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Chapter 5: Problem 27

A variety of opium poppy (Papaver somniferum) with lacerate leaves was crossed with a variety that has normal leaves. All the \(\mathrm{F}_{1}\) had lacerate leaves. Two \(\mathrm{F}_{1}\) plants were interbred to produce the \(\mathrm{F}_{2} .\) Of the \(\mathrm{F}_{2}, 249\) had lacerate leaves and 16 had normal leaves. Give genotypes for all the plants in the \(\mathrm{P}, \mathrm{F}_{1},\) and \(\mathrm{F}_{2}\) generations. Explain how lacerate leaves are determined in the opium poppy.

Short Answer

Expert verified
Lacerate is dominant; P: LL, ll; F1: Ll; F2: LL, Ll, ll (approx. 15.6:1 ratio observed).

Step by step solution

01

Determine Dominant and Recessive Traits

Since all the F1 generation had lacerate leaves after crossing a lacerate-leaved plant with a normal-leaved plant, we can infer that the lacerate leaf trait is dominant over the normal leaf trait.
02

Assign Genotypes

Let's assign 'L' for the dominant lacerate allele and 'l' for the recessive normal allele. In the P generation, the lacerate-leaved parent can be either homozygous (LL) or heterozygous (Ll), while the normal-leaved parent must be homozygous recessive (ll). Since the F1 offspring displayed the dominant trait uniformly, the lacerate-leaved parent must have been homozygous dominant (LL). This makes all F1 generation plants heterozygous (Ll).
03

Determine F2 Genotypes and Phenotypes

When two heterozygous (Ll) F1 plants are interbred, the F2 generation results in a 1:2:1 genotypic ratio (LL, Ll, ll) and a 3:1 phenotypic ratio (lacerate:normal). The observed counts in F2 of 249 lacerate and 16 normal support this prediction, with an approximately 15.6:1 ratio, suggesting some sampling error or selective advantage/disadvantage.
04

Summary of Genotypes

For the P generation, the genotypes are LL (lacerate) and ll (normal). For the F1 generation, all are Ll (lacerate). In the F2 generation, approximately 249 are either LL or Ll (lacerate) and 16 are ll (normal).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Dominant and Recessive Traits
In genetics, traits can be dominant or recessive in nature, which determines how they are expressed in offspring. A dominant trait is a characteristic passed down from a parent that appears in the organism whenever the dominant allele is present. In contrast, a recessive trait only appears when an organism has two copies of the recessive allele, one from each parent.

In the case of the opium poppy exercise, lacerate leaves are a dominant trait, while normal leaves are recessive. This conclusion comes from the observation that all offspring in the first generation (F1) had lacerate leaves, despite one parent having normal leaves. The presence of the dominant "L" allele causes the lacerate leaf phenotype to appear in the F1 generation.

This principle is crucial because it helps predict and understand inheritance patterns in various organisms. Dominant-recessive relationships are central to Mendelian genetics and play a role in how genetic traits pass across generations.
Genotypic Ratio
Genotypic ratios indicate the proportion of different genotypes in the offspring of a genetic cross. These ratios provide insight into the underlying genetic make-up, even if the physical traits (phenotypes) are not immediately visible.

In our opium poppy example, two heterozygous (Ll) F1 plants were crossed to produce the F2 generation. The resulting genotypic ratio is 1:2:1, meaning:

  • 1 portion will be homozygous dominant (LL)
  • 2 portions will be heterozygous (Ll)
  • 1 portion will be homozygous recessive (ll)
As the dominant allele "L" results in lacerate leaves, both LL and Ll genotypes exhibit this trait. Only the ll genotype will have the recessive normal-leaf phenotype.

Understanding genotypic ratios helps us grasp the chances of certain genetic combinations occurring. While phenotypes can give immediate visual cues, genotypic ratios reveal the genetic diversity present in any given offspring.
Phenotypic Ratio
The phenotypic ratio describes the relative number of offspring manifesting a particular trait after a genetic cross, revealing the expression of the dominant and recessive alleles. It focuses on the visible or observable characteristics rather than the gene configuration.

For our opium poppy case, the phenotypic ratio when two heterozygous plants are crossed generally predicts a 3:1 ratio. This means among every four offspring, three are expected to have lacerate leaves, and one is expected to have normal leaves. This is derived from the presence of the dominant allele "L" being expressed in both LL and Ll genotypes.

However, in the exercise, the F2 generation showed a slightly different ratio of approximately 15.6:1, suggesting an anomaly due to sampling error or natural variations. Despite these variations, the underlying principle remains straightforward: dominant traits will appear more frequently in the phenotype due to the expression of the dominant allele. Comprehending phenotypic ratios helps us predict how traits may appear in future generations.

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Most popular questions from this chapter

In the pearl-millet plant, color is determined by three alleles at a single locus: \(R p^{1}\) (red), \(R p^{2}\) (purple), and \(r p\) (green). Red is dominant to purple and green, and purple is dominant to green \(\left(R p^{1}>R p^{2}>r p\right)\). Give the expected phenotypes and ratios of offspring produced by the following crosses. a. \(R p^{1} / R p^{2} \times R p^{1} / r p\) b. \(R p^{1} / r p \times R p^{2} / r p\) c. \(R p^{1} / R p^{2} \times R p^{1} / R p^{2}\) d. \(R p^{2} / R p \times R p / R p\) e. \(R p / R p \times R p^{1} / R p^{2}\)

A summer-squash plant that produces disc-shaped fruit is crossed with a summer-squash plant that produces long fruit. All the \(\mathrm{F}_{1}\) have disc-shaped fruit. When the \(\mathrm{F}_{1}\) are intercrossed, \(\mathrm{F}_{2}\) progeny are produced in the following ratio: \(9 / 16\) disc- shaped fruit : \(6 / 16\) spherical fruit \(: 1 / 16\) long fruit. Give the genotypes of the \(\mathrm{F}_{2}\) progeny.

In rabbits, an allelic series helps to determine coat color: \(C\) (full color), \(c^{\mathrm{ch}}\) (chinchilla, gray color), \(c^{\mathrm{h}}\) (Himalayan, white with black extremities), and \(c\) (albino, all-white). The \(C\) allele is dominant to all others, \(c^{\mathrm{ch}}\) is dominant to \(c^{\mathrm{h}}\) and \(c, c^{\mathrm{h}}\) is dominant to \(c\), and \(c\) is recessive to all the other alleles. This dominance hierarchy can be summarized as \(C>c^{\mathrm{ch}}>c^{\mathrm{h}}>c .\) The rabbits in the following list are crossed and produce the progeny shown. Give the genotypes of the parents for each cross: Phenotypes of parents a. full color \(x\) albino b. Himalayan \(\times\) albino c. full color \(\times\) albino d. full color \(x\) Himalayan e. full color \(\times\) full color Phenotypes of offspring \(1 / 2\) full color, \(1 / 2\) albino \(1 / 2\) Himalayan, \(1 / 2\) albino \(1 / 2\) full color, \(1 / 2\) chinchilla \(1 / 2\) full color, \(1 / 4\) Himalayan, \(1 / 4\) albino \(3 / 4\) full color, \(1 / 4\) albino

In unicorns, two autosomal loci interact to determine the type of tail. One locus controls whether a tail is present at all; the allele for a tail \((T)\) is dominant to the allele for tailless \((t)\). If a unicorn has a tail, then alleles at a second locus determine whether the tail is curly or straight. Farmer Baldridge has two unicorns with curly tails: when he crosses them, \(1 / 2\) of the progeny have curly tails, \(1 / 4\) have straight tails, and \(1 / 4\) do not have a tail. Give the genotypes of the parents and progeny in Farmer Baldridge's cross. Explain how he obtained the 2: 1: 1 phenotypic ratio in his cross.

Palomino horses have a golden yellow coat, chestnut horses have a brown coat, and cremello horses have a coat that is almost white. A series of crosses between the three different types of horses produced the following offspring: $$ \begin{array}{ll} {\text { Cross }} & {\text { Offspring }} \\ \hline \text { palomino } \times & 13 \text { palomino, } 6 \text { chestnut, } 5 \\ \text { palomino } & \text { cremello } \\ \text { chestnut } \times \text { chestnut } & 16 \text { chestnut }\\\ \text { cremello } \times \text { cremello } & 13 \text { cremello }\\\ \text { palomino } \times \text { chestnut } & 8 \text { palomino, } 9 \text { chestnut }\\\ \text { palomino } \times \text { cremello } & 11 \text { palomino, } 11 \text { cremello }\\\ \text { chestnut } \times \text { cremello } & 23 \text { palomino }\\\ \end{array} $$ a. Explain the inheritance of the palomino, chestnut, and cremello phenotypes in horses. b. Assign symbols for the alleles that determine these phenotypes, and list the genotypes of all parents and offspring given in the preceding table.
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