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1 Chapter 14: Mendel and the Gene Idea Mendelian Genetics Mendel’s Experiment Mendel worked with garden peas, a good choice of study organism because: 1. They’re available in many varieties 2. Their fertilization is easily controlled 3. The characteristics of their many offspring can be quantified Mendel studied 7 characters, or heritable features, that occurred in alternative forms called traits. These 7 different traits turned out to be 7 different alleles on 7 different chromosomes. He worked exclusively with true-breeding pea plants. This means the plants he used were genetically pure and consistently produced the same traits. - For example, tall plants always produced tall plants; short plants always produced short plants To follow the transmission of these well-defined traits, Mendel performed hybridizations in which he cross-pollinated contrasting true-breeding varieties, and then allowed the next generation to self-pollinate. P generation (parental): the parental organisms involved in the first genetic cross; the truebreeding parental plants in Mendel’s experiment F1 generation (first filial): the offspring of the first cross F2 generation (second filial): the next generation from the self-cross of the F1 Inheritance of Traits Every trait – or expressed characteristic – is produced by hereditary factors known as genes. - A gene is a segment of a chromosome. There are many genes within a chromosome that control the inheritance of a particular gene. - The position of a gene on a chromosome is called a locus. Diploid organisms have 2 copies of a gene – or 2 alleles – one on each homologous chromosome. - Alleles = alternate forms of the same gene - An organism has 2 alleles for each inherited trait, one received from each parent - For example, there can be 2 alleles for the height of a plant: tall and short An allele can be dominant or recessive. - Dominant: an allele that masks the presence of a recessive allele of the same gene in a heterozygous organism determines the organism’s appearance - Recessive: an allele that’s masked and not expressed in a heterozygous organism has no observable effect on its appearance - A Punnett square can be used to predict the results of simple genetic crosses. - The dominant allele receives a capital letter and the recessive allele receives a lowercase. An organism is homozygous when an organism has 2 identical alleles for a given trait ©SarahStudyGuides 2 - For example, TT is homozygous dominant and tt is homozygous recessive An organism is heterozygous when an organism has two different alleles for a given trait - For example, Tt is heterozygous dominant Genotype is the genetic makeup of an organism Phenotype is the physical appearance of an organism How Mendel discovered his law of segregation: - For each of the 7 traits that Mendel studied, only one trait of each pair was visible in the F1 generation. He found that the F1 offspring didn’t show a blending of the parental traits. - However, in the F2 generation, the missing parental trait reappeared in the ratio of 3:1 – three offspring with the dominant trait to one offspring with the reappearing recessive trait - Allele pairs separate (or segregate) during the formation of gametes, so an egg or sperm carries only one allele for each inherited character. Mendel’s 3 Laws of Inheritance 1. Law of Segregation Bb B or b **Alleles are inherited separately and can segregate and recombine. -During meiosis: the alleles of each gene segregate during meiosis I when homologous chromosomes are divided among gametes 2. Law of Independent Assortment BB Bb Bb bb **Alleles can segregate and recombine independently of other alleles - During meiosis: the alleles for one trait randomly assort and divide among the gametes during meiosis I, independently of alleles for other traits - -When fertilization occurs, chromosomes – along with the alleles they carry – separate and get paired up in a new random combination -For example, gametes from an F1 hybrid generation (AaBb) contain four combinations of alleles in equal quantities (AB, Ab, aB, ab) Bb x Bb 3. Law of Dominance **One trait masks the effects of another trait - The presence of dominant alleles masks recessive alleles (except during incomplete and codominance) Laws of Probability - General rules of probability apply to the laws of segregation and independent assortment. The probability scale goes from 0 to 1: the probabilities of all possible outcomes add up to 1. ©SarahStudyGuides 3 - The probability of an event occurring is the number of times that event could occur over all the possible events. The outcome of independent events is not affected by previous or simultaneous trials. Monohybrid Crosses - The multiplication rule states that the probability that a certain combination of independent events will occur together is equal to the product of the separate probabilities of the independent events. The probability of a particular genotype being formed by fertilization = the probabilities of forming each type of gamete needed to produce that genotype If a genotype can be formed in more than one way, then the addition rule states that its probability equals the sum of the separate probabilities of the different, mutually exclusive ways the event can occur. For example, a heterozygote offspring can occur if the egg contains the dominant allele and the sperm the recessive (½ x ½ = ¼ probability) and vice versa ( ¼ ). Therefore, a heterozygote offspring would be the predicted result from a monohybrid cross half the time ( ¼ + ¼ = ½ ) Complex Genetic Crosses Fairly complex genetics problems can be solved by applying the multiplication and addition rules. The probability of a particular genotype arising from a cross can be solved by considering each gene involved as a separate monohybrid cross and then multiplying the probabilities of all the independent events involved in the final genotype. When more than one outcome is involved, the addition rule is also used. The larger the sample size, the more closely the results will conform to statistical predictions. Genetic Crosses Animals used in crosses include rabbits, mice, and fruit flies (=Drosophila Melanogaster) 1. 2. 3. 4. 5. High reproduction rate Easily maintained Many generations Many visible traits Indiscriminate mates Monohybrid Cross Monohybrid cross: genetic cross between monohybrids, or individuals that only differ in one character - For example, Mendel crossed plants that differed in only seed shape A monohybrid heterozygous cross will always result in the 3:1 ratio. - For example: Tt x Tt (T = tall; t = short) The ratio of phenotypes is 3 : 1 (3 tall: one short) The ratio of genotypes is 1 : 2 : 1 (one TT: 2 Tt: one tt) ©SarahStudyGuides 4 Dihybrid Cross Dihybrid cross: a genetic cross between dihybrids, or individuals that differ in 2 characters - Mendel made dihybrid crosses to follow the inheritance of 2 characters and determine whether the 2 characters were transmitted independently from the parent plants - Parents have 4 alleles and games have 2 alleles: P = Bbll 4 alleles It’s gametes are: Bl and bl 2 alleles A dihybrid heterozygous cross will always result in the 9:3:3:1 ratio. - For example: TtGg x TtGg (T = tall; t = short; G = green; g = yellow) Just memorize the ratio of phenotypes: 9 tall and green (Dominant-Dominant) 3 tall and yellow (D-R) 3 short and green (R-D) 1 short and yellow (R-R) Law of Probability: For dihybrid crosses, the probability that two or more independent events will occur simultaneously is equal to the product of the probability that each will occur independently - For example, to find the probability of a tall, yellow plant, just multiply the probabilities of each event. -If the probability of being tall is ¾ and the probability of being yellow is ¼, then the probability of being tall and yellow is ¾ x ¼ = 3/16 - For example: If you cross AaBbCCdd x AABbccDd, what is the probability of offspring with AABbCcdd? -Multiply the probabilities of each AA BB Cc dd ½ x ¼ 1 x ½ = 1/16 Test Cross A testcross uses a recessive organism to determine the genotype of an organism that expresses a dominant phenotype. - It’s used to determine if a plant appears dominant because it is homozygous (TT) or heterozygous (Tt). The only way to determine its genotype is to cross it with a recessive organism. Using the recessive plant, there are only 2 possibilities: 1) TT x tt - F1 = all Tt - So if none of the offspring is short, the original plant must be homozygous, TT. 2) Tt x tt - F1 = ½ Tt and ½ tt - But if even 1 short plant appears in the bunch, the original plant must be heterozygous, Tt. So it isn’t a purebred. ©SarahStudyGuides 5 Beyond Mendelian Genetics In complete dominance, the phenotype of the heterozygote (Tt) is the same as the dominant homozygote (TT). But not all patterns of inheritance obey the principles of Mendelian genetics. Incomplete dominance: - Incomplete dominance occurs when traits blend and are diluted - For example, if you cross a white snapdragon plant (dominant) with a red snapdragon plant (recessive), the resulting offspring will be pink. Codominance: - Codominance occurs when alleles are equally expressed For example, a palomino calf is speckled brown (dominant) and white (recessive). For example, hypercholesterolemia is codominant. For example, an individual can have an AB blood type. In this case, each allele – IA (the A allele) and the IB (the B allele) – is equally expressed. Blood Types: Phenotype Genotype A AA (IA), AO (IAi) B BB (IB), BB (IBi) AB AB (IAB) O OO (ii) Polygenetic/Multigene inheritance: - Polygenetic inheritance is when a trait results from the interaction of many genes. Each gene has a small effect on that particular trait. - A polygenic character may result in a normal distribution of the character within a population. For example, quantitative characters such as height, skin color, eye color, and weight are all examples of polygenetic traits. Multiple alleles: - Multiple alleles occur when traits are determined by many different alleles that occupy the same specific gene locus. - For example, the ABO blood group system is where 3 alleles determine blood type: IA, IB, and i -Each allele codes for an enzyme that adds a specific carbohydrate to the red blood cell. Epistasis - Epistasis is when the interaction between two gene products affects the expression of a trait. A gene at one locus may influence the expression of another gene at another locus. ©SarahStudyGuides 6 - - For example, 2 gene loci affect the coat color of mice. In one case, black (B) is dominant to brown (b). Yet at another gene locus, a pair of alleles (C and c) also affects coat color. If the mouse is homozygous recessive for the second locus (cc) then the coat is white (albino), regardless of the genotype of the black/brown locus. In this example, the recessive albino genotype is epistatic to the brown/black one – it influences it. F2 ratios that differ from the typical 9:3:3:1 often indicate epistasis. Pleiotropy - Pleiotropy is when a single allele exerts multiple effects on the genotype and can affect a number of characteristics of an organism. - For example, albinism in mice results in white fur and blindness. For example, in sickle-cell anemia, cystic fibrosis, and other hereditary symptoms, multiple symptoms are caused by a single pair of alleles. Environmental Impact on Phenotype - The phenotype of an individual is the result of complex interactions between its genotype and the environment. Genotypes have a phenotypic range called a norm of reaction within which the environment influences phenotypic expression. Polygenic characters are often multifactorial, meaning that a combination of genetic and environmental factors affects phenotype. Pedigree Analysis A family pedigree is a family tree with the history of a particular trait shown across generations. - Circles = females; squares = males Solid filled in symbols = individuals that have the trait (like attached earlobes) Parents joined by horizontal line; offspring listed below parents from left to right in order of birth The genotype of individuals in the pedigree can be deduced by following the patterns of inheritance. It looks like this: ©SarahStudyGuides 7 Chapter 15: The Chromosomal Basis of Inheritance Mendelian inheritance has its physical basis in the behavior of chromosomes. Mendel’s laws led to the chromosome theory of inheritance: -Genes occupy specific positions (loci) on chromosomes, and chromosomes undergo segregation and independent assortment during the process of meiosis in gamete formation. Morgan’s Experiment - T.H. Morgan worked with fruit flies, Drosophila Melanogaster. Fruits flies are prolific and rapid breeders. They have only 4 pairs of chromosomes; the sex chromosomes occur as XX in females and XY in males. The wild type is the normal phenotype found most commonly in nature for a character. Mutant phenotypes are alternative traits, assumed to have arisen as mutations. Morgan discovered a mutant white-eyed male fly that he mated with a wild-type red-eyed female. The F1 were all red-eyed. In the F2, however, all female flies were red-eyed, whereas half of the males were red-eyed and half were white-eyed. Morgan deduced that the gene for eye color was located on the X chromosome. Males have only one X, so their phenotype is determined by the eye-color allele they inherit from their mom Sex-Linked Traits Sex Chromosomes - Humans contain 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes. Autosomes code for many different traits. Sex chromosomes determine the sex of an individual. Females = XX chromosomes produce gametes (ova) with 1 X chromosome Males = X and Y chromosomes produce gametes (sperm) with either an X or Y There are some genes found only on the Y chromosome, like SRY, whose protein product regulates many other genes Inheritance of Sex-linked Genes Sex-linked traits are traits that are carried on sex-chromosomes Males inherit sex-linked alleles from their mothers. If a male has a defective X chromosome, he’ll express the sex-linked trait no matter what because he has one X and one Y chromosome. Recessive sex-linked traits are seen more often in males, since they are hemizygous for sexlinked traits. Females inherit sex-linked alleles from both parents. ©SarahStudyGuides 8 - A female with one defective X is a carrier. Although she doesn’t exhibit the trait, she can still pass it to her children. For her to express the trait, she has to inherit 2 defective X chromosomes. Sex-Linked Diseases Duchenne muscular dystrophy Colorblindness Hemophilia Hemophiliagene for blood clotting protein female H H X X XHXh XhXh male normal XHY XhY hemophilia -it’s very rare for a girl to have hemophilia -must have a mom that’s a carrier or has the disease and a father with the disease Barr Bodies Only one of the X chromosomes is fully active in most mammalian female somatic cells. The other X chromosome is condensed into a Barr body located inside the nuclear membrane. This means that both males and females have an equal dosage of X chromosome genes. - Females don’t have twice the amount of X chromosome genes (even though they have 2 Xs) because one of their X chromosomes is inactive in the form of a Barr body. Surprisingly, the X chromosome destined to be inactivated is randomly chosen in each cell. - A gene called XIST is active on the X chromosome that forms the Barr body. - Its RNA product may trigger DNA methylation and X-inactivation. Therefore, in every tissue in the adult female one X chromosome remains condensed and inactive. However, this X chromosome is replicated and passed on to a daughter cell. Linked Genes and Mapping Chromosomes Linked genes are genes located on the same chromosome that tend to be inherited together. - They stay together during assortment and move as a group. For example, the genes for blue eyes and blond hair are linked on the same chromosomes and show up together. How Linkage Affects Inheritance Morgan found the concept of linked genes: ©SarahStudyGuides 9 He performed a testcross of F1 dihybrid wild-type flies that were homozygous recessive for black bodies and vestigial wings. Wild type = traits found in nature; usually dominant He found that the offspring were not in the predicted 1:1:1:1 phenotypic classes. Instead, most of the offspring were the same phenotypes as the P generation parents – either wild type (gray, normal wings) or double mutant (black, vestigial). Morgan deduced that these traits were inherited together because their genes were located on the same chromosome. Genetic recombination results in offspring with combinations of traits that differ from those of the parents. Genetic Recombination and Linkage - - Parental types – have phenotypes like one or the other of the parents Recombinant types (recombinants) – have combinations of the 2 traits that are unlike the parents. A frequency of recombination greater than 50% occurs when 2 genes ARE NOT LINKED! They must be located on different chromosomes. In these cases, recombination simply results from the random alignment of homologous chromosomes at metaphase I and the resulting independent assortment of alleles. For example: a cross between a dihybrid heterozygote (YyRr) and a recessive homozygote (yyrr): - ½ will be parental types (YyRr, yyrr) - ½ of the offspring called recombinant types (Yyrr, yyRr) - This frequency of recombination is 50% the genes are not linked! Recombination of linked genes does occur, due to crossing over - Crossing over is the exchange of genes between nonsister (a maternal and a paternal) chromatids of synapsed homologous chromosomes during prophase of meiosis I. Linked genes do not assort independently, so crossing over is necessary for recombination. The percentage of recombinant offspring is called the recombination frequency. Recombination frequency = # of recombinant offspring Total # of offspring -For example: A wild type fruit fly (gray, normal wings) and a recessive fruit fly (black, vestigial wings) are mated -The offspring are: -965 wild type (gray-normal) IDENTICAL TO PARENT -944 black-vestigial IDENTICAL TO PARENT -206 gray-vestigial RECOMBINANT -185 black-normal RECOMBINANT -So the recombinant frequency = 206 + 185 x 100% = 17% or 17 map units 2300 Mapping Chromosomes A genetic map is an ordered list of genes on a chromosome. Because linked genes are found on the same chromosome, they can’t segregate independently. ©SarahStudyGuides 10 - The frequency of crossing over between any 2 linked alleles is proportional to the distance between them. The farther apart 2 linked genes are on a chromosome, the higher the frequency of crossing over. The closer together 2 genes are, the lower the frequency of crossing over. A recombination/linkage mapping is a mapping of linkage groups. ***1 map unit = 1% recombination. Genes may be located on the same chromosome, but too far apart to be linked genes. - Linkage can’t be determined if genes are so far apart that crossovers between them are almost certain. They would then have the 50% recombination frequency typical of unlinked genes. - Such genes are physically linked but genetically unlinked. - Distant genes on the same chromosome may be mapped by adding the recombination frequencies determined between them and intermediate genes. The sequence of genes on a chromosome can be determined by finding the recombination frequency. FOR EXAMPLE: if 2 linked genes, A and B, recombine with a frequency of 15% and B and C recombine with a frequency of 9%, and A and C recombine with a frequency of 24%, what is the sequence and the distance between them? - The sequence is A-B-C: A______15 map units______B___9 map units____ C The frequency of crossing over may vary along the length of a chromosome. A linkage map provides the sequence but not the exact location of genes on chromosomes. Cytogenetic maps locate gene loci in reference to visible chromosomal features. Inheritance Patterns Genomic Imprinting A few dozen traits in mammals depend on which parent supplied the alleles for the trait. Genomic imprinting determines whether an allele will be expressed or not in the offspring. - It causes the activation or inactivation of certain genes, which depends on the gene’s location on a chromosome and its parental origin. - It occurs during gamete formation. - For example: prader-willi syndrome results when genes on the paternal chromosome are deleted. Angelman syndrome results when genes on the maternal chromosome are deleted. When the next generation makes gametes, old maternal and paternal imprints are removed, and alleles are again imprinted according to the sex of each parent. - Most of the mammalian genes subject to imprinting identified so far are involved in embryonic development. The addition of methyl groups may inactivate the imprinted gene, assuring that the developing embryo has only one active copy ©SarahStudyGuides 11 Inheritance of Organelle Genes Exceptions to Mendelian inheritance are found in the case of extranuclear (or cytoplasmic) genes located on small circles of DNA in mitochondria and plant plastids, which are transmitted to offspring in the cytoplasm of the ovum. Some rare human disorders can be caused by mitochondrial mutations, and maternally inherited mitochondrial defects may contribute to diabetes, heart disease, and Alzheimer’s. Chi Square Problems Chi square = statistical analysis of date that allows the experimenter to determine if the results are due to change (random events) or another factor. Chi square formula: (**We have to memorize this!!**) X2 = £ (o – e)2 + (o – e)2 + … e e # of degrees of freedom = one less than the # of cases Null hypothesis = assumes there is no difference - Nonsignificant = accept Null hypothesis - Significant = reject Null hypothesis ©SarahStudyGuides 12 Genetic Disorders Disorders due to gene mutations A gene mutation results in a defective protein (a change in the sequence of genes causes a change in the shape of a protein) Autosomal-Recessive Disorders Only homozygous recessive individuals express the phenotype for the thousands of genetic disorders that are inherited as simple recessive traits Carriers of the disorder are heterozygotes, who are phenotypically normal but may transmit the recessive allele to their offspring. The likelihood of two mating individuals carrying the same rare deleterious allele increases when the individuals have common ancestors. Consanguineous matings, between close relatives, are indicated on pedigrees by double lines. Genetic diseases are unevenly distributed among groups of humans. - Tay-Sachs: most common in Jews - Cystic fibrosis: most common in people of European descent - Sickle cell disease: most common in African Americans. Autosomal-Dominant Disorders - A few human disorders are due to dominant genes. In achondroplasia, dwarfism is due to a single copy of a mutant allele. Dominant lethal alleles are more rare than recessive lethals because the harmful allele cannot be masked in the heterozygote. A late-acting lethal dominant allele can be passed on if the symptoms don’t develop until after reproductive age. An example of this is Huntington’s disease Mutlifactoral Disorders Many diseases have genetic (usually polygenic) and environmental components. These multifactorial disorders include heart disease, diabetes, cancer, and others. Genetic Testing The probability of a child having a genetic defect may be determined by considering the family history of the disease. A karyotype is a picture of what the chromosomes in a cell look like. Tests to identify carriers have been developed: 1) Amniocentesis – extracting amniotic fluid from the sac surrounding the fetus. - Allows you to get a karyotype after cells are cultured for several weeks - Biochemical tests can be performed immediately - Less invasive and easier, but less reliable 2) Chorionic villi sampling (CVS) - taking a sampling of placenta tissue - Allows immediate karyotyping and biochemical testing - More invasive, but more reliable ©SarahStudyGuides 13 An ultrasound is simple, noninvasive procedure that can reveal major abnormalities. Fetoscopy is the insertion of a needle thing viewing scope and light into the uterus which allows the fetus to be checked for anatomical problems. PGD (pregestational diagnosis) is another genetic test for the fetus. - It involves a long process: Scientists take eggs from a female and fertilize them in a petri plate and wait until the zygote develops into an embryo of 8 cells They then extract one of the cells (which doesn’t harm the embryo) They perform a DNA test to look for any genetic disorders They then implant the embryos that don’t carry disorders or mutations back into the female and destroy the ones with mutations or disorders Disorders due to chromosome mutations A chromosome mutation results in an abnormal number of chromosomes, either too many or too few. Chromosome mutations cause multiple things to go wrong, which is why these disorders are called syndromes. It is usually lethal. Abnormal Chromosome Number Nondisjunction is when homologous chromosomes don’t separate during meiosis. - As a result, a gamete receives either 2 or no copies of that chromosomes. Aneuploidy is a chromosome alteration where cells have too many or too few chromosomes. - Monosomy: only one chromosome of a pair is present (i.e. too few) - Trisomy: 3 chromosomes are present for one homologous pair, instead of 2 (i.e. too many) - Aneuploid organisms usually have a set of symptoms caused by the abnormal number of genes. - A disjunction during mitosis early in embryonic development is also likely to be harmful. Polypoidy is a chromosomal alteration where an organism has more than 2 complete chromosomal sets. - Triploidy (3n) is 3 chromosome sets; Tetraploidy is 3 chromosome sets. - Polyploidy is common in the plant kingdom and has played an important role in the evolution of plants. Alterations of Chromosome Structure Chromosome breakage can lead to four types of changes in chromosome structure. 1) Deletion is when chromosome fragments are lost 2) Duplication is when chromosome fragments join to a sister chromatid (or a nonsister chromatid) 3) Inversion is when fragments rejoin to the original chromosome in the reverse orientation ©SarahStudyGuides 14 4) Translocation is when chromosome fragments join a nonhomologous chromosome, and one chromosome gets stuck on another 12 13 A nonreciprocal crossover can result in a deletion and duplication in nonsister chromatids, caused by nonequal exchange between chromatids Human Disorders due to Chromosomal Alterations The frequency of aneuploid zygotes many be failry high in humans, but development is usually so disrupted that embryos spontaneously abort and die. Some genetic disorders, expressed as syndromes of characteristic traits, are the result of aneuploidy. 1. Down Syndrome (trisomy 21) 2. Turners Syndrome (XO) 3. Klinefelters Syndrome (XXY) Males with XYY and females with XXX usually do not exhibit any syndromes. Why do genetic disorders persist in a population? 1) Can’t distinguish a carrier from a homozygous normal 2) For humans, there are small numbers of offspring 3) Heterozygotes often have an advantage that allows them to survive Heterozygote advantage - Heterozygous carriers can sometimes be immune to or resistant to another disease - For example: heterozygote carriers for sickle cell anemia are resistant to malaria Genetic anticipation - Age of onset: when the severity of symptoms of a genetic disorder increases with age and symptoms show up earlier with each generation - For example: in Huntington’s disease, symptoms don’t appear till late 30s or 40s ©SarahStudyGuides 15 Human Genetic Disorders Disorder Gene/ Abnormality Chromosome mutation Most common in Sickle cell Gene African -sickle shaped Americans red blood cells Hemoglobin Autosomal recessive Hemophilia Gene *Sex-linked recessive Duschene Muscular Dystrophy Gene Tay-Sachs Gene *Sex-linked recessive Autosomal recessive Cystic Fibrosis Gene Autosomal recessive Characteristics Treatment of disorder Blood transfusions -can’t carry enough oxygen, clots blood vessels Life span Slightly less than normal Protein clotting factors Males -excessive bleeding and bruising Injection of normal clotting factor (rDNA =recombinant DNA) Dystrophin (muscle) Males -atrophy (=wasting away) of skeletal muscles Physical therapy (minimal effects) 20s Defective enzyme (that breaks down lipids) Jewish -Lipid buildup in brain None Less than 4 years Defective transport protein (affects chloride ions) -western European descent – Caucasian Antibiotics Late 20s and older -seizures, blindness, degenerate motor skills -thick mucus that clogs lungs, liver pancreas -difficulty breathing and digestive problems (=pleiotrophy) Lung transplant ©SarahStudyGuides 16 PKU Gene Autosomal recessive Huntington’s Gene Disorder Defective enzyme -mental retardation Diet free of Slightly phenylalanine less than normal Unknown Equal for all -significant deterioration of the nervous system None *symptoms don’t show up till 40s or 50s Less than normal *Autosomal dominant* Down syndrome (trisomy 21) Chromosome nondisjunction – an extra chromosome 21 (=trisomy) Children -characteristic facial features, short statute, heart defects, mental retardation Regular checkups, medicine, surgery, counseling Less than normal Turner’s syndrome (XO) Chromosome nondisjunction – monosomy X (=only one X chromosome) Females only -still feminine characteristics, webbed neck, reduced IQ Estrogen replacement therapy normal Klinefelter’s syndrome (XXY) Chromosome nondisjunctionan extra X chromosome in males Males only -tall and skinny, male sex organs, female characteristics, reduced IQ Fragile X syndrome Chromosome a change in a single gene, the Fragile X Mental Retardation 1 (FMR1) gene, which is found on the X chromosome Males -mental retardation Deletion of genes on paternal chromosome (genome Infancy or at birth Prader-Willi syndrome Chromosome normal -therapy normal Therapy normal -intellectual disabilities -This means that while most people have a single working copy of these ©SarahStudyGuides 17 genes, people with PWS have no working copy. imprinting) The paternal genes are deleted, but the maternal copy is silenced (=genomic imprinting). Angelman Syndrome Chromosome Deletion of genes on maternal chromosome (genome imprinting) -learning difficulties, obesity Equal for all -no working copy of the deleted maternal genes -intellectual and developmental delay, sleep disturbance, seizures, frequent laughter Medical, physical, behavioral therapy to help development normal ©SarahStudyGuides