Segregation of Alleles 



39 



parent is represented as a heterozygote. Be- 

 cause the trait is so rare, and because, bar- 

 ring mutation, both parents of a homozygous 

 WW child would have to have woolly hair, 

 WW probably does not occur. 



Finally, consider the genetic basis for cer- 

 tain kinds of anemia. Two special kinds 

 occur among native or emigrated Italians. 

 One type, severe and usually fatal in child- 

 hood, is called thalassemia major or Cooley's 

 anemia; the other type, a more moderate 

 anemia, is called thalassemia minor or micro- 

 cytemia. Pedigree and family studies show 

 that both parents of t. major children have t. 

 minor, and all the data support the hypoth- 

 esis that individuals with t. major are homo- 

 zygotes (tt) for a pair of genes, and that 

 persons with t. minor are heterozygotes (Tt ) 

 for this gene. More than 100,000 people in 

 Italy have been classified as TT. Tt, or tt. 



Notice that in the case of thalassemia neither 

 T nor / is completely dominant (nor com- 

 pletely recessive). 



Although with respect to phenotypic ex- 

 pression the relation between the alleles in 

 the heterozygote may involve complete, 

 partial, or no dominance, it should be re- 

 called that gene action ordinarily has no 

 effect upon either the integrity of the genes 

 or their segregation and recombination. 



One can study a large number of other 

 human traits by the pedigree and family 

 methods and apply the principles known 

 about genes to explain the data genetically, 

 using the simplest suitable explanations in 

 much the same way as was illustrated here 

 for albinism and other traits. Sometimes, 

 unfortunately, the data are insufficient and 

 the investigator is left with several equally 

 probable genetic explanations. 



SUMMARY AND CONCLUSIONS 



The gene is a unit or restricted portion of the total genetic material as discovered via 

 any operational procedure. The genes discovered in the present chapter were revealed 

 by recombination. 



Genes occur in pairs. When they are transmitted in sexual reproduction the mem- 

 bers of a pair segregate so that any offspring receives only one member of a pair from 

 each parent. The gene is uncontaminated by the type of gene that is its partner prior 

 to segregation, and enters the new individual uninfluenced by the allele being con- 

 tributed from the other parent. The hypothesis that the chromosomes comprise or 

 carry the genetic material can be made more specific — a recombinationally detected 

 gene may be represented by a short chromosome segment within which an exchange 

 leading to a chiasma cannot occur. 



Data furnished in pedigree and family studies provide evidence that a number of 

 human traits are based upon the action of a pair of segregating genes. 



REFERENCES 



Gates, R. R., Human Genetics, 2 Vols., New York: Macmillan, 1946. 



Mendel, G., 1866. "'Experiments in Plant Hybridization,*' translated in Sinnott, E. W., 

 Dunn, L. C, and Dobzhansky, Th... Principles of Genetics, 5th Ed.. New York: 

 McGraw-Hill, 1958. pp. 419-443; also in Dodson, E. O.. Genetics, the Modern 

 Science of Heredity, Philadelphia: Saunders. 1956. pp. 285-311: also in Classic 

 Papers in Genetics, Peters, J. A. (Ed.), Englewood Cliffs, N.J.: Prentice-Hall. 

 1959, pp. 1-20. 



