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+ wywy

When genes are linked, parental combinations outnumber recombination types.

W+ Y+ wywy W+ Y+ W+ Y+ wywy W+ Y+ Two parental types Two recombination types

W+ Y+ wywy W+ Y+ W+ Y+ wywy W+ Y+

Fig. 5.5 Autosomal genes can also exhibit linkage bb: blackcc: curved b c+ b c+ b+ c b c+ b+ c b+ c

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 Assume A and B genes are not linked. F1

Chi Square – Experiment 1 & 2  2 =  (observed – expected) 2 number expected  2 =  (31 – 25) 2 + (19 – 25) = 2.88  2 =  (62 – 50) 2 + (38 – 50) = 5.76 Experiment 1 Experiment 2

Table 5.1 A and B are not linked A and B are linked difference is significantdifference is non-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 (synaptonemal complex) anaphase

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 Unlinked genes show a recombination frequency of 50% ry ry+ tkvtkv+ ry tkv ry tkv+ ry+ tkv ry+ tkv+

Fig Unlinked genes show a recombination frequency of 50%

Locus: chromosomal position of a gene Mapping: the process of determining that locus

Fig Mapping genes by comparison of two-point crosses

The limitation of two-point cross 1.Gene order is difficult to determine if they are very close. 2.Actually distance do not always add up.

Fig Vestigial wings Black body Purple eye color

Vg to b: ( )/4197=0.177, 17.7% Vg to pr: ( )/4197=0.123, 12.3% B to pr: ( )/4197=0.064, 6.4%

Fig Three point-crosses allow correction for double cross-over

Vg to b (three-point cross): ( )/4197=0.187, 18.7% Vg to b (two-point cross): ( )/4197=0.177, 17.7%

For greatest accuracy, it is always best to construct a map using many genes separated by relative short distance.

Mapping genes at X chromosome by two-point cross

Fig RF between Y and W: /6823 X100=1.3 m.u. RF between m and W: /6823 X100=33.7 m.u RF between m and y: /6823 X100=35 m.u. y w m

The actual physical distance between genes does not always show a direct correspondence to genetic map distance 1.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 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 H HHtt hhTT HthT H h h T T t t (h) (H)

Fig 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 Tetrad analyses confirms that recombination occurs at the four-strand stage (A mistake model!)(a lot)

Fig How ordered tetrads form Arrangement of the four chromatids of each homologous chromosome pair

Fig (Cross-over between gene and centromere) Ordered Tetrads help locate genes in relation to the centromere

Fig Genetic mapping by ordered-tetrad analysis Thr-centromere: (1/2) ( )/105 x100=10 m.u. Arg-centromere:(1/2) ( )/105 x100=7.6 m.u.

Thr-Arg linkage: 3+(1/2)( )/105 X100=16.7 PD TTTNPD Double cross-over NPDTetrad Double cross-over Double cross-over Double cross-over

Fig Mitotic recombination Wild-type tissue: y sn+/Y+ sn: wild-type color and bristle y sn+/y sn+Y+ sn /Y+ sny sn+/y sn+ Y+ sn /Y+ sn

Fig. 5.26

Fig Mitotic recombination during growth of diploid yeast colonies can create sectors ADE2/ade2 ade2/ade2 Recombinatioin betweem ade2 and centromere A A a a A A a a Mitotic recombination

Fig. 5.28

Fig. 5a.p131

Fig. 5a.p143

Fig. 5.21

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