Ch. 19 final

Why is it that the range of resting blood pressures of humans is best represented by a bell-shaped curve covering a continuous range rather than by two populations, one with high blood pressure and one with normal blood pressure?

Blood pressure is controlled by a number of genes, not just one. The more genes that contribute to the trait, the more possible genotype combinations that are possible. Most people will be heterozygous for some of the alleles whereas only a few individuals will be dominant or recessive for all or nearly all of them. The more variables there are, the more likely that the distribution will be bell-shaped.

How is it that each parent is able to contribute only a single allele of each gene to his/her offspring, rather than both alleles?

During meiosis, haploid gamete cells are formed that should only contain a single chromosome from each chromosome pair. Therefore each human gamete cell should only contain 23 chromosomes, each one having a single allele of that parent's gene set.

Nondisjunction during meiosis can lead to the formation of gametes (sperm or eggs) with extra copies of one or more chromosomes. Normally, fertilizations involving extra copies of chromosomes simply do not result in a live birth because too many developmental events are altered in some way. The most common extra-chromosomal condition that does lead to live births is Down Syndrome (trisomy 21). Why do you suppose that trisomy 21 is more common than other trisomies?

If you take another look at the human karyotype (Figure 19.13) you'll see that chromosome 21 is one of the smallest chromosomes. It contains fewer genes than most chromosomes. Depending upon how many genes are involved in fetal development (perhaps most of them at one time or another) and precisely what each gene does, it could be hypothesized that trisomy 21 disrupts fewer critical developmental functions than other trisomies.

Geneticists often study the patterns of gene transfer in a variety of model organisms, including plants, fruit flies, and even worms. If they're really interested in patterns of in hesitance in humans, why don't they use humans or at least larger animals more similar to humans, such as pigs or even primates?

It would be unethical to conduct genetic experiments on humans. And although it would be possible to use matings between large animals to study the principles of human genetics, it is simply impractical.Large animals take years to reach maturity, are difficult and expensive to maintain, and produce relatively few offspring per mating compared to plants and simpler animal models. Fortunately, the general principles of genetic inheritance are the same across all sexually-reproducing species. In establishing the fundamental principles that govern human genetic inheritance, then, it is quite feasible to use simpler models than humans themselves.

What fraction of the offspring of two wavy-haired Caucasians would have wavy hair? Explain using a Punnett square. Hint: the allele for curly hair exhibits incomplete dominance over the allele for straight hair

Wavy hair appears only in heterozygous individuals. Two heterozygous parents could produce a homozygous dominant individual (curly hair), two heterozygous individuals (wavy hair) or a homozygous dominant individual (straight hair). So the answer is that half of their children would be likely to have wavy hair.

Can a father who is blood type A and a mother who is blood type B have a child who is blood type O?

Yes this is possible. Recall that there are three alleles for blood type (A, B, and O), but that any individual can only have two of them. The one important additional fact you must know, and that you might have picked up on from Chapter 7, is that A and B alleles are dominant over O. A father with genotype AO would be a blood type A. A mother with genotype BO would be would blood type B. If each contributed their O allele to the child, the child would be genotype OO and would be blood type O.

Hemophilia is a sex-linked trait mothers pass on to male offspring. Is it possible for females to have hemophilia too? How would such a thing happen?

Yes, it's theoretically possible. But the only way for a female to inherit hemophilia would be to inherit the hemophilia allele from BOTH parents. The father would have to carry the hemophilia allele on his only X chromosome, so he would already know he's a hemophiliac. Additionally, the mother would have to at least be a carrier for the trait. Even in this were the case, only half of their daughters be hemophiliacs; the other half would only carry the trait, like their mothers. Obviously, the couple would already know that the male would be contributing the hemophilia allele to any female offspring even before they had children. And being forwarned, they could seek genetic counseling to determine the liklihood that the female is a carrier.