Segregation of Alleles 



37 



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figure 3-3. A pedigree of albinism in man. 



65 



hypothesis that a genome is the material 

 equivalent of a gene. Nor can an entire 

 chromosome be identified with a gene, since 

 any given gametic chromosome is typically 

 constituted of some maternally-derived and 

 some paternally-derived parts, as a conse- 

 quence of the exchanges leading to chias- 

 mata. However, the possibility still remains 

 that the gene is associated with a particular 

 segment of a chromosome which is so short 

 that it cannot undergo an exchange leading 

 to a chiasma with a corresponding segment 

 on a homologous chromosome. Such a seg- 

 ment would always retain its pure maternal 

 or pure paternal constitution after meiosis. 

 Accordingly, the maximal size of such an 

 uncontaminable segment would equal the 

 maximal size of the gene. 



* Segregation of Alleles in Man 



The genetic principles discovered in garden 

 peas should also hold equally well for any 

 other sexually reproducing species, including 

 man. Because we naturally have great in- 

 terest in ourselves, let us consider some 

 human traits which may be based upon the 

 action of a single pair of genes. The study 

 of human genetics is complicated by the fact 

 that, unlike other species of plants and ani- 



* Throughout this book, the asterisk indicates an 

 optional chapter or section. 



mals. our species is not bred experimentally. 

 Because of this scientific difficulty special 

 methods of investigation have to be em- 

 ployed. These include the pedigree, family, 

 population, and twin methods. The present 

 discussion deals primarily with the first two 

 of these methods. 



The pedigree method uses phenotypic 

 records of families (family trees or gene- 

 alogies) extending over several generations. 

 In recording pedigrees certain symbols are 

 used by convention (Figure 3-2). In a 

 pedigree chart a square or { represents a 

 male, a circle or 9 represents a female: 

 filled-in symbols represent persons affected 

 by the anomaly under discussion. In con- 

 trast, the family method utilizes the pheno- 

 types only of parents and their offspring: 

 that is. it uses data that span only one gen- 

 eration. 



Albinism, or lack of melanin pigment, is 

 a rare disease which occurs approximately 

 once per 20.000 births. Studies of families, 

 and of pedigrees like the one in Figure 3-3. 

 yield the following facts, each of which is 

 discussed relative to the hypothesis that 

 albinism occurs in homozygotes for a reces- 

 sive gene. a. Alternative hypotheses that 

 albinism is due to a completely or a partiallv 

 dominant gene soon prove untenable: 



1. Both of the parents of albinos mav be 



