The Conditional Pregnancy

Years ago, women attempting to bear children waited to seek a pregnancy test until their second missed period. They then went in for the so-called rabbit test, and a positive result was often sufficient to warrant announcing the happy news. These days, the home tests available at most supermarkets or drug stores are accurate on the first day of the first missed period. However, earlier knowledge, coupled with increases in prenatal diagnostic techniques, has not always resulted in earlier...

Tdf Initiates Male Sexual Differentiation

Several lines of evidence argue that the TDF gene is both necessary and sufficient by itself to initiate male sexual differentiation. First, XX human beings are occasionally found in which a piece of the Y chromosome bearing the TDF gene has been appended (or translocated) onto the tip of one of the two X chromosomes, creating a chromosome that we will call X(TDF). Suppose a sperm bearing that translocated X(TDF) chromosome fertilizes an X-bearing egg Such XX(TDF) individuals will develop as a...

Most Cases Of Down Syndrome Are Due To Nondisjunction In The Mother

Most often, a baby with Down syndrome is found to have three copies of chromosome 21 if chromosomes in their cells are examined via traditional karyo-typing to classify the chromosomes present by size, centromere position, and banding pattern (Figure 22.2). Most cases of trisomy 21 are due to nondisjunction at the first meiotic division in the child's mother. We know this because we have developed several methods for determining which chromosome came from which parent, one of which makes use of...

What Passes From One Generation To The Next Is Information

In the first of Mendel's three conceptual breakthroughs, he separated the information that produced a given trait (which we will call the genotype) from the physical manifestation of the trait itself (which we will call the phenotype). In the case of the pea plant, the yellow-pod recipe (genotype) produces a seedpod that appears to our eyes to be yellow (phenotype). If we were cooking, the words of the cake recipe on the page of the cookbook would be the genotype, but the lemon flavor of the...

The Problem With Diagnosing An Inducible Phenotype

In the case of Ari's family, what we see looks a lot like recessive inheritance of a disease gene (Figure 17.2), but it can sometimes be hard to tell if someone has lactose intolerance if they are not exposed to a lot of lactose. Her father's family, with western European ancestry and a standard American diet full of dairy products, appears to be full of lactose-persistent people. However, it is actually harder to tell about people on her mother's side of the family, who consume a more standard...

Box 144 Manipulating Gene Expression To Treat Sickle Cell Anemia

Hemoglobins are the proteins in the red blood cells that carry oxygen to the tissues and carry carbon dioxide back to the lungs to be breathed out of the body. HbS is a damaged version of the HbA hemoglobin present in adult red blood cells. By itself, the sickle cell form of hemoglobin (HbS) forms abnormal biochemical strctures that cause the cell to sickle and become rigid. In heterozygotes, where HbA is also present, the abnormal hemoglobin structures that cause sickling don't take place....

Do We Have To Treat The Whole Body

In many cases, we would actually prefer to avoid treating the whole body if we can, partly to help limit the immune reactions going on, partly because treating fewer cells means less risk of rare side effects, and partly because in some cases there will be cell types in the body that actually need to not be expressing the gene we are trying to get into one specific organ or cell type. Even for genes that are expressed throughout the body, disease resulting from a defect is often specific to a...

Analysis Of Fetal Cells Karyotyping

A picture of a normal human karyotype is presented in Chapter 10. Such figures are obtained by taking dividing cells from the fetal sample and lysing them (breaking them open) on glass slides so that the individual mitotic chromosomes spread out in a loose field. After the slides are stained with dyes, the resulting clusters of chromosomes from each cell are photographed and examined. Skilled cytogenetic technicians begin with large photographs of each metaphase spread. Then they carefully cut...

Mutations In The Gene That Encodes The Androgen Receptor

Now imagine if a steroid hormone receptor in your body was not there or was not functional. Your hormones would continue to flow throughout your body, but when they arrived at the target cell, there would be no place on the cell surface for them to dock. If they don't dock with their receptor, the target cell cannot tell that the hormones are there and thus does not know that it needs to change which genes it is expressing and to change the levels of expression of some of the genes it is...

What Are The Biggest Problems With Gene Therapy

One of the distressing early findings in gene therapy efforts was that, in many cases, positive results from treatment ended up being transient. In cases in which a repeat effort at treatment was tried, often the result the next time was much reduced or even nonexistent. This turns out to be the result of the action of the immune system that normally protects us from infection by bacteria and viruses. The immune system is very good at rapidly mounting a defense against such an infection. In...