Chromosomes And Human Inheritance

Chromosomes andHuman InheritanceChapter 12

Impacts, Issues:Strange Genes, Tortured Minds Exceptional creativity often accompaniesneurobiological disorders such as schizophrenia,autism, chronic depression, and bipolar disorder Examples: Lincoln, Woolf, and Picasso

12.1 Human Chromosomes In humans, two sex chromosomes are thebasis of sex – human males have XY sexchromosomes, females have XX All other human chromosomes are autosomes– chromosomes that are the same in males andfemales

Sex Determination in Humans Sex of a child is determined by the father Eggs have an X chromosome; sperm have X or Y

Sex Determination in Humans The SRY gene on the Y chromosome is themaster gene for male sex determination Triggers formation of testes, which produce themale sex hormone (testosterone) Without testosterone, ovaries develop andproduce female sex hormones (estrogens)

Sexual Development in Humans

diploidgerm cellsin malediploidgerm cellsin femalemeiosis, gameteformation in bothfemale and male:eggsspermX YX Xfertilization:XXXXXXXYXYXYsex chromosome combinationspossible in the new individualFig. 12-2a, p. 186

Karyotyping Karyotype A micrograph of all metaphase chromosomes in acell, arranged in pairs by size, shape, and length Detects abnormal chromosome numbers andsome structural abnormalities Construction of a karyotype Colchicine stops dividing cells at metaphase Chromosomes are separated, stained,photographed, and digitally rearranged

Karyotyping

12.1 Key ConceptsAutosomes and Sex Chromosomes All animals have pairs of autosomes –chromosomes that are identical in length, shape,and which genes they carry Sexually reproducing species also have a pair ofsex chromosomes; the members of this pairdiffer between males and females

12.2 Autosomal Inheritance Patterns Many human traits can be traced to autosomaldominant or recessive alleles that are inheritedin Mendelian patterns Some of those alleles cause genetic disorders

Autosomal Dominant Inheritance A dominant autosomal allele is expressed inhomozygotes and heterozygotes Tends to appear in every generation With one homozygous recessive and oneheterozygous parent, children have a 50%chance of inheriting and displaying the trait Examples: achondroplasia, Huntington’s disease

Autosomal Recessive Inheritance Autosomal recessive alleles are expressed onlyin homozygotes; heterozygotes are carriers anddo not have the trait A child of two carriers has a 25% chance ofexpressing the trait Example: galactosemia

Autosomal Inheritance

Galactosemia

Neurobiological Disorders Most neurobiological disorders do not followsimple patterns of Mendelian inheritance Depression, schizophrenia, bipolar disorders Multiple genes and environmental factorscontribute to NBDs

12.3 Too Young to be Old Progeria Genetic disorder that results in accelerated aging Caused by spontaneous mutations in autosomes

12.2-12.3 Key ConceptsAutosomal Inheritance Many genes on autosomes are expressed inMendelian patterns of simple dominance Some dominant or recessive alleles result ingenetic disorders

12.4 Examples of X-Linked Inheritance X chromosome alleles give rise to phenotypesthat reflect Mendelian patterns of inheritance Mutated alleles on the X chromosome cause orcontribute to over 300 genetic disorders

X-Linked Inheritance Patterns More males than females have X-linkedrecessive genetic disorders Males have only one X chromosome and canexpress a single recessive allele A female heterozygote has two X chromosomesand may not show symptoms Males transmit an X only to their daughters, notto their sons

X-Linked Recessive Inheritance Patterns

Some X-Linked Recessive Disorders Hemophilia A Bleeding caused by lack of blood-clotting protein Red-green color blindness Inability to distinguish certain colors caused byaltered photoreceptors in the eyes Duchenne muscular dystrophy Degeneration of muscles caused by lack of thestructural protein dystrophin

Hemophilia A in Descendentsof Queen Victoria of England

Red-Green Color Blindness

Fig. 12-9c, p. 191

Fig. 12-9d, p. 191

12.4 Key ConceptsSex-Linked Inheritance Some traits are affected by genes on the Xchromosome Inheritance patterns of such traits differ in malesand females

12.5 Heritable Changesin Chromosome Structure On rare occasions, a chromosome’s structurechanges; such changes are usually harmful orlethal, rarely neutral or beneficial A segment of a chromosome may be duplicated,deleted, inverted, or translocated

Duplication DNA sequences are repeated two or moretimes; may be caused by unequal crossovers inprophase I

Deletion Loss of some portion of a chromosome; usuallycauses serious or lethal disorders Example: Cri-du-chat

Deletion: Cri-du-chat

Inversion Part of the sequence of DNA becomes orientedin the reverse direction, with no molecular loss

Translocation Typically, two broken chromosomes exchangeparts (reciprocal translocation)

Does Chromosome Structure Evolve? Changes in chromosome structure can reducefertility in heterozygotes; but accumulation ofmultiple changes in homozygotes may result innew species Certain duplications may allow one copy of agene to mutate while the other carries out itsoriginal function

Differences AmongClosely Related Organisms Humans have 23 pairsof chromosomes;chimpanzees, gorillas,and orangutans have24 Two chromosomesfused end-to-end

Evolution of X and Y Chromosomesfrom Homologous Autosomes

12.6 Heritable Changes inthe Chromosome Number Occasionally, new individuals end up with thewrong chromosome number Consequences range from minor to lethal Aneuploidy Too many or too few copies of one chromosome Polyploidy Three or more copies of each chromosome

Nondisjunction Changes in chromosome number can be causedby nondisjunction, when a pair ofchromosomes fails to separate properly duringmitosis or meiosis Affects the chromosome number at fertilization Monosomy (n-1 gamete) Trisomy (n 1 gamete)

Nondisjunction

Autosomal Change and Down Syndrome Only trisomy 21 (Down syndrome) allowssurvival to adulthood Characteristics include physical appearance,mental impairment, and heart defects Incidence of nondisjunction increases withmaternal age Can be detected through prenatal diagnosis

Trisomy 21

n 1n 1n 1n 1chromosomealignments atmetaphase INONDISJUNCTION alignments atAT ANAPHASE Imetaphase IICHROMOSOMENUMBERanaphase II IN GAMETESFig. 12-13b, p. 194

Down Syndrome and Maternal Age

Change in Sex Chromosome Number Changes in sex chromosome number mayimpair learning or motor skills, or be undetected Female sex chromosome abnormalities Turner syndrome (XO) XXX syndrome (three or more X chromosomes) Male sex chromosome abnormalities Klinefelter syndrome (XXY) XYY syndrome

Turner Syndrome XO (one unpaired Xchromosome) Usually caused bynondisjunction in thefather Results in femaleswith undevelopedovaries

12.5-12.6 Key Concepts: Changes inChromosome Structure or Number On rare occasions, a chromosome may undergoa large-scale, permanent change in its structure,or the number of autosomes or sexchromosomes may change In humans, such changes usually result in agenetic disorder

12.7 Human Genetic Analysis Charting genetic connections with pedigreesreveals inheritance patterns for certain alleles Pedigree A standardized chart of genetic connections Used to determine the probability that futureoffspring will be affected by a genetic abnormalityor disorder

Studying Inheritance in Humans Genetic studies canreveal inheritancepatterns or clues topast events Example: A linkbetween a Ychromosome andGenghis Khan?

Defining Genetic Disordersand Abnormalities Genetic abnormality A rare or uncommon version of a trait; notinherently life threatening Genetic disorder An inherited condition that causes mild to severemedical problems, characterized by a specific setof symptoms (a syndrome)

Some Human Genetic Disordersand Genetic Abnormalities

Stepped ArtTable 12-1, p. 196

Recurring Genetic Disorders Mutations that cause genetic disorders are rareand put their bearers at risk Such mutations survive in populations forseveral reasons Reintroduction by new mutations Recessive alleles are masked in heterozygotes Heterozygotes may have an advantage in aspecific environment

A Pedigree for Huntington’s Disease A progressive degeneration of the nervoussystem caused by an autosomal dominant allele

Constructing a Pedigree for Polydactyly

12.8 Prospects in Human Genetics Genetic analysis can provide parents withinformation about their future children Genetic counseling Starts with parental genotypes, pedigrees, andgenetic testing for known disorders Information is used to predict the probability ofhaving a child with a genetic disorder

Prenatal Diagnosis Tests done on an embryo or fetus before birth toscreen for sex or genetic problems Involves risks to mother and fetus Three types of prenatal diagnosis Amniocentesis Chorionic villus sampling (CVS) Fetoscopy

Amniocentesis

Fetoscopy

Preimplantation Diagnosis Used in in-vitro fertilization An undifferentiated cell is removed from the earlyembryo and examined before implantation

After Preimplantation Diagnosis When a severe problem is diagnosed, someparents choose an induced abortion In some cases, surgery, prescription drugs,hormone replacement therapy, or dietarycontrols can minimize or eliminate symptoms ofa genetic disorder Example: PKU can be managed with dietaryrestrictions

Genetic Screening Genetic screening (widespread, routine testingfor alleles associated with genetic disorders) Provides information on reproductive risksIdentifies family members with a genetic disorderUsed to screen newborns for certain disordersUsed to estimate the prevalence of harmfulalleles in a population

12.7-12.8 Key ConceptsHuman Genetic Analysis Various analytical and diagnostic proceduresoften reveal genetic disorders What an individual, and society at large, shoulddo with the information raises ethical questions

Autosomes and Sex Chromosomes All animals have pairs of autosomes – chromosomes that are identical in length, shape, and which genes they carry Sexually reproducing species also have a pair of sex chromosomes; the members of th