Question

In the plant Arabidopsis thaliana, every flower is constructed of four concentric whorls of modified leaves. The first whorl (whorl 1) consists of four green leaf-like sepals, whorl 2 is composed of four white petals, whorl 3 is made of six stamens bearing the pollen that houses the male gametes (sperm), and whorl 4 contains the two carpels, within which lie the ovules that hold the female gametes (eggs). As shown in the diagram that follows, a shorthand description of the wild-type flower pattern is: sepal, petal, stamen, carpel. Scientists wanted to understand how this pattern of whorls arises. They generated Arabidopsis strains homozygous for randomly induced mutations and screened them for mutant flowers with an abnormal order or selection of floral organs. The interesting mutants identified fell into three phenotypic classes: (1) carpel, stamen, stamen, carpel; (2) sepal, sepal, carpel, carpel; (3) sepal, petal, petal, sepal. The investigators found that all of the class $1 \mathrm{mu}-$ tants were alleles of the same gene which they called APETELA2 $(A P 2) .$ Class 2 mutants were alleles of either one of two genes, which were named APETELA $3(A P 3)$ and PISTILLATA $(P I) .$ Finally, the class 3 mutants represented a single gene, $A G A M O U S$ $(A G) .$ Molecular analysis showed that all four genes encode transcription factors. Based on the phenotypes of $A P 2, A P 3, P I,$ and $A G$ mutants (all of them are null alleles) and the fact that all four gene products are transcription factors, investigators formulated the following model for differentiation of the four flower whorls from four equivalent whorl precursor cell groups: AP2 protein b. Scientists tested the flower patterning model with RNA in situ hybridization experiments using cDNA probes for each of the four genes. The goal was to see whether each gene's mRNA was expressed in the precursor cells for each of the four whorls. What results would you predict with each probe on wild-type flowers? on $A P 2$ mutants? on $A G$ mutants? c. Another way the researchers tested their model for flower patterning was by making double mutants. What phenotypes does the model predict for each of the six double mutant combinations? [Hint: It will be helpful to expand the chart shown in part (a) for each possible double mutant. d. Are the roles of the four genes described in this problem more similar to those of the segmentation genes or to those of the homeotic genes from Drosophila described in the text?

Name: Problem 37, Chapter 19: Genetics: From Genes to Genomes
Uploaded: 2026-02-14T21:28:15-08:00
Duration: 5 min 8 s
Channel: James Kiss

In the plant Arabidopsis thaliana, every flower is constructed of four concentric whorls of modified leaves. The first whorl (whorl 1) consists of four green leaf-like sepals, whorl 2 is composed of four white petals, whorl 3 is made of six stamens bearing the pollen that houses the male gametes (sperm), and whorl 4 contains the two carpels, within which lie the ovules that hold the female gametes (eggs). As shown in the diagram that follows, a shorthand description of the wild-type flower pattern is: sepal, petal, stamen, carpel.
Scientists wanted to understand how this pattern of whorls arises. They generated Arabidopsis strains homozygous for randomly induced mutations and screened them for mutant flowers with an abnormal order or selection of floral organs. The interesting mutants identified fell into three phenotypic classes:
(1) carpel, stamen, stamen, carpel; (2) sepal, sepal, carpel, carpel; (3) sepal, petal, petal, sepal. The investigators found that all of the class $1 \mathrm{mu}-$ tants were alleles of the same gene which they called APETELA2 $(A P 2) .$ Class 2 mutants were alleles of either one of two genes, which were named APETELA $3(A P 3)$ and PISTILLATA $(P I) .$ Finally, the class 3 mutants represented a single gene, $A G A M O U S$ $(A G) .$ Molecular analysis showed that all four genes encode transcription factors.
Based on the phenotypes of $A P 2, A P 3, P I,$ and $A G$ mutants (all of them are null alleles) and the fact that all four gene products are transcription factors, investigators formulated the following model for differentiation of the four flower whorls from four equivalent whorl precursor cell groups: AP2 protein
b. Scientists tested the flower patterning model with RNA in situ hybridization experiments using cDNA probes for each of the four genes. The goal was to see whether each gene's mRNA was expressed in the precursor cells for each of the four whorls. What results would you predict with each probe on wild-type flowers? on $A P 2$ mutants? on $A G$ mutants?
c. Another way the researchers tested their model for flower patterning was by making double mutants. What phenotypes does the model predict for each of the six double mutant combinations? [Hint: It will be helpful to expand the chart shown in part (a) for each possible double mutant.
d. Are the roles of the four genes described in this problem more similar to those of the segmentation genes or to those of the homeotic genes from Drosophila described in the text?

Genetics: From Genes to Genomes

Leland Hartwell,… 6th Edition

Chapter 19, Problem 37

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Solved by Expert James Kiss

02/14/2026

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Based on the phenotypes of the mutants and the fact that all four gene products are transcription factors, we can create a model for the differentiation of the four flower whorls from four equivalent whorl precursor cell groups. We can represent the presence of Show more…

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In the plant Arabidopsis thaliana, every flower is constructed of four concentric whorls of modified leaves. The first whorl (whorl 1) consists of four green leaf-like sepals, whorl 2 is composed of four white petals, whorl 3 is made of six stamens bearing the pollen that houses the male gametes (sperm), and whorl 4 contains the two carpels, within which lie the ovules that hold the female gametes (eggs). As shown in the diagram that follows, a shorthand description of the wild-type flower pattern is: sepal, petal, stamen, carpel. Scientists wanted to understand how this pattern of whorls arises. They generated Arabidopsis strains homozygous for randomly induced mutations and screened them for mutant flowers with an abnormal order or selection of floral organs. The interesting mutants identified fell into three phenotypic classes: (1) carpel, stamen, stamen, carpel; (2) sepal, sepal, carpel, carpel; (3) sepal, petal, petal, sepal. The investigators found that all of the class $1 \mathrm{mu}-$ tants were alleles of the same gene which they called APETELA2 $(A P 2) .$ Class 2 mutants were alleles of either one of two genes, which were named APETELA $3(A P 3)$ and PISTILLATA $(P I) .$ Finally, the class 3 mutants represented a single gene, $A G A M O U S$ $(A G) .$ Molecular analysis showed that all four genes encode transcription factors. Based on the phenotypes of $A P 2, A P 3, P I,$ and $A G$ mutants (all of them are null alleles) and the fact that all four gene products are transcription factors, investigators formulated the following model for differentiation of the four flower whorls from four equivalent whorl precursor cell groups: AP2 protein b. Scientists tested the flower patterning model with RNA in situ hybridization experiments using cDNA probes for each of the four genes. The goal was to see whether each gene's mRNA was expressed in the precursor cells for each of the four whorls. What results would you predict with each probe on wild-type flowers? on $A P 2$ mutants? on $A G$ mutants? c. Another way the researchers tested their model for flower patterning was by making double mutants. What phenotypes does the model predict for each of the six double mutant combinations? [Hint: It will be helpful to expand the chart shown in part (a) for each possible double mutant. d. Are the roles of the four genes described in this problem more similar to those of the segmentation genes or to those of the homeotic genes from Drosophila described in the text?

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Key Concepts

Gene Expression Analysis using RNA in situ Hybridization

RNA in situ hybridization is a technique used to detect the spatial distribution of specific mRNA transcripts within tissues. In the context of flower patterning, it provides insight into where each gene is active, which is critical for testing models of organ specification. By comparing wild-type and mutant expression patterns, researchers can determine how mutations affect spatial gene regulation.

ABC Model of Flower Development

The ABC model explains how three classes of transcription factors (class A, B, and C) are expressed in overlapping domains in the developing flower to specify the identity of the four floral whorls. This combinatorial mechanism means that specific combinations of gene activities determine whether a cell will form a sepal, petal, stamen, or carpel. Loss-of-function mutations in these genes lead to predictable homeotic transformations of one floral organ type into another.

Transcription Factors in Development

Transcription factors are proteins that control the expression of target genes by binding to DNA regulatory sequences. In floral development, transcription factors such as those identified (AP2, AP3, PI, AG) guide the developmental fate of cells by activating or repressing sets of genes in a spatially regulated manner, thus ensuring that each whorl develops into the correct floral organ.

Double Mutant Analysis

Double mutant analysis involves generating organisms with mutations in two different genes simultaneously to study genetic interactions. This approach is key for uncovering the relationships between transcription factors in specifying organ identity. By analyzing the phenotypes of double mutants, scientists can infer whether genes act in parallel, in series, or have redundant functions in developmental pathways.

Homeotic Genes in Development

Homeotic genes are master regulatory genes that determine the identity of body parts or organs during development. In the context of floral organ identity, the genes studied (AP2, AP3, PI, AG) are homeotic because their disruption leads to the transformation of one type of floral organ into another, paralleling the role of homeotic genes in animals, such as those seen in Drosophila.

Comparison with Segmentation Genes

Segmentation genes function to divide the embryo into repeated body segments, while homeotic genes are responsible for determining the identity of these segments. The genes discussed in the flower patterning model function more like homeotic genes, since they specify the identity of each floral whorl rather than determining positional segments. This distinction highlights the different roles that developmental genes can play in organismal patterning.

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