Dihybrid Crosses, Gene Linkage and Recombination

Dihybrid Crosses & Gene Linkage Stephen Taylor 10.2 Dihybrid Crosses & Gene Linkage 1 http://sciencevideos.wordpress.com

Mendel’s Law of Independent Assortment “Can you remember it?” 10.2 Dihybrid Crosses & Gene Linkage 2 http://sciencevideos.wordpress.com

Mendel’s Law of Independent Assortment “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.” This only holds true for unlinked genes (genes on different chromosomes). 10.2 Dihybrid Crosses & Gene Linkage 3 http://sciencevideos.wordpress.com

Mendel’s Law of Independent Assortment “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.” Key to alleles: Y = yellow y = green S = smooth s = rough This only holds true for unlinked genes (genes on different chromosomes). meiosis sy Sy sY SY 10.2 Dihybrid Crosses & Gene Linkage 4 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous at both loci? Phenotype: F0 Heterozygous at both loci Heterozygous at both loci Genotype: Punnet Grid: F1 10.2 Dihybrid Crosses & Gene Linkage 5 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous at both loci? Phenotype: Smooth, yellow Smooth, yellow F0 Heterozygous at both loci Heterozygous at both loci SsYy SsYy Genotype: Punnet Grid: F1 10.2 Dihybrid Crosses & Gene Linkage 6 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous at both loci? Phenotype: Smooth, yellow Smooth, yellow F0 Heterozygous at both loci Heterozygous at both loci SsYy SsYy Genotype: Punnet Grid: F1 10.2 Dihybrid Crosses & Gene Linkage 7 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous at both loci? Phenotype: Smooth, yellow Smooth, yellow F0 Heterozygous at both loci Heterozygous at both loci SsYy SsYy Genotype: Punnet Grid: F1 Phenotypes: 9 Smooth, yellow : 3 Smooth, green : 3 Rough, yellow : 1 Rough, green 10.2 Dihybrid Crosses & Gene Linkage 8 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. Calculate the predicted phenotype ratio for: Phenotype: F0 Heterozygous for S, homozygous dominant for Y Heterozygous at both loci Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 9 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. Calculate the predicted phenotype ratio for: Phenotype: Smooth, yellow Smooth, yellow F0 Heterozygous for S, homozygous dominant for Y Heterozygous at both loci SsYY SsYy Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 10 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. Calculate the predicted phenotype ratio for: Phenotype: Smooth, yellow Smooth, yellow F0 Heterozygous for S, homozygous dominant for Y Heterozygous at both loci SsYY SsYy Genotype: Punnet Grid: 6 Smooth, yellow : 2Rough, yellow F1 Phenotypes: 3 Smooth, yellow : 1Rough, yellow Present the ratio in the simplest mathematical form. 10.2 Dihybrid Crosses & Gene Linkage 13 http://sciencevideos.wordpress.com

Dihybrid Crosses Common expected ratios of dihybrid crosses. SsYy SsYy SsYy SsYy Heterozygous at both loci Heterozygous at both loci Heterozygous at both loci Heterozygous at one locus, homozygous dominant at the other 3 : 1 9 : 3 : 3 : 1 Ssyy SsYy SSyy ssYY = All SsYy Heterozygous at both loci Heterozygous/ Homozygous recessive SSYY ssyy = all SyYy Ssyy ssYy = 1 : 1 : 1 : 1 4 : 3 : 1 10.2 Dihybrid Crosses & Gene Linkage 14 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A rough yellow pea is test crossed to determine its genotype. Phenotype: F0 Rough, yellow Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 15 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A rough yellow pea is test crossed to determine its genotype. Phenotype: F0 Rough, yellow ssYy Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 16 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A rough yellow pea is test crossed to determine its genotype. Phenotype: F0 Rough, yellow ssYy or ssYY Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 17 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A rough yellow pea is test crossed to determine its genotype. Phenotype: F0 Rough, yellow ssYy or ssYY ssyy Genotype: Punnet Grid: F1 Phenotypes: Remember: A test cross is the unknown with a known homozygous recessive. 10.2 Dihybrid Crosses & Gene Linkage 18 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A rough yellow pea is test crossed to determine its genotype. Phenotype: F0 Rough, yellow ssYy or ssYY ssyy Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 19 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A rough yellow pea is test crossed to determine its genotype. Phenotype: F0 Rough, yellow ssYy or ssYY ssyy Genotype: Punnet Grid: F1 Phenotypes: Some green peas will be present in the offspring if the unknown parent genotype is ssYy. No green peas will be present in the offspring if the unknown parent genotype is ssYY. 10.2 Dihybrid Crosses & Gene Linkage 20 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A smooth green pea is test crossed. Deduce the genotype. Smooth green = nine offspring. Rough green = one offspring. Phenotype: F0 Smooth, green Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 21 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A smooth green pea is test crossed. Deduce the genotype. Smooth green = nine offspring. Rough green = one offspring. Phenotype: F0 Smooth, green ssyy Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 22 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A smooth green pea is test crossed. Deduce the genotype. Smooth green = nine offspring. Rough green = one offspring. Phenotype: F0 Smooth, green SSyy ssyy Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 23 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A smooth green pea is test crossed. Deduce the genotype. Smooth green = nine offspring. Rough green = one offspring. Phenotype: F0 Smooth, green SSyy or Ssyy ssyy Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 24 http://sciencevideos.wordpress.com

Dihybrid Crosses Consider two traits, each carried on separate chromsomes (the genes are unlinked). Key to alleles: Y = yellow y = green S = smooth s = rough In this example of Lathyrusodoratus (sweet pea), we consider two traits: pea colourand pea surface. A smooth green pea is test crossed. Deduce the genotype. Smooth green = nine offspring. Rough green = one offspring. Phenotype: F0 Smooth, green SSyy or Ssyy ssyy Genotype: Punnet Grid: F1 Phenotypes: No rough peas will be present in the offspring if the unknown parent genotype is SSyy. The presence of rough green peas in the offspring means that the unknown genotype must be Ssyy. The expected ratio in this cross is 3 smooth green : 1 rough green. This is not the same as the outcome. Remember that each reproduction event is chance and the sample size is very small. With a much larger sample size, the outcome would be closer to the expected ratio, simply due to probability. 10.2 Dihybrid Crosses & Gene Linkage 25 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles*: C = colour c = albino A = agouti a = black R = round ears r = pointy ears L = long whiskers l = short whiskers S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty news story from the BBC: http://news.bbc.co.uk/2/hi/uk_news/wales/1048327.stm * C and A genes are real. The rest are made up for this story. 10.2 Dihybrid Crosses & Gene Linkage 26 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty has soft fur and sharp nails. In one of his matings with a rough-furred, smooth-nailed female, the following guinea piglets are produced: 6 x rough fur, sharp nails; 3 x soft fur sharp nails. DeduceSooty’s genotype. Phenotype: Rough fur, smooth nails Soft fur, sharp nails F0 Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 27 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty has soft fur and sharp nails. In one of his matings with a rough-furred, smooth-nailed female, the following guinea piglets are produced: 6 x rough fur, sharp nails; 3 x soft fur sharp nails. DeduceSooty’s genotype. Phenotype: Rough fur, smooth nails Soft fur, sharp nails F0 Genotype: ssnn Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 28 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty has soft fur and sharp nails. In one of his matings with a rough-furred, smooth-nailed female, the following guinea piglets are produced: 6 x rough fur, sharp nails; 3 x soft fur sharp nails. DeduceSooty’s genotype. Phenotype: Rough fur, smooth nails Soft fur, sharp nails F0 Genotype: ssnn SSNN or SsNN or SsNn Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 29 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty has soft fur and sharp nails. In one of his matings with a rough-furred, smooth-nailed female, the following guinea piglets are produced: 6 x rough fur, sharp nails; 3 x soft fur sharp nails. DeduceSooty’s genotype. Phenotype: Rough fur, smooth nails Soft fur, sharp nails F0 Genotype: ssnn SSNN or SsNN or SsNn Punnet Grid: F1 Soft fur Sharp nails Soft fur Smooth nails Rough fur Sharp nails Rough fur Smooth nails Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 30 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty has soft fur and sharp nails. In one of his matings with a rough-furred, smooth-nailed female, the following guinea piglets are produced: 6 x rough fur, sharp nails; 3 x soft fur sharp nails. DeduceSooty’s genotype. Phenotype: Rough fur, smooth nails Soft fur, sharp nails F0 Genotype: ssnn SSNN or SsNN or SsNn Punnet Grid: F1 Soft fur Sharp nails Soft fur Smooth nails Rough fur Sharp nails Rough fur Smooth nails Phenotypes: Only these two phenotypes have been produced. Sooty has only produced SN and sN gametes. 10.2 Dihybrid Crosses & Gene Linkage 31 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: S = soft fur s = rough fur N = sharp nails n = smooth nails Sooty has soft fur and sharp nails. In one of his matings with a rough-furred, smooth-nailed female, the following guinea piglets are produced: 6 x rough fur, sharp nails; 3 x soft fur sharp nails. DeduceSooty’s genotype. Phenotype: Rough fur, smooth nails Soft fur, sharp nails F0 Genotype: ssnn SSNN or SsNN or SsNn Punnet Grid: F1 Soft fur Sharp nails Soft fur Smooth nails Rough fur Sharp nails Rough fur Smooth nails Phenotypes: Only these two phenotypes have been produced. Sooty has only produced SN and sN gametes. It is most likely that his genotype is SsNN. 10.2 Dihybrid Crosses & Gene Linkage 32 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: R = round ears r = pointy ears L = long whiskers l = short whiskers DeduceSooty’s genotype. Offspring = five with pointy ears and long whiskers Phenotype: Pointy ears, short whiskers Pointy ears, long whiskers F0 Genotype: Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 33 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: R = round ears r = pointy ears L = long whiskers l = short whiskers DeduceSooty’s genotype. Offspring = five with pointy ears and long whiskers Phenotype: Pointy ears, short whiskers Pointy ears, long whiskers F0 Genotype: rrllrrLL or rrLl Punnet Grid: F1 Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 34 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: R = round ears r = pointy ears L = long whiskers l = short whiskers DeduceSooty’s genotype. Offspring = five with pointy ears and long whiskers Phenotype: Pointy ears, short whiskers Pointy ears, long whiskers F0 Genotype: rrllrrLL or rrLl Punnet Grid: F1 Pointy ears Long whiskers Pointy ears Short whiskers Phenotypes: 10.2 Dihybrid Crosses & Gene Linkage 35 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: R = round ears r = pointy ears L = long whiskers l = short whiskers DeduceSooty’s genotype. Offspring = five with pointy ears and long whiskers Phenotype: Pointy ears, short whiskers Pointy ears, long whiskers F0 Genotype: rrllrrLL or rrLl Punnet Grid: F1 Pointy ears Long whiskers Pointy ears Short whiskers Phenotypes: Only this phenotype has been produced. Sooty has only produced rL gametes. 10.2 Dihybrid Crosses & Gene Linkage 36 http://sciencevideos.wordpress.com

Sooty the Guinea Pig Key to alleles: R = round ears r = pointy ears L = long whiskers l = short whiskers DeduceSooty’s genotype. Offspring = five with pointy ears and long whiskers Phenotype: Pointy ears, short whiskers Pointy ears, long whiskers F0 Genotype: rrllrrLL or rrLl Punnet Grid: F1 Pointy ears Long whiskers Pointy ears Short whiskers Phenotypes: Only this phenotype has been produced. Sooty has only produced rL gametes. It is most likely that his genotype is rrLL. 10.2 Dihybrid Crosses & Gene Linkage 37 http://sciencevideos.wordpress.com

Gene Interaction The expression of one gene is dependent upon the prior expression of another. 10.2 Dihybrid Crosses & Gene Linkage 38 http://sciencevideos.wordpress.com

Gene Interaction The expression of one gene is dependent upon the prior expression of another. Key to alleles: C = colour c = albino A = agouti a = black In the case of guinea pigs, there is gene interaction for fur colour. The first gene, C, determines whether colour is present. The second gene, A, is only expressed if C is first expressed. It determines which colourwill be produced. 10.2 Dihybrid Crosses & Gene Linkage 39 http://sciencevideos.wordpress.com

Gene Interaction The expression of one gene is dependent upon the prior expression of another. Key to alleles: C = colour c = albino A = agouti a = black In the case of guinea pigs, there is gene interaction for fur colour. The first gene, C, determines whether colour is present. The second gene, A, is only expressed if C is first expressed. It determines which colourwill be produced. Genotypes ccAA ccAa ccaa CCAA CcAa CCaa Ccaa If the genotype ‘cc’ is present, there will be no expression of colour. A will also not be expresssed. 10.2 Dihybrid Crosses & Gene Linkage 40 http://sciencevideos.wordpress.com

Gene Interaction The expression of one gene is dependent upon the prior expression of another. Key to alleles: C = colour c = albino A = agouti a = black In the case of guinea pigs, there is gene interaction for fur colour. The first gene, C, determines whether colour is present. The second gene, A, is only expressed if C is first expressed. It determines which colourwill be produced. Phenotype ratios do not fit the normal 9 : 3 : 3 : 1 ratio. Genotypes ccAA ccAa ccaa CCAA CcAa CCaa Ccaa If the genotype ‘cc’ is present, there will be no expression of colour. A will also not be expresssed. 9 agouti : 3 black : 4 albino 10.2 Dihybrid Crosses & Gene Linkage 41 http://sciencevideos.wordpress.com

Autosomes and Sex Chromosomes Humans have 23 pairs of chromosomes in diploid somatic cells (n=2). 22 pairs of these are autosomes, which are homologous pairs. One pair is the sex chromosomes. XX gives the female gender, XY gives male. Karyotype of a human male, showing X and Y chromosomes: http://en.wikipedia.org/wiki/Karyotype SRY The X chromosome is much larger than the Y. X carries many genes in the non-homologous region which are not present on Y. The presence and expression of the SRY gene on Y leads to male development. Chromosome images from Wikipedia: http://en.wikipedia.org/wiki/Y_chromosome 10.2 Dihybrid Crosses & Gene Linkage 42 http://sciencevideos.wordpress.com

Autosomal Gene Linkage vs Sex-Linked Disorders Sex-linked disorders are carried on the non-homologous regions of the X chromosome. Alleles are expressed whether they are dominant or recessive, as there is no alternate allele carried on the Y chromosome. Gene-related disorders which are sex-linked include red-green colour blindness and hemophilia. Males are more frequently affected by sex-linked disorders. A B SCN5A a b (voltage-gated sodium channel) Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. Locus 1 Locus 2 PDCD10 Y X (programmed cell death) SOX2 (transcription factor - promoter region) There are about 2000 genes on X and 86 on Y. Gene linkage is therefore also common on X and Y. Chromosome 3 from: http://en.wikipedia.org/wiki/Chromosome_3_%28human%29 10.2 Dihybrid Crosses & Gene Linkage 43 http://sciencevideos.wordpress.com

Autosomal Gene Linkage Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. 10.2 Dihybrid Crosses & Gene Linkage 44 http://sciencevideos.wordpress.com

Autosomal Gene Linkage Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. SCN5A (voltage-gated sodium channel) The SCN5A, PDCD10 and SOX2 genes are all linked by being on chromosome 3. They are a linkage group, and alleles of each will therefore be inherited together. Independent assortment does not occurbetween linked genes. PDCD10 (programmed cell death) SOX2 (transcription factor - promoter region) Chromosome 3 from: http://en.wikipedia.org/wiki/Chromosome_3_%28human%29 10.2 Dihybrid Crosses & Gene Linkage 45 http://sciencevideos.wordpress.com

Autosomal Gene Linkage Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. Standard notation for linked genes: A B “heterozygous at both loci” SCN5A a b (voltage-gated sodium channel) The line denotes the chromosome, or the fact that the two genes are linked. Locus 1 Locus 2 The SCN5A, PDCD10 and SOX2 genes are all linked by being on chromosome 3. They are a linkage group, and alleles of each will therefore be inherited together. Independent assortment does not occurbetween linked genes. Syllabus examples of Linkage Groups: Sweet peas (Lathyrusodoratus): flower colour(P/p) linked with pollen grain shape (L/l) Corn (Zea mays): Kernel colour(C/c) linked with Waxiness of kernels (W/w) PDCD10 (programmed cell death) SOX2 (transcription factor - promoter region) Chromosome 3 from: http://en.wikipedia.org/wiki/Chromosome_3_%28human%29 10.2 Dihybrid Crosses & Gene Linkage 46 http://sciencevideos.wordpress.com

Notation of Gene Linkage Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. The genes A and B are linked. The genotype of an individual is AaBb (“heterozygous at both loci”.) 10.2 Dihybrid Crosses & Gene Linkage 47 http://sciencevideos.wordpress.com

Notation of Gene Linkage Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. The genes A and B are linked. The genotype of an individual is AaBb (“heterozygous at both loci”.) So in questions or problems you will be given the standard notation or enough information to be able to deduce which allele is on which chromosome. Confusing! Could be.. Standard notation: The line denotes the chromosome, or the fact that the two genes are linked. 10.2 Dihybrid Crosses & Gene Linkage 48 http://sciencevideos.wordpress.com

Notation of Gene Linkage Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome. The genes A and B are linked. The genotype of an individual is AaBb (“heterozygous at both loci”.) A B A b So in questions or problems you will be given the standard notation or enough information to be able to deduce which allele is on which chromosome. a b a B Confusing! Could be.. Standard notation: The line denotes the chromosome, or the fact that the two genes are linked. Locus 1 Locus 1 Locus 2 Locus 2 Alternative notation: Ab/aB Alternative notation: AB/ab 10.2 Dihybrid Crosses & Gene Linkage 49 http://sciencevideos.wordpress.com

Linkage Groups Are carried on the same chromosomeand are inherited together. They do not assort independently. In sweet peas (Lathyrusodoratus), the genes for flower colour and pollen grain shape are carried on the same chromosome. Plants which are heterozygous at both loci are test-crossed. What ratio of phenotypes is expected? Key to alleles: P = purple p = white L = long l = short Genotype: Phenotype: Image: 'Sweet Pea' http://www.flickr.com/photos/69166981@N00/3600419425 10.2 Dihybrid Crosses & Gene Linkage 50 http://sciencevideos.wordpress.com

Linkage Groups Are carried on the same chromosomeand are inherited together. They do not assort independently. In sweet peas (Lathyrusodoratus), the genes for flower colour and pollen grain shape are carried on the same chromosome. Plants which are heterozygous at both loci are test-crossed. What ratio of phenotypes is expected? Key to alleles: P = purple p = white L = long l = short p l Locus 1 Locus 2 p l Genotype: Phenotype: White; Short Image: 'Sweet Pea' http://www.flickr.com/photos/69166981@N00/3600419425 10.2 Dihybrid Crosses & Gene Linkage 51 http://sciencevideos.wordpress.com

Linkage Groups Are carried on the same chromosomeand are inherited together. They do not assort independently. In sweet peas (Lathyrusodoratus), the genes for flower colour and pollen grain shape are carried on the same chromosome. Plants which are heterozygous at both loci are test-crossed. What ratio of phenotypes is expected? Key to alleles: P = purple p = white L = long l = short P L p l Locus 1 Locus 1 Locus 2 Locus 2 p l p l Genotype: Phenotype: Purple; Long White; Short Image: 'Sweet Pea' http://www.flickr.com/photos/69166981@N00/3600419425 10.2 Dihybrid Crosses & Gene Linkage 52 http://sciencevideos.wordpress.com

Linkage Groups Are carried on the same chromosomeand are inherited together. They do not assort independently. In sweet peas (Lathyrusodoratus), the genes for flower colour and pollen grain shape are carried on the same chromosome. Plants which are heterozygous at both loci are test-crossed. What ratio of phenotypes is expected? Key to alleles: P = purple p = white L = long l = short P L p l Locus 1 Locus 1 Locus 2 Locus 2 p l p l Genotype: Phenotype: Purple; Long White; Short Punnet Grid: Phenotypes: Ratio: Image: 'Sweet Pea' http://www.flickr.com/photos/69166981@N00/3600419425 10.2 Dihybrid Crosses & Gene Linkage 53 http://sciencevideos.wordpress.com

Linkage Groups Are carried on the same chromosomeand are inherited together. They do not assort independently. In sweet peas (Lathyrusodoratus), the genes for flower colour and pollen grain shape are carried on the same chromosome. Plants which are heterozygous at both loci are test-crossed. What ratio of phenotypes is expected? Key to alleles: P = purple p = white L = long l = short P L p l Locus 1 Locus 1 Locus 2 Locus 2 p l p l Genotype: Phenotype: Purple; Long White; Short Punnet Grid: Purple; Long White; Short Phenotypes: Ratio: 1 : 1 Image: 'Sweet Pea' http://www.flickr.com/photos/69166981@N00/3600419425 10.2 Dihybrid Crosses & Gene Linkage 54 http://sciencevideos.wordpress.com

Linkage Groups Are carried on the same chromosomeand are inherited together. They do not assort independently. In sweet peas (Lathyrusodoratus), the genes for flower colour and pollen grain shape are carried on the same chromosome. Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. Key to alleles: P = purple p = white L = long l = short Image: 'Sweet Pea' http://www.flickr.com/photos/69166981@N00/3600419425 10.2 Dihybrid Crosses & Gene Linkage 55 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. Diploid cell Heterozygous at both loci 10.2 Dihybrid Crosses & Gene Linkage 56 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. Possible gametes: The test cross individual is homozygous recessive at both loci, so only one type of gamete is produced. Test individual: p l Heterozygous individual: Diploid cell Heterozygous at both loci Chromosomes replicate in Synthesis phase 10.2 Dihybrid Crosses & Gene Linkage 57 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. P L P L Possible gametes: The test cross individual is homozygous recessive at both loci, so only one type of gamete is produced. Test individual: p l Alleles segregate in meiosis, giving two possible gametes: Heterozygous individual: p l p l Diploid cell Heterozygous at both loci Chromosomes replicate in Synthesis phase 10.2 Dihybrid Crosses & Gene Linkage 58 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. P L Possible gametes: Test individual: p l Heterozygous individual: p l Diploid cell Heterozygous at both loci Chromosomes replicate in Synthesis phase Crossing Over Prophase I Alleles are exchanged Crossing-over occurs occasionally. It is more likely to happen between linked genes which are further apart. 10.2 Dihybrid Crosses & Gene Linkage 59 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. P L P l Possible gametes: Test individual: p l Heterozygous individual: p l Recombinants: Diploid cell Heterozygous at both loci Chromosomes replicate in Synthesis phase Crossing Over Prophase I Alleles are exchanged Sister chromatids are separated in anaphase II. Recombined gametes are produced. p L Crossing-over occurs occasionally. It is more likely to happen between linked genes which are further apart. 10.2 Dihybrid Crosses & Gene Linkage 60 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. P L P l Normal gametes (majority) Possible gametes: Test individual: p l Heterozygous individual: p l Recombinants: p L Crossing-over occurs occasionally. It is more likely to happen between linked genes which are further apart. 10.2 Dihybrid Crosses & Gene Linkage 61 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. P L P l Normal gametes (majority) Possible gametes: Test individual: p l Heterozygous individual: Purple; long White, short p l Recombinants: p L Crossing-over occurs occasionally. It is more likely to happen between linked genes which are further apart. 10.2 Dihybrid Crosses & Gene Linkage 62 http://sciencevideos.wordpress.com

Recombination of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes. Key to alleles: P = purple p = white L = long l = short Plants which are heterozygous at both loci are test-crossed. A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened. P L P l Normal gametes (majority) Recombinant gametes (small number) Possible gametes: Test individual: p l Heterozygous individual: White, long Purple; long White, short Purple; short p l Recombinants: p L Crossing-over occurs occasionally. It is more likely to happen between linked genes which are further apart. 10.2 Dihybrid Crosses & Gene Linkage 63 http://sciencevideos.wordpress.com

Crossing-Over Increases genetic variation through recombination of linked alleles. Synapsis Homologous chromosomes associate Chiasma Formation Neighbouring non-sister chromatids are cut at the same point. A Holliday junction forms as the DNA of the cut sections attach to the open end of the opposite non-sister chromatid. Recombination As a result, alleles are swapped between non-sister chromatids. 10.2 Dihybrid Crosses & Gene Linkage 64 http://sciencevideos.wordpress.com

Crossing-Over Increases genetic variation through recombination of linked alleles. 10.2 Dihybrid Crosses & Gene Linkage 65 http://sciencevideos.wordpress.com

Gene Linkage & Recombination The further apart a pair of alleles are on a chromosome, the more likely it is that crossing over may occur between them - leading to recombination. Knowing this, researchers can map the position of genes on a chromosome based on the frequency of recombination between gene pairs: the further apart they are, the more often they cross over. SCN5A (voltage-gated sodium channel) Crossing-over is more likely to occur between SCN5A and PDCD10 than between PDCD10 and SOX2. PDCD10 (programmed cell death) SOX2 (transcription factor - promoter region) Chromosome 3 from: http://en.wikipedia.org/wiki/Chromosome_3_%28human%29 Animation and quiz from: http://www.csuchico.edu/~jbell/Biol207/animations/recombination.html 10.2 Dihybrid Crosses & Gene Linkage 66 http://sciencevideos.wordpress.com

Gene Linkage & Recombination Which description best fits this image? Four chromosomes, four chiasmata Four chromatids, two chiasmata, two centromeres Two chromosomes, four chiasmata A pair of sister chromatids 10.2 Dihybrid Crosses & Gene Linkage 67 http://sciencevideos.wordpress.com

Gene Linkage & Recombination Which description best fits this image? Four chromosomes, four chiasmata Four chromatids, two chiasmata, two centromeres Two chromosomes, four chiasmata A pair of sister chromatids 10.2 Dihybrid Crosses & Gene Linkage 68 http://sciencevideos.wordpress.com

Gene Linkage & Recombination Which description best fits this image? chiasmata Sister chromatids Chromosome 1a Chromosome 1b Sister chromatids centromeres Four chromosomes, four chiasmata Four chromatids, two chiasmata, two centromeres Two chromosomes, four chiasmata A pair of sister chromatids 10.2 Dihybrid Crosses & Gene Linkage 69 http://sciencevideos.wordpress.com

Gene Linkage & Recombination The genes for kernel colour and waxiness are linked in the corn plant (Zea mays). In a cross between a plant that is homozygous dominant at both loci with a plant that is heterozygous at both loci (CW/cw), identify the following genotypes as: a: regularb:recombinantsc: impossible CcWwCCWwCcWW CCWW CCwwccWW 10.2 Dihybrid Crosses & Gene Linkage 70 http://sciencevideos.wordpress.com

Gene Linkage & Recombination The genes for kernel colour and waxiness are linked in the corn plant (Zea mays). In a cross between a plant that is homozygous dominant at both loci with a plant that is heterozygous at both loci (CW/cw), identify the following genotypes as: a: regularb:recombinantsc: impossible Key to alleles: C = coloured c = no colour W = waxy w = not waxy CcWwCCWwCcWW CCWW CCwwccWW C W C W Regular gametes (majority) Recombinant gametes (small number) 10.2 Dihybrid Crosses & Gene Linkage 71 http://sciencevideos.wordpress.com

Gene Linkage & Recombination The genes for kernel colour and waxiness are linked in the corn plant (Zea mays). In a cross between a plant that is homozygous dominant at both loci with a plant that is heterozygous at both loci (CW/cw), identify the following genotypes as: a: regularb:recombinantsc: impossible Key to alleles: C = coloured c = no colour W = waxy w = not waxy CcWwCCWwCcWW CCWW CCwwccWW C W C W c w C W Regular gametes (majority) Recombinant gametes (small number) 10.2 Dihybrid Crosses & Gene Linkage 72 http://sciencevideos.wordpress.com

Gene Linkage & Recombination The genes for kernel colour and waxiness are linked in the corn plant (Zea mays). In a cross between a plant that is homozygous dominant at both loci with a plant that is heterozygous at both loci (CW/cw), identify the following genotypes as: a: regularb:recombinantsc: impossible Key to alleles: C = coloured c = no colour W = waxy w = not waxy CcWwCCWwCcWWCCWWCCwwccWW C W C W c w C W Regular gametes (majority) Recombinant gametes (small number) 10.2 Dihybrid Crosses & Gene Linkage 73 http://sciencevideos.wordpress.com

Gene Linkage & Recombination The genes for kernel colour and waxiness are linked in the corn plant (Zea mays). In a cross between a plant that is homozygous dominant at both loci (CW/CW) with a plant that is heterozygous at both loci (CW/cw), identify the following genotypes as: a: regularb:recombinantsc: impossible Key to alleles: C = coloured c = no colour W = waxy w = not waxy CcWwCCWwCcWWCCWWCCwwccWW C C W C W w c w c W C W Regular gametes (majority) Recombinant gametes (small number) 10.2 Dihybrid Crosses & Gene Linkage 74 http://sciencevideos.wordpress.com

Gene Linkage & Recombination E m Two genes are linked as shown here e M The genes are far apart such that crossing-over between the alleles occurs occasionally. Which statement is true of the gametes? A. All of the gametes will be Em and eM B. There will be equal numbers of EM, EM, eM and em C. There will be approximately equal numbers of EM and eM gametes D. There will be more Em gametes than em gametes 10.2 Dihybrid Crosses & Gene Linkage 75 http://sciencevideos.wordpress.com

Gene Linkage & Recombination E m Two genes are linked as shown here e M The genes are far apart such that crossing-over between the alleles occurs occasionally. Which statement is true of the gametes? A. All of the gametes will be Em and eM B. There will be equal numbers of EM, EM, eM and em C. There will be approximately equal numbers of EM and eM gametes D. There will be more Em gametes than em gametes 10.2 Dihybrid Crosses & Gene Linkage 76 http://sciencevideos.wordpress.com

Gene Linkage & Recombination E m Two genes are linked as shown here e M The genes are far apart such that crossing-over between the alleles occurs occasionally. Which statement is true of the gametes? A. All of the gametes will be Em and eM B. There will be equal numbers of EM, EM, eM and em C. There will be approximately equal numbers of EM and eM gametes D. There will be more Em gametes than em gametes E m E M m e e M Regular gametes (majority) Recombinant gametes (small number) 10.2 Dihybrid Crosses & Gene Linkage 77 http://sciencevideos.wordpress.com

For more IB Biology resources: http://sciencevideos.wordpress.com This presentation is free to view. Please make a donation to one of my chosen charities at Gifts4Good and I will send you the editable pptx file. Click here for more information about Biology4Good charity donations. 10.2 Dihybrid Crosses & Gene Linkage 78 This is a Creative Commons presentation. It may be linked and embedded but not sold or re-hosted.

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