The Chromosomal Basis of Inheritance GENE MAPPING AP Biology/ Ms. Day Chapter 15 (Part 6) The Chromosomal Basis of Inheritance GENE MAPPING AP Biology/ Ms. Day

Linked Genes Remember… tend to be inherited together located near each other on same chromosome The CLOSER genes are linked LESS crossing over  tend to “travel” or segregate together into gamete

How Linkage Affects Inheritance Morgan did other experiments with fruit flies to see how linkage affects the inheritance of 2 different characteristics (genes) Body color and wing size

Morgan’s Dihybrid Cross In flies, gray body (b+) = dominant black body (b) = recessive long (normal) wings (vg+) = dominant vestigial wings (vg) = recessive What is expected phenotype ratio from a cross between a heterozygous fly (for both trait with a recessive fly for both traits? b+b vg+vg x bb vgvg (BbNn x bbnn) ¼ : ¼ : ¼ : ¼ ratio or a 1:1:1:1 ratio But what he got was…..

Recall…What is Mendel’s Law of Independent Assortment? states that allele pairs separate independently during meiosis Therefore, traits are transmitted to offspring independently of one another

What explains the non-Mendelian results? There was NO independent assortment! Traits are linked on the same chromosome If traits are linked, then all of offspring should resemble the parental types, black vestigial or gray normal. How did we get the recombined traits of black, normal and gray vestigial? Looks like parents

Crossing Over Linked genes are not inherited together every time. Chromosomes exchange homologous genes during meiosis Produces offspring with combinations of traits different from those found in EITHER parent

Morgan determined… Genes close together on SAME chromosome are linked and do not assort independently Unlinked genes are either on separate chromosomes or FAR apart on the same chromosome and assort independently (if crossing over separates the genes)

Produce recombination frequencies less than 50% Linked vs. Not Linked??? Linked genes Produce recombination frequencies less than 50% Frequency = # of recombinants x 100 total # of offspring

Linkage Mapping A genetic map an ordered list of the genetic loci (positions) of genes along a particular chromosome Can be developed using recombination frequencies

Using Recombination Frequency to Map Traits The farther apart genes are on a chromosome The more likely they are to be separated during crossing over Therefore… Recombination frequency is greater between genes further apart on a chromosome

Practice Problem #1 Suppose you cross a fly that is completely heterozygous with a completely recessive fly. AaBb (Wildtype) x aabb (mutant) These are your results. Expected Observed Wildtype 25 33 Mutant 25 33 Short wings, Wild 25 17 Wild, Black Body 25 17 What is the recombination frequency btw these genes? Are they linked? How far apart are these 2 alleles if linked?

Practice Problem #2 In pea plants, flower color and seed color are on the same chromosome. A plant w/ purple flowers and green seeds (AaBb) is crossed with one recessive for both traits (aabb) AaBb x aabb These are your results. Expected Observed Purple, green 25 47 White, green 25 47 purple, yellow 25 3 white, yellow 25 3 What is the recombination frequency btw these genes? Are they linked? How far apart are these 2 alleles if linked?

Practice Problem #3… Using Recombination Frequency to Map Traits The following 3 traits are linked to chromosome #1 Recombination frequency between black (b) and vestigial wing (vg) traits is 17% Recombination frequency between cn (cinnabar eyes) and black is 9% Recombination between cn and vg is 9.5% Map these traits on the chromosome. Chromosome #1

Distance between genes are called MAP UNITS = recombination frequency

Let’s Practice Again… Determine the order of genes along a chromosome based on the following recombination frequencies: A - B 8% A - C 28% A - D 25% B - C 20% B - D 33% __D___ __25mu___A_8 mu_B _ _20 mu__ C

Eukaryotic Chromosomes- How Do You Solve Gene Mapping Problems?

How do you tell if genes are linked or unlinked? Linked = no independent assortment Genes are on the SAME chromosome Unlinked = independent assortment occurs Genes are on SEPARATE chromosomes or VERY far apart on the same chromosome Use “expected” phenotype and genotype ratios (null hypothesis) If you do NOT get expected Punnett Square ratios, the genes are probably LINKED!

If genes are linked, how do you tell which are the parental offsr? Parental genotypes should be found MORE frequently (more often) in the offspring than the recombinant linkages 2 ways alleles can be linked in a heterozygous parent (--- = chromosome) Cis 2 dominant alleles on 1st chromosome homologue 2 recessive alleles on 2nd chromosome homologue OR… Trans 1 dominant, 1 recessive allele on 1st chromosome homologue (ex: A, b) 1 dominant, 1 recessive allele on 2nd chromosome homologue (ex: a, B) -----A------B---- -----a------b----  AaBb -----A------b---- -----a------B----  AaBb

Example #1… P Mating : AaBb x aabb (this is a testcross  ) A= Long antennae B = Green eyebrows a = Short antennae b = Blue eyebrows If genes are UNLINKED, what do you expect to see in the offspring (progeny)?  25% Long antennae, Green eyebrows  25% Long antennae, Blue eyebrows  25% Short antennae, Green eyebrow  25% Short antennae, Blue eyebrow  Assume your F1 offspring look like: Long, Green 850 Long, Blue 150 Short, Green 150 Short, Blue 850 Do these genes appear linked? ____ Parental genotypes are _________ x _________

How do you tell SINGLE from DOUBLE recombinants? Remember…recombinants do NOT look like parents Should be found MORE frequently in the offspring than the double recombinant linkages but LESS frequently in the parental linkages

Example #2… P Mating : AaBbCc x aabbcc (this is a testcross  ) A= Long antenna B = Green eyebrows C= Red eyes a = Short antennae b = Blue eyebrows c = White eyes If genes are UNLINKED, what do you expect to see in the offspring (progeny)? EQUAL # OF ALL OFFSPRING PHENOTYPE POSSIBILITIES  Assume your F1 offspring look like: Long, green, red 850 Long, green, white 175 Long, blue, red 50 Long, blue, white 150 Short, blue, white 850 Short, blue, red 50 Short, green, white 175 Short, green, red 150 Who are the single recombinants? Who are the double recombinants?

Example #3… Starting with pure breeding lines, Cross A= feathers, a= no feathers B= horns, b= no horns D= whiskers, d= no whiskers Starting with pure breeding lines, Cross AA BB DD x aa bb dd (P) mom dad AaBbDd (F1) The parental chromosomes in the F1 have to be ABD and abc From mom -----A------B------D----- -----a------b------d----- From dad F1 Genotype = AaBbDd

Cross (ABD abd) F1 progeny with (abd abd) testcross AaBbDd x aabbdd You expect 1/8 (12.5 %) for each phenotype (½ x ½ x ½) BUT….You get the F2 offspring below… Feathers, horns, whiskers 580 Feathers, horns, no whiskers 3 No feathers, no horns, whiskers 5 No feathers, no horns, no whiskers 592 Feathers, no horns, whiskers 45 Feathers, no horns, no whiskers 89 No feathers, horns, whiskers 94 No feathers, horns, no whiskers 40 So…genes are probably linked!

Which is which genotype? Feathers, horns, whiskers 580 *ABD Feathers, horns, no whiskers 3 ABd No feathers, no horns, whiskers 5 abD No feathers, no horns, no whiskers 592 abd Feathers, no horns, whiskers 45 AbD Feathers, no horns, no whiskers 89 Abd No feathers, horns, whiskers 94 aBD No feathers, horns, no whiskers 40 aBd A= feathers, a= no feathers B= horns, b= no horns D= whiskers, d= no whiskers **NOTE: the 2nd allele has to be recessive for each gene because one parent is a testcross So…ABD really means AaBbDd; Abd means Aabbdd Which is which genotype? Parental genotypes Single recombinants (1 crossing over event) Double recombinants (2 crossing over events)

A---B A---D B---D -----A------B---- -----a------b---- Ab aB Feathers, horns, whiskers 580 ABD Feathers, horns, no whiskers 3 ABd No feathers, no horns, whiskers 5 abD No feathers, no horns, no whiskers 592 abd Feathers, no horns, whiskers 45 AbD Feathers, no horns, no whiskers 89 Abd No feathers, horns, whiskers 94 aBD No feathers, horns, no whiskers 40 aBd

A---B A---D B---D -----A------B---- -----a------b---- Ab 45 + 89 aB 94 + 40 A---D -----A------D---- -----a------d---- Ad 3 + 89 aD 5 + 94 B---D -----B------D---- -----b------d---- Bd 3 + 40 bD 5 + 45 Feathers, horns, whiskers 580 ABD Feathers, horns, no whiskers 3 ABd No feathers, no horns, whiskers 5 abD No feathers, no horns, no whiskers 592 abd Feathers, no horns, whiskers 45 AbD Feathers, no horns, no whiskers 89 Abd No feathers, horns, whiskers 94 aBD No feathers, horns, no whiskers 40 aBd

A---B A---D B---D Linked genes Frequency = # of recombinants x 100 45 + 89 aB 94 + 40 = 268 A---D -----A------D---- -----a------d---- Ad 3 + 89 aD 5 + 94 = 191 B---D -----B------D---- -----b------d---- Bd 3 + 40 bD 5 + 45 = 103 Linked genes Produce recombination frequencies less than 50% Frequency = # of recombinants x 100 total # of offspring

A---B A---D B---D Frequency = # of recombinants x 100 1448 = 18.5 % = 18.5 mu A---D = 191 x 100 1448 = 6.4 % = 6.4 mu B---D = 103 x 100 1448 = 13.2 % = 13.2 mu Frequency = # of recombinants x 100 total # of offspring TOTAL # of offspring = 1448 Feathers, horns, whiskers 580 ABD Feathers, horns, no whiskers 3 ABd No feathers, no horns, whiskers 5 abD No feathers, no horns, no whiskers 592 abd Feathers, no horns, whiskers 45 AbD Feathers, no horns, no whiskers 89 Abd No feathers, horns, whiskers 94 aBD No feathers, horns, no whiskers 40 aBd

Now…map the genes on a chromosome! A---B = 18.5 mu A----D = 6.4 mu B----D = 13.2 B-----------------D------A 13.2 mu 6.4 mu 18.5 mu Chromosome