AUTOSOMAL LINKAGE Genes that do not obey Mendels
AUTOSOMAL LINKAGE Genes that do not obey Mendel’s Second Law © 2016 Paul Billiet ODWS
Reminder ¢ ¢ In monohybrid inheritance the F 2 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 F 2 generation gives a 9: 3: 3: 1 ratio because of Mendel 2 A test cross with a heterozygote for both pairs of alleles (Aa. Bb) gives a ratio of 1: 1: 1: 1 © 2016 Paul Billiet ODWS
Genes which do not obey Mendel's Second Law Sweet Characters Peas Flower colour Pollen grain shape Traits Alleles Purple F Red f Long L Round l © 2016 Paul Billiet ODWS http: //www. thegardenhelper. com/pixpg/sweetpeapix. html
Looked at from a simple dihybrid cross perspective P F 1 Phenotypes Purple Long Genotypes FFLL X ffll Phenotypes All purple long Genotypes Ff. Ll © 2016 Paul Billiet ODWS Red (Pure Round Bred) (Selfed)
Sweet peas, the second generation P Phenotypes Purple Long Genotypes F 1 F 2 X FFLL All purple long Genotypes Ff. Ll Phenotypes Purple Long Purple Round © 2016 Paul Billiet ODWS 9/16 56, 25% (Pure Bred) ffll Phenotypes Expected Ratios Red Round 3/16 18, 75% (Selfed) Red Long Red Round 3/16 18, 75% 1/16 6, 25%
What was expected? F 2 Phenotypes Expected Ratios Purple Long Purple Round Red Long Red Round 9/16 56, 25% 3/16 18, 75% 1/16 6, 25% Parental combination © 2016 Paul Billiet ODWS Parental combination
Not quite what was expected! F 2 Phenotypes Purple Long Purple Round Red Long Expected Ratios 9/16 56, 25% Observed numbers 296 19 27 85 Observed Ratios 69, 32% 4, 45% 6, 32% 19, 91% Parental combination © 2016 Paul Billiet ODWS 3/16 18, 75% Red Round 1/16 6, 25% Parental combination Total = 427
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 © 2016 Paul Billiet ODWS Telophase 2
Are there any other recombinants? Yes, hidden amongst the Purple Long plants ¢ The genotype Ff. Ll does not tell us enough about linked genes. Genotypes for linked genes can be shown as: ¢ o This is an example of a parental combination This genotype would give the same phenotype as: o 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 1 Phenotypes Genotypes © 2016 Paul Billiet ODWS F L f l All Purple Long
The Genetic Diagram for Linked Genes F 1 Phenotypes All Purple Long (Selfed) Genotypes crossing over in meiosis I Gametes F L f l Parental Combinations © 2016 Paul Billiet ODWS F l f L Recombinants
The Genetic Diagram for Linked Genes F 2 Genotypes F L f l F l f L © 2016 Paul Billiet ODWS F L f l F l f L
The ratios Phenotypes Expected Ratio if the genes were only linked © 2016 Paul Billiet ODWS Purple Long Red Round Purple Round Red Long 3/4 75% 1/4 25% 0 0
The ratios Phenotypes Expected Ratio if the genes were only linked Observed Ratio © 2016 Paul Billiet ODWS Purple Long Red Round Purple Round Red Long 3/4 75% 1/4 25% 0 0 69, 32% 19, 91% 4, 45% 6, 32%
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 F 1 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 Normal E Ebony e Body colour © 2016 Paul Billiet ODWS
CALCULATING THE CROSS OVER VALUE Phenotypes Gametes Heterozygous wild type Bb. Ee BE, Be, b. E, be If these genes are linked Be and b. E could only be produced by crossing over. © 2016 Paul Billiet ODWS X Ebony Bent bbee be
CALCULATING THE CROSS OVER VALUE Phenotypes Wild type Normal Ebony Bent Normal Bent Ebony Genotypes Bb. Ee Bbee bb. Ee bbee 83 82 76 71 25% 25% Numbers Approx. Ratio © 2016 Paul Billiet ODWS Parental combination Recombinants = 50% of the offspring Parental combination
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 Eye colour Red P Pink p Normal E Ebony e Body colour © 2016 Paul Billiet ODWS Alleles
CALCULATING THE CROSS OVER VALUE Phenotypes Heterozygous wild type Genotypes Pp. Ee ppee PE, Pe, p. E, pe pe Gametes X Pink Ebony Phenotypes Wild type Red Ebony Pink Normal Pink Ebony Genotypes Pp. Ee Ppee pp. Ee ppee © 2016 Paul Billiet ODWS
CALCULATING THE CROSS OVER VALUE Phenotypes Wild type Red Ebony Pink Normal Pink Ebony Genotypes Pp. Ee Ppee pp. Ee ppee 601 3 4 584 Numbers Parental combinations Recombinants < 50% Parental combinations 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 = o This value tells us how far apart the loci of the genes are o Cross over values from several pairs of genes permit a geneticist to plot a gene map of the chromosome. © 2016 Paul Billiet ODWS
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