Chapter 19

1. Transplantation experiments with amphibian embryos showed that in the early amphibian embryo, cell fate is not yet determined and the environment in which a cell is placed can result in determination. In the later embryo, cell fate has been determined and the environment has no effect.

2. Stem cells in adult body tissues are typically multipotent, while stem cells in the embryo are typically pluripotent.

3. One could analyze mRNA in egg cells, in the parent differentiated cells, and in the reprogrammed cells. This could be done by reverse transcriptase PCR or by gene expression arrays.

4. In animals, cloning has been used to reproduce valuable organisms and increase populations of rare species. In humans, reproductive cloning could be used by couples who want offspring that are genetically related to one or both of them but who cannot produce eggs or sperm. It could allow a lesbian couple to have children that are genetically related to them or gay male couples to have genetically related offspring (using a surrogate mother to carry the child). It could allow a couple to have a child who is genetically the same as a child who is dying.

1. Unequal distribution of a cytoplasmic component in the fertilized egg can result in only one of two daughter cells receiving the component after cell division. This asymmetry can be maintained by the cytoskeleton. It can mean that only one of the two cells will express certain genes, resulting in a difference in cell fate determination.

2. The anchor cell would not be able to send a differentiation signal to the overlying surface tissue, so the latter would not develop into the vulva.

3. The p21 gene would be activated and the cell cycle would be blocked; in the presence of other factors, muscle cells would form.

1. Organ identity genes specify different organs of the flower in plants. They are identified by loss-of-function mutations (mutations that result in a missing organ) or gain-of-function mutations (a promoter for the gene specifying one organ can be artificially spliced to a gene specifying a different organ, causing the latter gene to specify the first organ).

   Which particular organ will be generated depends not just on the expression of a single organ identity gene but on the combination of the expression of other genes. For instance, the whorl that generates petals (whorl 2) and the whorl that generates stamens (whorl 3) both express class B genes. The difference between these whorls lies in the other organ identity genes that they also express, with whorl 2 also expressing class A genes and whorl 3 also expressing class C genes.

2. A mutation that caused expression of class A genes instead of class C genes would lead to an AB combination instead of AC, and petals would develop instead of stamens.

3. All neuronal precursors might undergo apoptosis, and no neurons would form. The Hunchback protein gradient would not form properly, and the embryo would not establish its anterior–posterior axis.

1. Substituting the fruit fly genetic switch for eye formation in the mouse embryo for the mouse genetic switch still resulted in eye formation in the mouse, and vice versa. This indicates that the genetic switches for development are conserved in evolution.

2. BMP4 is involved with cartilage development.

3. The observations are consistent with the hypothesis that there has been selection in some human populations for mutations of the enhancer that controls expression of the glycoprotein in red blood cells. This genetic change would be expected to have a selective advantage in human populations that are exposed to malaria at high levels, because the mutation confers greater resistance to malaria in humans who carry it.

1. The developmental gene Dll controls the formation of legs in arthropods. Insects have three pairs of legs on their three thoracic segments, whereas centipedes have many legs on both thoracic and abdominal segments. In insects, the expression of Dll is repressed by the gene product of Ubx. Ubx is expressed in the abdominal segments of all arthropods, but it has different effects in different species. In centipedes, Ubx is co-expressed with Dll to promote the formation of legs. During the evolution of insects, a mutation in the Ubx gene sequence resulted in a modified Ubx protein that represses Dll expression in abdominal segments.

2. Marine sticklebacks have bony structures that protect them from predatory marine fish. In freshwater sticklebacks, these structures are not prominent. The differences are due to the expression of a developmental regulatory gene, Pitx1, which encodes a transcription factor normally expressed in regions of the developing embryo that in marine sticklebacks form the head, trunk, tail, and pelvis. However, in several freshwater stickleback populations, the Pitx1 gene is no longer expressed in the pelvis, and spines do not develop. This same change in regulatory gene expression has evolved to produce similar phenotypic changes in several independent populations and is an example of parallel evolution.

1. In an RCT, the only difference between treated and untreated subjects is the response to the variable under investigation. Genetic and other differences between experimental and control groups are assumed to be randomized and equal between the groups.

2. The difference in the score for lameness in the stem cell group at 60 days was –1.11. This is greater than the difference in the experimental group (–0.33). So there was more improvement in the stem cell group. A statistical test to show the significance of the differences would be the t-test.

   A-20

3. Plot with Excel. The beneficial effect of stem cell treatment was evident after 30 days.

   

1. The two species differ with respect to how long the transcription factor expression remains high, with the duration of high expression being longer in Species 1. Thus this difference in timing is an example of heterochrony. A reasonable hypothesis is that the morphological difference between the species arises from differential timing of the transcription factor’s expression; specifically, a longer period of high expression (as in Species 1) leads to longer bristles.

2. A direct test of the hypothesis would involve experimental manipulation of the transcription levels in the two species. Increasing the duration of high expression of hairball in Species 2 should result in a bristle size more like that of Species 1. Decreasing the duration of high expression of hairball in Species 1 should result in a bristle size more like that of Species 2.

3. The differences in the presence of bristles in the other segments are likely due to the expression of hairball in those segments. Given that hypothesis, the prediction is that expressing hairball appropriately in those other segments should result in bristles in those segments in Species 1 and 2. This is an example of heterotopy.

4. You could express the Hox gene from Species 3 in Species 1 and Species 2 and eliminate the expression of their normal Hox gene. If the differences in the Hox gene are responsible for the bristle pattern difference, then the flies of Species 1 and Species 2 with Species 3 Hox expression should have a bristle pattern like that of Species 3. Similarly, expressing the Species 1 Hox gene in Species 3 should result in a Species 1 bristle pattern in Species 3.