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: Germ-cell mutations: in gametes 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 upside-down 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)

Heterozygous: produced both normal and mutated forms of hemoglobin 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