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Chapter 12 Inheritance Patterns and Human Genetics 12-1 Chromosomes and Inheritance Remember… • DNA in chromosomes has instructions for protein synthesis • Chromosomes are transmitted from one generation to the next Sex Determination • 1900s: Thomas Hunt Morgan bred Drosophila melanogaster (fruit flies) • Determined: – Females: XX – Males: XY • “Y” chromosome is smaller and hook-shaped • Autosome: all other chromosomes • Sex chromosomes segregate (form pairs) during meiosis • SO! Each sex cell either has an “X” or a “Y” • Male gametes: can contain “X” OR “Y” • Female gametes: ONLY “X” Why???? Sex Linkage • Morgan also hypothesized that more genes are carried by X chromosome than the Y • X-linked: genes carried on the X chromosome • Y-linked: genes found on the Y chromosome • Sex linkage: presence of gene on sex chromosome Sex Linked in Drosophila • Normal: Red eyes • Mutation: WHITE eyes • P1 x P1 F1 – Red eye female x White eye male ALL RED EYES • F1 x F1 F2 – Red female x red male 3(red):1(white) – HOWEVER! All white eyes on MALES ONLY No White Eyed Females • Trait for eye color is carried on X • Red eye female: XR XR • White eye male: Xr Y Linkage Group • Thousands more genes than chromosomes many genes on same chromosome • Linkage group: multiple genes on same chromosome • Linked genes tend to be inherited together Why? Linked Groups in Drosophila • Morgan looked at fly body color and wing size – Gray body, Long wing x Black body, Short wing – BBLL x bbll • If on separate chromosomes: – 9:3:3:1 • If on the same chromosome: – 3:1 So, what actually happened? • Results: – several Gray, long (Ggll) – Several black, short (bbLl) • HOW???? Crossing Over • Exchange of pieces of DNA between homologous chromosomes • Accounts for unexpected phenotypes in the F2 generation of Morgan’s experiment Chromosome Mapping • Likelihood of genes crossing over depends on distance from each other on chromosome • Farther apart the genes, the more likely that they will cross • If more offspring show the new combination of traits, the farther the genes are on a chromosome • Scientist conduct breeding experiments to determine how frequently genes separate from one another • THUS! They measure distance of genes on chromosomes • THUS! They prepare chromosome maps: diagram used to show linear sequence of gene on chromosome Alfred H. Sturtevant • Morgan’s student • Used crossing over data to construct chromosome map of Drosophila • Genes that are separated by crossing over 1% of the time are 1 map unit apart Mutation • MANY Different types: – Chromosomal: Whole chromosome – Genetic: Single nucleotide • Germ-cell mutations: in gametes – Do NOT affect organism – Affect organism’s offspring • Somatic mutations: in body cells – affect organism (skin cancer, leukemia) – NOT passed on to offspring Chromosome Mutations • Deletion: loss of a piece of chromosome due to chromosomal breakage – All info of that chromosome lost • Inversion: piece of chromosome broken off and reattached upsidedown • Translocation: piece of chromosome breaks off and attaches to another, nonhomologous chromosome • Nondisjunction: failure of chromosome to separate from homologue during meiosis – Result: one gamete receives EXTRA COPY of a chromosome Gene Mutations • Point mutation: single based mutated – Substitution, deletion, insertion • Substitutions: single nucleotide replaced with a different nucleotide ( ATC TTC) * REMEMBER: DNA mRNA amino acid • Results: 1- base codes for same A.A. NO CHANGE 2- base codes for different A.A. Protein change 3- base code for STOP codon Protein change Sickle Cell Anemia • Point substitution mutation • Substitutes adenine for thymine • Protein changed: hemoglobin • Result: irregular shaped red blood cells – Circulatory problems: heart, brain, lungs and many other organs and tissues damaged – Widespread in African Americans ( 1 of 500 in US) • In US 1 of every 10 African Americans are heterozygous for sickle cell anemia • Heterozygous: produced both normal and mutated forms of hemoglobin – Usually healthy Frame Shift Mutations • Deletion: one or more nucleotide deleted from sequence • Insertion: one or more nucleotides inserted • Frame shift mutation: one or more nucleotide deleted or inserted • MUCH more serious effects • Entire rest of sequence mutated • More DNA mutated if deletion/insertion occurs closer to beginning of sequence – More amino acids inaccurately coded for 12-2 Human Genetics Quick Look: • 23 pairs of chromosomes • 100,000 genes • Scientists focus on disease causing genes – Easily traced from one generation to the next Pedigree Analysis • Pedigree: a family record that shows how a trait is inherited over several generations • Study phenotypes of family to see how traits are inherited Patterns of Inheritance • Certain phenotypes usually repeat in predictable patterns • Carriers: individuals who have one recessive autosomal allele Genetic Disorders • Diseases or condition that has a genetic basis Single-allele Traits • Controlled by single allele of a gene • 200+ Huntington’s Disease (HD) *“Autosomal-dominant pattern of inheritance” – Caused by dominant allele on autosome • Symptoms: mild forgetfulness; irritability • Results: loss of muscle control; uncontrollable physical spasms; severe mental illness; death • On-set: 40’s or later *Usually unknown until after individuals have children *THUS! Usually passed on without knowing Genetic Marker • Short section of DNA that is known to have close association with particular gene • If marker is present, USUALLY gene is present • Sampling can show presence of gene (disorder) – Marker present: 96% chance of HD Homozygous Recessive Single-allele Traits • • • • Expressed ONLY in homozygous state (aa) Show autosomal-recessive patt. of inherit. 250+ Ex: Cystic fibrosis (CF) and sickle cell anemia Multiple-allele Traits • Controlled by three or more alleles of the same gene that code for a single trait • Ex: ABO blood groups – Three alleles: IA, IB, i – Codominant: IA, IB Polygenic Traits • Traits controlled by two or more genes • Show many degrees of variation • Skin color; 3-6 genes • Eye color; – Light blue: very little melanin – Dark brown: a lot of melanin • Many influenced by environment – Height: also influenced by nutrition and disease X-Linked Traits • Not all are diseases • Ex: Colorblindness, Hemophilia Sex-Influenced Traits • When presence of female or male hormones influence expression of certain human traits • Ex: Pattern Baldness – Male and female: BB lose hair – Male: BB’ lose hair due to high testosterone – Female: B’ does NOT lose hair • Genes that code for trait on autosomes NOT gametes! Due to Nondisjunction • Lack a chromosome OR have an extra • Monosomy: only one copy of a chromosome – 45 instead of 46 • Trisomy: an extra copy of a chromosome – 47 instead of 46 Down Syndrome • AKA Trisomy-21 • Extra chromo. 21 • Severe mental retardation, characteristic facial features, muscle weakness, heart defects, short stature Nondisjunction in Sex Chromosome • Klinefelters syndrome: XXY – Female appearance; mental retardation; infertile • Turner’s syndrome: XO – Female; do not mature sexually; infertile • Just Y CANNOT survive • X contains info vital to survival Detecting Human Genetic Disorders • Family history of genetic disorders Genetic screening before becoming a parent – Examination of genetic makeup • Make karyotype • Test blood for certain proteins • Genetic counseling: medical guidance to inform parents about problems that could affect offspring Amniocentesis • Physician removes small amount of amniotic fluid from amnion (sac that surrounds fetus) – Fetal cells and proteins analyzed – Karyotype prepared Chronic villi Sampling • Physician takes sample of chronic villi (tissue that grow between mother’s uterus and placenta) – Produce karyotype • Has same genetic make up as fetus Sample Taken After Birth • Immediately after birth genetic testing done of fetus • Ex: PKU (phenylketonuria) – Disorder when infant cannot metabolize the amino acid phenylalanine – Excess phenylalanine can cause brain damage – Infants put on diet without phenylalanine