A 5 1 Linked Genes Do Not Assort Independently Genetics Chapter 5 Outline 1 Principle of Segregation each individual diploid organism possesses two alleles at a locus that separate in meiosis with one allele going into each gamete 2 Principle of Independent Assortment the process of separation when the two alleles at a locus act independently of alleles at other loci 3 Recombination the sorting of alleles into new combinations It may create combinations the same as the parents or different combinations 4 Linked Genes Genes located close together on the same chromosome that belongs to the same linkage group They travel together eventually arriving at the same location They do not assort independently B 5 2 Linked Genes Segregate Together and Crossing Over Produces Recombination Between Them 1 Genes occasionally switch from one homologous chromosome to the other through the process of crossing over a Results in recombination s and breaks up the association of genes that are close together on the same chromosome i Linkage keeps particular genes together ii Crossing Over mixes them up 2 Notation for Crosses with Linkage A B A B ab ab a Each line represents one of the two homologous chromosomes Inheriting one chromosome from each parent The F1 progeny will have the following genotype A B ab b Once chromosome has two dominate alleles and the other has two recessive alleles So the notation can be simplified with the understanding that genes located on the same side of the line lie on the same chromosome A B a b c Remember that two alleles at a locus are always located on different homologous chromosomes and therefore must lie on the opposite sides of the line and they must keep the same order of the genes on both sides of the line so it will never be written A B A a ba ba 3 Complete Linkage Compared with Independent Assortment a With complete linkage no crossing over occurs and only the gametes of the parents can be produced No new combinations occur because the genes affecting the two traits are completely linked and are inherited together b Nonrecombinant gamete or parental gametes contain only original combinations c Nonrecombinant progeny display the orginial combinations of traits present in i New combinations can only arise if the physical connection between the of alleles present in the parents the P generation two genes was broken d Recombinant gametes gametes with new combinations of alleles i With independent assortmen nonrecombinant and recombinant gametes are produced in equal proportions e Recombinant progeny the progeny with new combinations of traits formed from recombinant gametes 4 Crossing Over with Linked Genes a Genes that exhibit crossing over are independently linked i For closely linked genes crossing over does not take place in every ii meiosis In meiosis when there is no crossing over only nonrecombinant gametes are produced In meiosis when there is a crossover only half of the gametes are recombinants and half are nonrecombinants Because it only affects two of the four chromatids 1 Thus the frequency of recombinant gametes is always half the frequency of crossing over and the maximum proportion is 50 5 Calculating Recombination Frequency a Recombination frequency the percentage of recombinant progeny produced in a cross recombinant frequency number of recombinant progeny total number of progeny 100 6 Coupling and Repulsion a p wild type p normal phenotype b Coupling or cis configuration arrangement in which wild type alleles are found on one chromosome and mutant alleles are found on the other chromosome b pb p c Repulsion or trans configuration arrangement in which each chromosome contains one wild type and one mutant allele p b p b It is essential to know the arrangement of the alleles on the chromosomes to accurately predict the outcome of crosses in which genes are linked Interchromosomal recombination between genes on different chromosomes It arises from independent assortment Intrachromomsomal recombination between genes located on the same chromosome It arises from crossing over d e f 7 Predicting the Outcome of Crosses with Linked Genes a Knowing the arrangement of alleles on a chromosome allows us to predict the types of progeny that will result from a cross entailing linked genes and to determine which of these types will be the most numerous b To determine the proportions of the types of offspring you need to know the recombinant frequency It tells you how often the alleles in the gametes appear in new combinations and allows us to predict the proportions of offspring phenotypes that will result from a specific cross with linked genes i Example smooth fruit t is recessive to warty fruit T and glossy fruit T D t d d is recessive to dull fruit D These genes exhibit a recombination frequency of 16 t d t d 1 Four types gametes will produced by the heterozygous parent Two types of nonrecombinant genes T D and t d and two types of recombinant genes T d and t D 2 The recombinant frequency tells us that 16 of the gametes produced by the heterozygote s will be recombinants and because there are two types of recombinant gametes so the frequency will be 8 or 0 08 16 2 3 All other gametes will be nonrecombinants so their frequency will be 100 16 84 Because there are two types of nonrecombinants they each should arise with a frequency of 84 42 or 0 42 2 4 The other parent is homozygous and therefore only produces a single type of gamete t d with the frequency of 100 or 1 0 5 The progeny from the cross result from the union of two gametes producing four types of progeny The expected proportion can be determined by using the multiplication rule and multiplying together the probability of each uniting gamete 6 Testcross progeny with warty and dull fruit will have the frequency 42 42 1 0 and the genotype T D t d 8 Testing for Independent Assortment a In some crosses genes are obviously linked because there are clearly more nonrebombinant progeny than recombinant b Must answer the question Is the inheritance of alleles at one locus independent of alleles at a second locus It yes genes are assorting independently It no the genes are probably linked c A fast method for testing for independent in genotypes is with a chi square test of independence I skipped because he didn t do it in the notes if interested on pages 116 119 9 Gene Mapping with Recombination Frequencies a Genetic maps chromosome maps calculated by using the genetic phenomenon of recombination the
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