18 



( MAI'I'ER 3 



nonalbino, i.e., normally pigmented. This 

 may be explained bv the occurrence of .1 

 homozygous albino child (aa) from the 

 marriage of two nonalbino heterozygotes 

 (Aa ■ Aa). 



2. The trait appears most frequentlv in 

 progenj sharing a common ancestor. In 

 Sweden and Japan, although the percentage 

 of all marriages between cousins is less than 

 595 in the general population, it is 20 to 

 509? among the parents of albino children. 

 Since albinos are rare, so is the a gene. Ac- 

 cordingly, the chance of obtaining homozy- 

 gous, aa. individuals is relatively slight if 

 the parents are unrelated, because even when 

 the first parent is A a or aa, the second parent 

 is most likely to be A A. On the other hand. 

 if once again the first parent is A a or aa, 

 marriage with a related individual makes it 

 much more likely that the second parent 

 carries an a received from the ancestor held 

 in common with the first parent. 



3. What relative frequencies of nonalbino 

 and albino children are expected when one 

 tallies all the children in two-child families 

 in which albinism appears in the progeny 

 even though both parents are nonalbino? 

 Based on the hypothesis under consideration, 

 the parents must be Aa X A a. From such 

 a marriage, the chance that a given child is 

 nonalbino is :: 4 and that it is albino 1 4- 

 Each child produced from such a marriage 

 has these same chances for nonalbinism and 

 albinism, chances which are independent of 

 the genotypes (or phenotypes) of the chil- 

 dren preceding or following it in the family. 

 Accordingly, of all two-child families whose 

 parents are Aa, % have the first child non- 

 albino. and of these % also have the second 

 child nonalbino. Thus, % 6 of all two-child 

 families from heterozygous parents are ex- 

 cluded from our sample, since both children 

 are normally pigmented. Our sample in- 

 cludes the following, however: families 

 whose first child is normal (% ) and second 

 child is albino ( ', ). making up % 6 ( % of 



;; , ) o{ all tWO-child families; families where 

 the reverse is true ( :; , o\' ',). comprising 

 another ; ,,, oi all two-child families; and 

 families in which both children are albino 

 ( ' 1 of ' , ). which make up ' , o\ all two- 

 child families. On the average, then, everv 

 seven albino-containing families scored 

 should contain six normal children (three 

 from each of the two kinds of families con- 

 taining one albino) and eight albinos (three 

 from each of the two kinds of families con- 

 taining one albino, and two from each family 

 containing two albinos), so that the ratio 

 expected is 3:4 as nonalbino: albino. The 

 ratio actually observed closely approximates 

 the expected one. 



The observed proportions of nonalbino 

 and albino children in families of three, or 

 of four, or of more children from normal 

 parents also fit the expected proportions cal- 

 culated in a similar manner. 



4. Marriages between two albinos pro- 

 duce only albino children, as expected ge- 

 netically from aa X aa. 



5. Twins arising from the same zygote 

 (monozygotic or identical twins) are either 

 both albino or nonalbino. Since ordinarily 

 such twins are genetically identical, both arc 

 expected to be normal, AA or Aa, or albino. 

 aa. Twins arising from different zygotes 

 (dizygotic, nonidentical, or fraternal twins) 

 are no more likely to be the same with re- 

 spect to albinism than any two children of 

 the same parents. 



These evidences offer clear proof that an 

 albino person is usually homozygous reces- 

 sive for a single pair of segregating genes. 



The anomaly of woolly hair is a rare trait 

 in Norwegians. After studying pedigrees, 

 woolly hair can be attributed to the presence 

 of a rare dominant gene, represented by W. 

 When woolly-haired individuals (Ww) 

 marry normal-haired individuals (tin), it 

 is expected and found that approximate!} 

 50% of children have woolly hair and 50% 

 have normal hair. Note that the affected 



