Heredity and Development Second Edition (1972) / Chapter Skim
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4 The Chromosomes and Inheritance
4 The Chromosomes and Inheritance
Pages 70-86
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From page 70... ...
Boveri thought otherwise. He believed that chromosomes differed from one another, and that a complete set of 36 was necessary for normal development in the sea urchin.
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From page 71... ...
In order to understand the complications we should first review normal fertilization. In the case of an embryo entered by a single sperm, the sperm brings in a centrosome, which is the region containing the centriole.
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From page 72... ...
According to Boveri, these data correspond fairly well with the chance expectation that normal larvae will come from embryos that begin development with a normal set of chromosomes in each of the cells formed at the first division. He interpreted the data to mean that for normal development every cell of the embryo must have the regular set of 36 chromosomes.
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From page 73... ...
He concluded that constant size was an attribute of the individual chromosomes. After these eight mitotic divisions, the cells undergo the usual two meiotic divisions.
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From page 74... ...
This conclusion inevitably raises the question whether there is also a physiolog ical individuality, i.e., whether the chromosomes represent respectively differ ent series or groups of qualities or whether they are merely different-sized aggregations of the same material and, therefore, qualitatively alike. On this question my observations do not furnish direct evidence.
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From page 75... ...
By the light of this conception we are enabled to see an explanation of that hitherto problematical process, synapsis, in the provision which it makes that the two chromosomes representing the same specific characters shall in no case enter the nucleus of a single spermatid or mature egg. I may finally call attention to the probability that the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reducing division as indicated above may constitute the physical basis of the Mendelian law of heredity.
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From page 76... ...
A pure-breeding roundyellow plant would be symbolized as RRYY indicating that it has a pair of chromosomes carrying the round gene and another pair with the yellow gene. Similarly, a wrinkled-green plant would be rryy.
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From page 77... ...
HEREDITY AND DEVELOPMENT: SECOND EDITION 77 4–2 Diagram of chromosome distributions in Mendel's cross of a roundyellow ×wrinkled-green pea on the basis of Sutton's hypothesis.
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From page 78... ...
If we are to use the genes-are-parts-of-chromosomes hypothesis, it will be necessary to find a parallel between all types of genetic behavior and chromosome behavior. Any variations in chromosomal phenomena from the usual condition must be reflected in the genetic results.
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From page 79... ...
The unusual behavior might be a difference in reaction of the chromosome to stains, meiotic movements that were not synchronous, or the presence of ‘extra' or ‘accessory chromosomes.' The term accessory chromosome referred to those cases where there was one chromosome without a mate, in contrast to the usual situation of all chromosomes being in morphologically similar pairs. The analysis of accessory chromosomes led to important results concerning the role of chromosomes in heredity.
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From page 80... ...
and the others lack an X (d) (H.Henking, ‘Untersuchungen über die ersten Entwicklungsvorgänge in den Eiern der Insekten,' Zeit.
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From page 81... ...
The other daughter cell contained only the 11 dyads. At the second meiotic division of the cell with the X, the X dyad and the 11 regular dyads were divided so each of the resulting cells contained an X chromosome plus 11 of the regular chromosomes.
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From page 82... ...
Because of the association of X chromosomes with sex they were called sex chromosomes. All other chromosomes were called autosomes.
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From page 84... ...
but the observations on the relation of sex to the sex chromosomes, and especially Sutton's remarkable theoretical analysis, made it a far more useful hypothesis. Inheritance was so poorly understood before 1900 that one could not design specific experiments that might uncover its cellular basis.
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From page 85... ...
Sutton had shown how the observable behavior of chromosomes could account for this otherwise obscure phenomenon: the chromosomes of diploid cells exist in pairs and one could carry a dominant gene and the other member of the pair could carry the recessive gene; during meiosis these homologous chromosomes would be separated and half of the gametes would have the dominant gene and the other half the recessive gene; normal meiosis, therefore, could account for the purity of the gametes. Before 1900, one might find the hypothesis ‘the genes are parts of chromosomes' to be probable but it was very difficult to see how it might be tested.
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From page 86... ...
5. In species in which the male has XY sex chromosomes and the female XX, speculate on the relation between the chromosomes and sex.
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