BSCI222 Genetics Week 3 Chapter 4 and 6 Notes
BSCI222 Genetics Week 3 Chapter 4 and 6 Notes BSCI222
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This 10 page Class Notes was uploaded by Colin Fields on Friday September 16, 2016. The Class Notes belongs to BSCI222 at University of Maryland taught by Dr. Paczolt in Fall 2016. Since its upload, it has received 6 views. For similar materials see Genetics in Biology at University of Maryland.
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BSCI 222 Genetics Chapter 4 Sex Determination and Sex-Linked Characteristics 4.1 Sex is Determined by a Number of Different Mechanisms 1. Sex: sexual phenotype, determines gametes produced a. Mechanism that determines is sex determination b. Hermaphroditism: phenotypic expression of both sexual phenotypes i. Monoecious: organisms that bear both male and female reproductive structures ii. Dioecious: organisms that bear either male or female reproductive structures 2. Chromosomal Sex-Determining Systems a. Autosomes: Nonsex chromosomes which are present in both sexes b. XX-XO Sex Determination i. Females posses two X chromosomes while males only poses one sex chromosome, also an X ii. Because males of these species produce different types of gametes (one has 1 less chromosome) they are called the heterogametic sex iii. Females that produce gametes that are all the same are called the homogametic sex c. XX-XY Sex Determination i. The Y chromosome is typically acrocentric ii. X and Y chromosomes are not usually homologous but they do pair and segregate as though they were a homologous pair 1. They can pair because they are homologous in small regions called the pseudoautosomal regions 2. This region contains the same genes in both chromosomes 3. In humans, this region is at the tips of the chromosomes d. ZZ-ZW Sex Determination i. Female is heterozygomatic while male is homozygomatic ii. Z and W are used to distinguish from the XX-XY system but do not indicate a different structure for the chromosomes iii. Found in Birds, Snakes, Butterflies, and some Amphibians 3. Genic Sex Determination a. Organisms that do not have sex chromosomes but instead genes for sex at different loci that determine the sex of the individuals have genic sex determination i. Has been observed in some plants, fungi, protozoans, and fish 4. Environmental Sex Determination a. Sex can be determined fully or in part by environmental factors b. Sequential hermaphroditism: each organism can be both male and female but not at the same time c. Temperature can also determine sex and even override heterogametic sexual determination 5. Sex Determination in Drosophila melanogaster a. Fruit fly i. 8 chromosomes, 3 autosomes and 1 pair of sex chromosomes ii. Sex is determined by genes on the sex chromosomes whose expression is partially regulated by autosomes 1. Autosomes either shorten or lengthen developmental stages producing sexual phenotypes 2. Female -> Metafemale -> Intersex -> Metamale -> Male 6. Sex Determination in Humans a. Presence of the SRY gene on the Y chromosome determines maleness b. Turner Syndrome i. 1/3000 female births are missing an X chromosome and assigned the genotype XO ii. Frequently have underdeveloped secondary sexual characteristics iii. Lack of an individual with no X chromosome indicates that at least one X is necessary for human development c. Klinefelter Syndrome i. 1/1000 male births ii. 1 or more Y chromosomes and multiple X chromosomes iii. Tall and sterile with underdeveloped or express sexual characteristics d. Poly-X Females i. 1/1000 female births ii. 3 X chromosomes (triplo-X syndrome) iii. Tall and thin iv. Slightly higher rate of intellectual disability v. Disability increases with more and more X chromosomes e. The role of sex chromosomes i. X chromosome contains information essential for human development ii. Male-gene is on the Y chromosome and is dominant iii. Lack of Y usually results in a female phenotype iv. Genes for fertility are located on both X and Y chromosomes v. Additional copies of X chromosomes upset normal development in both males and females f. The Male-Determining Gene in Humans i. There were some males that were XX but it was found that part of the Y chromosome was attached to another chromosome ii. 6 weeks after fertilization, a region of the Y chromosome becomes active that differentiates the gonads iii. This is the sex-determining region Y (SRY) g. Androgen-Insensitivity Syndrome i. Females with an XY genotype and testes and a vagina ii. SRY encoded proteins need to bind to androgen receptors to have an effect but this is defective in these females 4.2 Sex-Linked Characteristics are Determined by Genes on the Sex Chromosomes 1. Sex-linked characteristics: characteristics determined by genes located on the sex chromosomes a. X linked and Y linked 2. X-Linked White Eyes in Drosophila a. Hemizygous: males are this for X-linked loci because they only have 1 (half of normal) X chromosome 3. Nondisjunction and the Chromosome Theory of Inheritance a. Strange and not by chance occurrence of random white eyes b. Bridges’s Explanation i. Nondisjunction: failure of failure of chromosomes to separate during anaphase I ii. Specific strain of fly was a XXY with two white alleles on the Xs iii. Nondisjunction occurred in 10% of cases c. Confirmation of Bridges’s Hypothesis i. He examined the chromosomes of his flies and found his predicted chromosomal makeup 4. X-Linked Color Blindness in Humans a. Pigments for Red and Green are encoded close to each other on the X chromosome b. Genes in the pseudoautosomal regions are not sex linked as they are truly homologous regions of the sex chromosomes 5. Symbols for X-Linked Genes a. Superscript of the phenotypic allele is added the end of the chromosome (X or Y) denoting the sex-linked trait 6. Z-Linked Characteristics a. Exactly like X linked except males are the homogametic sex b. Females act like the males of XX-XY 7. Y-Linked Characteristics a. Holandric traits b. Only males have these because only they have the Y chromosome c. Evolution of the Y Chromosome i. A pair of autosomes diverged into the X and Y chromosomes ii. One of the chromosomes acquired a gene that determines maleness iii. Mutations occurred on that proto-Y chromosome that were only beneficial to males iv. Crossing over of the X and Y chromosomes was suppressed except for the pseudoautosomal region d. Characteristics of the Human Y Chromosome i. 2/3 of chromosome contains small repeated sequences that contain no active genes ii. Other 1/3 has about 350 genes that do things iii. Most genes here affect the testes and sexual development iv. 8 massive stretches of palindromic sequences 1. Facilitate internal crossover to prevent loss of genetic material and accumulation of mutations e. The use of Y-Linked Genetic Markers i. Mutations that occur in nonexpressing regions of the Y chromosome can be analyze to trace male ancestry 4.3 Dosage Compensation Equalizes the Amount of Protein Produced by X-Linked and Autosomal Genes in Some Animals 1. Dosage compensation: mechanisms by which organisms compensate for only having one X chromosome in males which would in theory result in less proteins from genes than those on the 2 autosomal chromosomes 2. In placental mammals, both sexes has double expression of the X chromosome but one of the X chromosomes in females is suppressed 3. Lyon Hypothesis a. Barr Bodies: inactivated X chromosomes in the nuclei of cells b. Occur whenever there is more than 1 X present so only 1 X gets expressed c. This means that females cells express X linked genes in a 50:50 manner making them mosaics 4. Mechanisms of Random X Inactivation a. Step 1 i. Cell some how counts the number of X chromosomes b. Step 2 i. A random X chromosome is selected to remain active and the others are deactivated c. Repression of X is mediated by the Xist gene that expresses a 17,000 nucleotide long RNA sequence i. This sequence coats the chromosome and inactivates it ii. The X that remains active has other genes that suppress Xist Lecture Only 1. Organelle DNA inheritance a. Mitochondria i. Included in both sperm and eggs ii. Sperm mitochondria are programmed for destruction after fertilization iii. Only inherited from mother 1. Can be used to trace maternal lineages iv. During replication mutations can occur 1. During cytokinesis mitochondria are randomly distributed to daughter cells 2. This means mosaics can be created inters of the mitochondria a cell has if it gets all mutant mitochondria BSCI222 Genetics Chapter 6 Pedigree Analysis, Applications, and Genetic Testing 6.1 The Study of Genetics in Humans in Constrained by Special Features of Human Biology and Culture 1. People keep extended records of family characteristics and pedigrees 2. People are not good model organisms 3. Controlled mating is not possible a. No test crossing b. No pure breeding 4. Long generation time 5. Small number of progeny 6.2 Geneticists Often Use Pedigree to Study the Inheritance of Characteristics in Humans 1. Pedigree: pictorial representation of a family history/tree 2. Analysis of Pedigree a. Pedigrees are insufficient to identify mendalian ratios so patterns of inheritance are studied to identify modes of inheritance 3. Autosomal Recessive Traits a. Normally appear with equal frequency in both sexes b. Can skip generations due to parent heterozygoticity c. Consanguinity: mating between closely related people i. More likely to pass on recessive rtaits 4. Autosomal Dominant Traits a. Appear in both sexes with equal frequency b. Can’t skip generations i. Unaffected individuals cannot pass on a gene they don’t have 5. X-Linked Recessive Traits a. More frequent in males than females since males only have one X chromosome b. Can skip generations 6. X-Linked Dominant Traits a. Appear more frequently in females than males i. 2 Xs means 2 chances to have bad allele b. Does not skip generations c. Cannot be passed from father to son d. Males usually more severely affected by phenotypes 7. Y-Linked Traits a. Only males, always father to son b. Does not skip generations c. Neither dominant nor recessive since there is only one Y chromosome 6.3 Studying Twins and Adoptions Can Help Assess the Importance of Genes and Environment 1. Types of Twins a. Dizygotic twins: nonidentical, two separate eggs b. Monozygotic twins: identical, single egg splits after fertilization into two separate embryos c. Dizygotic twinning tends to run in families i. Both genetic and environmental factors 2. Concordance in twins a. Concordant: both twins have a trait b. Discordant: only one twin has a trait c. Concordance: the percentage of twin pairs that are concordant for a trait i. High concordance in monozygotic twins compared to dizygotic twins indicates a genetic basis for a trait 3. A Twin Study of Asthma a. Environmental factors can induce it b. Frequently accompanied by allergies i. This indicates it to be an immune condition c. Study on twins revealed that monozygotic twins were almost twice as concordant as dizygotic twins but both had a very significant concordance suggesting environmental factors 4. Adoption Studies a. Distinguishes between genetic and environmental influences b. Assumes that environment of adopted child and parent are different from biological parent c. Traits observed in adoptive pair but not biological pair are considered environmentally caused or associated 6.4 Genetic Counseling and Genetic Testing Provide Information to Those concerned about Genetic Diseases and Traits 1. Genetic Counseling a. Provides information to patients who are concerned about hereditary conditions b. Correct diagnosis of the condition is the first and key step c. Counselor then discusses results with patients and helps them interpret them 2. Genetic Testing a. Ultrasonography i. High frequency sound is used to gain a picture of the physical structure of a fetus ii. Can reveal morphological conditions b. Amniocentesis i. Removal of fetal cells by sampling amniotic fluid ii. Genetic testing of cells is then done iii. 1/400 result in complications c. Chorionic Villus Sampling i. Small portion of the chorion (outer layer of the placenta) is removed and tested ii. Can produce limb deformities if done before 10 weeks d. Maternal Blood Screening Tests i. Sample the chemical composition of blood from the mother ii. Can reveal SOME conditions but only based on their expression e. Noninvasive Prenatal Genetic Diagnosis i. Directly examine fetal DNA in the mother’s blood f. Preimplantation Genetic Diagnosis i. In vitro fertilization ii. Only uses fertilized cells that don’t have mutations g. Newborn Screening i. Testing of newborns for select genetic conditions h. Presymptomatic Testing i. Testing of healthy people for genes that may cause future problems i. Heterozygote Screening i. Testing of a population for people who are carriers for a disease 3. Interpreting Genetic Tests a. Some times there are many genes that can cause a disease b. Sometimes penetrance is not complete c. Sometimes environmental stimuli is necessary for disease onset 4. Direct-to-Consumer Genetic Testing a. Consumers frequently not equipped to interpret results b. Provider greater access to testing 5. Genetic Discrimination and Privacy a. People at risk avoid testing out of fear of repercussions for health care and such b. Law passed outlawing this type of discrimination