688 General and Applied Biology 



pear black-short-rough are not aHke as far as their gene content is concerned. 

 When we group together all those whose gene content is the same, we have the 

 so-called genotype ratio (based on the gene content). 



A very useful method in determining the number and difTerent types of gametes 

 (sex cells) produced by a particular organism is known as the bracket method. 

 In the case of the trihybrid cross of guinea pigs considered above, the Fi off- 

 spring, which is black-short-rough and with genes (BbSsRr), the gametes may 

 be ascertained by the use of the bracket as shown in Fig. 337. 



BbSsRr 



(Parent genes) 



B 



hil" 



— BSR 

 BSr 



< 



\ 



I P.— -BsR 



Bsr 



r R— .bSR 



bSr 



{I:: 



--bsR 

 bsr 



Fig. 337. — The so-called bracket method of determining the number and types 

 of gametes (sex cells) produced by a parent containing the genes BbSsRr. 



By using a plant, such as the pea, and the following characters with their sym- 

 bols, the different crosses (monohybrid, dihybrid, trihybrid) may be illustrated: 



T, tall plant 

 t, dwarf plant 

 R, round seed 

 r, wrinkled seed 

 Y, yellow seed 

 y, green seed 



When the dominant (ex- 

 pressed by the capital 

 letter) is present, it 

 expresses itself even 

 though the opposite or 

 recessive is present. 



Monohybrid Cross (Pea) 



Parents (P) 



Gametes produced 



Offspring (Fi) 



Gametes for both male and female 



Offspring (Fz) shown by the Punnet 

 square or checkerboard 



Tall (TT) X Dwarf (tt) 



I i 



T t 



\ ^ 



Tall (Tt) 



T t 



Male 

 gametes 



Female 



Thus we have produced 1 TT (tall), 2 Tt (tall), and 1 tt (dwarf), or a ratio 

 of 3 tall to 1 dwarf. This is a phenotype ratio of 3:1 for a monohybrid cross. 



