Theory question 104
Mendelian Inheritance(a)(i) What is the difference between the genotype and phenotype of an individual?
(ii) State two factors each which determine each of the terms in 4(a)(i).
(b) In a breeding experiment, a homozygous black dog (BB) was crossed with a homozygous white dog (bb) to produce black puppies in the first filial generation (F1).
The second filial generation produced a total of two hundred and forty (240) black and white puppies. With the aid of a genetic diagram, determine:
(i) the number of white puppies; (ii) the number of black puppies in the second filial generation.
Show the model answer
(a)(i) Genotype is the genetic make-up (the alleles) of an individual; phenotype is its observable characteristics.
(ii) Genotype: the genes/alleles inherited from the parents; mutation; (crossing over/independent assortment). Phenotype: the genotype; the environment (e.g. nutrition, climate); interaction of dominant and recessive alleles.
(b) Parents BB × bb → gametes B and b → F1 all Bb (black).
F1 cross Bb × Bb → gametes B, b × B, b → F2: BB, Bb, Bb, bb → 3 black : 1 white.
(i) White = ¼ × 240 = 60 puppies.
(ii) Black = ¾ × 240 = 180 puppies.
The F2 of a monohybrid cross gives a 3 : 1 ratio.