LEQ: What are some of the LEQ: What are some of the diseases we inherit diseases we inherit

genetically?genetically?

Recessively Inherited Recessively Inherited DisordersDisorders

Many genetic disorders are inherited in a Many genetic disorders are inherited in a recessive mannerrecessive manner

These range from relatively mild to life-These range from relatively mild to life-threateningthreatening

Recessively inherited disorders show up only in Recessively inherited disorders show up only in individuals homozygous for the alleleindividuals homozygous for the allele

CarriersCarriers are heterozygous individuals who carry are heterozygous individuals who carry the recessive allele but are phenotypically the recessive allele but are phenotypically normal; most individuals with recessive normal; most individuals with recessive disorders are born to carrier parentsdisorders are born to carrier parents

Albinism is a recessive condition characterized Albinism is a recessive condition characterized by a lack of pigmentation in skin and hairby a lack of pigmentation in skin and hair

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Figure 14.16Figure 14.16

Parents

NormalAa

Sperm

Eggs

NormalAa

AANormal

AaNormal(carrier)

AaNormal(carrier)

aaAlbino

A

A

a

a

If a recessive allele that causes a disease is If a recessive allele that causes a disease is rare, then the chance of two carriers meeting rare, then the chance of two carriers meeting and mating is lowand mating is low

Consanguineous matings (i.e., matings between Consanguineous matings (i.e., matings between close relatives) increase the chance of mating close relatives) increase the chance of mating between two carriers of the same rare allele - between two carriers of the same rare allele - “INBREEDING”“INBREEDING”

Most societies and cultures have laws or taboos Most societies and cultures have laws or taboos against marriages between close relativesagainst marriages between close relatives

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Cystic FibrosisCystic Fibrosis

Cystic fibrosisCystic fibrosis is the most common lethal is the most common lethal genetic disease in the United States, striking genetic disease in the United States, striking one out of every 2,500 people of European one out of every 2,500 people of European descent descent

The cystic fibrosis allele results in defective or The cystic fibrosis allele results in defective or absent chloride transport channels in plasma absent chloride transport channels in plasma membranes leading to a buildup of chloride ions membranes leading to a buildup of chloride ions outside the celloutside the cell

Symptoms include mucus buildup in some Symptoms include mucus buildup in some internal organs and abnormal absorption of internal organs and abnormal absorption of nutrients in the small intestinenutrients in the small intestine

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Sickle-Cell Disease: A Sickle-Cell Disease: A Genetic Disorder Genetic Disorder with with

Evolutionary ImplicationsEvolutionary ImplicationsSickle-cell diseaseSickle-cell disease affects one out of 400 affects one out of 400

African-AmericansAfrican-Americans

The disease is caused by the substitution of a The disease is caused by the substitution of a single amino acid in the hemoglobin protein in single amino acid in the hemoglobin protein in red blood cellsred blood cells

In homozygous individuals, all hemoglobin is In homozygous individuals, all hemoglobin is abnormal (sickle-cell)abnormal (sickle-cell)

Symptoms include physical weakness, pain, Symptoms include physical weakness, pain, organ damage, and even paralysisorgan damage, and even paralysis

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Fig. 14-UN1

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Heterozygotes (said to have sickle-cell trait) are Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptomsusually healthy but may suffer some symptoms

About one out of ten African Americans has sickle About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele cell trait, an unusually high frequency of an allele with detrimental effects in homozygoteswith detrimental effects in homozygotes

Heterozygotes are less susceptible to the malaria Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being parasite, so there is an advantage to being heterozygousheterozygous

Dominantly Inherited Dominantly Inherited DisordersDisorders

Some human disorders are caused by dominant Some human disorders are caused by dominant allelesalleles

Dominant alleles that cause a lethal disease are Dominant alleles that cause a lethal disease are rare and arise by mutationrare and arise by mutation

AchondroplasiaAchondroplasia is a form of dwarfism caused by is a form of dwarfism caused by a rare dominant allelea rare dominant allele

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Figure 14.17Figure 14.17

Parents

DwarfDd

Sperm

EggsDd

Dwarfdd

Normal

DdDwarf

ddNormal

D

d

d

d

Normaldd

Huntington’s Disease: A Huntington’s Disease: A Late-Onset Lethal DiseaseLate-Onset Lethal Disease The timing of onset of a disease significantly affects The timing of onset of a disease significantly affects

its inheritanceits inheritance

Huntington’s diseaseHuntington’s disease is a degenerative disease is a degenerative disease of the nervous systemof the nervous system

The disease has no obvious phenotypic effects until The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of agethe individual is about 35 to 40 years of age

Once the deterioration of the nervous system Once the deterioration of the nervous system begins the condition is irreversible and fatalbegins the condition is irreversible and fatal

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Multifactorial DisordersMultifactorial Disorders

Many diseases, such as heart disease, diabetes, Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have alcoholism, mental illnesses, and cancer have both genetic and environmental componentsboth genetic and environmental components

Little is understood about the genetic Little is understood about the genetic contribution to most multifactorial diseasescontribution to most multifactorial diseases

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Genetic Testing and Genetic Testing and CounselingCounseling

Genetic counselors can provide information to Genetic counselors can provide information to prospective parents concerned about a family history prospective parents concerned about a family history for a specific diseasefor a specific disease

Using family histories, genetic counselors help Using family histories, genetic counselors help couples determine the odds that their children will couples determine the odds that their children will have genetic disordershave genetic disorders

Probabilities are predicted on the most accurate Probabilities are predicted on the most accurate information at the time; predicted probabilities may information at the time; predicted probabilities may change as new information is availablechange as new information is available

For a growing number of diseases, tests are available For a growing number of diseases, tests are available that identify carriers and help define the odds more that identify carriers and help define the odds more accuratelyaccurately

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Fetal TestingFetal Testing

In In amniocentesisamniocentesis, the liquid that bathes the , the liquid that bathes the fetus is removed and testedfetus is removed and tested

In In chorionic villus sampling (CVSchorionic villus sampling (CVS), a sample ), a sample of the placenta is removed and testedof the placenta is removed and tested

Other techniques, such as Other techniques, such as ultrasoundultrasound and and fetoscopyfetoscopy, allow fetal health to be assessed , allow fetal health to be assessed visually in uterovisually in utero

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Figure 14.19Figure 14.19

(a) Amniocentesis (b) Chorionic villus sampling (CVS)

Ultrasound monitor

Amnioticfluidwithdrawn

FetusPlacentaUterus Cervix

Centrifugation

Fluid

Fetal cells

Several hours

Severalweeks

Several weeks

Biochemicaland genetic

tests

Karyotyping

Ultrasoundmonitor

Fetus

Placenta

Chorionic villi

Uterus

Cervix

Suctiontubeinsertedthroughcervix

Several hours

Fetal cells

Several hours

1

1

2

2

3

Newborn ScreeningNewborn Screening

Some genetic disorders can be detected at birth Some genetic disorders can be detected at birth by simple tests that are now routinely performed by simple tests that are now routinely performed in most hospitals in the United Statesin most hospitals in the United States

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