The Life Sciences Recent Progress and Application to Human Affairs The World of Biological Research Requirements for the Future (1970) / Chapter Skim
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Heredity and Evolution
Heredity and Evolution
Pages 126-132
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From page 126... ...
This was provided in the formulation of the rules of inheritance by Gregor Mendel, based on his observations with garden peas, which led to understanding that heredity is particulate in nature. Shortly thereafter, observations of the behavior of chromosomes, particularly the manner in which the chromosome number is halved during the formation of sperm and egg, made it apparent that the chromosomes were the carriers of these particulate genetic factors, later called "genes." In the 1920's, as a result of a large body of ingenious, elegant experimentation with the fruit fly Drosophila melanogaster, genes were shown to be arranged in single file on the chromosomes.
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From page 127... ...
It was recognition of the fact that harmful recessive genes usually produce their effects only when present in duplicate and that the occurrence of such duplication is enormously increased by inbreeding, as evident in the occurrence of human hereditary disorders, that led to objections to inbreeding on scientific rather than social grounds. Observational and experimental population genetics started with field studies of natural populations.
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From page 128... ...
Unbalanced chromosome combinations arise because of an accident in the process of meiosis, the reduction division that yields egg or sperm cells containing half the normal complement of chromosomes of somatic cells. Such accidents are not common but neither are they exceedingly rare, and whenever they have been systematically sought they have been found.
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From page 129... ...
This suggested an approach to finding other chromosomal diseases, and the study of patients with mental retardation and other superficially unrelated abnormalities led almost immediately to the discovery of two other trisomic types, both new conditions previously unknown to clinical medicine. But nc other examples of trisomic types have been found in several years of subsequent study.
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From page 130... ...
In confirmation, a study of miscarried human embryos indicated that more than a third of spontaneous abortions are caused by tnsomy, monosomy, and more complex chromosome anomalies. An exception to the rule that chromosomes occur in identical pairs is found in the sex chromosomes X and Y (Figure 30~.
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From page 131... ...
is somehow selectively advantageous in comparison with either form of homozygote (A,A, or ALA, j, then both types of genes persist in the population in a stable equilibrium determined by the relative fitnesses of the two homozygotes. This has been demonstrated amply with many forms; fruit flies have been most thoroughly studied, but polymorphisms are found throughout the animal and plant kingdoms.
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From page 132... ...
The fact that the genetic code is a constant from bacteria to man to higher plants is the most cogent available argument supporting the concept that all living forms derive from a single common ancestor. At the same time, by tracing the amino acid sequences of specific proteins with essentially identical functions, it has been possible to illustrate how the process of mutation causes divergence from an original form.
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