weeks. This fluid contains living fetal cells (from the skin and the mouth cavity) and can be karyotyped. Some chemical tests can be performed on the fluid itself (Figure 9.10A).B.Chorionic villus samplinginvolves removing tissue from the fetal side of the placenta nurturing a fetus, at 8–10 weeks. These cells are rapidly dividing and can be immediately karyotyped. Some biochemical tests can be performed (Figure 9.10B).C.Ultrasound imagingof the fetus provides a noninvasive view inside the womb (Figure 9.10C, D).D.Fetoscopy provides a more direct view of the fetus through a needle-thin viewing scope inserted into the uterus.E.Amniocentesis, chorionic villus sampling, and fetoscopy carry a small risk and are reserved for situations with higher probabilities of disorders (for example, older parents or situations where genetic counseling has uncovered a higher risk).
F.Analysis of the mother’s blood can detect abnormal levels of certain hormones (HCG and estriol) or proteins produced by the fetus (alpha-fetoprotein). Abnormal levels may indicate that the fetus has Down syndrome or a neural tube defect.G.If fetal testing suggests that there is a problem that cannot be helped by routine surgery or other therapy, the difficult choice must be made between terminating a pregnancy by abortion or carrying a defective baby to term.II.Variations on Mendel’s PrinciplesModule 9.11The relationship of genotype to phenotype is rarely simple.A.The inheritance of many characteristics among all eukaryotes follows the principles that Mendel discovered.NOTE: Discussing these principles first has allowed us to focus on the conventions and basic functioning of the system that underlies inheritance patterns.B.However, most characteristics are inherited in ways that follow more complex patterns.C.Before looking at the chromosomal explanation of Mendel’s principle of independent assortment, we will look at four such complex patterns: incomplete dominance, multiple alleles at a gene locus, pleiotropy, and polygenic inheritance.D.These patterns are extensions of Mendel’s principles, not exceptions to them.Module 9.12Incomplete dominance results in intermediate phenotypes.A.Incomplete dominancedescribes the situation where one allele is not completely dominant in the heterozygote; the heterozygote usually exhibits characteristics intermediate between both homozygous conditions.B.Snapdragon color is a good example of how this works. Note that the possibilities of each genotype are the same as in a case of complete dominance, but the phenotypic ratios are different (Figure 9.12A).C.Another example: the inheritance of alleles that relate to hypercholesterolemia. Normal individuals, HH,have normal amounts of LDL receptor proteins; hhindividuals (rare in the population, about 1 in 1 million) have no receptors and five times the amount of blood cholesterol; Hhindividuals (1 in 500) have half the number of receptors and twice the amount of blood cholesterol (Figure 9.12B).Module 9.13Many genes have more than two alleles in the population.A. The