92 Monohybrids 



hornless. The hh females can be differentiated from the H 

 females by the small growths, or scurs, and not depressions, 

 which they have in place of horns. Hornless (HH) crossed with 

 Dorset horns [H'H') gives only hornless females {HH'), and 

 hornless by Merino {hh) gives only hornless females {Hh). 

 When Dorset is crossed with Merino, the females are horned 

 like the Dorset parent and are heterozygous {H'h) (Fig. 31). 

 Other examples of multiple alleles are found in later chapters. 



Any hornless Dorset 



breed x horns 



HH H'H' 



HH' 



Females hornless 

 and heterozygous 



Dorset horns 

 H'H' 



Merino 

 hh 



H'h 



Females Dorset 

 and heterozygous 



Any hornless 



breed X Merino 



HH hh 



Hh 



Females hornless 

 and heterozygous 



Heterozygous 

 HH' female x 



HH 



Hh 



Females 

 hornless 



Merino 



hh 



H'h 



Females 

 Dorset 



Fig. 31. Diagram of the inheritance of horns in sheep (females only). 

 The hornless type results from the dominant gene H, Dorset horns from 

 H', and Merino horns from h. These three genes form a series of mul- 

 tiple alleles. 



QUESTIONS AND PROBLEMS 



1. In Phlox Drummondii, salver-shaped corolla is dominant over 

 funnel-shaped. If a plant with funnel-shaped flowers is crossed with 

 one homozygous for the genes for salver-shaped flowers, what are the 

 genotypes and phenotypes of the Fi and F2? What are the phenotypes 

 and genotypes of the offspring of a cross between the Fi and the salver- 

 shaped parent and of that between the Fi and the funnel-shaped parent ? 

 What would be the nature of the offspring (F3 generation) of each F2 

 plant ? 



2. Looking over a field of F2 plants it would occur to you that some 

 of those with salver-shaped flowers were homozygous and some were 

 heterozygous. How could you tell which was which (a) by inspection 

 and (6) by breeding? Might it ever be important to know? Why? 



3. In rabbits, long hair {I) is recessive to short (L). If a homozy- 

 gous short-haired female is mated with a long-haired male, what are the 

 phenotype and genotype of the Fi and of the F2 and of the backcross 



