قراءة كتاب Inheritance of Characteristics in Domestic Fowl
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Inheritance of Characteristics in Domestic Fowl
the products of two Y-combed individuals of F1 used as parents, will exhibit the following proportions: median element, 25 per cent; split comb, 50 per cent; and no median element, 25 per cent. These proportions will show themselves, whatever the generation to which the Y-combed parents belong, whether both are of generation F1, or F2, or F3, or one parent of one generation and the other of another. Other combinations of parental characters should give the proportions in the progeny shown in table 1.
On the second hypothesis, on the other hand, the proportions of the different kinds occurring in the progeny will vary with the generation of the parents. This hypothesis assumes the existence in each germ-cell of the original parent of two comb allelomorphs, M and l in single-combed birds and m and L in the Polish fowl, the capital letter standing for the presence of a character (Median element or Lateral element) and the small letter for the absence of that character. Consequently, after mating, the zygote of F1 contains all 4 factors, MmLl, and the soma has a Y comb; but in the germ-cells, which contain each only 2 unlike factors, these factors occur in the following 4 combinations, so that there are now 4 kinds of germ-cells instead of the 2 with which we started. These are ML, Ml, mL, and ml. Furthermore, since in promiscuous mating of birds these germ-cells unite in pairs in a wholly random fashion, 16 combinations are possible, giving 16 F2 zygotes (not all different) as shown in table 2.
Table 2.
| [A] This convenient form of zygotic formulæ, using a subscript 2 instead of doubling the letter, is proposed by Prof. W. E. Castle. |
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| Type. | Zygotic constitution. | Soma. |
| a | M2L2[A] | Y |
| b | M2Ll | Y |
| b | M2Ll | Y |
| c | MmL2 | Y |
| d | MmLl | Y |
| e | M2Ll | Y |
| f | M2l2 | I |
| g | MmLl | Y |
| h | Mml2 | I |
| i | mLML | Y |
| k | mLMl | Y |
| l | m2L2 | oo |
| m | m2Ll | oo |
| n | mlML | Y |
| o | mlMl | I |
| p | m2Ll | oo |
| q | m2l2 | Absent |
It is a consequence of this second hypothesis that, in F2, of every 16 young 9 should have the Y comb; 3 the I comb; 3 the oo comb, and 1 no comb at all. It follows further that the progeny of two F2 parents will differ in different families. Thus if a Y-combed bird of type a be mated with a bird of any type, all of the progeny will have the Y comb.
From Y-combed parents of various types taken at random 4 kinds of families will arise having the following percentage distribution of the different types of comb:
- 1. Y comb, 100 per cent.
- 2. Y comb, 75 per cent; I comb, 25 per cent.
- 3. Y comb, 75 per cent; oo comb, 25 per cent.
- 4. Y comb, 56.25 per cent; I comb, 18.75 per cent; oo comb, 18.75 per cent; absent, 6.25 per cent.
Again, mating two extracted I combs of F2 should yield, in F3, two types of families in equal frequency as follows:
- 1. I comb, 100 per cent.
- 2. I comb, 75 per cent; no comb, 25 per cent.
Again, mating two extracted oo combs of F2 should yield, in F3, two types of families in equal frequency, as follows:
- 1. oo comb, 100 per cent.
- 2. oo comb, 75 per cent; no comb, 25 per cent.
Single comb × Y comb should give families of the types:
- 1. Y comb, 100 per cent.
- 2. Y comb, 50 per cent; I comb, 50 per cent.
- 3. Y comb, 50 per cent; oo comb, 50 per cent.
- 4. Y comb, 25 per cent; I comb, 25 per cent; oo comb, 25 per cent; absent, 25 per cent.
Mating oo comb and Y comb should give the family types:
- 1. Y comb, 100 per cent.
- 2. Y comb, 50 per cent; oo comb, 50 per cent.
- 3. Y comb, 50 per cent; I comb, 50 per cent.
- 4. Y comb, 25 per cent; oo comb, 25 per cent; I comb, 25 per cent; no comb, 25 per cent.
Finally, I comb and oo comb should give the following types of families:
- 1. Y comb, 100 per cent.
- 2. I comb, 100 per cent.
- 3. Y comb, 50 per cent; oo comb, 50 per cent.
- 4.