Genetics Problems – Dihybrid Crosses

GENETICS PROBLEMS – DIHYBRID CROSSES 1. In humans, brown eyes (B) are dominant over blue eyes (b) and dark hair (D) is dominant over blonde hair (d). A) Determine the genotypes of a woman with blue eyes (bb) and hybrid dark hair( Dd) and a man with hybrid dark hair (Dd)and hybrid brown eyes (Bb)( known as a DIHYBRID individual and could simply be described as a man who is hybrid for both traits). Ddbb X DbBb B) Determine the possible gametes for the parents. Ddbb DdBb Db DB Db Db db dB db db Vertical Horizontal C) Show the Punnett square that represents the cross between these two parents. Db Db db db DB DDBb DDBb DdBb DdBb Db DDbb DDbb Ddbb Ddbb db Ddbb Ddbb ddbb ddbb db ddbb ddbb ddbb ddbb D) Determine the phenotypic ratio for this cross. 2 -DDBb 2- DbBb 2- DDbb 4- Ddbb 6- ddbb 2. In the garden pea, Mendel found that yellow seed colour (Y) was dominant to green (yy) and round seed shape (R) was dominant to wrinkled (rr). A) What phenotypic ratio would be expected in the F1 from a cross of a pure yellow pure round and a green wrinkled? YYRR X yyrr RrYy B) What phenotypic ratio would be expected in the F2? F2 ====> RrYy X RrYy 9: 3: 3: 1 C) What is the F2 ratio of yellow: green? YR Yr yR yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr Yyrr yyRr yyrr 12 Yellow : 4 Green or 3 Yellow: 1 Green D) What is the F2 ratio of round: wrinkled? 12 Round: 4 Wrinkled or 3 Round: 1 wrinkled E) What do you notice about the results of C) and D) in relation to monohybrid crosses? 3. Tall tomato plants are produced by the dominant allele (D) and dwarf plants by its recessive allele (d). Hairy stems are produced by a dominant allele (H) and hairless stems are produced by its recessive allele (h). A dihybrid tall hairy plant is test crossed. Draw the Punnett square to show this cross. Hairy stems (Hh) Hair less ( hh) Tall tomato plant ( DD) dwarf plant ( dd) DdHh X ddhh DH Dh dH dh dh DdHh Ddhh ddHh ddhh dh DdHh Ddhh ddHh ddhh dh DdHh Ddhh ddHh ddhh dh DdHh Ddhh ddHh ddhh 4. Bronze turkeys have at least one dominant allele (R). Red turkeys are homozygous for its recessive allele (r). Another dominant allele (H) produces normal feathers and the recessive genotype (hh) produces feathers lacking webbing, a condition known as hairy. In a cross between a homozygous bronze hairy bird and a red, pure normal-feathered bird, what proportion of the F 2 progeny will be Bronze turkeys (RR) Red turkeys ( rr) normal feathers ( HH) Hairy ( hh) A) B) C) D) E) genotype Rrhh phenotype red normal-feathered genotype RrHh phenotype bronze hairy genotype RRhh F) G) H) I) J) phenotype bronze normal-feathered genotype rrhh phenotype red hairy genotype RRHh genotype RRHH (a) F1 Rh Rh Rh Rh rH RrHh RrHh RrHh RrHh rH RrHh RrHh RrHh RrHh rH RrHh RrHh RrHh RrHh rH RrHh RrHh RrHh RrHh F2 RH Rh rH rH RH RRHH RRHh RrHH RrHh Rh RRHh RRhh RrHh Rrhh rH RrHH RrHh rrHH rrHh rh RrHh Rrhh rrHh rrhh 2/ 16 Rrhh 5. Normal leg size, characteristic of the Kerry type of cattle, is produced by the homozygous genotype DD. Short-legged Dexter type cattle possess the heterozygous Dd. The homozygous genotype dd is lethal, producing stillbirths called ‘bulldog calves’. The presence of horns in cattle is produced by its recessive allele p, and the absence of horns (called ‘polled’) is due to the dominant allele P. In a mating between polled Dexter cattle (DdPp), what phenotypic ratio is expected in the ADULT progeny?