Question

1. Understanding monogenic ratios and the results from crosses, one can work backwards to predict unknown parents. Consider plant height; you are told that tall is dominant (S) and short is a (s) condition recessive mutant (remember, the alleles are named for the mutant condition, [s]) In the Punnett below, what are the genotypes of the missing parents that justify this outcome? What is the phenotypic ratio of the offspring? Does it differ from the genotypic ratio? If so, how?

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Answer 1

• Cross of a single genetic locus using the two alleles determine one genetic locus.
• Monohybrid or monogenic crosses involve the use of single trait.
• Example: Cross between pea plants determined by single trait of height of plant. Two alleles tall and short may be used to determine one genetic trait of height.
• Monohybrid cross is used to determine the phenotypical expression of homozygoric individuals generating its heterozygous genotypic form in the offspring.
• This help in determining the hypothesis of dominance.
• Dominant allele is one which expresses the phenotype when present in heterozygous form or along with the recessive allele.
• Thus if S is dominant, the effect of S will be observed as SS and Ss.
• Similar hypothesis was determined by Mendel, using pea plants, but alleles were then described as factors. The hypothesis was concluded as law of dominance.

Representation:

Trait: Height

S= dominant= tall, genotype= SS or Ss

s= recessive= short

• If offspring are all tall- parents are true breeding tall and short with genotype: SS and ss
• Cross: SS xss

	s	s
S	Ss	Ss
S	Ss	Ss

F1: All are of genotype = Ss = Tall phenotype

• If offspring are all tall and short both- parents are heterozygous with genotype Ss
• Ss x Ss

	S	s
S	SS	Ss
s	Ss	ss

Phenotype Ratio = Tall: short=3:1

Genotype ratio = SS:Ss:ss=1:2:1

• Phenotype is the observed character, which is tall or short. Since tall is dominant over short, the phenotype of homozygous SS and heterozygous Ss are observed as tall.
• However, genotype ratio indicates genetic composition of homozygous dominant (SS), heterozygous dominant (Ss), and homozygous recessive (ss).
• Thus, phenotypic ratio is different from genotypic ratio.