Page 45 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Solved Problems 37

solutions. The more problems you do, the easier they be- Answer
come. In solving problems, you will not only solidify your The solution to this problem requires an understanding of
understanding of genetic concepts, but you will also de- dominance/recessiveness, gamete formation, and the inde-
velop basic analytical skills that are applicable in many pendent assortment of alleles of two genes in a cross.
disciplines. First make a representation of the known information:
Note that some of the problems at the end of each
chapter are designed to introduce supplementary but im- Mothers: solid, short-haired solid, long-haired
portant concepts that expand on the information in the text. Cross: cat 1 cat 2
You can nonetheless answer such problems using logical patches, long-haired × solid, short-haired
inferences from your reading. What genotypes can you assign? Any cat showing a reces-
Solving genetics problems requires much more than sive phenotype must be homozygous for the recessive al-
simply plugging numbers into formulas. Each problem is lele. Therefore the long-haired cats are ss; solid cats are pp.
unique and requires thoughtful evaluation of the informa- Cat 1 is long-haired, so it must be homozygous for the re-
tion given and the question being asked. The following are cessive allele (ss). This cat has the dominant phenotype of
general guidelines you can follow in approaching these patches and could be either PP or Pp, but because the
word problems: mother was pp and could only contribute a p allele in her
gametes, cat 1 must be Pp. Cat 1’s full genotype is Pp ss.
a. Read through the problem once to get some sense of Similarly, cat 2 is solid-colored, so it must be homozygous
the concepts involved. for the recessive allele (pp). Because this cat is short-
b. Go back through the problem, noting all the informa- haired, it could have either the SS or Ss genotype. Its mother
tion supplied to you. For example, genotypes or phe- was long-haired (ss) and could only contribute an s allele in
notypes of offspring or parents may be given to you her gamete, so cat 2 must be heterozygous Ss. The full
or implied in the problem. Represent the known infor- genotype is pp Ss.
mation in a symbolic format—assign symbols for al- The cross is therefore between Pp ss (cat 1) and pp Ss
leles; use these symbols to indicate genotypes; make a (cat 2). To determine the types of kittens, first establish the
diagram of the crosses including genotypes and phe- types of gametes that can be produced by each cat and then
notypes given or implied. Be sure that you do not as- set up a Punnett square to determine the genotypes of the
sign different letters of the alphabet to two alleles of offspring. Cat 1 (Pp ss) produces Ps and ps gametes in
the same gene, as this can cause confusion. Also, be equal proportions. Cat 2 (pp Ss) produces pS and ps gam-
careful to discriminate clearly between the upper- and etes in equal proportions. Four types of kittens can result
lowercases of letters, such as C(c) or S(s). from this mating with equal probability: Pp Ss (patches,
c. Now, reassess the question and work toward the solu- short-haired), Pp ss (patches, long-haired), pp Ss (solid,
tion using the information given. Make sure you short-haired), and pp ss (solid, long-haired).
answer the question being asked!
d. When you finish the problem, check to see that the Cat 1
answer makes sense. You can often check solutions P s p s
by working backwards; that is, see if you can recon-
struct the data from your answer. p S Pp Ss pp Ss
e. After you have completed a question and checked your Cat 2
answer, spend a minute to think about which major
concepts were involved in the solution. This is a criti- p s Pp ss pp ss
cal step for improving your understanding of genetics.

For each chapter, the logic involved in solving two or three The following table demonstrates that you could also work
types of problems is described in detail. through this problem using the product rule of probability

I. In cats, white patches are caused by the dominant al- instead of a Punnett square. The principles are the same:
lele P, while pp individuals are solid-colored. Short Gametes produced in equal amounts by either parent are
hair is caused by a dominant allele S, while ss cats combined at random.
have long hair. A long-haired cat with patches Cat 1 Cat 2
whose mother was solid-colored and short-haired gamete gamete Progeny
mates with a short-haired, solid-colored cat whose
mother was long-haired and solid-colored. What 1/2 P s × 1/2 p S 1/4 Pp Ss patches, short-haired
1/2 P s
×
1/4 Pp ss patches, long-haired
1/2 p s
kinds of kittens can arise from this mating, and in 1/2 P s × 1/2 p S 1/4 pp Ss solid-colored, short-haired
what proportions? 1/2 P s × 1/2 p s 1/4 pp ss solid-colored, long-haired