Human Genetics Exam Study Guide
Human Genetics Exam Study Guide Biology 355-01
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This 6 page Study Guide was uploaded by Emily Erffmeyer on Friday February 19, 2016. The Study Guide belongs to Biology 355-01 at Grand Valley State University taught by Avisa Aledavood in Winter 2016. Since its upload, it has received 84 views. For similar materials see Human Genetics in Biology at Grand Valley State University.
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Date Created: 02/19/16
EXAM #2 MAIN CONCEPTS • Centromeres: ◦ Determine the shape of the chromosome ◦ Metacentric-Centromere at the middle producing two EQUAL arms ◦ Submetacentric- Centromere off the middle producing two UNEQUAL arms ◦ Acrocentric- Centromere closer to on end producing two UNEQUAL arms ◦ Do NOT contain genes ◦ Structure is required for a normal division • Chromosomal Groups ◦ Grouping based on the similarity in the sizes of the group of chromosomes ◦ GroupsAthrough G ◦ X chromosome is in group C ◦ Y chromosome is in group G • KaryotypeAnalysis ◦ Used as a clinical tool to provide information such as the total number of chromosomes, sex chromosome content, presence/absence of individual chromosomes, and the nature/extent of chromosomal aberrations ◦ RBC, cells that can divide, cells trapped in G0, and/or dead can be used for analysis ◦ Purpose of staining is to differentiate a chromosome type from other types ◦ Identify the potential abnormalities in the number or structures of chromosomes • G-Banding ◦ Giemsa is the dye that creates the dark regions as it attaches to the undigested proteins ◦ Common method of staining ◦ Trypsin is a protease that digests proteins in the less compacted regions of the chromosome creating the light bands. ◦ Banding makes chromosomes with the same length or shape identifiable by “number” ◦ Two sister chromatids of a replicated chromosome, and the two copies of each pair have the same banding pattern ◦ CANNOT BE USED FOR: small structural abnormalities of chromosomes like gene deletions, point mutations ◦ Not useful in diagnosing genetic diseases that are caused by gene mutations • FISH ◦ Fluorescent in Situ Hybridization ◦ Uses DNAsequences (DNAprobes) attached to fluorescent dyes ◦ Each probe is a single stranded DNAcomplementary to a specific nucleotide sequence in DNAand when presented with the DNAprobe, the probe makes a hydrogen bond with its complementary regions ◦ Using DNAprobes of various chromosome segments creates a unique colored pattern for each of the 24 types of chromosomes ◦ Some chromosomal and gene abnormalities that cannot be detected by G-banding are identified by using FISH • Karyotype is Used to Analyze: Congential malformations (birth defects), infertility, history of habitual abortions, mental retardation, cancer, and abnormalities of sex determination • Amniocentesis ◦ Approximately weeks 15-16 ◦ Method of collecting fetus cells ◦ Occurs when the mother is worried about the fetus having an abnormal chromosome to determine if they want to terminate the pregnancy ◦ Removal of about 20 mL of amniotic fluid containing suspended cells ◦ After collecting of the fluid, kayotyping, g-banding, and so on can take place • Chorionic Villi Sampling ◦ CVS ◦ During weeks 8-10 of pregnancy ◦ Placental cells are used because they have the same chromosome constitution as the embryo ◦ CVS is another way to collect fetus cells ◦ Uses the outer placental cells • Conditions that May Suggest the Use ofAmniocentesis/CVS ◦ Advanced maternal age (>35) because the woman's primary oocyte is older which may lead to more chromosome abnormalities or nondisjunction ◦ Previous child with chromosomal aberration because the chances of another pregnancy with chromosomal abnormalities increases if a child already has it. ◦ Embryos of parents with previously identified structural abnormalities of a chromosome because one parent could have an abnormality that doesn't affect them but may affect the zygote resulting in many miscarriages. • Polyploidy ◦ Change in the number of CHROMOSOME SETS or all types of chromosomes exist in extra copies ◦ Tetraploidy- Presence of 2 extra sets of chromosomes (4 copies of all types) (4n) ◦ Triploidy- Presence of 1 extra set (3n) ◦ Adiploid gamete has 2n • Aneuploidy ◦ Change in the number of INDIVIDUAL CHROMOSOMES in diploid or haploid cells ◦ Trisomy, nuclisomy, disomy, monosomy ◦ Autosomes: Disomy is normal for 2n cells ◦ Sex Chromosomes: Trisomies, monosomies, tetrasomies can be seen in live births • Individuals affected with a type of chromosomal abnormality have the abnormal karyotype in all or most of their body cells • Errors of mitosis occur in mitotically dividing cells and produce abnormal embryonic, fetal, or somatic cells • Errors of meiosis produce abnormal gametes which may live long enough to participate in fertilization. • Causes of Aneuploidy ◦ Diploid cells: nondisjunction or chromosomal loss/lagging in mitosis and/or meiosis 1 and 2 ◦ Haploid cells: nondisjunction or chromosomal loss/lagging in mitosis and/or meiosis 1 and 2 ◦ Nondisjunction: ▪ In meiosis Disomic gametes (n+1) → (2n+1 or trisomic) ▪ In meiosis Nulisomic gametes (n-1) → (2n-1 or monosomic) ◦ Chromosomal Loss/Lagging: ▪ Meiosis 1: produces two normal and two nulisomic gametes with 22 chromosomes ▪ Meisosis : produces three normal and one nulisomic gamete ▪ Mitosis: produces one cell with the same chromosome number as the parent cell, and one cell with one chromosome less than the parent cell • Causes of Polyploidy ◦ Cell division errors: failure of nuclear division in mitosis or meiosis; failure of cytokinesis in meiosis or mitosis ◦ Fertilization errors ◦ Failure of nuclear division ▪ In meiosis: produces a diploid gamete and a diploid gamete produces triploid zygote and embryo ▪ In mitosis: produces a generation of tetraploid embryonic cells and a tetraploid embryonic ◦ Fertilization errors: produces triploidy=69 chromosomes • All autosomal monosomies are fatal in diploid cells. Monosomic cell may live long enough to produce 2 more monosomic cells through mitosis. • Adiploid cell which is monosomic for an autosomal chromosome should be assumed dead upon production • Trisomies of most autosomal chromosomes are fatal and should be assumed dead upon production except chromosomes 13,18, and 21. • No abnormal gamete should be assumed dead: only if the gamete has only a Y chromosome without an X chromosome. • The chance a newborn is affected with a chromosome abnormality is less than 2%. • Numerical abnormalities are more common than the structural aberrations. • Aneuploidies are more common than polyploidies. • Triploid Infant: 69, XXX ◦ Does not originate from errors of mitosis ◦ Large head, syndactyly, malformations of eye, mouth, and genitalia • B-Tetraploidy (4n) ◦ Very rare in live births ◦ Usually results from a single cell division error ◦ The source of error in tetraploidy is never meiosis because it always occurs after fertilization. • Mosaicism ◦ If no error occurs, mitosis generates cells that are identical to the parental cells. If the parental cell is abnormal, the daughter cells will also be abnormal. ◦ If a person is mosaic for an autosome, the nondisjunction occurred after the first mitotic division ◦ If a nondisjunction or chromosomal lagging of a sex chromosome occurs in the first mitotic division, it may lead to production of a mosaic individual for sex chromosomes. ◦ Chromosomal mosaic- an individual with two chromosomal distinct cell populations ◦ If only a small portion of the body cells are altered, health may not be affected ◦ Chromosomal mosaic for a trisomy may have a mild version of the condition • Trisomy 21: Down Syndrome ◦ 47, XY+21 ◦ Mental retardation is found in all ◦ Most have short stature, epicanthic fold, simian crease, dark spots on the irises, thick and protruding tongue, susceptibility to leukemia and heart defects, early onsetAlzheimer's ◦ Maternal age is the major risk factor ◦ 90% of cases the extra 21 is maternal due to nondisjunction in meiosis of 1 of oocytes ◦ About 80% of all Down syndrome babies are born to mothers younger than 35 ◦ Mosaic down syndrome is the result of a nondisjunction after the first mitotic division • Trisomy 18: Edwards Syndrome ◦ 47, XX+18 ◦ Average survival after birth is 2-4 months ◦ malformation of brain, heart, hands, and feet ◦ Mostly have clenched fists and club foot ◦ Growth Retardation ◦ Advanced maternal age is a risk factor • Trisomy 13: Patau Syndrome ◦ 47, XY+13 ◦ Severe malformations of brain, nervous system, and heart, cleft lip, eye defects, extra digits, large protruding heels ◦ Fatal (mean survival age is one month) ◦ Some mosaic ◦ Correlated with advanced maternal age • Common Sex ChromosomeAneuploidies are results of cell division errors in spermatogenesis because the X and Y chromosomes are pseudo homologous so they don't make enough synapses within a tetrad • No cell can ever survive without at least one X chromosome • Turner Syndrome ◦ 45, X ◦ 9% of the affected embryos or fetuses are not born ◦ Short stature, not mentally retarded, infertility, underdevelopment of the uterus, webbed neck, course facial features, under developed secondary sexual features at puberty ◦ Can be a result of fusion of a nulisomic and a normal gamete: a meiotic error ◦ Can be the result of a partial large deletion in the X chromosomes • Klinefelter Syndrome ◦ 47,XXY ◦ Most common genetic or chromosomal cause of male infertility ◦ Learning disablities, underdeveloped sexual features after puberty, tall stature, long arms and legs, large hands and feet, may develop breast tissue ◦ Meiotic Error: ▪ Disomic sperm fertilizes a normal egg ▪ Disomic egg fuses with a normal sperm ◦ Mitotic Error: nondisjunction of the X chromosome in a male zygote • Jacobs Syndrome ◦ 47, XYY ◦ Above average in height, lots of acne, not mentally retarded, not recognizable by phenotype ◦ Origin of error is in father's meiosis which leads to formation of a disomic sperm for the Y chromosome ◦ “Super male syndrome” • X-Polysomy ◦ 47, XXX (Triplo-X): Usually average IQ; Usually low fertility ◦ 48, XXXX (Tetrasomy of the X Chromosome): mental retardation, no fertility ◦ 49, XXXXX (Penta X Syndrome): Severe mental retardation, no fertility • Causes of StructuralAberrations ◦ Chromosomal Breakages followed by abnormal enzymatic repair ▪ may occur through exposure to radiation, chemicals, viruses causing two spots on one chromosome to break and the repair enzymes link the wrong ends ◦ Unequal Crossing Over between two chromosomes in a tetrad • Deletion ◦ Terminal Deletion requires one break and internal deletion requires two break ◦ Deletions that are present in all or part of the body cells of an embryo usually cannot be passed to the next generations ◦ Deletion of more than a small piece of chromosome is detrimental to developing embryo ◦ Deletion of an entire autosomal chromosome is fatal ◦ FISH can detect tiny deletions that cannot be identified by G-banding • Duplications ◦ Genes may be repeated ◦ Associated with a change in the amount of genetic material and cannot be passed on to the next generation usually • Inversion ◦ Middle segment of a chromosome changes directions ◦ Requires 2 breaks ◦ Doesn't change the length of the chromosome but may change the shape ◦ May or may not cause gene mutation ◦ Can be passed onto the next generations • Translocation ◦ Exchange of nonhomologous chromosome segments ◦ Two types: Reciprocal and Robertsonian ◦ Reciprocal ▪ two nonhomologous chromosomes exchange terminal segments ▪ no DNAgain or loss but gene mutation may occur ▪ If an abnormal gamete participates in fertilization it may result in miscarriage, still birth, or birth defects in the offspring ▪ If a balanced gamete with two translocated chromosomes participates in fertilization, the offspring will be a balanced carrier of the translocation ◦ Robertsonian ▪ Centromeres of 2 acrocentric chromosomes break and fuse (one short arm with one large arm) (short ends fuse; large ends fuse) ▪ Chromosome rearrangement with a translocation associated with a deletions ▪ Genetic material is lost but no genetic information for production of proteins is lost • Consequences of StructuralAberrations ◦ Pregnancy loss ◦ Birth defects ◦ Cancer ◦ Infertility • First trimester: about 12 weeks and organs form • Second trimester: organs mature • Third trimester: growth of the fetus • Birth defects can be genetic, non-genetic, multifactorial, or unknown • Genetic Birth Defects ◦ Chromosome or gene abnormality is present in all or most body cells ◦ Can be passed on to the next generation ◦ May be a history of defects in other family members • Non-Genetic Birth Defects ◦ Development of only some organs is disrupted ◦ Cannot be passed onto future generations • Teratogens ◦ Physical, biological, or chemical agents that produce embryonic or fetal abnormalities and result in prenatal death or defects ◦ Mostly affect regulation of the gene expression, therefore the function of the gene as the source of information for normal development is impaired ◦ Radiation, infectious agents, chemicals, maternal metabolic problems • Chromosomal Sex (Genetic) ◦ Determined at the time of fertilization by the presence or absence of Y chromosome ◦ Males have Y ◦ Females have XX • Gonadal Sex Determination ◦ No sexual differentiation in the first 6 to 7 weeks ◦ Two undifferentiated gonads ◦ Both male and female reproductive duct systems are present ◦ Undifferentiated gonads can become either ovaries or testes ◦ Mullerian duct produces male reproductive system ◦ Wolffian duct produces female reproductive system ◦ Gonadal sex is the result of gene's actions and interactions ◦ SRY gene: located near the end of the short arm of the Y chromosome and plays a major role in causing the undifferentiated gonad to develop into a testis • Phenotypic Sex Determination ◦ Cells in the testes secrete two hormones which determine phenotypic sex ◦ Testosterone: steroid hormone also known as the male sex hormones ▪ Stimulates the Wolffian ducts to form the male internal reproductive structures ▪ Partly converts to DHT to form the external reproductive structures ◦ MIH: hormone that causes breakdown of the Mullerian ducts in the embryo and stops further development of female reproductive structures
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