Mistakes Happen

DNA is the genetic material ofliving organisms and is located inthe chromosomes of each cell.

What happens if a mistake is madewhen DNA is copied?

Random errors can occur when DNA iscopied, or damage can be caused byphysical and chemical agents known asmutagens.

• A mutation is a change in the hereditarymaterial of an organism.

• Although usually the processes of DNAreplication and meiosis happen withoutmistakes, mutations can happen in any celland in any gene.• They also can be deliberately caused in thelaboratory by scientists.

Some mutations involve a change inthe structure or number ofchromosomes.

• For instance, during meiosis one or morepairs of chromosomes may fail to separate.• Sex cells with extra sets of chromosomesmay be produced.

A mutation in thenumber ofchromosomes.

In plants, a complete extra set ofchromosomes can cause desirable traits.• Some varieties of strawberries are bredwith extra sets of chromosomes.• This causes the berries to grow extra large.

A genetic disorder is an abnormalcondition that an organism inheritsfrom its parents.• Genetic disorders can result from mutation of a single gene or mutation of the chromosomes.• In order to be passed on to offspring, the mutation must be present in the sex cells.

Some mutations are helpful while others are harmful.• Genetic disorders are a harmfuleffect of mutations.

• One chromosome pair has a mutated extra chromosome.

Some genetic disorders result fromtoo many or too few chromosomes.• Down’s syndrome, forexample, is a geneticdisorder in which aperson’s cells have anextra copy ofchromosome 21.• People with Down’ssyndrome have somemental and physicallimitations. .• However, they canlead normal,productive lives

Some genetic disorders result when amutation causes the product of a singlegene to be altered or missing.• An example of thiskind of disorder iscystic fibrosis.• Cystic fibrosisaffects about30,000 children andadults in the UnitedStates.

A recessive allele causes the body to producean abnormally thick, sticky mucus that clogsthe lungs and leads to life-threatening lunginfections.• Thick mucus alsoclogs the organsof the digestivesystem and oftenleads to digestiveproblems andliver damage.

Another example ofthis type of geneticdisorder ishemophilia, aninherited bleedingdisorder in whichblood doesn’t clotnormally.

A gene mutation involves a change inone of the bases in the sequence along a gene.

• A change in the base sequence changes one of the three-letter codes for an amino acid.• This may cause the cell to produce thewrong protein.

In the example below, one of the basesin the sequence is substituted foranother.• The mutation causes sickle cell anemia, a blood disorder.

Sickle cells often cause blockages in theblood vessels of people who have them.• This disease, calledsickle cell anemia, ispassed from parent tooffspring.• It has a higherincidence in Africansbecause the gene alsoprotected againstmalaria.

Normal hemoglobin has disc-shaped redblood cells that are soft (like a bag ofjelly), which enables them to easily flowthrough small blood vessels.• Diseased red blood cellshave a sickle-shape, arehard (like pieces ofwood), and often getstuck in small bloodvessels and stop the flowof blood.

Watch A Mutation Story

• http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html

Discussion Questions• What effect did the sickle cell gene have onthe people who were carriers of the mutation?• Why has the sickle cell gene persisted evenwhen sickle cell anemia is so debilitating?• What are the odds that the child of parentswho each carry one normal gene and onesickle cell mutation gene will have sickle cellanemia?• What are the odds that a child of two carrierparents will also be a carrier and, thus, beprotected from malaria?

To find out if a baby will have a geneticdisorder, doctors use a procedure calledamniocentesis.• In amniocentesis, the doctorremoves a small amount ofthe fluid that surrounds thedeveloping baby.• That fluid contains cells fromthe baby.• Next, the chromosomes fromthe cells are analyzed to lookfor abnormal genes orchromosome numbers.

Watch the One Wrong Letter Video Clip

http://www.pbs.org/wgbh/nova/genome/media/2809_q056_03.htmlDiscussion Questions

• What causes Tay-Sachs disease?

• What are the odds that a child born to two carrier parents will inherit Tay-Sachs disease?

Dominant Disorders: A 50-50 Chance• The affected parenthas a single defectivegene (D), whichdominates its normalcounterpart (n).• Each child has a 50percent risk ofinheriting the faultygene and the disorder.

Recessive Disorders: One Chance in Four• Both parents carry asingle defective gene(d) but are protectedby the presence of anormal gene (N),which is generallysufficient for normalfunction.• Two defective copiesof the gene arerequired to produce adisorder.

Each child has a 50%chance of being acarrier like both parentsand a 25% risk ofinheriting the disorder.