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Updated: June 6, 2025
If a heterozygote is bred with a pure recessive the offspring are half heterozygote and half recessive. The heterozygote individual in typical cases shows the dominant character. In the formation of its gametes when the reduction division of the chromosomes takes place, half of them receive the dominant character, half the recessive.
When two such gametes unite in fertilisation the resulting organism will be a homozygous recessive, and the corresponding character will be absent. In this way we can conceive the origin of albino individuals from a coloured race, supposing the colour was due to a single factor. In Bateson's opinion the origin of a new dominant is a much more difficult problem.
Thus supposing M to represent a modification acquired in the male and m the absence of the modification, such as the feathered antenna of a moth, and the sex-chromosomes to be X and Y, then we should have in the gametocytes Male Female MM mm Gametes MX, MX: mXmY Zygotes MmXX male, MmXY female,
The process of cell-division, whereby the germ-cells or gametes are made, is called gameto-genesis. Somewhere in its course there occurs the capital fact discovered by Mendel and called by him segregation. A cell divides into two which are the final gametes. One of these will definitely contain the Mendelian factor, and the other will be as definitely without it.
It was formerly stated that no process was known or could be conceived by which modifications produced in the soma by external stimuli could affect the determinants in the gametes in such a way that the modifications would be inherited.
The biological value of the association which imposes this additional obligation may be found perhaps in the fact that a large group is not so readily eaten by an enemy as an individual cell; but it is clearer that the process of reproduction, which consists of the fusion of small "gametes," or nucleated fragments produced by diverse or similar parents, must be greatly facilitated by the occurrence of gamete-forming individuals in one and the same colony.
Mating two HA hybrids, the H and A of course split apart in the formation of the gametes, as the HH and AA did in the previous generation; so that we get an equal number of single H and A factors.
It is not even certain that the black or white hair or feathers are giving off special hormones which would or could influence the gametes. The hormone theory only postulates such influence from hormones issuing from tissues modified by external stimuli. It is quite certain that the black colour in Leghorns or guinea-pigs is not due to any external stimulus or influence.
If segregation takes place at the reduction divisions we might suppose that half the gametes formed are sperms and half are ova, and that in the male the latter do not survive but perish and disappear.
When we compare the results reached by the mutationists with those obtained by the Mendelians we find that they tend to two different conceptions of the relation between the gametes and the organism developed from them. The effect of a change in the determinants of the gametes according to the mutationists is evident in every part of the plant.
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