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604 Chapter 18 NEL

A 3:1 phenotypic ratio occurs when two heterozygous individuals are crossed, so we

know that the parents must be heterozygous. Since only ^14 of the progeny had wrinkled

seeds, this is the recessive phenotype and must be determined by two copies of the

recessive allele. The parents were heterozygous, so their genotype was Rr. Check the

answer using a Punnett square (Figure 6).

Practice

3.A fish breeder has a red male cichlid of unknown parentage. Red colour is

dominant to yellow in the fish. He must know whether the fish is heterozygous

for these colours. Suggest a way the fish breeder might find out the genotype

of his red male. Use a Punnett square to explain your answer.

4.A neighbour gives a home gardener some seeds that he collected last year

from his red carnations. The gardener plants 50 of the seeds and is surprised

to find 12 of the plants have white flowers. Assuming that all the seeds came

from one cross, what was the genotype of the parents?

rr

round

RR Rr

round wrinkled

round

Rr

round parent

r

R

R r

round parent

Figure 6
The observed phenotypic ratio is
the same as the ratio predicted by
the Punnett square.

Section 18.2 Questions

1. In Dalmatian dogs, the spotted condition is dominant to
   non-spotted.
   (a) Using a Punnett square, show the cross between two
   heterozygous parents.
   (b) A spotted female Dalmatian dog has six puppies sired
   by an unknown male. From their appearance, the
   owner concludes that the male was a Dalmatian. Three
   of the pups are spotted and three are not. What is the
   genotype and phenotype of the puppies’ father?

2.For Mexican hairless dogs, the hairless trait is dominant to

hairy. A litter of eight pups is found; six are hairless and

two are hairy. What are the genotypes of their parents?

3.Test crosses are valuable tools for plant and animal

breeders.

(a) Provide two practical examples of why a cattle rancher

might use a test cross.

(b) Why are most test crosses performed using bulls

rather than cows?

• By using a Punnett square, the expected phenotypic ratio and genotypic ratio of
  the offspring of a cross can be determined.


• Probability,P
  

Probability values can be used to predict the likelihood that a particular

phenotype will appear in a cross.

• A test cross is the cross of an individual of unknown genotype to an individual
  with a fully recessive genotype.

number of ways that a given outcome can occur



total number of possible outcomes

Probability and Inheritance of Single

SUMMARY Traits