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STUDENT DIGITAL NEWSLETTER ALAGAPPA INSTITUTIONS |
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Jennifer Breckler PhD
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This combination of characters would have made it difficult for the hatchlings to move about or feed themselves anxiety lump in throat generic 75 mg imipramine amex, and supports the inference that adults of this species of sauropod might have cared for their young anxiety disorder definition generic imipramine 75mg fast delivery. The hands contained three fingers and were held in the same position as human hands-palms inward anxiety symptoms test buy imipramine 75mg free shipping. These theropods used their hands in a clapping motion to seize prey and hold it while they dismembered it with their jaws anxiety symptoms of the heart discount 75 mg imipramine with mastercard. Derived theropod dinosaurs (coelurosaurs) included three general types of animals (Figure 19. The evolutionary history of tyrannosaurs proceeded in three stages starting in the Middle Jurassic. In the first stage, the skull was strengthened, the anterior upper teeth were serrated and the jaw muscles were powerful. The second stage, in the Early Cretaceous, produced a miniature tyrannosaur: Raptorex kriegsteini was only 3 m long (compared with 15 m for Tyrannosaurus rex), but it had nearly all the distinctive features of tyrannosaurs, including a large skull, tiny forelimbs, and long hindlimbs. In the third stage, which extended throughout the rest of the Cretaceous, tyrannosaurs increased in size. The arms were too short to reach the mouth, and the third finger had been lost, yet the arm bones were robust and the fingers were tipped with large claws. The teeth of large tyrannosauroids were as long as 15 cm, dagger shaped with serrated edges, and driven by powerful jaw muscles. The pelvis of a horned dinosaur (Triceratops) found in Montana bears dozens of bite marks from a Tyrannosaurus rex, some as deep as 11. Fossilized feces (coprolites) deposited by a Tyrannosaurus rex in Saskatchewan, Canada, contained fragments of crushed bone from a juvenile ornithischian, indicating that tyrannosaurs could crush bone-a phenomenon unique to derived tyrannosaurs among diapsids. Featherlike structures (filamentous protofeathers) appeared long before birds (the evolution of feathers is discussed in Chapter 21). New discoveries are pushing the origin of protofeathers back, and feathers may even be an ancestral character of Dinosauria. Not all dinosaurs had feathers, however; they are known from only a few ornithischians, and they are not universally present in saurischians. No featherlike structures have been identified in sauropods, but protofeathers and feathers are widespread among theropods, from coelurosaurs (the lineage that gave rise to birds; see Chapter 21) onward. Filaments described as protofeathers are present in Dilong and Yutyrannus, two basal species of tyrannosauroids from the Early Cretaceous, but the skins of Tyrannosaurus rex and several other derived tyrannosaurids from the Late Cretaceous were covered by scales. Tyrannosauroides Early tyrannosauroids were small and lightly built with long arms and legs. Although derived tyrannosaurs were very large, the distinctive features of the lineage appeared in Ornithomimisauria the ornithomimisaurians were lightly built, cursorial (specialized for running) coelurosaurs of the Cretaceous. Bipedality, an ancestral character of theropods, was retained throughout the lineage. Feathers (protofeathers) may also have been ancestral, and some (but probably not all) species in the lineages shown here had feathers, at least on the arms, legs, or tail. Birdlike characters (reduction or complete loss of teeth and development of a beak) appeared independently in ornithomimids, oviraptosaurs, Archaeopteryx, and Aves (birds). Like ostriches, Ornithomimus were probably omnivorous and fed on fruits, insects, small vertebrates, and eggs. Quite possibly they lived in groups, as do ostriches, and their long legs suggest that they inhabited open regions rather than forests. Maniraptora Maniraptorans were active predators with long, slender legs and long arms with grasping fingers (their name means "predators that grasp with hands"). They were feathered- the body had a covering of downy feathers, and long pennaceous feathers were present on the arms, legs, and tail of many species. They were toothless, had horny beaks, and were probably herbivorous or omnivorous. Oviraptosaurids had boxlike skulls, and most species had prominent crests on the head that appear to have been sexually dimorphic (presumably larger in males than in females). These crests were probably brightly colored and identified the species and sex of an individual. Ornithomimus was Sinauer Associates Morales Studio ostrichlike in size, shape, and probably ecology as well. Ornithomimus had a small skull on a long neck, and its toothless jaws were covered with a horny bill. The inner digit was opposable and the wrist was flexible, making the hand an effective organ for capturing small 19. The impressive talon on the second toe led to the suggestion that Deinonychus hunted in packs and attacked large dinosaurs, disemboweling them with their talons. It probably pinned prey down with its claws, as modern falcons and hawks do, using its jaws to kill the prey and flapping its feathered arms for balance as the prey struggled. Deinonychosaur means "terrible claw lizard," and these maniraptorans deserved that name. Ranging in size from less than 1 m to at least 6 m, they were consummate predators, with large eyes and large brains. The claw on the second toe was sicklelike and was held off the ground during locomotion; only the third and fourth toes touched the ground. The arms and hands were long, with three long, claw-tipped fingers, one of which was a semiopposable thumb. Although deinonychosaurs retained grasping hands, they used their feet to subdue prey. Focusing on the sickle-shaped claws, early interpretations proposed that Deinonychus used its claw to slash prey, perhaps hunting in packs and attacking sauropods and ornithopods many times its size. Tests with fossilized claws have shown that they are Pough Vertebrate Life 10E not effective at slicing, however, so the claws probably were Sinauer Associates not used to slash. It seems likely that deinonychosaurs were solitary hunters, attacking prey smaller than themselves. Like falcons, they may have used their body weight to pin prey to the ground while tearing at it with their jaws. Some medium-size mammalian predators are more social; wolves, coyotes, and African hunting dogs live and hunt in family groups, for example, and smaller species of theropods, especially ornithomimids, might have been social. More than 20 juvenile Sinornithomimus dongi ranging from 1 to 7 years old were trapped in mud in a drying pond in western Mongolia, and 160 juvenile and adult individuals of Avimimus appear to have drowned while crossing a river in southern Mongolia. Avimimus was probably herbivorous, making it unlikely that these animals were drawn together by a temporary abundance of food. Nesting and parental care by theropods Recognition of parental care by theropods lagged behind discoveries of nests of ornithischian dinosaurs because of a mistaken identification in 1923. The fossil of a theropod dinosaur that apparently died while attending a nest of eggs was discovered in the Gobi Desert, but its significance was not recognized until 70 years later. The eggs, which were about 12 cm long and 6 cm in diameter, were thought to have been deposited by the small ceratopsian Protoceratops andrewsi because adults of that species were by far the most abundant dinosaurs at the site. The theropod was assumed to have been robbing the nest and was given the name Oviraptor philoceratops, which means "egg seizer, lover of ceratops. Adults of these species hunt individually, and groups form only when prey are concentrated in a small area-for example, Kodiak bears gather during the upstream migrations of salmon. Additional fossils of adult Troodon and Deinonychus, which like Oviraptor were maniraptorans, have subsequently been found sitting on eggs with their legs folded, arms extended, and bellies in contact with the eggs-the same posture that extant ground-nesting birds use when incubating eggs (Figure 19. Male parental care appears to be ancestral for coelurosaurs, and is retained in the most primitive extant birds, paleognaths such as emus and ostriches. Young maniraptorans may have remained with their male parent for extended periods, as do young emus and ostriches. With the benefit of hindsight, it is apparent that the adult dinosaur found in 1923 had been resting on its own nest, apparently trying to shelter its eggs from the sandstorm that buried the adult and the (A) 19. The discovery that many dinosaurs had feathers has added a new dimension to the discussion, because insulation is one of the functions of feathers. With a half-century of hindsight, it has become clear that the relationship between high metabolic rates and high body temperatures that is characteristic of extant vertebrates is not applicable to dinosaurs. Body temperatures and thermoregulatory mechanisms are closely related to body size, and most dinosaurs were much larger than any extant tetrapods. As a result, the link between high body temperatures and high metabolic rates seen among extant vertebrates does not apply to dinosaurs. Gigantothermy is a form of thermoregulation characteristic of large animals that have low metabolic rates but nonetheless maintain body temperatures higher than their surroundings as a result of having low surface/volume ratios.
Hydnocarpus kurzii (Chaulmoogra). Imipramine.
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Pedigree Analysis anxiety in relationships order imipramine 50mg online, Applications anxiety symptoms for hiv imipramine 25 mg without a prescription, and Genetic Testing 139 A number of human metabolic diseases are inherited as autosomal recessive traits anxiety scale cheap 25 mg imipramine free shipping. Gradually anxiety symptoms red blotches order 25 mg imipramine overnight delivery, their physical and neurological conditions worsen, leading to blindness, deafness, and, eventually, death at 2 to 3 years of age. Heterozygotes have only one normal copy of the allele encoding hexosaminidase A and produce only about half the normal amount of the enzyme. Autosomal Dominant Traits Autosomal dominant traits appear in both sexes with equal frequency, and both sexes are capable of transmitting these traits to their offspring. Every person with a dominant trait must have inherited the allele from at least one parent; autosomal dominant traits therefore do not skip generations (Figure 6. Exceptions to this rule arise when people acquire the trait as a result of a new mutation or when the trait has reduced penetrance. If an autosomal dominant allele is rare, most people displaying the trait are heterozygous. When one parent is affected and heterozygous and the other parent is unaffected, approximately 1/2 of the offspring will be affected. If both parents have the trait and are heterozygous, approximately 3 /4 of the children will be affected. Unaffected people do not transmit the trait to their descendants, provided that the trait is fully penetrant. Because homozygotes are more severely affected than heterozygotes, familial hypercholesterolemia is said to be incompletely dominant. However, homozygotes are rarely seen, and the common heterozygous form of the disease appears as a simple dominant trait in most pedigrees. An affected person has an affected parent (unless the person carries new mutations), and the trait does not skip generations. X-Linked Recessive Traits X-linked recessive traits have a distinctive pattern of inheritance (Figure 6. First, these traits appear more frequently in males than in females because males need inherit only a single copy of the allele to display the trait, whereas females must inherit two copies of the allele, one from each parent, to be affected. Second, because a male inherits his X chromosome from his mother, affected males are usually born to unaffected mothers who carry an allele for the trait. Because the trait is passed from unaffected female to affected male to unaffected female, it tends to skip generations (see Figure 6. A trait that is usually considered to be autosomal dominant is familial hypercholesterolemia, an inherited disease in which blood cholesterol is greatly elevated owing to a defect in cholesterol transport. Cholesterol is transported throughout the body in small soluble particles called lipoproteins (Figure 6. All daughters of an affected man, however, will be carriers (if their mother is homozygous for the normal allele). When a woman displays an X-linked recessive trait, she must be homozygous for the trait, and all of her sons also will display the trait. Affected sons are usually born to unaffected mothers who are carriers of the gene for the trait; thus X-linked recessive traits tend to skip generations. I Unaffected female carrier Affected male 5 6 7 8 X-linked recessive traits appear more frequently in males. An example of an X-linked recessive trait in humans is hemophilia A, also called classic hemophilia. The complex process of blood clotting consists of a cascade of reactions that includes more than 13 different factors. For this reason, there are several types of clotting disorders, each due to a glitch in a different step of the clotting pathway. People with hemophilia A bleed excessively; even small cuts and bruises can be life threatening. Spontaneous bleeding occurs in joints such as elbows, knees, and ankles, producing pain, swelling, and erosion of the bone. The inheritance of hemophilia A is illustrated by the family of Queen Victoria of England (Figure 6. X-Linked Dominant Traits X-linked dominant traits appear in males and females, although they often affect more females than males. Each person with an X-linked dominant trait must have an affected parent (unless the person possesses a new mutation or the trait has reduced penetrance). As with X-linked recessive traits, a male inherits an X-linked dominant trait only from his mother; the trait is not passed from father to son. This fact is what distinguishes X-linked dominant inheritance from autosomal dominant inheritance, in which a male can inherit the trait from his father. A female, on the other hand, inherits an X chromosome from both her mother and her father; so females can receive an X-linked trait from either parent. Affected males must have affected mothers (unless the males possess a new mutation), and they pass the trait on to all their daughters. A male affected with an X-linked dominant trait will have what proportion of offspring affected with the trait All sons and no daughters What features of a pedigree would distinguish between a Y-linked trait and a trait that is rare, autosomal dominant, and sex-limited to males An example of an X-linked dominant trait in humans is hypophosphatemia, or familial vitamin-D-resistant rickets. People with this trait have features that superficially resemble those produced by rickets: bone deformities, stiff spines and joints, bowed legs, and mild growth deficiencies. This disorder, however, is resistant to treatment with vitamin D, which normally cures rickets. X-linked hypophosphatemia results from the defective transport of phosphate, especially in cells of the kidneys. People with this disorder excrete large amounts of phosphate in their urine, resulting in low levels of phosphate in the blood and reduced deposition of minerals in the bone. As is common with X-linked dominant traits, males with hypophosphatemia are often more severely affected than females. The major characteristics of autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked traits are summarized in Table 6. The trait appears only in males, and so autosomal dominant and autosomal recessive modes of inheritance are unlikely because traits with these modes appear equally in males and females. Additionally, autosomal dominance can be eliminated because some affected persons do not have an affected parent. The trait is observed only among males in this pedigree, which might suggest Y-linked inheritance. When both parents are heterozygous, approximately one-fourth of the offspring will be affected. Affected offspring must have an affected parent unless they possess a new mutation. When one parent is affected (heterozygous) and the other parent is unaffected, approximately half of the offspring will be affected. Affected sons are usually born to unaffected mothers; thus, the trait skips generations. Both males and females are usually affected; often more females than males are affected. Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father. Affected mothers (if heterozygous) will pass the trait on to half of their sons and half of their daughters. X-linked recessive traits often appear more commonly in males, and affected males are usually born to unaffected female carriers; the pedigree shows this pattern of inheritance. For an X-linked trait, about half the sons of a heterozygous carrier mother should be affected. Another important characteristic of an X-linked recessive trait is that it is not passed from father to son. For additional practice, try to determine the mode of inheritance for the pedigrees in Problem 25 at the end of the chapter. Types of Twins Twins are of two types: dizygotic (nonidentical) twins arise when two separate eggs are fertilized by two different sperm, producing genetically distinct zygotes; monozygotic (identical) twins result when a single egg, fertilized by a single sperm, splits early in development into two separate embryos. Like other siblings, dizygotic twins may be of the same sex or of different sexes. The only difference between dizygotic twins and other siblings is that dizygotic twins are the same age and shared the same uterine environment.
Bacteria with the a+ gene are selected anxiety medication 05 mg generic imipramine 50mg with visa, and the percentage of cells with cotransduced b+ and c+ genes are recorded anxiety early pregnancy imipramine 75mg low price. The phage lysate from the bacterial cells is collected and used to infect a second strain of bacteria that are leu- gal+ pro- venom separation anxiety purchase imipramine 50 mg otc. One strain has a mutant host range anxiety symptoms edu buy discount imipramine 25 mg, is temperature sensitive, and produces clear plaques (genotype is h st c); another strain carries the wild-type alleles (genotype is h+ st+ c+). The genotypes of the progeny phages are given here: Donor leu gal pro + - + Recipient leu- gal+ pro- Which genes are closest, leu and gal or leu and pro To determine whether these mutations occur at the same functional gene, he simultaneously infects E. As a summer project, a microbiology student independently isolates two mutations in E. The student wants to know whether these two mutants are at the same functional unit. Outline a procedure that the student could use to determine whether these two gly- mutations occur within the same functional unit. A group of genetics students mix two auxotrophic strains of bacteria: one is leu+ trp+ his- met- and the other is leu- trp- his+ met+. After mixing the two strains, they plate the bacteria on minimal medium and observe a few prototrophic colonies (leu+ trp+ his+ met+). How can they determine whether the transfer of genes is due to conjugation, transduction, or transformation Their features were so similar, in fact, that he felt that they might easily be mistaken as children of the same family. Down did not understand the cause of their retardation, but his original description faithfully records the physical characteristics of this most common genetic form of mental retardation. As early as the 1930s, geneticists suggested that Down syndrome might be due to a chromosome abnormality, but not until 1959 did researchers firmly establish the cause of Down syndrome: most people with the disorder have three copies of chromosome 21, a condition known as trisomy 21. In a few rare cases, people having the disorder are trisomic for just specific parts of chromosome 21. In spite of this exciting finding, the genetics of Down syndrome appears to be more complex than formerly thought. Mouse breeders have developed several strains of mice that are trisomic for most of the genes found on human chromosome 21 (the equivalent mouse genes are found on mouse chromosome 16). These mice display many of the same anatomical features found in people with Down syndrome, as well as altered behavior, and they are considered an animal model for Down syndrome. This gene appears to be responsible for at least some of the Alzheimer-like features observed in older Downsyndrome people. Taken together, findings from these studies suggest that Down syndrome is not due to a single gene but is instead caused by complex interactions among multiple genes that are affected when an extra copy of chromosome 21 is present. Research on Down syndrome illustrates the principle that chromosome abnormalities often affect many genes that interact in complex ways. Nevertheless, variations in chromosome number-such as the extra chromosome 21 that leads to Down syndrome-do periodically arise. Variations may also arise in chromosome structure: individual chromosomes may lose or gain parts and the order of genes within a chromosome may become altered. These variations in the number and structure of chromosomes are termed chromosome mutations, and they frequently play an important role in evolution. We begin this chapter by briefly reviewing some basic concepts of chromosome structure, which we learned in Chapter 2. We then consider the different types of chromosome mutations, their definitions, features, phenotypic effects, and influence on evolution. M Chromosome Morphology Each functional chromosome has a centromere, to which spindle fibers attach, and two telomeres, which stabilize the chromosome (see Figure 2. The centromere is located approximately in the middle, and so the chromosome has two arms of equal length. The centromere is near one end, producing a long arm and a knob, or satellite, at the other end. The complete set of chromosomes possessed by an organism is called its karyotype and is usually presented as a picture of metaphase chromosomes lined up in descending order of their size (Figure 9. Karyotypes are prepared from actively dividing cells, such as white blood cells, bonemarrow cells, or cells from meristematic tissues of plants. After treatment with a chemical (such as colchicine) that prevents them from entering anaphase, the cells are chemically preserved, spread on a microscope slide, stained, and photographed. The photograph is then enlarged, and the individual chromosomes are cut out and arranged in a karyotype. For human chromosomes, karyotypes are often routinely prepared by automated machines, which scan a slide with a video camera attached to a microscope, looking for chromosome spreads. A karyotype for a male is shown here; a karyotype for a female would have two X chromosomes. Types of Chromosome Mutations Chromosome mutations can be grouped into three basic categories: chromosome rearrangements, aneuploids, and polyploids (Figure 9. Chromosome rearrangements alter the structure of chromosomes; for example, a piece of a chromosome might be duplicated, deleted, or inverted. In aneuploidy, the number of chromosomes is altered: one or more individual chromosomes are added or deleted. Some organisms (such as yeast) possess a single chromosome set (1n) for most of their life cycles and are referred to as haploid, whereas others possess two chromosome sets and are referred to as diploid (2n). A polyploid is any organism that has more than two sets of chromosomes (3n, 4n, 5n, or more). C D E takes a picture of the chromosomes, the image is digitized, and the chromosomes are sorted and arranged electronically by a computer. Preparation and staining techniques help to distinguish among chromosomes of similar size and shape. Duplications, trisomy, and autotriploids are examples of each category of mutation. G A B C D Rearranged chromosome (d) Translocation A D E F G M N O P Q R S B C D E F G (b) Deletion A B C In a chromosome deletion, a segment of the chromosome is deleted. M N O P E F S In a translocation, a segment of a chromosome moves from one chromosome to a nonhomologous chromosome (shown here) or to another place on the same chromosome (not shown). The four basic types of rearrangements are duplications, deletions, inversions, and translocations (Figure 9. The pairing and synapsis of homologous regions require that one or (a) Normal chromosome A B C D E F G Duplications A chromosome duplication is a mutation in which part of the chromosome has been doubled (see Figure 9. This type of duplication, in which the duplicated region is immediately adjacent to the original segment, is called a tandem duplication. If the duplicated segment is located some distance from the original segment, either on the same chromosome or on a different one, the chromosome rearrangement is called a displaced duplication. F E F G Alignment in prophase I of meiosis (b) A B C D E F G A B C D E F G E F Effects of chromosome duplications An individual homozygous for a duplication carries the duplication on both homologous chromosomes, and an individual heterozygous for a duplication has one normal chromosome and one chromosome with the duplication. Chromosome Variation 243 both chromosomes loop and twist so that these regions are able to line up (Figure 9. The appearance of this characteristic loop structure in meiosis is one way to detect duplications. Among fruit flies (Drosophila melanogaster), for example, a fly having a Bar mutation has a reduced number of facets in the eye, making the eye smaller and bar shaped instead of oval (Figure 9. The Bar mutation results from a small duplication on the X chromosome that is inherited as an incompletely dominant, X-linked trait: heterozygous female flies have somewhat smaller eyes (the number of facets is reduced; see Figure 9. Occasionally, a fly carries three copies of the Bar duplication on its X chromosome; for flies carrying such mutations, which are termed double Bar, the number of facets is extremely reduced (see Figure 9. The Bar mutation arises from unequal crossing over, a duplication-generating process (Figure 9. After all, gene sequences are not altered by duplications, and no genetic information is missing; the only change is the presence of additional copies of normal sequences.
Diseases
On the other hand anxiety herbs buy imipramine 25mg without a prescription, the study of human genetic characteristics presents some major obstacles anxiety service dog order 75 mg imipramine with mastercard. With other organisms anxiety or heart problem cheap imipramine 25 mg, geneticists carry out specific crosses to test their 136 hypotheses about inheritance anxiety 4 hereford bull buy 75mg imipramine mastercard. We have seen, for example, how the testcross provides a convenient way to determine whether an individual organism having a dominant trait is homozygous or heterozygous. Unfortunately (for the geneticist at least), matings between humans are usually determined by romance, family expectations, or-occasionally-accident rather than by the requirements of a geneticist. Human reproductive age is not normally reached until 10 to 14 years after birth, and most people do not reproduce until they are 18 years of age or older; thus, generation time in humans is usually about 20 years. This long generation time means that, even if geneticists could control human crosses, they would have to wait on average 40 years just to observe the F2 progeny. Observation of even the simple genetic ratios that we learned in Chapter 3 would require a substantial number of progeny in each family. When parents produce only 2 children, the detection of a 3: 1 ratio is impossible. Even an extremely large family of 10 to 15 children would not permit the recognition of a dihybrid 9: 3: 3: 1 ratio. Although these special constraints make genetic studies of humans more complex, understanding human heredity is tremendously important. So geneticists have been forced to develop techniques that are uniquely suited to human biology and culture. Pedigree Analysis, Applications, and Genetic Testing 137 Sex unknown Male Female or unspecified Unaffected person 6. A pedigree is a pictorial representation of a family history, essentially a family tree that outlines the inheritance of one or more characteristics. When a particular characteristic or disease is observed in a person, a geneticist often studies the family of this affected person by drawing a pedigree. Person affected with trait Obligate carrier (carries the gene but does not have the trait) Asymptomatic carrier (unaffected at this time but may later exhibit trait) Multiple persons (5) 5 5 5 Symbols Used in Pedigrees the symbols commonly used in pedigrees are summarized in Figure 6. A horizontal line drawn between two symbols representing a man and a woman indicates a mating; children are connected to their parents by vertical lines extending downward from the parents. Persons who exhibit the trait of interest are represented by filled circles and squares; in the pedigree of Figure 6. Each generation in a pedigree is identified by a Roman numeral; within each generation, family members are assigned Arabic numerals, and children in each family are listed in birth order from left to right. The limited number of offspring in most human families means that clear Mendelian ratios in a single pedigree are usually impossible to discern. Pedigree analysis requires a certain amount of genetic sleuthing, based on recognizing patterns associated with different modes of inheritance. For example, autosomal dominant traits should appear with equal frequency in both sexes and should not skip generations, provided that the trait is fully penetrant (see p. If we observe that a trait 138 Chapter 6 Each generation in a pedigree is indentified by a Roman numeral. In the following sections, the traits discussed are assumed to be fully penetrant and rare. Autosomal Recessive Traits Autosomal recessive traits normally appear with equal frequency in both sexes (unless penetrance differs in males and females) and appear only when a person inherits two alleles for the trait, one from each parent. If the trait is uncommon, most parents of affected offspring are heterozygous and unaffected; consequently, the trait seems to skip generations (Figure 6. Frequently, a recessive allele may be passed for a number of generations without the trait appearing in a pedigree. Whenever both parents are heterozygous, approximately one-fourth of the offspring are expected to express Autosomal recessive traits usually appear equally in males and females. In the rare event that both parents are affected by an autosomal recessive trait, all the offspring will be affected. When a recessive trait is rare, persons from outside the family are usually homozygous for the normal allele. A recessive trait is more likely to appear in a pedigree when two people within the same family mate, because there is a greater chance of both parents carrying the same recessive allele. Affected children are commonly born to unaffected parents who are carriers of the gene for the trait, and the trait tends to skip generations. Recessive traits appear more frequently among the offspring of consanguine matings. Dizygotic twinning often runs in families and the tendancy to produce dizygotic twins is influenced by heredity, but there appears to be little genetic tendency for producing monozygotic twins. If both members of a twin pair have a trait, the twins are said to be concordant; if only one member of the pair has the trait, the twins are said to be discordant. Because identical twins have 100% of their genes in common and dizygotic twins have on average only 50% in common, genetically influenced traits should exhibit higher concordance in monozygotic twins. However, when a dizygotic twin has epilepsy, the other twin has epilepsy only 19% of the time (19% dizygotic concordance). The higher concordance in the monozygotic twins suggests that genes influence epilepsy, a finding supported by the results of other family studies of this disease. Concordance values for several additional human traits and diseases are listed in Table 6. The hallmark of a genetic influence on a particular trait is higher concordance in monozygotic twins compared with concordance in dizygotic twins. High concordance in monozygotic twins by itself does not signal a genetic influence. Twins normally share the same environment-they are raised in the same home, have the same friends, attend the same school-and so high concordance may be due to common genes or to common environment. If the high concordance is due to environmental factors, then dizygotic twins, who also share the same environment, should have just as high a concordance as that of monozygotic twins. When genes influence the trait, however, monozygotic twin pairs should exhibit higher concordance than that of dizygotic twin pairs, because monozygotic twins have a greater percentage of genes in common. Monozygotic twins develop from a single egg, fertilized by a single sperm, that splits into two embryos; they have 100% percent of their genes in common. Multiple sclerosis Why are monozygotic twins genetically identical, whereas dizygotic twins have only 1/2 of their genes in common on average Monozygotic twins develop from a single embryo, whereas dizygotic twins develop from two embryos. Concordance in Twins Comparisons of dizygotic and monozygotic twins can be used to assess the importance of genetic and environmental factors in producing differences in a characteristic. Pedigree Analysis, Applications, and Genetic Testing 145 any discordance among monozygotic twins is usually due to environmental factors, because monozygotic twins are genetically identical. The use of twins in genetic research rests on the important assumption that, when concordance for monozygotic twins is greater than that for dizygotic twins, it is because monozygotic twins are more similar in their genes and not because they have experienced a more similar environment. The degree of environmental similarity between monozygotic twins and dizygotic twins is assumed to be the same. Because they look alike, identical twins may be treated more similarly by parents, teachers, and peers than are nonidentical twins. Evidence of this similar treatment is seen in the past tendency of parents to dress identical twins alike. In spite of this potential complication, twin studies have played a pivotal role in the study of human genetics. Asthma is characterized by constriction of the airways and the secretion of mucus into the air passages, causing coughing, labored breathing, and wheezing (Figure 6. Asthma is a major health problem in industrialized countries and appears to be on the rise. The incidence of childhood asthma varies widely; the highest rates (from 17% to 30%) are in the United Kingdom, Australia, and New Zealand. A number of environmental stimuli are known to precipitate asthma attacks, including dust, pollen, air pollu- tion, respiratory infections, exercise, cold air, and emotional stress. Allergies frequently accompany asthma, suggesting that asthma is a disorder of the immune system, but the precise relation between immune function and asthma is poorly understood.
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