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Chapter 5. Linkage, recombination, and the mapping of genes on chromosomes. Fig. 5.1. linkage. recombination. w Y+. w Y+. W+ y. w Y+. w Y+. W+ y. w Y+. W+ y. W+ Y+. w y. When genes are linked , parental combinations outnumber recombination types. W+ Y+. W+ Y+.
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Chapter 5 Linkage, recombination, and the mapping of genes on chromosomes
Fig. 5.1 linkage recombination
w Y+ w Y+ W+ y w Y+ w Y+ W+ y w Y+ W+ y W+ Y+ w y
When genes are linked, parental combinations outnumber recombination types.
W+ Y+ W+ Y+ w y W+ Y+ W+ Y+ w y Two parental types Two recombination types
W+ Y+ W+ Y+ w y W+ Y+ W+ Y+ w y
Fig. 5.5 Autosomal genes can also exhibit linkage b c+ b c+ b+ c b+ c b c+ b+ c bb: black cc: curved
The Chi square test pinpoints the probability that experimental results are evident for linkage Chi test measures the “goodness of fit”: How often an experimentally observed deviation from the prediction of a particular hypothesis will occur solely by chance.
Fig. 5.6 F1 Assume A and B genes are not linked.
c2 = S(31 – 25)2 +(19 – 25)2 25 25 c2 = S(62 – 50)2 +(38 – 50)2 50 50 = 2.88 = 5.76 Chi Square – Experiment 1 & 2 c2 = S(observed – expected)2 number expected Experiment 1 Experiment 2
Table 5.1 A and B are linked A and B are not linked difference is non-significant difference is significant
Recombination results when crossing-over during meiosis separates linked genes
Fig. 5.7 Evidence that recombination results from reciprocal exchange between homologous chromosomes X chromosome
Fig. 5.8 Recombination through the light microscope anaphase (synaptonemal complex)
Fig. 5.9 Recombination frequencies are the basis of genetic map RF: recombination frequency; 1% RF= 1 Centimorgan (cM)=1 map unit (m.u.)
Fig. 5.10 Unlinked genes show a recombination frequency of 50% ry ry+ tkv tkv+ ry ry+ ry+ ry tkv tkv tkv+ tkv+
Fig. 5.10 Unlinked genes show a recombination frequency of 50%
Locus: chromosomal position of a gene Mapping: the process of determining that locus
Fig. 5.11 Mapping genes by comparison of two-point crosses
The limitation of two-point cross Gene order is difficult to determine if they are very close. Actually distance do not always add up.
Fig. 5.12 Vestigial wings Black body Purple eye color
Vg to b: (252+ 241+131+118)/4197=0.177, 17.7% Vg to pr: (252+241+13+9)/4197=0.123, 12.3% B to pr: (131+118+13+9)/4197=0.064, 6.4%
Fig. 5.13 Three point-crosses allow correction for double cross-over
Vg to b (two-point cross): (252+ 241+131+118)/4197=0.177, 17.7% Vg to b (three-point cross): (252+241+131+118+13+13+9+9)/4197=0.187, 18.7%
For greatest accuracy, it is always best to construct a map using many genes separated by relative short distance.
Fig. 5.14 y w m RF between Y and W: 49+41+1+2/6823 X100=1.3 m.u. RF between m and W: 1203+1092+2+1/6823 X100=33.7 m.u RF between m and y: 1203+1029+49+41+2+2+1+1/6823 X100=35 m.u.
The actual physical distance between genes does not always show a direct correspondence to genetic map distance • Recombination is not uniform over the length of a single chromosome, Hot spot. • The existence of double, triple, or even more cross-overs.
Rates of recombination differ from species to species In human, 1 m.u. is = 1 million base In yeast, 1 m.u. is 1500 base pairs In Drosophila, meiotic recombination only occurs in female.
Fig. 5.15 Linkage groups:
Fig. 5.16a The life cycle of the yeast Saccharomyces cerevisiae stress
Fig. 5.16b The life cycle of the bread mold Neurospora crassa Bread mold
Fig. 5.17ab How meiosis can generate three kinds of tetrads
Fig. 5.17cde When PD=NPD, two genes are unlinked
Four types of gametes when genes on different chromosome hhTT HHtt Ht hT H T T (h) H (H) t h t h
Fig. 5.18 When genes are linked, PDs exceed NPDs
Fig. 5.19abc How crossovers between linked genes generate different tetrads
Fig. 5.19def Rare!
How to calculate the recombination frequency between two linked genes in the tetrad analyses? RF=(NPD+1/2 T)/total tetrads x100 RF= 3+(1/2)(70)/200 x100=19 m.u. (tetrads) = (4X3)+ (2X70)/800 x100=19 m.u. (spores)
Fig. 5.20 Tetrad analyses confirms that recombination occurs at the four-strand stage (A mistake model!) (a lot)
Fig. 5.22 How ordered tetrads form Arrangement of the four chromatids of each homologous chromosome pair
Fig. 5.23 (Cross-over between gene and centromere) Ordered Tetrads help locate genes in relation to the centromere
Fig. 5.24 Genetic mapping by ordered-tetrad analysis Thr-centromere: (1/2) (16+2+2+1)/105 x100=10 m.u. Arg-centromere:(1/2) (11+2+2+1)/105 x100=7.6 m.u.
PD T T T NPD NPD PD Double cross-over Double cross-over Double cross-over Double cross-over Thr-Arg linkage: 3+(1/2)(16+11+2)/105 X100=16.7 NPD Tetrad
Fig. 5.25 Mitotic recombination Wild-type tissue: y sn+/Y+ sn: wild-type color and bristle y sn+/y sn+ y sn+/y sn+ Y+ sn /Y+ sn Y+ sn /Y+ sn
Fig. 5.27 Mitotic recombination during growth of diploid yeast colonies can create sectors ADE2/ade2 Recombinatioin betweem ade2 and centromere ade2/ade2 A A Mitotic recombination A a a A a a
