then Mendel’s law of independent assortment does not apply.  Genes, in this case, are not passed on independent of another gene. Sunday, June 05, 201613

14. Recombination of Genes  Recombination of linked genes is only possible when crossing over occurs during meiosis. Sunday, June 05, 201614

15. Recombination  In meiosis, genetic material can move from one chromosome to another?  It results in offspring having chromosomes that are not identical to parental chromosomes.

16. Morgan and Crossing Over  Morgan proposed that the chiasmata visible on chromosomes were regions of crossing over.  Occurs between non-sister chromatids.  Crossing over occurs in prophase of meiosis 1, where homologous chromosomes break at identical locations and rejoin with their non-sister chromatids.

17. Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings. Mechanism of crossing-over

18. Recombination  You can see that the probability of genes that are close together (a and b) being separated during crossing over is less than that of genes that are further apart (a and c)

19. Recombination  In the example below, the mother is homozygous at both loci, while the father is heterozygous at both loci. GG aa G g a A  The codes found on the parental chromosomes: Ga, gA  If crossing over occurs in maternal meiosis, there is no change, as the alleles are the same on each. HOWEVER, if there is crossing over in the paternal chromosomes, and the G/g alleles swap places, there will be RECOMBINANT offspring with chromosomes with the following allelic combinations: ga or GA

20.  Determining Whether Genes are Linked or Unlinked DOT Point: Analyse the outcome of dihybrid crosses with independently inherited and linked traits Sunday, June 05, 201620

21. Test Cross I  The test cross allows scientists to determine whether genes are linked or unlinked.  In order to perform a test cross one parent must be heterozygous while the other must be homozygous recessive. Sunday, June 05, 201621

22. Test Cross II  If the majority of the offspring have a genotype similar to one of the parents, then the genes are linked.  If the majority of the offspring have a recombinant genotype, then the genes are unlinked. Sunday, June 05, 201622

23. Test Cross Example Parent #1  BbVv  Grey with normal wings Parent #2  bbvv  Black with vestigial wings Sunday, June 05, 201623

24. Test Cross BVbvBvbV bvBbVvbbvvBbvvbbVv Expect ed Results 575 Actual Results 965944206185  Parental Genotypes  965 (42%) +944 (41%) = 1909  1909/2300 = 83%  Recombinant Genotypes  206 (9%)+185 (8%) = 391  391/2300 = 17%  If independent assortment was to occur, the percentages would be 25% a piece.  Based on the data, the recombinants arose because of crossing over 24

25.  Cross breeding DOT Point: Explain how cross- breeding experiments can identify the relative position of linked genes Sunday, June 05, 201625

26. Gene Mapping  As pointed out by T. H. Morgan’s student, Alfred Sturtevant, who produced the first Drosophila gene map in 1913.  Each gene is found at a fixed position on a particular chromosome.  The basis of gene mapping is that since crossing over occurs at random locations, the closer two genes are to each other, the less likely it is that a crossover will occur between them.

27. Recombination Frequency  Amount of recombination between two genes reflects the distance between them  The greater the distance, the greater the recombination frequency  Greater chance of crossover between genes

28. Genetic Maps  Recombination frequencies used to determine relative locations on a chromosome Linkage map for genes a, b, and c:

29.  For example, the recombination rate between the genes B and P is 30.7% BP 0.030.7 Sunday, June 05, 201629 Genetic Maps Example

30.  The recombination rate between the genes P and R is 3.0%, so R is closer to P than B. BR?PR? 0.027.730.733.7 Sunday, June 05, 201630 Genetic Maps Example

31.  The recombination rate between the genes B and R is 35.5%, so B is further from R than P. BPR 0.030.733.7 Sunday, June 05, 201631 Genetic Maps Example

32.  30.7% + 3.0% do not equal 35.5%, this is because recombinations are random.  Short distances are more accurate than long distances. BPR 0.030.733.7 Sunday, June 05, 201632 Genetic Maps

33. Example: Genes A, B and C are linked. In a cross breeding experiment, ABC was crossed with abc and the percentages of recombinations were observed. AB x ab Results: AB and ab 85% Ab and aB 15% (recombined through crossing over) BC x bc Results: BC and bc 95% Bc and bC 5% (recombinations) AC x ac Results: AC and ac 90% Ac and aC 10% (recombinations) Sunday, June 05, 201633

34. A and B are 15 units apart A ________________15 _______________ B B and C are 5 units apart B ____5___C A and C are 10 units apart A ________10 _________C The resulting map is: A __________10__________C______5_____B Sunday, June 05, 201634

35.  Use this diagram to answer Questions  In which gene map is the probability of crossing-over between A and D greatest? Sunday, June 05, 201635 C

36.  Use this diagram to answer Questions  In which gene map is the probability of crossing-over between A and D the least? Sunday, June 05, 201636 D

37.  Use this diagram to answer Questions  In which map are genes C and D most closely linked? Sunday, June 05, 201637 B

38.  Use this diagram to answer Questions  In map D, which genes are least likely to cross over? Sunday, June 05, 201638 C and B

39.  Chromosome mapping DOT Point: Discuss the role of chromosome mapping in identifying relationships between species Sunday, June 05, 201639

40.  If the two species are very closely related, then their chromosome maps will be very similar.  Chimpanzees and humans have DNA similarities of more than 98%.  This suggests that humans and chimpanzees had a common ancestor in quite recent times. Sunday, June 05, 201640 Genetic Maps

41.  A chromosome map that has been produced by percentages of recombinations of genes during crossing over does not completely represent a whole chromosome.  Chromosome maps based on recombinations can only be used to produce the relative positions of genes, not the absolute ones. Sunday, June 05, 201641 Genetic Maps