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AUTOSOMAL LINKAGE Genes that do not obey Mendel’s Second Law © 2007 Paul Billiet ODWSODWS
Reminder In monohybrid inheritance the F 2 generation gives a 3:1 ratio because of Mendel 1 A back cross with a heterozygote (Aa) gives a 1:1 ratio In dihybrid inheritance the F 2 generation gives a 9:3:3:1 ratio because of Mendel 2 A back cross with a heterozygote for both pairs of alleles (AaBb) gives a ratio of 1:1:1:1 © 2007 Paul Billiet ODWSODWS
Genes which do not obey Mendel's Second Law Sweet Peas CharactersTraitsAlleles Flower colourPurpleF Redf Pollen grain shape LongL Roundl © 2007 Paul Billiet ODWSODWS
Looked at from a simple dihybrid cross perspective PPhenotypesPurple Long XRed Round (Pure Bred) Genotypes FF LL ff ll F1F1 PhenotypesAll purple long Genotypes FfLlFfLl (Selfed) © 2007 Paul Billiet ODWSODWS
Sweet peas, the second generation PPhenotypesPurple Long XRed Round (Pure Bred) GenotypesFFLLffll F1F1 PhenotypesAll purple long GenotypesFfLl(Selfed) F2F2 PhenotypesPurple Long Purple Round Red LongRed Round Expected Ratios 9/16 56,25% 3/16 18,75% 3/16 18,75% 1/16 6,25% © 2007 Paul Billiet ODWSODWS
What was expected? F2F2 PhenotypesPurple LongPurple Round Red LongRed Round Expected Ratios 9/16 56,25% 3/16 18,75% 3/16 18,75% 1/16 6,25% Parental combination © 2007 Paul Billiet ODWSODWS
Not quite what was expected! F2F2 PhenotypesPurple LongPurple Round Red Long Red Round Expected Ratios 9/16 56,25% 3/16 18,75% 3/16 18,75% 1/16 6,25% Observed numbers Total = 427 Observed Ratios 69,32%4,45%6,32%19,91% Parental combination © 2007 Paul Billiet ODWSODWS
Linked genes The parental combinations of alleles (purple long and red round) seem to be inherited as almost a 3:1 ratio As though they were behaving as a single character These genes are called LINKED GENES That is the LOCI for these genes are linked on the same chromosome © 2007 Paul Billiet ODWSODWS
Crossing over and recombinants BUT if these genes were perfectly linked together they would stay in their parental combinations (purple & long or red & round) There would be no Purple Round or Red Long These combinations have come about because of CROSSING OVER between the linked alleles on their chromosomes during Meiosis 1 These are called RECOMBINANTS © 2007 Paul Billiet ODWSODWS
Meiosis & Crossing over Early prophase 1 Anaphase 1 Telophase 2 © 2007 Paul Billiet ODWSODWS
Are there any other recombinants? Yes, hidden amongst the Purple Long plants The genotype FfLl does not tell us enough about linked genes. Genotypes for linked genes can be shown as: This is an example of a parental combination This genotype would give the same phenotype as: BUT this is an example of a recombinant © 2007 Paul Billiet ODWSODWS
Recombinants Note: recombinants are any combination of alleles that are not the same as the parental combinations This is not exclusive to the crossing over of linked genes © 2007 Paul Billiet ODWSODWS
PPhenotypesPurple LongXRed Round(Pure Bred) Genotypes Gametes F Lf l F1PhenotypesAll Purple Long Genotypes The Genetic Diagram for Linked Genes © 2007 Paul Billiet ODWSODWS
F1PhenotypesAll Purple Long(Selfed) Genotypes crossing over in meiosis I GametesF Lf lF lf L Parental CombinationsRecombinants The Genetic Diagram for Linked Genes © 2007 Paul Billiet ODWSODWS
F2GenotypesF Lf lF lf L F L f l F l f L The Genetic Diagram for Linked Genes F1GametesF Lf lF lf L Parental CombinationsRecombinants © 2007 Paul Billiet ODWSODWS
The ratios PhenotypesPurple Long Red Round Purple Round Red Long Expected Ratio if the genes were only linked 3/4 75% 1/4 25% 00 © 2007 Paul Billiet ODWSODWS
The ratios PhenotypesPurple Long Red Round Purple Round Red Long Expected Ratio if the genes were only linked 3/4 75% 1/4 25% 00 Observed Ratio69,32%19,91%4,45%6,32% © 2007 Paul Billiet ODWSODWS
How do we know which of the Purple Long plants are Parental Combinations and which are Recombinants? How often does crossing over occur? Answer: Test Cross the F 1 with a double recessive for both characters © 2007 Paul Billiet ODWSODWS
CALCULATING THE CROSS OVER VALUE DrosophilaCharactersTraitsAlleles Wing shapeNormalB Bentb Body colourNormalE Ebonye © 2007 Paul Billiet ODWSODWS
CALCULATING THE CROSS OVER VALUE PhenotypesHeterozygous wild type XEbony Bent GenotypesBbEebbee GametesBE, Be, bE, bebe If these genes are linked Be and bE could only be produced by crossing over © 2007 Paul Billiet ODWSODWS
CALCULATING THE CROSS OVER VALUE PhenotypesWild typeNormal Ebony Bent Normal Bent Ebony GenotypesBbEeBbeebbEebbee Numbers Approx. Ratio25% Parental combination Recombinants = 50% of the offspring Parental combination © 2007 Paul Billiet ODWSODWS
CALCULATING THE CROSS OVER VALUE These results are typical of non-linked genes The recombinants are in the same frequency as the parental combinations Note: In this example bent wing flies are a bit crippled so their offspring are not so viable. This accounts for their low numbers © 2007 Paul Billiet ODWSODWS
CALCULATING THE CROSS OVER VALUE DrosophilaCharactersTraitsAlleles Eye colourRedP Pinkp Body colourNormalE Ebonye © 2007 Paul Billiet ODWSODWS
PhenotypesHeterozygous wild type XPink Ebony GenotypesPpEePpEeppee GametesPE, Pe, pE, pepepe PhenotypesWild typeRed Ebony Pink NormalPink Ebony GenotypesPpEePpEePpeeppEeppee © 2007 Paul Billiet ODWSODWS
PhenotypesWild typeRed Ebony Pink Normal Pink Ebony GenotypesPpEePpEePpeeppEeppee Numbers Parental combinations Recombinants < 50%Parental combinations The frequency of the recombinants is less than 50% This is an example of linkage © 2007 Paul Billiet ODWSODWS
The % recombination in a test cross is called the CROSS OVER VALUE (cov) The cross over value between ebony and pink = This value is important as it tells us how far apart the loci of the genes are Cross over values from several pairs of genes permit a geneticist to plot a gene map of the chromosome THE CROSS OVER VALUE © 2007 Paul Billiet ODWSODWS
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