A&P II: Heredity (Week 14)

A cell from a person with trisomy 21 (Down Syndrome) contains __________.

A cell from a person with trisomy 21 (Down Syndrome) contains a nondisjunction. A nondisjunction is an abnormal segregation of chromosomes during meiosis, resulting in gametes receiving two or no copies of a particular parental chromosome. It is more common in female meiosis than in male meiosis. If the abnormal gamete participates in fertilization, the resulting zygote will have an abnormal chromosomal complement (monosomy or trisomy) for that particular chromosome (as in Down syndrome).

Which of the following processes may separate linked genes during meiosis?

A chiasma, or crossover, may separate linked genes during meiosis. In the hypothetical example shown in the figure, the genes for hair and eye color are linked. The paternal chromosome contains alleles coding for blond hair and blue eyes, and the maternal alleles code for brown hair and brown eyes. In the crossover, or chiasma, shown, the break occurs between these linked genes, resulting in one gamete with alleles for blond hair and brown eyes and another with alleles for brown hair and blue eyes. As a result of the crossover, two of the four chromatids present in the tetrad end up with a mixed set of alleles—some maternal and some paternal. This means that when the chromatids segregate, each gamete will receive a unique combination of parental genes.

What type of allele will be expressed if both dominant and recessive alleles are present for a given trait?

A dominant type of allele will be expressed if both dominant and recessive alleles are present for a given trait. Sometimes, one allele masks or suppresses the expression of its partner. Such an allele is said to be dominant, whereas the allele that is masked is said to be recessive. By convention, a dominant allele is represented by a capital letter (for example, J), and a recessive allele by the lowercase form of the same letter (j). Dominant alleles are expressed, or make themselves "known," when they are present in either single or double dose. For recessive alleles to be expressed, they must be present in double dose, that is, the homozygous condition. Returning to our thumb example, a person whose genetic makeup includes either the gene pair JJ (the homozygous dominant condition) or the gene pair Jj (the heterozygous condition) will have double jointed thumbs. The combination jj (the homozygous recessive condition) is needed to produce tight thumb ligaments.

A karyotype is a complete __________.

A karyotype is a complete display of homologous chromosome pairs. The complete human karyotype (kar′e-o-tīp), or diploid chromosomal complement displayed in homologous pairs, is illustrated in the figure below. The diploid genome (je′nōm), or genetic (DNA) makeup, represents two sets of genetic instructions—one from the egg and the other from the sperm.

A person who inherits the B and the O blood type alleles will possess which blood type?

A person who inherits the B and the O blood type alleles will possess B type blood. Although we inherit only two alleles for each gene, some genes exhibit more than two allele forms, leading to a phenomenon called multiple-allele inheritance. For example, three alleles determine the ABO blood types in humans: IA, IB, and i. Each of us receives two of these. The IA and IB alleles are codominant, and both are expressed when present, resulting in the AB blood type. The i allele is recessive to the other two. Genotypes determining the four possible ABO blood groups are shown in the table.

A person without a Y chromosome will __________.

A person without a Y chromosome will always show female characteristics. Inherited traits determined by genes on the sex chromosomes are said to be sex-linked. The X and Y sex chromosomes are not homologous in the true sense. The Y, which contains the gene that determines maleness, is much smaller than the X chromosome. The X bears over 1400 genes, and a disproportionately large number of them code for proteins important to brain function. Because the Y carries only about 200 genes, it lacks many of the genes present on the X. For example, genes coding for certain clotting factors, cone pigments, and even testosterone receptors are present on X but not on Y. A gene found only on the X chromosome is said to be X-linked. When a male inherits an X-linked recessive allele—for example, for hemophilia or for red-green color blindness—its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

Match the genetic term with its correct characteristic: Genotype.

A person's actual genetic makeup is referred to as his or her genotype (jēn′o-tīp). The way that genotype is expressed in the body is called one's phenotype (fe′no-tīp). For example, the double-jointed condition is the phenotype produced by a genotype of JJ or Jj.

Match the genetic term with its correct characteristic: Phenotype.

A phenotype is the way the genotype is expressed (for example, skin color). A person's genetic makeup is referred to as his or her genotype (jēn′o-tīp). The way that genotype is expressed in the body is called one's phenotype (fe′no-tīp). For example, the double-jointed condition is the phenotype produced by a genotype of JJ or Jj.

A sperm cell is produced by __________.

A sperm cell is produced by meiosis. Each pair of replicated homologous chromosomes synapses during meiosis I, forming a tetrad. This happens during both spermatogenesis and oogenesis. Because chance determines how the tetrads align (line up) on the meiosis I metaphase spindle, maternal and paternal chromosomes are randomly distributed to daughter nuclei.

In a recessive disorder like cystic fibrosis, if the mother is heterozygous and the father is heterozygous, what is the likely percentage of offspring that will be affected?

After completing a Punnet square if the mother is heterozygous and the father is heterozygous, 25% will be affected. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. In the example shown, both parents have normal pigmentation but carry the recessive allele for albinism (a), the lack of the pigment melanin. This means each parent is heterozygous for this trait and has the genotype Aa. The alleles of one parent are written along one side of the Punnett square, and the alleles for the other parent are shown along an adjacent side. The alleles are then combined down and across to determine the possible gene combinations (genotypes) and their expected frequency in the offspring of these two parents. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism.

In a sex-linked disorder like hemophilia, if the mother is heterozygous and the father is affected, what is the likely percentage of offspring that will be affected?

After completing a Punnet square, if the mother is heterozygous and the father is affected, the likely percentage of offspring that will be affected is 25% affected female, 25% normal female carrier, 25% normal male, 25% affected male. In the example shown, if a female heterozygous for red-green color blindness and a male with normal color vision have offspring, there is a 50% chance that their sons will be color blind and a 50% chance that their daughters will be carriers. The N allele determines normal color vision and the n allele is recessive. Two examples of X-linked conditions are hemophilia and red-green color blindness. When a male inherits an X-linked recessive allele for one of these conditions, its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though it is present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

In a dominant disorder like Huntington's, if the mother is heterozygous and the father is heterozygous, what is the likely percentage of offspring that will be affected?

After completing a Punnet square, if the mother is heterozygous and the father is heterozygous, 75% of the offspring will be affected. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. In the example shown, both parents have normal pigmentation but carry the recessive allele for albinism (a), the lack of the pigment melanin. This means each parent is heterozygous for this trait and has the genotype Aa. The alleles of one parent are written along one side of the Punnett square, and the alleles for the other parent are shown along an adjacent side. The alleles are then combined down and across to determine the possible gene combinations (genotypes) and their expected frequency in the offspring of these two parents. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism.

In a sex-linked disorder like hemophilia, if the mother is heterozygous and the father is normal, what is the likely percentage of offspring that will be affected?

After completing a Punnet square, if the mother is heterozygous and the father is normal, the likely percentage of offspring that will be affected is 25% normal non-carrier female, 25% normal female carrier, 25% normal male, 25% affected male. In the example shown, if a female heterozygous for red-green color blindness and a male with normal color vision have offspring, there is a 50% chance that their sons will be color blind and a 50% chance that their daughters will be carriers. The N allele determines normal color vision and the n allele is recessive. Two examples of X-linked conditions are hemophilia and red-green color blindness. When a male inherits an X-linked recessive allele for one of these conditions, its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though it is present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

In a sex-linked disorder like hemophilia, if the mother is homozygous dominant and the father is affected, what is the likely percentage of offspring that will be affected?

After completing a Punnet square, if the mother is homozygous dominant and the father is affected, the likely percentage of offspring that will be affected is 50% normal female carrier and 50% normal males. In the example shown, if a female heterozygous for red-green color blindness and a male with normal color vision have offspring, there is a 50% chance that their sons will be color blind and a 50% chance that their daughters will be carriers. The N allele determines normal color vision and the n allele is recessive. Two examples of X-linked conditions are hemophilia and red-green color blindness. When a male inherits an X-linked recessive allele for one of these conditions, its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though it is present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

In a recessive disorder like cystic fibrosis, if the mother is homozygous dominant and the father is heterozygous, what is the likely percentage of offspring that will be affected?

After completing a Punnet square, if the mother is homozygous dominant and the father is heterozygous, 0% of the offspring will be affected. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. In the example shown, both parents have normal pigmentation but carry the recessive allele for albinism (a), the lack of the pigment melanin. This means each parent is heterozygous for this trait and has the genotype Aa. The alleles of one parent are written along one side of the Punnett square, and the alleles for the other parent are shown along an adjacent side. The alleles are then combined down and across to determine the possible gene combinations (genotypes) and their expected frequency in the offspring of these two parents. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism.

In a recessive disorder like cystic fibrosis, if the mother is homozygous dominant and the father is homozygous dominant, what is the likely percentage of offspring that will be affected?

After completing a Punnet square, if the mother is homozygous dominant and the father is homozygous dominant, 0% of the offspring will be affected. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. In the example shown, both parents have normal pigmentation but carry the recessive allele for albinism (a), the lack of the pigment melanin. This means each parent is heterozygous for this trait and has the genotype Aa. The alleles of one parent are written along one side of the Punnett square, and the alleles for the other parent are shown along an adjacent side. The alleles are then combined down and across to determine the possible gene combinations (genotypes) and their expected frequency in the offspring of these two parents. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism.

What is the chance the offspring will possess these traits or diseases: albinism, if both parents are heterozygous for the albino gene?

After completing a Punnet square, one will find that if both parents are heterozygous for the albino gene, there is a 25% chance that a child will have a recessive trait. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism. The Punnett square predicts only the probability of a particular genotype (and phenotype). The larger the number of offspring, the greater the likelihood that the ratios will conform to the predicted values—just as the chances of getting heads half the time and tails half the time increase with the number of tosses of a coin. If we toss only twice, we may well get heads both times. Likewise, if the couple in our example had only two children, it would not be surprising if both children had the genotype Aa.

What is the probability of having a child with a recessive trait if both parents are heterozygous for the trait?

After completing a Punnet square, one will find that if both parents are heterozygous there is a 25% chance that a child will have a recessive trait. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism. The Punnett square predicts only the probability of a particular genotype (and phenotype). The larger the number of offspring, the greater the likelihood that the ratios will conform to the predicted values—just as the chances of getting heads half the time and tails half the time increase with the number of tosses of a coin. If we toss only twice, we may well get heads both times. Likewise, if the couple in our example had only two children, it would not be surprising if both children had the genotype Aa.

In a dominant disorder like Huntington's, if the mother is homozygous recessive and the father is homozygous recessive, what is the likely percentage of offspring that will be affected?

After completing a Punnett square, if the mother is homozygous recessive and the father is homozygous recessive 0% of the offspring will be affected. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes (Figure 29.4). In the example shown, both parents can roll their tongue into a U because both are heterozygous for the dominant allele (T) that confers this ability. In other words, each parent has the genotype Tt. The alleles of one parent are written along one side of the Punnett square, and the alleles for the other parent are shown along an adjacent side. The alleles are then combined down and across to determine the possible gene combinations (genotypes) and their expected frequency in the offspring of these two parents. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (TT) is 25% (1 out of 4); of producing a heterozygous child (Tt), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The TT and Tt offspring will be tongue rollers. Only the tt offspring will not be able to roll their tongues. The Punnett square predicts only the probability of a particular genotype (and phenotype). The larger the number of offspring, the greater the likelihood that the ratios will conform to the predicted values—just as the chances of getting heads half the time and tails half the time increase with the number of tosses of a coin. If we toss only twice, we may well get heads both times. Likewise, if the couple in our example had only two children, it would not be surprising if both children had the genotype Tt.

In a recessive disorder like Cystic Fibrosis, if the mother is homozygous recessive and the father is homozygous recessive, what is the likely percentage of offspring that will be affected?

After completing a Punnett square, if the mother is homozygous recessive and the father is homozygous recessive, 100% of the offspring will be affected. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes (Figure 29.4). In the example shown, both parents can roll their tongue into a U because both are heterozygous for the dominant allele (T) that confers this ability. In other words, each parent has the genotype Tt. The alleles of one parent are written along one side of the Punnett square, and the alleles for the other parent are shown along an adjacent side. The alleles are then combined down and across to determine the possible gene combinations (genotypes) and their expected frequency in the offspring of these two parents. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (TT) is 25% (1 out of 4); of producing a heterozygous child (Tt), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The TT and Tt offspring will be tongue rollers. Only the tt offspring will not be able to roll their tongues. The Punnett square predicts only the probability of a particular genotype (and phenotype). The larger the number of offspring, the greater the likelihood that the ratios will conform to the predicted values—just as the chances of getting heads half the time and tails half the time increase with the number of tosses of a coin. If we toss only twice, we may well get heads both times. Likewise, if the couple in our example had only two children, it would not be surprising if both children had the genotype Tt.

Match the genetic term with its correct characteristic: Allele.

Alleles represent an alternative gene form. Because chromosomes are paired, it follows that the genes in them are paired as well. Consequently, each of us receives two genes, one from each parent (for the most part), that interact to dictate each trait. Matched genes, which are at the same locus (location) on homologous chromosomes, are called alleles (ah-lēlz′) of each other. Alleles may code for the same or for alternative forms of a given trait. For example, one allele might code for tight thumb ligaments and the other for loose ligaments (the double-jointed thumb condition). When the two alleles controlling a trait are the same, a person is said to be homozygous (ho-mo-zi′gus) for that gene. When the two alleles are different, the individual is heterozygous (het″er-o-zi′gus) for the gene.

Alternative forms of genes are called __________.

Alternative forms of genes are called alleles. Because chromosomes are paired, it follows that the genes in them are paired as well. Consequently, each of us receives two genes, one from each parent (for the most part), that interact to dictate each trait. Matched genes, which are at the same locus (location) on homologous chromosomes, are called alleles (ah-lēlz′) of each other. Alleles may code for the same or for alternative forms of a given trait. For example, one allele might code for tight thumb ligaments and the other for loose ligaments (the double-jointed thumb condition). When the two alleles controlling a trait are the same, a person is said to be homozygous (ho-mo-zi′gus) for that gene. When the two alleles are different, the individual is heterozygous (het″er-o-zi′gus) for the gene.

Amniocentesis and chorionic villus sampling are both examples of a category of genetic screening called __________.

Amniocentesis and chorionic villus sampling are both examples of a category of genetic screening called fetal testing. Fetal testing is used when there is a known risk of a genetic disorder. The most common type of fetal testing is amniocentesis (am″ne-o-sen-te′sis), in which a wide-bore needle is inserted into the amniotic sac through the mother's abdominal wall, and about 10 ml of fluid is withdrawn. Because there is a chance of injuring the fetus before ample amniotic fluid is present, this procedure is not normally done before the 14th week of pregnancy. Using ultrasound to visualize the position of the fetus and the amniotic sac has dramatically reduced the risk of this procedure. The fluid is checked for enzymes and other chemicals that serve as markers for specific diseases, but most tests are done on the sloughed-off fetal cells in the fluid. These cells are cultured in laboratory dishes over a period of several weeks. Then the cells are examined for DNA markers of genetic disease and karyotyped to check for chromosomal abnormalities. Chorionic villus sampling (CVS) suctions off bits of the chorionic villi from the placenta for examination. A small tube is inserted through the vagina and cervical canal and guided by ultrasound to an area where a piece of placental tissue can be removed. CVS allows testing at 8 weeks but waiting until after the 10th week is usually recommended. Karyotyping can be done almost immediately on the rapidly dividing chorionic cells, much earlier than in amniocentesis. Both of these procedures are invasive, and they carry with them an inherent risk to both fetus and mother. (For example, increased fetal risk of finger and toe defects is linked to CVS.) These tests are routinely ordered for pregnant women over 35 (because of the higher risk of Down syndrome), but they are performed on younger women when the probability of finding a severe fetal disorder is greater than the probability of doing harm during the procedure. Advances in DNA sequencing are about to make both of these invasive procedures obsolete. An easily obtained maternal blood sample contains free-floating maternal and fetal DNA released from the placenta. Maternal blood samples are currently being used in the clinic to test for fetal chromosomal abnormalities such as Down syndrome. The same techniques can provide detailed information about the fetus's entire genome, including mutations that increase the likelihood of certain diseases. The technology already exists and the potential for its widespread use is just around the corner.

An allele that is able to mask the expression of its partner allele is said to be __________.

An allele that is able to mask the expression of its partner allele is said to be dominant. Sometimes, one allele masks or suppresses the expression of its partner. Such an allele is said to be dominant, whereas the allele that is masked is said to be recessive. By convention, a dominant allele is represented by a capital letter (for example, J), and a recessive allele by the lowercase form of the same letter (j). Dominant alleles are expressed, or make themselves "known," when they are present in either single or double dose. For recessive alleles to be expressed, they must be present in double dose, that is, the homozygous condition. Returning to our thumb example, a person whose genetic makeup includes either the gene pair JJ (the homozygous dominant condition) or the gene pair Jj (the heterozygous condition) will have double jointed thumbs. The combination jj (the homozygous recessive condition) is needed to produce tight thumb ligaments.

Any two matched genes that are __________ are called alleles.

Any two matched genes that are at the same locus on homologous chromosomes are called alleles. Because chromosomes are paired, it follows that the genes in them are paired as well. Consequently, each of us receives two genes, one from each parent (for the most part), that interact to dictate each trait. Matched genes, which are at the same locus (location) on homologous chromosomes, are called alleles (ah-lēlz′) of each other. Alleles may code for the same or for alternative forms of a given trait. For example, one allele might code for tight thumb ligaments and the other for loose ligaments (the double-jointed thumb condition). When the two alleles controlling a trait are the same, a person is said to be homozygous (ho-mo-zi′gus) for that gene. When the two alleles are different, the individual is heterozygous (het″er-o-zi′gus) for the gene.

A method that is likely to make invasive fetal testing methods obsolete is ______________.

DNA sequencing is a method that will make invasive fetal testing methods obsolete. Advances in DNA sequencing are about to make invasive fetal testing procedures obsolete. An easily obtained maternal blood sample contains free-floating maternal and fetal DNA released from the placenta. Maternal blood samples are currently being used in the clinic to test for fetal chromosomal abnormalities such as Down syndrome. The same techniques can provide detailed information about the fetus's entire genome, including mutations that increase the likelihood of certain diseases. The technology already exists and is in increasingly wide use. Chorionic villus sampling, as wel as amniocentesis, are invasive fetal testing methods that have the potential to harm the fetus and that may be made obsolete by DNA sequencing.

Match the mode of genetic expression with its correct characteristic: Dominant-recessive inheritance.

Dominant-recessive inheritance is a type of genetic expression where one allele can mask the other allele. Dominant-recessive inheritance reflects the interaction of dominant and recessive alleles. A simple diagram, called the Punnett square, is used to figure out, for a single trait, the possible gene combinations that would result from the mating of parents of known genotypes. As the completed Punnett square shows, the probability of these parents producing a homozygous dominant child (AA) is 25% (1 out of 4); of producing a heterozygous child (Aa), 50% (2 out of 4); and of producing a homozygous recessive child, 25% (1 out of 4). The AA and Aa offspring will have normal pigmentation. Only the aa offspring will have albinism.

For most genes, both the maternal and paternal alleles are expressed __________.

For most genes, both the maternal and paternal alleles are expressed at the same time. Althought this is true for most genes, for about 300 genes, this isn't the case and only one of the two parental alleles is expressed in every body cell. The addition of a methyl (—CH3) group during gamete formation silences one of the alleles and marks it as coming from the mother or the father. This process is called genomic imprinting. During gametogenesis, the "old" imprinting tags are stripped away and a new set of tags are added to each gene, marking it as maternal (in ova) or paternal (in sperm).

Genes that are located on the same chromosome are said to be __________.

Genes that are located on the same chromosome are said to be linked. One aspect of gamete variation results from the crossing over and exchange of chromosomal parts during meiosis I. Genes are arranged linearly along a chromosome's length, and genes on the same chromosome are said to be linked because they are transmitted as a unit to daughter cells during mitosis. However, chromosomes can break and precisely exchange gene segments with their homologous counterparts during meiosis. This exchange gives rise to recombinant chromosomes that have mixed contributions from each parent.

Genetic variation is not enhanced through __________.

Genetic variation is not enhanced through mitosis. Each pair of replicated homologous chromosomes synapses during meiosis I, forming a tetrad. This happens during both spermatogenesis and oogenesis. Because chance determines how the tetrads align (line up) on the meiosis I metaphase spindle, maternal and paternal chromosomes are randomly distributed to daughter nuclei. As illustrated in the figure below, this simple event leads to an amazing amount of variation in gametes. One aspect of gamete variation results from the crossing over and exchange of chromosomal parts during meiosis I. Genes are arranged linearly along a chromosome's length, and genes on the same chromosome are said to be linked because they are transmitted as a unit to daughter cells during mitosis. At any point in time, gametogenesis is turning out gametes with all variations possible from independent assortment and random crossovers. Fertilization compounds this variety because a single human egg will be fertilized by a single sperm on a totally haphazard, or random, basis. If we consider variation resulting only from independent assortment and random fertilization, any offspring represents one out of the close to 72 trillion (8.5 million × 8.5 million) zygotes possible.

Match the following type of inheritance with its correct phenotype: Polygene inheritance.

Height is governed by a type of inheritance known as polygene inheritance. Most phenotypes depend on several gene pairs at different locations acting in tandem. Such polygene inheritance results in continuous, or quantitative, phenotypic variation between two extremes and explains many human characteristics. Examples of polygene traits in humans include skin color, height, metabolic rate, and intelligence. Skin color, for instance, is controlled by at least three separately inherited genes, each existing in two allelic forms: A, a; B, b; C, c. The A, B, and C alleles confer dark skin pigment, and their effects are additive. The a, b, and c alleles confer pale skin tone. An individual with an AABBCC genotype would be about as dark-skinned as a human can get, while an aabbcc person would be very fair. However, when individuals heterozygous for at least one of these gene pairs mate, a broad range of pigmentation is possible in their offspring. Such polygene inheritance results in a distribution of genotypes and phenotypes that, when plotted, yields a bell-shaped curve.

Match the type of inheritance with the correct phenotype: Sex-linked.

Hemophilia is a sex-linked disorder. When a male inherits an X-linked recessive allele—for example, for hemophilia or for red-green color blindness—its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

Heterozygous individuals that can pass on recessive, abnormal conditions even if they do not express the disease are referred to as __________.

Heterozygous individuals that can pass on recessive, abnormal conditions even if they do not express the disease are referred to as carriers. Recessive genetic disorders are more frequent than disorders caused by dominant alleles because those who carry a single recessive allele for a recessive genetic disorder do not themselves express the disease. However, they can pass the gene on to offspring and so are called carriers of the disorder.

Humans have __________ pairs of chromosomes.

Humans have 23 pairs of chromosomes. The nuclei of all human cells except gametes contain the diploid number of chromosomes (46), consisting of 23 pairs of homologous chromosomes. Recall that homologous chromosomes are pairs of chromosomes—one from the father (sperm) and one from the mother (egg)—that look similar and carry genes for the same traits, but do not necessarily bring about the same expressions of those traits. Two of the 46 chromosomes are sex chromosomes (X and Y), which determine genetic sex (male = XY; female = XX). The other 44 are the 22 pairs of autosomes that guide the expression of most other traits.

If a male inherits a sex-linked gene for color blindness __________.

If a male inherits a sex-linked gene for color blindness it will always be expressed. Inherited traits determined by genes on the sex chromosomes are said to be sex-linked. The X and Y sex chromosomes are not homologous in the true sense. The Y, which contains the gene that determines maleness, is much smaller than the X chromosome. The X bears over 1400 genes, and a disproportionately large number of them code for proteins important to brain function. Because the Y carries only about 200 genes, it lacks many of the genes present on the X. For example, genes coding for certain clotting factors, cone pigments, and even testosterone receptors are present on X but not on Y. A gene found only on the X chromosome is said to be X-linked. When a male inherits an X-linked recessive allele—for example, for hemophilia or for red-green color blindness—its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

If the allele for brown hair is represented as "B," it would mean that __________.

If the allele for brown hair is represented as "B," it would mean that brown hair is a dominant trait. Sometimes, one allele masks or suppresses the expression of its partner. Such an allele is said to be dominant, whereas the allele that is masked is said to be recessive. By convention, a dominant allele is represented by a capital letter (for example, J), and a recessive allele by the lowercase form of the same letter (j). Dominant alleles are expressed, or make themselves "known," when they are present in either single or double dose. For recessive alleles to be expressed, they must be present in double dose, that is, the homozygous condition. Returning to our thumb example, a person whose genetic makeup includes either the gene pair JJ (the homozygous dominant condition) or the gene pair Jj (the heterozygous condition) will have double jointed thumbs. The combination jj (the homozygous recessive condition) is needed to produce tight thumb ligaments.

What will the offspring's blood type be if the mother is type B, and father is type AB?

If the mother is type B, and father is type AB, the offspring's blood type will be type A, type B, or type AB. Although we inherit only two alleles for each gene, some genes exhibit more than two allele forms, leading to a phenomenon called multiple-allele inheritance. For example, three alleles determine the ABO blood types in humans: IA, IB, and i. Each of us receives two of these. The IA and IB alleles are codominant, and both are expressed when present, resulting in the AB blood type. The i allele is recessive to the other two. Genotypes determining the four possible ABO blood groups are shown in the table.

What will the offspring's blood type be if the mother is type O, and father is type O?

If the mother is type O, and father is type O, the offspring's blood type will be type O only. Although we inherit only two alleles for each gene, some genes exhibit more than two allele forms, leading to a phenomenon called multiple-allele inheritance. For example, three alleles determine the ABO blood types in humans: IA, IB, and i. Each of us receives two of these. The IA and IB alleles are codominant, and both are expressed when present, resulting in the AB blood type. The i allele is recessive to the other two. Genotypes determining the four possible ABO blood groups are shown in the table.

In females, one of the X chromosomes is inactivated by __________.

In females, one of the X chromosomes is inactivated by epigenetic marks. Epigenetic marks (epi = "over, above") form the third layer of gene controls. This continually changing information is stored in the proteins and chemical groups that bind to the DNA and in the way chromatin is packaged in the cell, is continually changing. Within cells, chemical tags such as methyl and acetyl groups bound to DNA segments and to histones determine whether the DNA is available for transcription (acetylation) or silenced (methylation). Epigenetic marks also account for the inactivation (by methylation) of one of the female's X chromosomes in the early embryo.

Match the mode of genetic expression with the correct characteristic: Incomplete dominance.

In incomplete dominance, a heterozygote has a phenotype that is intermediate between the homozygous dominant and recessive phenotypes. In dominant-recessive inheritance, one allele variant completely masks the other. Some traits, however, exhibit incomplete dominance. In such instances, the heterozygote has a phenotype intermediate between those of homozygous dominant and homozygous recessive individuals. Perhaps the best human example is inheritance of the sickling gene (s), which causes a substitution of one amino acid in the beta chain of hemoglobin. Hemoglobin molecules containing the abnormal beta chains crystallize when blood oxygen levels are low, causing the erythrocytes to assume a sickle shape. Individuals with a double dose of the sickling allele (ss) have sickle-cell anemia, and any condition that lowers their blood oxygen level, such as respiratory difficulty or excessive exercise, can precipitate a sickle-cell crisis. The deformed erythrocytes jam up and fragment in small capillaries, causing intense pain. Individuals heterozygous for the sickling gene (Ss) have sickle-cell trait. They make both normal and sickling hemoglobin, and as a rule, these individuals are healthy. However, they can suffer a crisis if there is prolonged reduction in blood oxygen levels, as might happen when traveling in high-altitude areas, and they can transmit the sickling gene to their offspring.

In the human blood type AB, the alleles are __________.

In the human blood type AB, the alleles are codominant. Although we inherit only two alleles for each gene, some genes exhibit more than two allele forms, leading to a phenomenon called multiple-allele inheritance. For example, three alleles determine the ABO blood types in humans: IA, IB, and i. Each of us receives two of these. The IA and IB alleles are codominant, and both are expressed when present, resulting in the AB blood type. The i allele is recessive to the other two. Genotypes determining the four possible ABO blood groups are shown in the table.

Which of the following is false regarding the procedure depicted in "B."

It is false that chorionic villus sampling creates very little risk to the fetus. Chorionic villus sampling (CVS) suctions off bits of the chorionic villi from the placenta for examination. A small tube is inserted through the vagina and cervical canal and guided by ultrasound to an area where a piece of placental tissue can be removed. CVS allows testing at 8 weeks but waiting until after the 10th week is usually recommended. Karyotyping can be done almost immediately on the rapidly dividing chorionic cells, much earlier than in amniocentesis. Both of these procedures are invasive, and they carry with them an inherent risk to both fetus and mother. (For example, increased fetal risk of finger and toe defects is linked to CVS.) These tests are routinely ordered for pregnant women over 35 (because of the higher risk of Down syndrome), but they are performed on younger women when the probability of finding a severe fetal disorder is greater than the probability of doing harm during the procedure. Advances in DNA sequencing are about to make both of these invasive procedures obsolete. An easily obtained maternal blood sample contains free-floating maternal and fetal DNA released from the placenta. Maternal blood samples are currently being used in the clinic to test for fetal chromosomal abnormalities such as Down syndrome. The same techniques can provide detailed information about the fetus's entire genome, including mutations that increase the likelihood of certain diseases. The technology already exists and the potential for its widespread use is just around the corner.

Which of the following statements is true?

It is true that it takes relatively few genes to build a human being. Mendel's writings underlie mainstream thinking about heredity, but some genetic outcomes do not fit his rules. Among these nontraditional types of inheritance are influences due to RNAonly genes, to chemical groups attached to DNA or histone proteins, and to extranuclear inheritance conferred by mitochondrial DNA. Small RNAs control timing of programmed cell death during development and can also prevent translation of another gene. Mutations in these RNA-only areas have already been linked to several conditions including prostate and lung cancers and schizophrenia. It takes relatively few genes to build a human being. Our complexity is the result of the small RNAs that control the expression of genes, especially during growth and differentiation. Nucleotide sequences of these RNA-specifying DNA areas are now worked out and biochemical companies are investing heavily in gene therapy research. They are especially hot on synthesizing RNA interference drugs to silence or shut down particular genes to treat age-related macular degeneration, Parkinson's disease, cancer, and a host of other disorders.

Which statement is false with regard to alleles J and j?

JJ would indicate a homozygous recessive condition is false because JJ would indicate a homozygous dominant condition. Sometimes, one allele masks or suppresses the expression of its partner. Such an allele is said to be dominant, whereas the allele that is masked is said to be recessive. By convention, a dominant allele is represented by a capital letter (for example, J), and a recessive allele by the lowercase form of the same letter (j). Dominant alleles are expressed, or make themselves "known," when they are present in either single or double dose. For recessive alleles to be expressed, they must be present in double dose, that is, the homozygous condition. For example, a person whose genetic makeup includes either the gene pair JJ (the homozygous dominant condition) or the gene pair Jj (the heterozygous condition) will have double-jointed thumbs. The combination jj (the homozygous recessive condition) is needed to produce tight thumb ligaments.

Linked genes __________.

Linked genes are on the same chromosome. One aspect of gamete variation results from the crossing over and exchange of chromosomal parts during meiosis I. Genes are arranged linearly along a chromosome's length, and genes on the same chromosome are said to be linked because they are transmitted as a unit to daughter cells during mitosis. However, chromosomes can break and precisely exchange gene segments with their homologous counterparts during meiosis. This exchange gives rise to recombinant chromosomes that have mixed contributions from each parent.

Males tend to inherit more sex-linked conditions because __________.

Males tend to inherit more sex-linked conditions because there is no corresponding allele on their Y chromosome. When a male inherits an X-linked recessive allele—for example, for hemophilia or for red-green color blindness—its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

Most human traits are determined by __________.

Most human traits are determined by multiple alleles. A few human phenotypes can be traced to a single gene pair, but most such traits are very limited in nature, or reflect variation in a single enzyme. Most human traits are determined by multiple alleles or by the interaction of several gene pairs.

Pedigree analysis is most useful __________________.

Pedigree analysis is most useful for screening and genetic counseling before conception. There are two major avenues for identifying carriers of detrimental genes: pedigrees and blood tests. A pedigree traces a genetic trait through several generations and helps predict the future. For prospective parents, a genetic counselor collects phenotype information on as many family members as possible and uses it to construct the pedigree (often called the family tree). By working backward from current individuals and applying the rules of dominant-recessive inheritance, a counselor can deduce the genotypes of the parents and figure out the genotypes of the other individuals in their parents' generation.

Red-green color blindness exhibits __________ inheritance.

Red-green color blindness exhibits sex-linked inheritance. Inherited traits determined by genes on the sex chromosomes are said to be sex-linked. The X and Y sex chromosomes are not homologous in the true sense. The Y, which contains the gene that determines maleness, is much smaller than the X chromosome (Figure 29.5). The X bears over 1400 genes, and a disproportionately large number of them code for proteins important to brain function. Because the Y carries only about 200 genes, it lacks many of the genes present on the X. For example, genes coding for certain clotting factors, cone pigments, and even testosterone receptors are present on X but not on Y. A gene found only on the X chromosome is said to be X-linked. When a male inherits an X-linked recessive allele—for example, for hemophilia or for red-green color blindness—its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

Match the mode of genetic expression with the correct characteristic: Sex-linked inheritance.

Sex-linked inheritance ocurrs when a gene is found only on the X chromosome. Inherited traits determined by genes on the sex chromosomes are said to be sex-linked. The X and Y sex chromosomes are not homologous in the true sense. The Y, which contains the gene that determines maleness, is much smaller than the X chromosome. The X bears over 1400 genes, and a disproportionately large number of them code for proteins important to brain function. Because the Y carries only about 200 genes, it lacks many of the genes present on the X. For example, genes coding for certain clotting factors, cone pigments, and even testosterone receptors are present on X but not on Y. A gene found only on the X chromosome is said to be X-linked. When a male inherits an X-linked recessive allele—for example, for hemophilia or for red-green color blindness—its expression is never masked or damped, because there is no corresponding allele on his Y chromosome. Consequently, the recessive gene is always expressed, even though present only in single dose. In contrast, females must have two X-linked recessive alleles to express such a disorder, and as a result, very few females exhibit any X-linked conditions.

The 46 chromosomes of a zygote come from __________.

The 46 chromosomes of a zygote come from the mother and the father. The nuclei of all human cells except gametes contain the diploid number of chromosomes (46), consisting of 23 pairs of homologous chromosomes. Recall that homologous chromosomes are pairs of chromosomes—one from the father (sperm) and one from the mother (egg)—that look similar and carry genes for the same traits, but do not necessarily bring about the same expressions of those traits. Two of the 46 chromosomes are sex chromosomes (X and Y), which determine genetic sex (male = XY; female = XX). The other 44 are the 22 pairs of autosomes that guide the expression of most other traits.

Identify the the correct answer for the procedure depicted by "A."

The correct answer for the procedure depticted by "A", i.e., amniocentisis, is that it is a procedure that is not normally done before the 14th week of pregnancy. Fetal testing is used when there is a known risk of a genetic disorder. The most common type of fetal testing is amniocentesis (am″ne-o-sen-te′sis), in which a wide-bore needle is inserted into the amniotic sac through the mother's abdominal wall, and about 10 ml of fluid is withdrawn. Because there is a chance of injuring the fetus before ample amniotic fluid is present, this procedure is not normally done before the 14th week of pregnancy. Using ultrasound to visualize the position of the fetus and the amniotic sac has dramatically reduced the risk of this procedure. The fluid is checked for enzymes and other chemicals that serve as markers for specific diseases, but most tests are done on the sloughed-off fetal cells in the fluid. These cells are cultured in laboratory dishes over a period of several weeks. Then the cells are examined for DNA markers of genetic disease and karyotyped to check for chromosomal abnormalities.

Match the event at "A."

The event at "A" is chromatid segments exchange, forming a chiasma.

Match the event at "B."

The event at "B" is the chromatids forming the chiasma break, and the broken-off ends join their corresponding homologues.

Match the event at "C."

The event at "C" is the chromatids forming the chiasma break, and the broken-off ends join their corresponding homologues.

Match the event at "D."

The event at "D" is two of the chromosomes carry new combinations of genes.

The expression of genes is called the __________.

The expression of genes is called the phenotype. A person's genetic makeup is referred to as his or her genotype (jēn′o-tīp). The way that genotype is expressed in the body is called one's phenotype (fe′no-tīp). For example, the double-jointed condition is the phenotype produced by a genotype of JJ or Jj.

Which of the following statements regarding dominant and recessive alleles is false?

The following statement is false: Dominant and recessive refer to the frequency of an allele in the population. Sometimes, one allele masks or suppresses the expression of its partner. Such an allele is said to be dominant, whereas the allele that is masked is said to be recessive. Dominant alleles are expressed, or make themselves "known," when they are present in either single or double dose. For recessive alleles to be expressed, they must be present in double dose, that is, the homozygous condition. Dominant and recessive do not refer to how frequently an allele occurs in the population. A dominant allele that is lethal, for example, will be very rare. Even dominant alleles for relatively harmless conditions are often not widespread.

amniocentesis and chorionic villus sampling

The following statement is true: Environmental factors can override or influence gene expression. In many situations environmental factors override or at least influence gene expression. Our genotype (discounting mutations) is as unchanging as a rock, but our phenotype is more like clay. If this were not the case, we would never get a tan, women bodybuilders would never be able to develop bulging muscles, and there would be no hope for treating genetic disorders. Sometimes, maternal factors such as drugs or pathogens alter normal gene expression during embryonic development. Mutations are permanent, transmissible changes in the DNA, whereas phenocopies are environmentally produced phenotypes that can mimic genetic mutations.

The form of inheritance in which the heterozygous state is expressed as an intermediate is __________.

The form of inheritance in which the heterozygous state is expressed as an intermediate is incomplete dominance. In dominant-recessive inheritance, one allele variant completely masks the other. Some traits, however, exhibit incomplete dominance. In such instances, the heterozygote has a phenotype intermediate between those of homozygous dominant and homozygous recessive individuals. Perhaps the best human example is inheritance of the sickling gene (s), which causes a substitution of one amino acid in the beta chain of hemoglobin. Hemoglobin molecules containing the abnormal beta chains crystallize when blood oxygen levels are low, causing the erythrocytes to assume a sickle shape. Individuals with a double dose of the sickling allele (ss) have sickle-cell anemia, and any condition that lowers their blood oxygen level, such as respiratory difficulty or excessive exercise, can precipitate a sickle-cell crisis. The deformed erythrocytes jam up and fragment in small capillaries, causing intense pain. Individuals heterozygous for the sickling gene (Ss) have sickle-cell trait. They make both normal and sickling hemoglobin, and as a rule, these individuals are healthy. However, they can suffer a crisis if there is prolonged reduction in blood oxygen levels, as might happen when traveling in high-altitude areas, and they can transmit the sickling gene to their offspring.

Match the genetic term with its correct characteristic: Genome.

The genome is the set of all genes. The complete human karyotype (kar′e-o-tīp), or diploid chromosomal complement displayed in homologous pairs, is illustrated in the figure below. The diploid genome (je′nōm), or genetic (DNA) makeup, represents two sets of genetic instructions—one from the egg and the other from the sperm.

The number of different gametes that can be produced in a male, based on independent assortment alone, equals __________.

The number of different gametes that can be produced in a male, based on independent assortment alone, equals 223. Note that the number of gamete types increases dramatically as the chromosome number increases. A cell with six pairs of homologues would produce 26, or 64, kinds of gametes. In a man's testes, the number of gamete types that can be produced on the basis of independent assortment alone is 223, or about 8.5 million—an incredible variety. The number of different gamete types produced in a woman's ovaries is substantially less because her ovaries complete at most 500 reduction divisions in her lifetime. Still, each ovulated oocyte will most likely be novel genetically because of independent assortment.

How many chromosome pairs are in a normal diploid human cell?

There are 23 pairs of chromosomes in a normal diploid human cell. The nuclei of all human cells except gametes contain the diploid number of chromosomes (46), consisting of 23 pairs of homologous chromosomes. Recall that homologous chromosomes are pairs of chromosomes—one from the father (sperm) and one from the mother (egg)—that look similar and carry genes for the same traits, but do not necessarily bring about the same expressions of those traits. Two of the 46 chromosomes are sex chromosomes (X and Y), which determine genetic sex (male = XY; female = XX). The other 44 are the 22 pairs of autosomes that guide the expression of most other traits.

Traits that display continuous phenotypic variation are usually determined by this form of inheritance.

Traits that display continuous phenotypic variation are usually determined by polygene inheritance. Most phenotypes depend on several gene pairs at different locations acting in tandem. Such polygene inheritance results in continuous, or quantitative, phenotypic variation between two extremes and explains many human characteristics. Examples of polygene traits in humans include skin color, height, metabolic rate, and intelligence. Skin color, for instance, is controlled by at least three separately inherited genes, each existing in two allelic forms: A, a; B, b; C, c. The A, B, and C alleles confer dark skin pigment, and their effects are additive. The a, b, and c alleles confer pale skin tone. An individual with an AABBCC genotype would be about as dark-skinned as a human can get, while an aabbcc person would be very fair. However, when individuals heterozygous for at least one of these gene pairs mate, a broad range of pigmentation is possible in their offspring. Such polygene inheritance results in a distribution of genotypes and phenotypes that, when plotted, yields a bell-shaped curve.

Match the genetic term with its correct characteristic: Homozygous.

Two identical alleles for a gene are called homozygous. Because chromosomes are paired, it follows that the genes in them are paired as well. Consequently, each of us receives two genes, one from each parent (for the most part), that interact to dictate each trait. Matched genes, which are at the same locus (location) on homologous chromosomes, are called alleles (ah-lēlz′) of each other. Alleles may code for the same or for alternative forms of a given trait. For example, one allele might code for tight thumb ligaments and the other for loose ligaments (the double-jointed thumb condition). When the two alleles controlling a trait are the same, a person is said to be homozygous (ho-mo-zi′gus) for that gene. When the two alleles are different, the individual is heterozygous (het″er-o-zi′gus) for the gene.

When the two alleles controlling a trait are different, the individual is __________ for the trait.

When the two alleles controlling a trait are different, the individual is heterozygous for the trait. Because chromosomes are paired, it follows that the genes in them are paired as well. Consequently, each of us receives two genes, one from each parent (for the most part), that interact to dictate each trait. Matched genes, which are at the same locus (location) on homologous chromosomes, are called alleles (ah-lēlz′) of each other. Alleles may code for the same or for alternative forms of a given trait. For example, one allele might code for tight thumb ligaments and the other for loose ligaments (the double-jointed thumb condition). When the two alleles controlling a trait are the same, a person is said to be homozygous (ho-mo-zi′gus) for that gene. When the two alleles are different, the individual is heterozygous (het″er-o-zi′gus) for the gene.

Use the following information to solve the problem using a Punnett square: Assume that the dominant "A" allele encodes normal pigmentation and the recessive "a" allele encodes albinism. An individual with the genotype aa has children with an individual of the genotype Aa. What is the percent chance that they would have children with albinism?

When we assume that the dominant "A" allele encodes normal pigmentation and the recessive "a" allele encodes albinism. An individual with the genotype aa who has children with an individual of the genotype Aa has a 50% chance a child will have albinism.

Chromosome Segregation and Independent Assortment (Independent Assortment)

• Independent assortment •Alleles of two different traits on two different chromosomes are distributed independently of each other •Example: Hh is on one chromosome, and Aa is on another chromosome, so possibilities of inheritance are: HA, Ha, hA, and ha •Whether you inherit a H or h is independent of whether you inherit a A or a •How much variation does this produce? •number of gamete types is 2ⁿ •n = number of homologous pairs -In gametes, 2²³ = 8.5 million possible combinations!

Vocabulary of Genetics (Alleles)

•Alleles •Gene pairs •genes that occur at same locus (location) on homologous chromosomes •DNA sequence can be the same or different •Homozygous -alleles are the same for single trait(DNA sequence is same on both homologous chromosomes) •Heterozygous •alleles are different for single trait(DNA sequence is different on one homologous chromosome than other)

Carrier Recognition (Blood Tests)

•Blood tests •Simple blood tests are used to screen for sickling gene in heterozygotes •Sophisticated blood chemistry tests and DNA probes can detect presence of other unexpressed recessive genes •Carriers of Tay-Sachs and cystic fibrosis genes can be identified with such tests

Vocabulary of Genetics (Chromosomes)

•Diploid number (46) of chromosomes in all human cells except gametes •23 pairs of homologous chromosomes •1 pair of sex chromosomes •determine genetic sex (XX = female, XY = male) •22 pairs of autosomes -most other traits •Karyotype •diploid chromosomal complement displayed in homologous pairs •Genome •genetic (DNA) makeup -two sets of genetic instructions (maternal and paternal)

Vocabulary of Genetics (Dominance)

•Dominance •One allele masks expression of its recessive partner •Dominant allele is denoted by capital letter and recessive by same letter, but in lowercase •Example: widow's peak is a dominant trait, designated as A ; straight hair line is a recessive trait designated as a •Example: hitchhikers thumb is recessive, designated as h, a straight thumb is dominant and designated as H •Note the letters are not "permanently" assigned to the trait. If they were we could only talk about 26 traits. •Dominant trait is expressed even if other allele codes for recessive trait •Example: AA or Aa will result in widow's peak •Recessive trait is expressed only if both alleles are recessive •Example: straight hair line would occur only if person has aa •What about hitchhikers thumb?

Sexual Sources of Genetic Variation

•Each person is genetically unique as a result of three events •Independent assortment of chromosomes •Crossover of homologues •Random fertilization of eggs by sperm

Polygene Inheritance (Skin Color)

•Example of polygenic inheritance for skin color •Alleles for dark skin (ABC) are incompletely dominant over those for light skin (abc) •First-generation offspring of AABBCC ´ aabbcc cross would result in all heterozygotes with intermediate pigmentation •Second-generation offspring would have even wider variation in possible pigmentations, which, if charted, would lead to a bell-shaped curve

Dominant-Recessive Inheritance (Example-Albinism)

•Example: albinism •Dominant allele: A (normal pigmentation) •Recessive allele: a (albinism) •AA and aa are homozygous; Aa is heterozygous •Probability of genotypes from mating two heterozygous parents for albinism •Results: •25% AA (normal pigmentation) -50% Aa (normal pigmentation) •25% tt (albinism)

Human Gene Therapy

•Gene therapy may alleviate or cure disorders •Genetic engineering has potential to replace defective gene, including: •Defective cells infected with genetically engineered virus containing functional gene •Inject "corrected" gene directly into cells •Prohibitively expensive, as well as controversial •Many ethical, religious, and societal questions

Introduction to Genetics

•Genes •segments of DNA •contain the "recipe," or blueprints, for synthesis of proteins •Gene expression can be controlled by other genes •Genetics: study of the mechanism of heredity •Basic principles of genetics were proposed in mid-1800s by Mendel, who studied inherited characteristics that were either all or none •Human traits are much more complex than that •Human Genome Project •determined human DNA sequence •Used in genetic research and genetic screening

Genetic Screening

•Genetic screening and genetic counseling provide information and options for prospective parents •Newborn infants are routinely screened for •Congenital hip dysplasia, imperforate anus, PKU, and other metabolic disorders Other examples: screening adult children of parents with Huntington's disease; testing fetus of 35-year-old woman for trisomy-21 (Down syndrome

Genotype and Phenotype

•Genotype: genetic makeup of a person for a trait •For the hitchhikers thumb gene example, person can have three possible genotypes: HH, Hh or hh •Phenotype: physical expression of genotype •For double-jointed example: •Person with genotypes HH or Hh will have straight thumbs (H is dominant) •Person with genotype hh will have hitchhiker's thumbs •What would be the phenotype for a person with the genotype Aa in our widows peak example?

Environmental Factors' Effect on Gene Expression

•In many situations, environment can override or influence gene expression •Maternal factors (example: drugs, pathogens) can alter normal gene expression during embryonic development •Example: thalidomide •Embryos developed phenotypes not directed by their genes, but by the drug •Example of phenocopies: environmentally produced phenotypes that mimic conditions caused by genetic mutations •Environmental factors can also influence gene expression after birth •Poor nutrition can affect brain growth, body development, and height •Childhood hormonal deficits can lead to abnormal skeletal growth and proportions So the answer to nature vs nature is yes!

Dominant-Recessive Inheritance Incomplete Dominance (Sickle Cell Anemia)

•Incomplete dominance •Heterozygous individuals have intermediate phenotype •May have symptoms, but usually not as intense as those experienced by homozygous individuals •Example: sickling gene •SS = normal hemoglobin (Hb) made •Ss = sickle-cell trait: both mutated and normal Hb are made; person can suffer sickle-cell crisis under prolonged reduction in blood O2 •ss = sickle-cell anemia: makes only mutated Hb; person is more susceptible to sickle-cell crisis even with short O2 reduction

Dominant-Recessive Inheritance (Punnett Square)

•Inheritance involves an interaction between dominant and recessive alleles •Punnett square -Diagram used to predict possible gene combinations resulting from mating of parents of known genotypes •Predictions are just the probability of offspring inheriting a particular genotype (and thus phenotype) •Larger number of offspring would increase likelihood of ratios conforming to predicted values •Example: if you toss a coin only 2 times, you may get heads both times, but if you toss coin 1000 times, you would probably end up with predicted probability of heads 50% of the time

Crossover of Homologues and Gene Recombination

•Linked Genes •On same chromosome so can be passed to daughter cells as one unit •During crossover (or chiasma) •Homologous chromosomes can break •May between linked genes •Precise exchange of gene segments results in recombinant chromosomes •Chromosomes are now a mixture of contributions from each parent -Results in tremendous variability

Dominant-Recessive Inheritance Multiple-Allele Inheritance (ABO Blood Typing)

•Multiple-allele inheritance •Genes that exhibit more than two allele forms •Example: ABO blood groups have three alleles: I A, I B, and iThe combination of two out of the three alleles determine a person's ABO blood type •I A and I B are codominant: both are expressed if present (type AB) •i is recessive type O •Heterozygotes express dominant A or B •So a person with OA is type A blood •A person with OB is type B blood •Homozygote person with ii results in OO, so type O blood

Dominant-Recessive Inheritance Sex-Linked Inheritance

•Sex-linked inheritance •Inherited traits determined by genes on sex chromosomes •X chromosomes bear over 1400 genes (many code for proteins important for brain function), and Y chromosomes carry about 200 genes •Few regions can participate in crossover •Genes found only on X chromosome are called X-linked genes

Random Fertilization Increases Variation!

•Single egg is fertilized by a single sperm in a random manner •Independent assortment and random fertilization together result in ~72 trillion zygote possibilities •Egg possibilities × sperm possibilities = 8.5 million × 8.5 million = ~72 trillion •Additional variations introduced by crossover would increase this number of possibilities exponentially •The odds that you are who you are less about 1 in 72,000,000,000,000(72 trillion) raised to the power of the number of crossover events!

Recessive Trait Examples!

•Some recessive genes result in the more desirable condition •Example: normal endochondral ossification is a recessive trait, whereas achondroplasia (abnormal endochondral ossification is a dominant trait) •Most genetic disorders are inherited as simple recessive traits •Examples: albinism, cystic fibrosis, and Tay-Sachs disease •Heterozygotes are carriers of trait, meaning they do not express trait but can pass it on to offspring

Polygene Inheritance

•Traits that are result of actions of several gene pairs at different locations •The more genes are involved in a trait, the more phenotypic variation will be seen •Results in continuous (quantitative) phenotypic variation between two extremes •Examples: skin color, height, intelligence, and metabolic rate

Chromosome Segregation and Independent Assortment (Segregation)

•Two important separations of traits occur in meiosis I of gametogenesis: •Segregation •Two alleles of one particular trait will be separated and distributed to two different daughter cells •Example: for Hh —allele H will go in one daughter cell, and allele h will go in other •Errors in segregation can lead to cancer, infertility, and Down syndrome

Carrier Recognition (Pedigrees)

•Two major ways to identify carriers of detrimental genes (pedigrees and blood tests) • Pedigrees •Traces a genetic trait through several generations to help predict future risks •Genetic counselor collects phenotype information on as many family members as possible to construct the pedigree (family tree) •Genetic counselor can work backward, applying rules of dominant-recessive traits to deduce genotypes of parents and others

Dominant-Recessive Inheritance How Sex-Linked Works

•X-linked recessive alleles are always expressed in males and are never masked or damped because there is no Y counterpart •Females must have recessive alleles on both X chromosomes in order to express an X-linked condition •X-linked recessive conditions are passed from mothers to sons such as hemophilia or red-green color blindness •Can also be passed from mothers to daughters, but females require two alleles to express

Dominant Trait Examples!

•widow's peaks, freckles, and dimples, double-jointed thumbs, ability to roll tongue , etc. •Dominant disorders are uncommon as most are lethal, and death occurs before reproductive age •Exception is Huntington's disease, caused by delayed-action gene that is not activated until ~ age 40 •Offspring of individual with Huntington's have a 50% chance of also having disease