Genes that do not obey Mendel’s Second Law AUTOSOMAL LINKAGE Genes that do not obey Mendel’s Second Law © 2016 Paul Billiet ODWS

Reminder In monohybrid inheritance the F2 generation gives a 3:1 ratio because of Mendel 1 A test cross with a heterozygote (Aa) gives a 1:1 ratio In dihybrid inheritance the F2 generation gives a 9:3:3:1 ratio because of Mendel 2 A test cross with a heterozygote for both pairs of alleles (AaBb) gives a ratio of 1:1:1:1 © 2016 Paul Billiet ODWS

Genes which do not obey Mendel's Second Law Sweet Peas Characters Traits Alleles Flower colour Purple F Red f Pollen grain shape Long L Round l © 2016 Paul Billiet ODWS http://www.thegardenhelper.com/pixpg/sweetpeapix.html

Looked at from a simple dihybrid cross perspective Phenotypes Purple Long X Red Round (Pure Bred) Genotypes FFLL ffll F1 All purple long FfLl (Selfed) © 2016 Paul Billiet ODWS

Sweet peas, the second generation Phenotypes Purple Long X Red Round (Pure Bred) Genotypes FFLL ffll F1 All purple long FfLl (Selfed) F2 Purple Round Red Long Expected Ratios 9/16 56,25% 3/16 18,75% 1/16 6,25% © 2016 Paul Billiet ODWS

What was expected? F2 Phenotypes Purple Long Purple Round Red Long Red Round Expected Ratios 9/16 56,25% 3/16 18,75% 1/16 6,25% Parental combination © 2016 Paul Billiet ODWS

Not quite what was expected! F2 Phenotypes Purple Long Purple Round Red Long Red Round Expected Ratios 9/16 56,25% 3/16 18,75% 1/16 6,25% Observed numbers 296 19 27 85 Total = 427 Observed Ratios 69,32% 4,45% 6,32% 19,91% Parental combination © 2016 Paul Billiet ODWS

Linked genes The parental combinations of alleles (purple long and red round) 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. © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

Meiosis & Crossing over Early prophase 1 Anaphase 1 Telophase 2 © 2016 Paul Billiet ODWS

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: This is an example of a recombinant. © 2016 Paul Billiet ODWS

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. © 2016 Paul Billiet ODWS

The Genetic Diagram for Linked Genes P Phenotypes Purple Long X Red Round (Pure Bred) Genotypes Gametes F L f l F1 All Purple Long © 2016 Paul Billiet ODWS

The Genetic Diagram for Linked Genes Phenotypes All Purple Long (Selfed) Genotypes crossing over in meiosis I Gametes F L f l F l f L Parental Combinations Recombinants © 2016 Paul Billiet ODWS

The Genetic Diagram for Linked Genes Genotypes F L f l F l f L © 2016 Paul Billiet ODWS

The ratios Phenotypes Purple Long Red Round Purple Round Red Long Expected Ratio if the genes were only linked 3/4 75% 1/4 25% © 2016 Paul Billiet ODWS

The ratios Phenotypes Purple Long Red Round Purple Round Red Long Expected Ratio if the genes were only linked 3/4 75% 1/4 25% Observed Ratio 69,32% 19,91% 4,45% 6,32% © 2016 Paul Billiet ODWS

COMBINATIONS OR RECOMBINANTS? 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 F1 with a double recessive for both characters. © 2016 Paul Billiet ODWS

CALCULATING THE CROSS OVER VALUE Drosophila Characters Traits Alleles Wing shape Normal B Bent b Body colour E Ebony e © 2016 Paul Billiet ODWS

CALCULATING THE CROSS OVER VALUE Phenotypes Heterozygous wild type X Ebony Bent Genotypes BbEe bbee Gametes BE, Be, bE, be be If these genes are linked Be and bE could only be produced by crossing over. © 2016 Paul Billiet ODWS

CALCULATING THE CROSS OVER VALUE Phenotypes Wild type Normal Ebony Bent Normal Bent Ebony Genotypes BbEe Bbee bbEe bbee Numbers 83 82 76 71 Approx. Ratio 25%   Parental combination Recombinants = 50% of the offspring © 2016 Paul Billiet ODWS

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: Bent wing flies are a bit crippled so their offspring are not so viable. This accounts for their low numbers.

CALCULATING THE CROSS OVER VALUE Drosophila Characters Traits Alleles Eye colour Red P Pink p Body colour Normal E Ebony e © 2016 Paul Billiet ODWS

Heterozygous wild type CALCULATING THE CROSS OVER VALUE Phenotypes Heterozygous wild type X Pink Ebony Genotypes PpEe ppee Gametes PE, Pe, pE, pe pe Wild type Red Ebony Pink Normal Ppee ppEe © 2016 Paul Billiet ODWS

Parental combinations CALCULATING THE CROSS OVER VALUE Phenotypes Wild type Red Ebony Pink Normal Pink Ebony Genotypes PpEe Ppee ppEe ppee Numbers 601 3 4 584   Parental combinations Recombinants < 50% The frequency of the recombinants is less than 50% This is an example of linkage. © 2016 Paul Billiet ODWS

THE CROSS OVER VALUE The % recombination in a test cross is called the CROSS OVER VALUE (cov) The cross over value between ebony and pink = This value 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. © 2016 Paul Billiet ODWS