34 



CHAPTER 5 



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FIGURE 5-2. /i pedigree of albinism in man. 



The anomaly of woolly hair is a rare trait 

 in Norwegians and can be attributed, after 

 a study of pedigrees, to the presence of a rare 

 dominant gene, call it W. For, when woolly- 

 haired individuals (Ww) marry normal- 

 haired individuals (uu), it is predicted and 

 found that approximately 509c of children 

 have woolly and 509c normal hair. Note that 

 the affected parent is represented as a hetero- 

 zygote, the trait being so rare that the homo- 

 zygote WW is probably nonexistent, since, 

 barring mutation, an individual with this 

 genotype would have to have parents both of 

 whom had woolly hair. 



A study of pedigrees was instrumental in 

 clarifying the nature of certain ataxias, which 

 involve a lack of neuromuscular coordination, 

 found in certain famihes in Sweden. In some 

 families, affected people had parents who 

 were apparently unrelated, whereas other 

 affected people had parents who were first 

 cousins. From the rarity of this disease it 

 was suggested that the ataxia was being 

 caused by the presence of a dominant gene in 

 heterozygous condition in those cases where 

 the parents were unrelated, and by the pres- 

 ence of a recessive gene in homozygous 

 condition in those cases where the parents 

 were related. When careful clinical tests 

 were made by a neurologist it was found that, 

 indeed, there were differences in symptoms 



in the cases where the parents were and were 

 not related. In this way a combination of 

 pedigree and medical studies established the 

 genetic basis and nature of two kinds of 

 ataxia. 



Numerous family studies have been made 

 regarding blood type. However, before dis- 

 cussing their genetic meaning, we shall need 

 to know just what is meant by a blood type 

 or blood group. 



Human blood contains red blood corpus- 

 cles (cells) carried in a fluid medium, the 

 plasma. The corpuscles carry on their sur- 

 faces substances called antigens, while the 

 plasma contains, or may form, substances 

 called antibodies. An antibody is a very 

 specific kind of molecule, which is capable 

 of reacting with and binding a specific antigen. 

 This reaction may be visualized as a lock 

 (antibody) which holds or binds a particular 

 key (antigen). If a rabbit is injected with a 

 suitable antigenic material, in the form of 

 foreign red blood cells to which it has never 

 before been exposed, certain antibody-pro- 

 ducing cells of the rabbit will manufacture an 

 abundance of antibodies which will appear 

 in its plasma, some of which will be used to 

 react specifically with the antigenic compo- 

 nent of the foreign red blood cells. If, on some 

 later occasion, the same antigen is injected 

 into the rabbit's blood stream there will be, 



