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5 5 1 GENETIC LINKAGE AND MAPPING Genetic Linkage So far we have considered traits that are affected by one or two genes and if there are two genes we have assumed that they assort independently However It should be obvious that there are many more genes than there are chromosomes in all organisms In this segment of the course you will learn how to determine if genes are linked on the same chromosome and how to determine how far apart the genes are This is the basis of genetic mapping First let s revisit the independent assortment of genes that are located on different chromosomes Mendel s Law of Independent Assortment during gamete formation segregation of one gene pair is independent of other gene pairs derived because the traits he studied were determined by genes on different chromosomes First consider two genes each with two alleles A a and B b on separate chromosomes A NEW NOTATION represents the two chromatids of a metaphase chromosome bearing allele A of the A a locus represents the same chromosome in the single chromatid stage So the cross AA BB aa bb could be represented as Female P Gametes A A Egg A B A F1 after the S phase Male B B a a b b Sperm a b B a b A B a b Gametes non homologous chromosomes assort independently at anaphase Gamete Expected Genotype Proportion A B Parental AB 1 4 a b Parental ab 1 4 A b Recombinant Ab 1 4 a B Recombinant aB 1 4 So when genes are on different chromosomes 50 of the gametes produced by a doubly heterozygous individual are recombinant when compared to the gametes produced by its parents The other 50 are parental If two genes occur on the same chromosome they may not assort independently at anaphase of meiosis These genes are said to be linked and demonstrate linkage in genetic crosses Linkage is present when fewer than 50 of the gametes produced by a double heterozygote are recombinant Now consider a case where the two genes are on the same chromosome A and B are linked Female A B A B P Gametes F1 Male some X a b a b Egg A B Sperm a b A B a b A B A B X a b a b and some Gametes resulting from no crossover Gametes resulting from crossover Expected Prob A B Parental A B When F1s produce gametes some will be produced with crossing over and some will be produced without Expected Prob 1 4 A B Parental 1 4 Parental 1 4 a b Parental 1 4 a b Parental 1 4 A b Recombinant 1 4 a b Parental 1 4 a B Recombinant 1 4 If there were no crossing over all the alleles on a single chromosome would segregate together and would end up in the same gamete depicted on the left above But with crossing over we get recombination of alleles on the same chromosomes depicted on the right above Since crossover occurs in the 4 strand stage of meiosis and involves only two of the four chromatids each crossover event results in 50 recombinant gametes and 50 parental gametes So Genes on different chromosomes 50 recombinant gametes after meiosis Genes on the same chromosome 50 recombinant gametes after meiosis 5 2 Genetic Mapping Genes with recombination frequencies less than 50 are present in the same chromosome linked Two genes that undergo independent assortment indicated by a recombination frequency of 50 percent are either on nonhomologous chromosomes or are located far apart in a single chromosome However crossing over does not occur between linked genes in every meiotic event especially when the positions of the genes on the chromosome are very near one another The frequency with which crossing over occurs between any two linked genes is proportional to the distance between the loci along the chromosome 1 At very small distances crossover is very rare and most gametes are parental 2 As the distance between two genes increases crossover frequency increases More recombinant gametes fewer parental gametes 3 When genetic loci are very far apart on the same chromosome crossing over nearly always occurs and the frequency of recombinant gametes approaches 50 percent How do we determine how much crossing over occurs between linked genes and therefore get an idea of how far apart two loci are The answer is critical because this is the first step in constructing a genetic map The following shows the use of a testcross to determine how much crossing over occurs between the genes for brown eyes bw and heavy veined hv in the fruit fly Drosophila melanogaster bw hv bw hv P1 bw hv GAMETES bw hv bw hv bw hv F1 TEST CROSS x F1 bw hv bw hv bw hv X bw hv x double heterozygote bw hv double recess homozyg A test cross bw hv If there is no crossover event in the F1 double heterozygote the following offspring are produced in the testcross RESULT OF TESTCROSS IF NO CROSSOVER bw hv bw hv bw hv bw hv bw hv bw hv bw hv bw hv All Parental If there is a crossover event in the double heterozygote half the offspring produced by the gametes resulting from crossing over will be like those above bw hv bw hv bw hv bw hv bw hv bw hv bw hv bw hv and the other half will be RESULT OF TESTCROSS WITH CROSSING OVER IN 4 STRAND STAGE So half the offspring are recombinant with respect to the parental combinations of alleles at the two loci Another example this time using a hemizygous male to detect recombination on the X chromosome white eyes miniature wings w w m m on D melanogaster X chromosome w m w m x w m w m Parents w m Gametes w m w m w m x Two kinds of offspring No crossing over all offspring are parental w m w m w m w m w m w m With crossing over 1 2 of offspring will be parental Essentially a testcross no crossing over in male Drosophila 1 2 will be recombinant w m w m w m w m w m w m Lets say we do this cross and find 62 8 non recombinant offspring and 37 2 recombinant offspring 37 2 is the frequency of recombination 5 3 Map units When large numbers of mutations are available for a species genes on the same chromosome will show evidence of linkage to one another 50 recombination frequency Genes will fall into LINKAGE GROUPS The number of linkage groups will equal the haploid number of chromosomes The linkage of genes on a chromosomes can be represented on a genetic map linkage map chromosome map A genetic map shows the linear order of the genes along a chrom with distance proportional to the frequency of recombination Unit of distance in linkage map is a map unit 1 map unit is equal to 1 percent recombination A map unit is also equivalent to the physical distance along a chromosome which will …


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UIUC IB 201 - GENETIC LINKAGE AND MAPPING

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