GENETICS 9: 1-12 (Jan 1924)
VARIATIONS OF LINKAGE IN RATS AND MICE
W. E. CASTLE AND W. L. WACHTER1

Bussey Institution, Harvard University, Forest Hills, Boston, Massachusetts
Received June 18, 1923

1The observations on which this paper is based were made chiefly by the junior author; both authors have shared in the statistical analysis of the data, and the senior author has formulated this statement of results.

In the common brown rat (Mus norvegicus) there occurs a linkage system of three genes; that is, there occur in this species three characters which show with relation to one another -the phenomena of coupling and repulsion in heredity. Two characters are said to be coupled in heredity when they have a tendency to stay together generation after generation, and they are said to exhibit repulsion when they have a tendency to keep apart. In general, two linked characters show coupling when they are inherited from the same parent in the same gamete and repulsion when they are inherited from different parents and so in different gametes. It is supposed that linked genes lie in the same chromosome. If this explanation is correct, it follows that in the brown rat there occur, in a common chromosome, genes for the three recessive characters, albinism, red-eyed yellow and pink-eyed yellow. With a view to learning more about the behavior of linked genes in heredity, an intensive study has been made of this case.

Everyone is familiar with the common white rat, an albino variety of Mus norvegicus, which has long been in captivity as a pet and is now extensively used as a laboratory animal. Its coat is practically without pigment (a creamy white) and its eyes are pink. Albinism is a simple Mendelian recessive character. Consequently albino rats produce only albino offspring.

The two yellow varieties are of more recent origin. They were discovered in England only a few years ago. They are identical with each other in appearance except for the eye color, which in one variety is similar to the albino, being practically without pigment and so showing a pink reflection from the blood in the retina. This variety is known as pinkeyed yellow. In the other variety the eye contains considerable black pigment (though less than is found in black or gray rats), consequently the eye has a dull red color and the variety is called red-eyed yellow. Fanciers call it black-eyed yellow.

The yellow varieties behave in exactly the same way as the albino in crosses with the fully pigmented black or gray varieties. Each is a simple recessive character and so breeds true as soon as isolated.

2Crosses of the two yellow varieties with each other produce lighter grays than either cross of albino with yellow.

When the albino variety is crossed with one of the yellow varieties, or when the yellow varieties are crossed with each other, gray or black offspring are obtained fully or nearly2 as heavily pigmented as wild rats. This result shows that the three recessive variations are all different in nature and are really complementary.

When an F2 generation is produced from one of these crosses it contains more of the recessive (grandparental) types and fewer of the dark type of F1 than the percentage expected when two recessive characters are independent in their inheritance. This suggests linkage, and when more exact tests are made it is indeed found that linkage occurs in every case, though its strength is different in each case.

The linkage between albinism and red-eyed yellow is strongest of all. If an albino is crossed with a red-eyed yellow rat, gray F1 young are obtained. These mated with each other produce an F2 generation of albino, gray, and red-eyed yellow rats. Now if albinism and red-eyed yellow were independent (not linked), half the F2 albinos should transmit yellow and so would produce yellow young when crossed with yellows. Also half of the F2 yellow rats should transmit albinism and so would produce albino young when mated with albinos. In reality, we had to test several hundred F2 albinos and red-eyed yellows before we found one that transmitted both albinism and red-eyed yellow. From such "crossover" individuals a stock of double-recessive individuals was in time obtained which enabled us to determine more precisely the crossover percentage. The procedure was as follows. An F1 individual was mated with a double-recessive individual or individuals and the young were examined at birth as to their eye color, note being made of how many had light eyes (destined to be either albinos or red-eyed yellows) and how many had dark eyes (the F1 type). Those having dark eyes would theoretically equal half the total number of crossover gametes produced by the F1 parent. For, if we designate the albino character by c and the red-eyed yellow character by r, then the gametes which unite to form the F1 individual are cR and Cr. The F1 individual will form gametes ordinarily of these same two types, but exceptionally (as crossovers) of the types CR and cr. The double-recessive individuals mated with the F1 individual will produce only gametes cr in formula, which, united with the four kinds of gametes produced by the F1 mate, will produce a double-dominant combination necessary for the production of dark eyes, only when united with the CR crossover type of gamete. In matings of F1 with double recessive individuals, 12,587 young have been produced, of which 18 were dark-eyed, indicating the occurrence of twice 18, or 36 crossover gametes, a percentage of 0.28 (see table 1). In testing F2 individuals for evidence of crossing over (CASTLE 1919, DUNN 1920), in the preliminary work, the occurrence was noted of 6 crossovers among 704 gametes which entered into the production of 352 tested individuals. This is less than one percent of crossovers. Combining the data from the F2 tests, with those obtained from back-crosses, we have 42 indicated crossovers in 13,291 gametes, or 0.31 0.41 percent, which in round numbers is one-third of one percent of crossovers. The numbers are large enough to show that crossing over between these two genes is a rare event.

TABLE 1
Summary of data on linkage in rats and mice.

GENES CONCERNED

GAMETES
TESTED

CROSSOVER
GAMETES

PERCENT
CROSSOVERS

NATURE OF CROSS

SOURCE OF DATA

Albinism and red-eyed yellow in rats

434
270
12,587

1
5
36

0.46 1.62
1.85 2.05
0.28 0.42

Repulsion
Repulsion

CASTLE 1919
DUNN 1920
New data

Total

13,291

42

0.31 0.41

   

Red-eyed yellow and pink-eyed yellow in rats

4,746

870

18.33 0.69

Repulsion and coupling

CASTLE 1919

Albinism and pink-eyed yellow in rats

90
32,735

19
6438

21.10 3.55
19.66 0.26

Repulsion
Repulsion

CASTLE 1919
New data

Total

32,825

6457

19.67 0.26

   

Albinism and pink-eye in mice

3,418
3,331
3,880

482
503
568

14.10 0.81
15.10 0.82
14.64 0.76

Repulsion
Coupling
Coupling

CASTLE 1919
DUNN 1920
New data

Total

10,629

1553

14.61 0.46

   

A word might be said as to the statistical significance in this case of the calculated crossover percentage, which is slightly less than its probable error. Does this mean that doubt exists as to the occurrence of crossing over between the two genes? By no means. The demonstration of the existence of a single F2 individual, which transmitted in the same gamete both yellow and albinism, would constitute incontrovertible proof that crossing over had occurred. Six such individuals were obtained. The probable error in this case indicates merely the likelihood that, on the repetition of the experiment, crossovers would again be obtained in comparable numbers.

The probable errors throughout this paper are based on the convenient tables prepared and distributed privately by Dr. R. A. EMERSON in 1922. In the use of these tables in our work, the probable error has been based, not as ordinarily on the total population studied, but on half the total population, for the reason that the observed number of crossovers was really that of the half population, since one of the two theoretically equal classes of crossovers was not identified. This procedure increases considerably the probable error, over what it would otherwise be in populations of the size recorded.

Between the genes for pink-eyed yellow and for red-eyed yellow, the crossover percentage is 18.33 0.69 (data of CASTLE 1919), based on tests of 4746 gametes produced by F1 males and females.

The linkage is least between the genes for albinism and pink-eyed yellow. Back-crosses of F1 animals of both sexes with double recessive individuals have been studied very extensively in this case. They indicate that of 32,735 gametes produced by F1 animals, 6438 were crossovers. This is a crossover percentage of 19.66 0.26. Combining with these figures data reported by CASTLE (1919) on tests of 90 F2 animals, which gave an indicated crossover percentage of 21.10 3.55, we have for this linkage relation, data on 32,825 gametes produced by F1 animals, giving a crossover percentage of 19.67 0.26.

These figures clearly indicate that the order of the genes, if linear, is (1) albinism, (2) red-eyed yellow and (3) pink-eyed yellow, since the greatest crossover percentage occurs between albinism and pink-eyed yellow, which makes them the end genes of the series. Before we discuss the evidence for the linearity or non-linearity of the arrangement of the three genes, it will be necessary to consider the relation of sex to the crossover percentage, since in the data already given in summary fashion, male and female parents were unequally represented.

In all the rat linkages, as also in the linkage studied in mice, F1 females give a higher crossover percentage than males give. See table 2. In the case of albinism and red-eyed yellow, F1 females have given a crossover percentage of 0.53 0.78, while males have given 0.18 0.50 percent.. In the cross of albino with pink-eyed yellow, F1 females have given 21.93 0.44 percent of crossovers, whereas males have given 18.39 0.32, a difference of more than three percent between the sexes. As regards the relation of pink-eyed yellow to red-eyed yellow, F1 females have given 20.46 0.92 percent crossovers, and males have given 15.55 1.04 percent. The probable errors are greater in the last case than in either of the others because fewer young were produced.

TABLE 2
Data on the frequency of crossing over in the two sexes of rats and mice.

GENES CONCERNED

SEX OF
RAREST

GAMETES
TESTED

CROSSOVER
GAMETES

PERCENT
CROSSOVERS

NATURE OF CROSS

SOURCE OF DATA

Albinism and red-eye in rats

Female

3,759

20

0.53 0.78

Repulsion

New data

Male

8,828

16

0.18 0.50

Repulsion

New data

Total

 

12,587

36

0.28 0.42

   

Red-eye and pink-eye in rats

Female

2,683

549

20.46 0.92

Repulsion and coupling

CASTLE 1919

Male

2,063

321

15.55 1.04

Repulsion and coupling

CASTLE 1919

Total

 

4,746

870

18.33 0.69

   

Albinism and pink-eye in rats

Female

11,480

2518

21.93 0.44

Repulsion

New data

Male

21,255

3920

18.39 0.32

Repulsion

New data

Total

 

32,735

6438

19.66 0.26

   

Albinism and pink-eye in mice

Female

2,789

444

15.92 0.90

Repulsion and coupling

DUNN 1920

Male

3,683

503

13.65 0.78

Repulsion and coupling

DUNN 1920

Female

556

106

19.06 2.02

Coupling

New data

Male

3,324

462

13.89 0.82

Coupling

New data

Female

3,345

550

16.44 0.82

 

Combined data

Male

7,037

965

13.77 0.57

 

Combined data

Total

 

10,352

1515

14.63 0.47

   

The data obtained from mice are entirely consistent with those given by rats. In the cross of albino with pink-eyed colored mice, F1 females have given a crossover percentage of 16.44 0.82 and F1 males 13.77 0.57 percent, a difference of nearly three percent. All the data are consistent with the view that in rats and mice a pair of chromosomes containing genes for albinism and pink-eyed colored coat cross over somewhat more freely in females than in males. This being the case, it is legitimate to compare only crossover percentages obtained from the same sex.

On the theory of linear arrangement of the genes, the crossover percentage between the most distant genes in a linkage system should approximate the sum of the intermediate linkages from gene to gene. That is, AC should equal AB+BC, when the order of the genes is ABC. The facts in the present case are in harmony with this theory. Calling the three genes c (albinism), r (red-eyed yellow), and p (pink-eyed yellow), we should have cr + rp = cp. Substituting the observed, we have for males, 0.18+15.55=18.44, which is greater than the actual sum of the first two terms (15.73) by 2.71 percent, but the sum of the three probable errors is 1.86 percent, more than half of the observed deviation. In the case of females the equation would read 0.53+20.46=21.93, which again is greater than the true sum of the first two terms (20.99), although the discrepancy is not so great, being 0.94, the sum of the three probable errors being 2.14. In each case AC>AB+BC, yet the difference is no greater than random sampling might produce. The linear arrangement theory is tenable in the case.

A literal interpretation of the chromosome theory might lead one to the conclusion that in this case homologous chromosomes are longer in females than in males, but such a suggestion can scarcely be considered seriously. There is a difference in the physiology of maturation in ova and in spermatogonia. In Drosophila no crossing over occurs in males, though the chromosomes certainly have appreciable length in the male. On the contrary, in silk-moths crossing over occurs only in males, though the chromosomes are probably quite as long in the egg as in the sperm-cell. In rats and mice it is evident, at least in the case of one pair of chromosomes, that crossing over occurs a little more freely in females than in males, but there is no reason to think that the arrangement or spacing of the genes is different in the two sexes.

Aside from sex, we have investigated two other factors which might conceivably influence the amount of crossing over, namely, the age of the parent, and seasonal conditions, in particular, temperature. Both these investigations have yielded negative results. There was some indication of a rise in the crossover percentage in litters born in the warmer months, July, August and September, but fuller investigation failed to substantiate the suggestion. Age of the parent and size of the litter also exert no discoverable effect on the crossover percentage. Differences in the crossover percentages given by different male F1 individuals have also been sought but largely without success. The crossover percentages of individual male parents fluctuate in a typical curve of error, and selection of extreme individuals has had no racial effect. Hence, we conclude that such fluctuation has no genetic significance, but is due solely to random sampling.

TABLE 3
Crossover percentages between albinism and pink-eyed yellow among the young sired by males of known age in months.

AGE OF SIRE 
IN MONTHS

YOUNG WITH 
LIGHT EYES

YOUNG WITH
DARK EYES

TOTAL
YOUNG

PERCENT
CROSSOVERS

P. E.

DEVIATION
FROM AVERAGE

DEV.
P.E

3

880

78

958

16.28

1.54

-1.81

1,17

4

1963

194

2157

17.98

1.02

- .11

.10

5

2081

209

2290

18.24

.99

+ .15

15

6

2029

215

2244

19.16

1.00

+1.07

1.07

7

2220

249

2469

20.17

0.96

+2.08

2.16

8

1838

187

2025

18.47

1.05

+ .36

.34

9

1658

181

1839

19.68

1.11

+159

1.43

10

1302

126

1428

17.64

1.24

- .45

.36

11

1231

104

1335

15.58

1.31

-2.51

1.91

12

927

81

1008

16.07

1.50

-2.02

1.34

13

759

69

819

14.65

1.67

-3.44

2.06

14

567

64

631

20.28

1.90

+2.19

1.15

15

400

30

430

13.95

2.30

-4.04

1.74

16

263

32

295

21.69

2.78

+3.60

1.29

17

176

10

186

10.75

3.50

-7.34

2.97

18

69

4

73

10.95

5.55

-7.14

.1.28

19

29

3

32

18.75

8.43

+ .66

.08

20

22

3

25

24.00

9.50

+ 5.91

.62

21

5

2

7

Total

18,419

1832

20,251

18.09

0.33

Table 3 shows the crossover percentage among the young sired by F1 males of known age from three to twenty-one months old. The average crossover percentage among the 20,251 young, was 18.09 0.33. It was slightly lower than the average when the sires were three and four months old, then above the average for the next five months, again below the average for the next four months, and then oscillated between high and low average values. No consistency is shown in the deviations for young or old sires. In no month does the deviation from the general average reach three times the probable error. In only three months does it exceed twice the probable error, and these are widely scattered, being in the seventh, thirteenth and seventeenth months, respectively. Accordingly, the observed deviations are without statistical significance, being satisfactorily accounted for as the results of random sampling alone.

TABLE 4
Crossover percentages between albinism and pink-eyed yellow among the young of F1 females of known age in months.

AGE IN
MONTHS

DARK-EYED 
YOUNG

LTGHT-EYED
YOUNG

TOTAL
YOUNG

PERCENT
CROSSOVERS

P.E.

DEVIATION
FROM AVERAGE

DEV. P.E.

3

435

71

506

28.06

2.12

+6.50

3.06

4

920

110

1030

21.36

1.49

-0.20

0.14

5

1260

121

1381

17.52

1.28

-4.04

3.15

6

986

117

1103

21.21

1.44

-0.35

0.24

7

1148

157

1305

24.06

1.32

+2.50

1.89

8

961

117

1078

21.70

1.45

+ .14

0.09

9

834

108

942

22.92

1.55

+1.36

0.90

10

665

84

749

22.43

1.74

+0.87

0.50

11

600

67

667

20.09

1.85

-1.47

0.79

12

420

50

470

21.27

2.20

-0.29

0.13

13

407

34

441

15.42

2.27

-6.14

2.70

14

327

44

371

23.72

2.47

+2.46

0.87

15

188

24

212

22.64

3.28

+1.08

0.32

16

161

24

185

25.94

3.51

+4.38

1.24

17

91

9

100

18.00

4.77

-3.36

0.74

18

47

5

52

19.23

6.61

-2.33

0.35

19

5

1

6

20

0

Total

9460

1143

10,603

21.56

0.46

Table 4 contains similar data on the crossover percentage given by F1 females. Neither old nor young females give percentages consistently high or low. The deviations from the average crossover percentage reach three times the probable error in two months and exceed twice the probable error in a third month, but are of doubtful statistical significance in any case.

TABLE 5
Summary of the young sired by individual F1 males heterozygous for albinism and pink-eyed yellow.

MALE BRED IN
CAGE NUMBER

TOTAL
YOUNG

PERCENT
CROSSOVERS

P.E.

DEVIATION
FROM AVERAGE

DEV. P. E.

151

83

28.91

5.20

+10.52

2.02

153

387

24.28

2.42

+ 5.89

2.43

155

548

17.51

2.04

- 0.88

0.43

157

451

18.18

2.24

- 0.21

0.09

159

245

19.59

3.04

+ 1.20

0.39

183

62

22.58

6.06

+ 4.19

0.69

185

498

20.40

2.14

+ 2.01

0.93

187

251

17.52

3.00

- 0.87

0.29

189

535

16.82

2.06

- 1.57

0.76

191

389

14.91

2.41

- 3.48

1.44

193

387

21.18

2.42

+ 2.79

1.15

195

509

16.89

2.11

- 1.50

0.71

200

166

24.09

3.70

+ 5.70

1.64

241

522

13.02

2.09

- 5.37

2.56

243

410

24.87

2.36

+ 6.48

2.73

245

177

18.07

3.57

- 0.32

0.09

247

376

15.42

2.46

- 2.97

1.20

249

386

15.02

2.43

- 3.37

1.38

251

425

14.11

2.31

- 4.28

1.85

253

291

21.30

2.79

+ 2.91

1.04

255

376

19.14

2.46

+ 0.75

0.30

257

351

21.65

2.54

+ 3.26

1.28

259

487

19.30

2.16

+ 0.91

0.42

261

504

16.66

2.12

- 2.33

1.09

265

367

13.62

2.49

- 4.77

1.91

267

227

23.78

3.16

+ 5.39

1.70

269

433

19.87

2.29

+ 1.48

0.64

271

214

23.37

3.26

+ 4.98

1.52

273

502

19.12

2.13

+ 0.73

0.34

275

231

18.18

3.13

- 0.21

0.06

277

361

17.72

2.51

- 0.67

0.26

279

362

23.20

2.51

+ 4.81

1.90

303

432

18.98

2.29

+ 0.59

0.25

305

241

22.40

3.07

+ 4.01

1.30

307

307

18.89

2.72

+ 0.50

0.18

309

331

18.12

2.62

- 0.27

0.10

311

305

14.42

2.73

- 3.97

1.45

313

177

9.03

3.57

- 9.36

2.62

315

478

23.43

2.18

+ 5.04

2.31

329

262

13.74

2.95

- 4.65

1.57

331

365

17.53

2.49

- 0.86

0.34

333

309

18.12

2.71

- 0.27

0.10

335

367

13.62

2.49

- 4.77

1.91

337

451

16.85

2.24

- 1.54

0.68

341

287

12.54

2.81

- 5.85

2.08

343

30

13.33

. 8.71

- 5.06

0.58

343,

90

15.55

5.03

- 2.84

0.56

345

323

19.19

2.65

+ 0.80

0.30

347

551

21.41

2.03

+ 3.02

1.48

349

591

17.25

1.96

- 1.14

0.58

351

562

17.08

2.01

- 1.31

0.65

353

230

11.30

3.14

- 7.09

2.25

355

342

16.95

2.58

- 1.44

0.55

357

184

16.30

3.52

- 2.09

0.59

359

298

26.17

2.76

+ 7.78

2.81

361

581

14.11

1.98

- 4.28

2.16

363

371

18.86

2.47

+ 0.47 .

0.19

365

305

13.11

2.73

- 5.28

1.93

369

475

32.42

2.19

+14.03

6.40

263

341

15.83

2.58

- 2.56

0.99

493

137

17.51

4.06

- 0.88

0.21

Total

21,324

18.39

0.32

TABLE 6
Summary of the young produced by groups of 3 or 4 F1 females heterozygous for albinism and pink-eyed yellow.

REARED IN
CAGE NUMBER

TOTAL
YOUNG

PERCENT
CROSSOVERS

P E.

DEVIATION
FROM AVERAGE

DEV. P. E.

161

219

9.13

3.22

-12.56

3.90

162

252

27.72

3.00

+ 6.03

2.01

163

220

21.81

3.22

+ 0.12

0.03

164

312

28.20

2.70

+ 6.51

2.41

165

174

12.74

3.62

- 8.95

2.47

166

208

21.15

3.31

- 0.44

0.13

167

252

19.84

3.00

+ 0.15

0.05

168

119

20.16

4.35

- 1.53

0.35

169

127

23.62

4.22

+ 1.93

0.45

170

222

19.81

3.20

- 1.88

0.58

171

505

24.55

2.12

+ 2.86

1.34

172

208

19.23

3.31

- 2,46

0.74

174

193

24.87

3.42

+ 3.18

0.92

175

228

21.05

3.16

- 0.64

0.20

176

190

24.21

3.46

+ 2.52

0.72

177

272

22.05

2.89

+ 0.36

0.12

178

225

28.44

3.17

+ 6.75

2.13

179

183

19.67

352

- 2.02

0.57

180

249

17.67

3.02

- 4.02

1.33

281

174

19.54

3.62

- 2.15

0.59

282

253

15.01

2.99

- 6.68

2.23

283

112

14.28

4.51

- 7.41

1.64

284

84

14.28

5.20

- 7.41

1.42

285

225

24.00

3.17

+ 2.31

0.73

286

88

11.36

5.08

-10.33

2.03

287

48

16.66

6.88

- 5.03

0.73

288

301

21.26

2.74

- 0.43

0.15

289

59

20.33

6.16

- 1.36

0.22

290

323

24.42

2.65

+ 2.73

1.03

291

82

24.39

5.27

+ 2.70

0.51

292

106

7.54

4.63

-14.15

3.05

295

185

16.21

3.50

- 5.48

1.56

296

303

25.08

2.74

+ 3.39

1.23

297

265

22.66

2.92

+ 0.97

0.33

298

183

22.95

3.52

+ 1.26

0.35

299

109

22.01

4.55

+ 0.32

0.07

300

302

21.19

2.74

- 0.50

0.18

301

252

23.80

3.00

+ 2.11

0.70

302

112

17.85

4.31

- 3.84

0.85

318

219

23.74

3.22

+ 2.05

0.63

319

185

24.86

3.50

+ 3.17

0.90

319

63

12.69

5.96

- 9.00

1.51

320

332

15.66

2.62

- 6.03

2.30

325

230

21.73

3.14

+ 0.04

0.01

333

69

26.08

5.70

+ 4.39

0.77

371

338

22.48

2.59

+ 0.79

0.30

372

294

25.16

2.78

+ 3.47

1.24

373

287

20.20

2.47

- 1.49

0.60

374

140

17.14

4.03

- 4.55

1.12

375

140

15.71

4.03

- 5.98

1.48

376

68

23.52

5.78

+ 1.83

0.31

377

68

29.41

5.78

+ 7.72

1.33

378

281

32.02

2.84

+10.33

3.63

379

253

28.45

2.99

+ 6.76

2.26

380

295

22.37

2.77

+ 0.68

0.24

Total

11,186

21.69

0.45

-

Table 5 shows the variation in crossover percentage among individual F1 parents. Sixty-one different males included in this table produced from 30 to 591 young each. The crossover percentages among these young range from 9.03 to 32.42, average 18.39 0.32. The deviations from the average are less than the probable error in 31 cases, lie between once and twice the probable error in 19 cases, lie between twice and three times the probable error in 10 cases, and exceed three times the probable error in only one case, that of the male bred in cage 369 which sired 475 young. He is credited with a very large number of crossovers, 6.4 times the probable error. His case alone suggests a possibly significant deviation from the average. As soon as it was recognized that this male was giving an unusually high crossover percentage, he was given additional mates and subsequent litters of young were examined with especial care. It was found that he was heterozygous, not for ordinary albinism but for its allelomorph, ruby-eye, and one of his double-recessive mates also contained one dose of ruby-eye with one of ordinary albinism. Homozygous ruby-eyed young resulted from this mating and it is possible that in the earlier litters, which were destroyed at birth, some of these rubyeyed young may have been classed as dark-eyed. Accordingly, all litters of this female were excluded from the total, but still the percentage of dark-eyed young credited to this male was abnormally high. To see whether the peculiarity of this male would be inherited by his descendants, several of his doubly heterozygous sons were mated with double recessive females. Such matings produced a total of 351 young with an indicated crossover percentage of 17.37 3.00, a perfectly normal result. Apparently the male had not transmitted his peculiarity to any of his sons. The question whether ruby-eye gives a crossover percentage different from that of its allelomorph, ordinary albinism, is being investigated further.

An F1 male which in his earlier litters had given only seven percent of crossovers was also made an object of special study, but two months later the percentage had risen to fifteen and it showed subsequently no tendency to return to its earlier low amount. Random sampling seems an adequate explanation of the peculiar early result.

SUMMARY

  1. Three genes of the common rat, which belong to a common linkage system, are apparently linear in their arrangement. The order of the genes is crp.
  2. The crossover percentages for these genes are regularly higher among the gametes of F1 females than of F1 males. The same is true for two linked genes of the house mouse, possibly homologous with genes c and p of the rat.
  3. No evidence has been obtained that the crossover percentage varies with age of the parent, or temperature or other seasonal conditions, either in rats or in mice.
  4. Variations in crossover percentages given by different male parents are capable of explanation as due to chance alone, except possibly in the case of one individual which, however, did not transmit the peculiarity to his sons.

LITERATURE CITED