United States or Saint Kitts and Nevis ? Vote for the TOP Country of the Week !
In the maturation divisions reduction does not occur, N chromosomes passing to one gamete, none to the other, and the latter perishes so that the sperms all contain N chromosomes. When fertilisation occurs the zygote therefore contains 2N chromosomes and becomes female. Here then we have no segregation of Fxf in the ova.
But in this case it would be the whole of the chromosomes coming from the original female gamete which would disappear, and the spermatozoon would be incapable of transmitting characters derived from the female parent of the individual in which the spermatozoa were formed. An individual could never inherit character from its paternal grandmother.
We have, it is true, some evidence that one cell affects the other by some chemical action, as for instance in the fact that the mobile male gametes of a fern are attracted to a tube containing malic acid, but this may be merely an influence on the direction of movement of the male gamete, while there are cases in which neither cell is actively mobile.
In Mendelian experiments, a heterozygote individual is one arising from gametes containing opposite members of a pair of characters, in other words, from the union of a gamete carrying a dominant with another carrying a recessive. A pure recessive individual is one arising from the union of two gametes both carrying recessives.
If such a gamete is fertilised by a normal gamete the organism developed from the zygote will be heterozygous, and segregation will take place in its gametes between the chromosome carrying the factor and the other without it, so that there will now be many gametes destitute of the factor in question.
What we know in higher animals and plants is that each gamete contains in its nucleus half the number of chromosomes found in the other cells of the parent, and that in the fertilised ovum the chromosomes of both gametes form the new nucleus, in which therefore the original number of chromosomes is restored.
When two gametes unite, the specific number is restored. Since the male gamete is very small and seems to contribute to the zygote almost nothing except the chromosomes, which carry with them all the characters of the male parent, it seems a necessary conclusion that the chromosomes alone determine the character of the adult. There are, however, facts which point to an opposite conclusion.
At the same time it must be borne in mind that since the factor, whether a portion of a chromosome or not, is transmitted in heredity as a part of a single cell, the gamete, and since every cell of the developed individual is derived by division from the single zygote cell formed by the union of the two gametes, the factor or determinant must be contained in every cell of the soma, except in cases where differential division, or what is called somatic segregation, takes place.
Other cases occur in which the number of chromosomes in the somatic cells is double the ordinary number namely, 28 and others in which the number is 21. The normal number in the gamete, 7, is considered the simple or haploid number, and therefore the number 28 is called tetraploid. This doubling of the somatic number of chromosomes is now known in a number of plants and animals.
Wilson points out that in the bee, where fertilised eggs develop into females and unfertilised into males, we should have to assume that the X chromosome in the female gamete is a female determiner which meets a recessive male determiner in the X chromosomes of the sperm. Dr.
Word Of The Day