Occasionally, a genetic error occurs during egg or sperm cell formation.
Weismann has also ingeniously argued from the structure of the egg-cell and sperm-cell, and from the way in which, and the period at which, they are derived in the course of the growth of the embryo from the egg - from the fertilized egg-cell - that it is impossible (it would be better to say highly improbable) that an alteration in parental structure could produce any exactly representative change in the substance of the germ or sperm-cells.
Each egg and sperm cell contains only one copy of chromosome and, therefore, only one copy of the RB1 gene.
A child conceived with such an egg or sperm cell may inherit an incorrect number of chromosomes.
Often, the genetic material is missing due to an error in replication of an egg or sperm cell.
When a baby is conceived by combining one sperm cell with one egg cell, the baby receives 23 chromosomes from each parent, for a total of 46 chromosomes.
Less than 25 percent of Down syndrome cases occur due to an extra chromosome in the sperm cell.
In the first place, the continued study of human population has thrown additional light on some of the questions involved, whilst the progress of microscopical research has given us a clear foundation as to the structural facts connected with the origin of the egg-cell and sperm-cell and the process of fertilization.
of Their causes are extremely difficult to trace in detail, but it appears that they are largely due to a " shaking up " of the living matter which constitutes the fertilized germ or embryo-cell, by the process of mixture in it of the substance of two cells - the germcell and the sperm-cell - derived from two different individuals.
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