Chapter 14

Thus, a __ or other multicharacter cross is equivalent to two or more independent __ crosses occurring simultaneously.

The reason __ counted so many offspirng from his crosses is that he understood this statistical feature of inheritance and had a keen sense of the rules of chance.

In the 20th century, geneticists extended __ principles not only to diverse organisms, but also to patterns of __ more complex than those described by Mendel. For the work that led to two __, Mendel chose pea plant characters that turn out to have a relatively simple genetic basis: __ (phrase)

But these conditions are not met by all heritable characters,and the relationship between __ and __ is rarely so simple. Mendel himself realized that he could not xplain the more complicated patterns he observed in crosses involving other pea characters or other plant species. This does not diminish the utility of __ (also called __), however, because the basic principles of __ and __ apply even to more complex patterns of inheritance.

THe inheritance of characters determined by a single genes deviates from __ patterns when alleles are not completely __ or __, when a particular gene has more than two __, or when a single gene produces multiple __.

__ can show different degrees of dominace and recessiveness in relation to each other. In Mendel's classic pea crosses, the __ offspring always looked like one of the two parental varietes because one allele in a pair showed __ over the other. In such situations, the __ of the heterozygote and the dominant homozygote are indistinguishable.

For some genes, however, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties. This phenomenon is called __.

sistuation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable

Situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele

the situation in which the phenotype of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate distinguishable ways

At first glance, __ of either allele seems to provide evidence for the __ of inheritance, which would predict that the red or white trait could never be retrieved from the pink hybrids. In fact, interbreeding F1 hybrids produces _ offspring with a phenotypic ration of one to two. (Because heterozygotes have a separate phenotype, the genotypic and phenotypic rations for the __ generation are the same, __.)

True or False:
Inheritance is particulate.
True False

Another variation on dominance relationships between alleles is called __; in this variation, the two alleles both affect the phenotype in separate, distinguishable ways.

It is important to understand that an allele is not termed __ because it somehow subdues a reessive allele. When a dominant allele coexists with a recessive allele in a __, they do not actually interact at all. It is in the pathway from genotype to phenotype that __ and __ come into play.

A closer look at the relationship between dominance and phenotype reveals an intriguing fact: __ (phrase)

__, an inherited disorder in humans, provides an example. The brain cells of a child with this disease cannot metabolize certain lipids because a crucial enzyme does not work properly. As these lipids accumulate in brain cells, the child begins to suffer seizures, blindness and degeneration of motor and mental performance and dies within a few years.

Only children who inherit two copies of the __ allele (homozygotes) have the disease. Thus, at the __ level, this disease's allel qualifies as recessive. However, the activity level of the lipid- metabolizing enzyme in heterozygotes is intermediate between that in individuals homozygous for the normal allele and that in individuals with this disease. The intermediate phenotype observed at the __ level is characteristic of incomplete dominance of either allele. Fortunately, the heterozygote condition does not lead to disease symptoms, apparently because 1/2 the normal enzyme activity is sufficient to prevent lipid accumulation in the brain. Extending our analysis to yet another level, we find that heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. Thus at the __ level, the normal allele and the __ allele are __. As you can see, whether alleles appear to be completely __, __, or __ depends on the level at which the phenotype is analyzed.

TRUE or FALSE:
Although you might assume that the dominat allele for a particular character would be more common in a population than the recessive allele for that character, this is not necessarily the case.
-(One out of 400 babies born with polydactyly (extra fingers/toes). Some cases of this are caused by the presence of a dominant allele. The low frequency of polydactyly indicates that the recessive allele, which results in five digits per appendage, is far more prevalent than the dominant allele in the population.)
True False

True or False:
Most genes exist in more than two allelic forms.
- A person's blood group (phenotype) may be one of four types: __?__.
These refer to __.
True False

the ability of a single gene to have multiple phenotypic effects
-->ex: a sickle- cell disease has multiple symptoms all due to a single defective gene
a gene at one locus alters the phenotypic expression of a gene at a second locus -->ex: an individual may have genes for heavy skin pigmentation, but if a separate gene that produces the pigment is defective, the genes for pigment deposition will not be expressed, leading to albinism

__, __ and __ all have to do with the effects of alleles of a single gene.

Mendel studied characters that could be classified on an either-or basis. But for many characters, an either-or classification is impossible because the characters vary in population along a continuum (in gradations). These are called __. Quantitative variation usually indicates __, an additive effect of two or more genes on a single phenotypic character (the converse of __, where a single gene affects several phenotypic characters.)

Enviromental factors, such as exposure to the sun, also affect the skin-color __.

Another departure from simple Mendelian genetics arises when the phenotype for a character depends on __ as well as genotype.

Whether human characteristics are more influenced by genes or the environemnt- __ or __- is a very old and hotly contested debate. We can say that a genotype generally is not associated with a rigidly defined phenotype, but rather with a range of phenotypic possibilities due to environment influences. This phenotypic range is called the __ for a genotype. For soome characters, the norm of reaction has no breadth whatsoever; that is, a given genotype mandates a very specific phenotype. Other characteristics vary a bit (like blood count of RBCs and WBCs), dependinng on such factors as the __, the __ and the __.

Generally, norms of reaction are broadest for __. Environment contributes to the __ of these characters. Geneticists refer to such characters as __, meaning that many factors, both genetic and environmental, collectively influence phenotype.

The key to integrating these refinements (dominance as well as multiple alleles, pleiotrophy, epistasis, polygenic inheritance, and the phenotypic impact of the enviroment) into a comprehensive theory of Mendelian genetics is to make the transition from the __ emphasis on single genes and phenotypic characters to the emergent properties of teh organisms as a whole.

The term __ can refer not only to specific characters, like color, but also to an organism in its entirety- all aspects of its physical appearance, internal anatomy, physiology, and behavior.

The term __ can refer to an organism's enire genetic makeup, not just its alleles for a single genetic locus. In most causes, a gene's impact on __ is affected by others genes and by the environment. In this integrated view of __ and __, an organism's phenotype reflects its overall genotype and unique environmental history.

Mendel's two laws, __ and __, explain heritable variations in terms of alternative forms of genes (hereditary "particles" now known as the alleles of genes) that are passed along, generation after generation, according to simple rules of __. This theory of __ is equally valid for peas, flies, etc.- indeed for any organism with a sexual life cycle. Furthermore, by extending the principles of __ and __ to help explain such hereditary patterns as __ and __, we begin to see how broadly Mendelism applies.

The human generation span is about __ years, and human parents produce relatively few offspring compared to peas and most other species.

Unable to manipulate the mating patterns of people, geneticists must do what?
They do so by collection info about a family's history for a particular trait and assembling this info into a family tree describing the traits of parents and children across the generations- the family __, a diagram that shows the relationship between parents and offspirng across two or more generations.

An important application of a __ is to help us calculate the probability that a child will have a particular genotype and phenotype. __ are a more serious matter when the alleles in question cause disabling or deadly disease instead of innocuous human variations. Howerver, for disorders inherited as simple __, the same techniques of __ analysis apply.

Thousands of genetic disorders are known to be inherited as simple __ traits. THese disorders range in severity from relatively mild, such as __ (lack of pigmentation, which results in susceptibility to skin cancers and vision problems), to life threatening, like __.

How can we account for the behavior of alleles that cause recessively inherited disorders?
-- An __ that causes a genetic disorder codes either for a malfunctioning protein or for no protein at all.

In the case of disorders classified as recessive, __ are normal in phenotype because one copy of the normal allele prodcues a sufficient amount of the specific protein. Thus, a recessively inherited disorder shows up only in the __, who inherit one recessive allele from each parent. Although phenotypically normal with regard to the disorder, __ may transmit the recessive allele to their offspring and thus care called __.

Most people who have __ are born to parents who are carriers of the disorder but themselves have a normal __. A mating between two __ corresponds to a Medelian __, so the predicted genotypic ratio for the offspring is 1 AA: 2Aa: 1aa. Thus, each child has a 1/4 chance of inheriting a double dose of the recessive allele. From the genotypic ratio, we also can see that out of three offspring with the normal phenotype, two are predicted to be __ carriers, a 2/3 chance.

Recessive _ could also result from As xaa and aa x aa matings, but if the disorder is lethal before reproductive age or results in __, no aa individuals will reproduces. Even if recessive __ are able to reproduces, such individuals will still account for a much smaller percentage of the population than __ carriers.

TRUE or FALSE
In general, genetic disorders are not evenly distributed among all groups of people.
True False

When a disease- causing recessive allele is rare, it is relatively unlikely that two carriers of the same harmful allele will meet and mate. HOwever, if the man and woman are close relatives, the probability of passing on recessive traits increases greatly. These are called __ ("same blood") matings, and they are indicated in __ by double lines. BEcause people with recent common ancestors are more likely to carry the same __ than are unrelated people, it is more likely that a mating of close relatives will produce offspring homozygous for _- including harmful ones. SUch effects can be observed in many types of domesticated and zoo animals that have become inbred.

There is debate among __ about the extent to which human __ increases the risk of inherited diseases. Many __ alleles have such severe efects that a homozygous embryo spontaneously aborts long before birth. Still most societies and cultures have laws or taboos forbidding marriages between close relatives. These rules may have evolved out of empirical observation that in most populations, stilllbirths and birth defects are more common when parents are closely related. Social and economic actors have also influenced the development of custons and laws against __ marriages.

most common lethal genetic disease in the US is __, whichs trikes one of every 2500 people of EUro descent, though it is much rarer in other groups. Among people of Euro descent, one of 25 carry the allele. The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluid. These chloride transport channels anre defective or absent in the plasma membrane of children who inherit two recessive alleles for __. The result is an abnormally high concentration of extracellular chloride, which causes the mucus that coats certain cells tobecome thicker and stickier than normal. The mucus builds up in the pancreas, lungs, digestive tract, etc. leading to multiple (__) effects, including poor absorption of nutrients from the intestines, chronic bronchitis, and recurrent bacterial infections.

REcent research indicates that the high concentration of extracellular chloride also contributes to infection by disabling a natural antibiotic made by some body cells. When immune cells come to the rescue, their remains add to the mucus, creating a __.

If untreated, most kids with __ die before 5. But daily doses of antibiotics to prevent infection, gentle pounding on the chest to clear mucus from clogged airways, and other preventive treatments can prolong life.

The most common inherited disorder among people of African descent is __, which affects one of 300 Afr. A. __ is caused by the substitution of a single amin acid in the __ protein of RBCs. When the oxygen content of an affected individual's blood is low, the __ hemoglobin molecules aggregate into long rods that deform the red cells into a sickle shape. These cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including physical weakness, pain, organ damage and even paralysis. The multiple effects of a double does of this allele, are another example of __. Regular blood transfusions can ward off brain damage in children with this disease, and new drugs can help prevent or treat other problems, but there is no cure.

Althoug two sickle-cell alleles are necessary, the presence of one can affect the __.At the organismal level, the normal allele is __ to the sickle-cell allele. Heterozygotes are usually healthy, but may suffer some symptoms during prolonged periods of reduced blood oxygen content. At the molecular level, the two alleles are __; both normal and abnormal hemoglobins are made in heterozygotes.

AAlthough many harmful alleles are recessive, a # of human disorders are due to dominant alleles. One example is __, a form of dwarfism that occurs in one of every 25000 ppl. Heterozygous individuals have the dwarf phenotype. All ppl who aren't __ dwarfs- 99.99%- are __ for the recessive allele. __ is a trait for which the recessive trait is much more prevalent than the corresponding dominant allele.

Dominant alleles that cause a lethal disease are much less common than recessive alleles that do so. All lethal alleles are by __ in cells that produce sperm and eggs; presumably, such __ occur equally often whether the mutant allele is dominant or recessive. However, if a lethal dominant allele causes the death of offspring before they mature and can reproduce, the allele will not be passed on to future generations. In contrast, a lethal recessive allele can be perpetuated from gen. to gen. by heterozygous carriers who have normal phenotypes, since only homozygous recessive offspring will have a lethal disease.

A lethal dominant allele can escape elimination if it causes death only after an individual who carries the allele has reached a relatively advanced age. By the time the symptoms become evident, the individual may have already transmitted the lethal allele to his or her children. For example, __, a degerative disease of the nervous system, is caused by a lethal dominant allele that has no obvious phenotypic effect until the age of 35-45. Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal. Any child born to a parent who has the allele for this disease has a 50% chance of inheriting the allele and the disorder.

BEfore, the symptoms was the only way to determine if __ was inherited. Now, by analyzing DNA samples from a large family with incidence of the disorder, geneticists tracked the __ to locus near the tip of chromosome __, and the gene has now been sequenced. This info led to development of a test tha can detect the presence of the __ in an individual's genome. The availability of this test poses an agonizing dilemma for those with a family history of __.

Many more ppl are susceptible to diseases that have a __ basis- a genetic component plus a significant environmental influence. Heart disease, diabetes, cancer, etc. are many other diseases are __. In many cases, the hereditary component is __. No matter what our genotype, however, our lifestyle has a trememdous effect on phenotype for cardiovascular health and other __ characters. So little is understood about the genetic contributions to most __ diseases that the best public health strategy is educating ppl about the importance of environmental factors and promote healthful behaviors.

True or False
Test can be run to see whether you're at risk of inheriting a disease before you're born.
True False

When we use __laws to predict possible outcomes of matings, it is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings.

BEcause most kids with __ disorders are born to parents with normal phenotypes, the key to accurately assessing the genetic risk for a part. disease is determining whether the prospective parents are __ for the recessive allele. For an increasing number of heritable disorders, tests are available that can distinguish individuals of normal phenotype who are dominant __ from those who are __.

Tests performed in conjunction with a technique known as __ can determine, beginning at the 14-16th week of pregnancy, whether the developing fetus has a certain genetic disease. To perform this procedure, a doctor inserts a needle into the uterus and extracts about 10 mL of amniotic fluid, the liquid bathing the fetus. Some genetic disorders can be detected from the presence of certain chemicals in the amniotic fluid itself. Tests for other disorders, including Tay- Sachs, are performed on cells cultured in the lab, descendants of the fetal cells sloughed off into the amniotic fluid. These cultured cells can be used for karyotyping to identify certain chromosomal defects.

In an alternative technique called __, a physician inserts a narrow tube through the cervix into the uterus and suctions out a tiny sample of tissue from the placenta. The cells of the chorionic villi of the placenta, the portion sampled, are derived from the fetus and have the same genotype as the new individual. These cells are proliferating rapidly enough to allow karyotyping to be carried out immediately. This rapid analysis is an advantage over __, in which the cells must be cultured for several weeks before karyotyping. Another advantage of __is that it can be performed as early as the 8-10th week of pregnancy. However it is not suitable for tests requiring amniotic fluid. Recently, medical scientists have developed methods for isolating fetal cells that have excaped into the mother's blood. Although very few in number, these cells can be cultured and then tested.

Imaging techniques allow a physician to examine a fetus directly for major anatomical abnormalitites. In the __ technique, sound waves are used to produce an image of the fetus by a simple noninvasive procedure. In __, a needle-thin tube containing a viewing scope and fiber optics (to transmit light) is inserted into the uterus.

__ has no known risk to either mother or fetus, while the other procedures can cause complications in a small % of cases. Previously, __ or __ for diagnostic testing was generally offered only to women over age 35, due to risk of bearing kid with Down-Syndrome. In 2007, however, reassessment of the risks and possible benefits led to a change in the recommended practice and now such testing is offered to all pregnant women. If the fetal tests reveal a serious disorder, teh parents face the difficult choice of terminating the pregnancy or preparing to care for a child with a genetic disorder.

Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the US. One common screening program is for __, a recessively inherited disorder that occurs in one of every 10-15000 births in the US.

Children with __ cannot properly metabolize the amino acid __. This compound and its by-product, __, can accumulate to toxic levels in the blood, causing mental retardation. However, if the deificiency is detected in the newborn, a special diet low in __ will usually allow normal development and prevent retardation. Unfortunately, very few other genetic disorders are treatable at the present time.

Screening of newborns and fetuses for serious inherited diseases, tests for identifying carriers, and genetic counseling all rely on the Mendelian model of __. We owe the __- the concept of particulate heritable factors transmitted according to simple rules of chance- to the elegant quantitative experiments of Gregor Mendel. The importance of his discoveries was overlooked by most biologists until early in the 20th century, several decades after his findings were reported.