Segregation in Man — Multiple A I lei ism 



O NORMAL FEMALE 



FIGURE 5-1. Symbols used in human pedigrees. 



33 



S- 



I I NORMAL MALE 



\X UNKNOWN 



W AFFECTED Q 



H AFFECTED Q 

 (^ I I I Marriage Line 



ChO 



6<> 



have been excluded from our sample, since 

 both children will be normally pigmented. 

 Our sample will include the following, how- 

 ever: all those families whose first child is 

 normal (X) and second child is albino ('!). 

 making up %& (% of %) of all families; those 

 families where the reverse is true (% of K), 

 comprising another Y^ of all families; and 

 those families in which both children are al- 

 bino (V4 of %), which make up Me of all fami- 

 lies. So every seven albino-containing families 

 scored will give, on the average, 6 normal 

 children (three from each of the two kinds of 

 families containing one albino) and 8 albinos 

 (three from each of the two kinds of families 

 containing one albino and two from each 

 family containing two albinos), so that the 

 ratio expected is 3 : 4 as nonalbino : albino. 



Offspring Line 



Offspring, in order 

 of birth ( I. to r. ) 



Dizygotic Twins* 



Monozygotic Twins* 



* See Chapter 11. 



It may be noted that the observed propor- 

 tions of nonalbino and albino children in 

 families of three, or of four, or of more 

 children from normal parents also fit the 

 expected proportions calculated in a similar 

 manner. 



4. Marriages between two albinos produce 

 only albino children, as expected genetically 

 from aa X aa. 



5. Twins arising from the same zygote 

 (monozygotic or identical twins) are either 

 both albino or nonalbino. Since such twins 

 are genetically identical they would be ex- 

 pected to be both normal, AA or Aa, or both 

 albino, aa. 



These evidences offer clear proof that 

 human albinism is usually the result of a 

 single pair of genes. 



