HEREDITY 



587 



Multigenic traits 



Up to this point we have assumed that 

 there is one gene in the gamete for each trait 

 exhibited by the adult, and this was for 

 many years supposed to be the case. But 

 now we know that many traits are de- 

 termined by more than one gene. Several 

 genes may work together in the development 

 of a trait. When two or more genes are 

 necessary for the development of a trait, it 

 may be called a multigenic (polygenic) 

 trait. A modifying gene is one which 

 modifies in some way a trait determined by 

 another gene. For instance, in man the gene 

 for brown eyes is dominant to blue eyes; 

 therefore, the difference between brown 

 eyes and nonbrown in a given family prob- 

 ably depends on a single gene difference. 

 But differences in amounts, kinds, and ar- 

 rangement of pigment account for green, 

 hazel, and brown eyes, which in turn appear 

 to depend on other independent genes. 



Lack of dominance 



In all of Mendel's pea experiments, 

 dominance was essentially complete so that 

 there was no appreciable difference between 

 the heterozygous and the homozygous in- 

 dividuals in the expression of a dominant 

 trait. For example, (Ss) tall peas could not 

 be easily distinguished from (SS) tall peas. 

 However, there are many instances of lack 

 of dominance, that is, the heterozygote is 

 quite different from both the homozygous 

 dominant and the homozygous recessive in- 

 dividuals. 



One of the best illustrations of lack of 

 dominance is provided by the blue Andalu- 

 [iian fowl. The "blue" appearance is due 

 to very fine alternating white and black 

 stripes. This type of fowl (Fi in Fig. 415) 

 is a hybrid (heterozygote) between black 

 and white-splashed homozygous parents. 

 When blue Andalusians (Fi) are interbred, 

 the offspring (F2) are 1 white-splashed, 2 

 blue Andalusians, and 1 black. The number 



of visible types (phenotypes) here obtained 

 is 3 instead of 2, which would have been 

 true with complete dominance. Poultry 

 breeders long tried by inbreeding to obtain 

 pure blue Andalusian fowls, that is, fowls 

 when bred together would produce only 

 blue offspring. This is obviously impossible 

 in the light of modern genetic information. 

 The expression incomplete dominance or 

 partial dominance is also used to describe 

 cases of distinguishable heterozygotes, es- 

 pecially where the hybrids tend to resemble 

 one parent much more than the other. 



Examples of lack of dominance in man 



In man it is probably rather common for 

 heterozygotes to differ from both homozy- 

 gotes. The form of the hair, with reference 

 to curly and straight, as found in Caucasian 

 people of European descent, illustrates a 

 trait showing lack of dominance. The result 

 of a mating involving curly and straight hair 

 is offspring which have wavy hair, that is, 

 hair intermediate in form between the 

 homozygous parents. Certain of the blood 

 groups are known to be inherited this way; 

 and some diseases are known to have this 

 mode of inheritance. Skin color in Negro- 

 white crosses, which involve at least two 

 pairs of genes (Fig. 406), is another case in 

 which dominance is lacking. The Fi's of a 

 homozygous black and a homozygous white 

 cross are all mulattoes of an intermediate 

 color. The color of the skin of the offspring 

 of mulattoes depends on the number of 

 genes for black pigmentation distributed to 

 each child, and may range from the extremes 

 of white to black, although most of the 

 progeny will show intermediate shades of 

 color. 



Penetrance 



Penetrance refers to the regularity with 

 which a gene produces a detectable effect. 

 If every individual possessing a dominant 

 gene develops the trait, the gene is said to 

 have 100 per cent penetrance or complete 



