Updated April 2006Created by C. Ippolito April 2006 Chapter 12 Patterns of Inheritance.

Updated April 2006 Created by C. IppolitoApril 2006

Chapter 12

Patterns of Inheritance


Gene and Allele• Gene - sequence of DNAsequence of DNA found in a

specific site (locus) on a chromosome.– DeterminesDetermines a trait or characteristiccharacteristic in a

human.• AlleleAllele - alternate forms - alternate forms of a gene

that occupy the same loci on a chromosome.– The interaction of 2 alleles on a

chromosome pair determine a person’s trait.

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• Sex of a child is determined by the male parentmale parent.– Sex cells of parents contain one member

of each pair.– The eggegg will always have an X spermsperm can

have an X or Y.

• Female - egg is fertilized by a sperm bearing an X chromosomeX chromosome.

• Male - egg is fertilized by a sperm bearing an Y chromosomeY chromosome.

Sex Determination

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Mendel’s Laws

• GregorGregor MendelMendel – Austrian monk – first to explain how characteristics

are passed– Experimented with pea plants


Law of Segregation• Each trait is determined by twotwo

factorsfactors.– In sex cells factorsfactors separateseparate.– Zygote gets one factor from each

parent.


Law of Dominance• Mendel determined that certain

factors overpowered the appearance of others.– DominantDominant – overpowers

• presencepresence of factor forcesforces traittrait to appearappear regardless of the form of the other factor on its companion chromosome.

– RecessiveRecessive - is overpowered• Both chromosomes must possesschromosomes must possess this

factor for trait to appear


Genotype• the actualactual genegene pairspairs or allelesalleles

present to cause a trait.– There are two genotypes:

• Homozygous– Genotype with twotwo identicalidentical allelesalleles causing a

trait.» TTTT - homozygous dominant» tttt - homozygous recessive - the onlyonly wayway a

recessiverecessive can showshow• Heterozygous

– Genotype with twotwo differentdifferent allelesalleles causing a trait.

» TtTt - only dominant appears


Phenotype• the physicalphysical appearanceappearance

caused by the alleles.– Both TTTT and TtTt have samesame

appearanceappearance - tallness


Law of Independent Assortment

• Factor inherited in one gene does not affect the factor that will be inherited in another gene– not true if genes are linked -

found on same chromosome


Polygenic (Multifactorial ) Inheritance

• Single trait controlled by two or more sets of alleles.– Traits show continuous variation “bell curve”– The greater the number of alleles the more perfect the

bell curve will be.• Known polygenic traits:

– Skin Color– Height

• Possible polygenic traits– Cleft Palate– Club Foot– Hypertension– Diabetes– Schizophrenia– Allergies

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Multiple Alleles• Trait controlled one gene pair

with more than two distinct alleles.– Human Blood types

• IA - codominant -causes A blood• IB - codominant -causes B blood• i - recessive -cause O blood

• Incomplete Dominance– Alleles for A and B do not

dominate so the heterozygote IAIB

has what is known as Type AB blood

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Blood Types

• Type AIAi - heterozygous for AIAIA - homozygous for Aproduces anti-Bcan be given A or O blood

• Type BIBi - heterozygous for BIBIB - homozygous for Bproduces anti-Acan be given B or O blood

• Type ABIAIB - heterozygous for ABproduces no antibodyUniversal Receivercan be given any blood

• Type Oii - homozygous for Oproduces anti-A and anti-BUniversal Donorcan be given only O blood

Protein marker (antigen) on membrane of the RBC.

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Pleiotropy• One gene affects many traits.• Sickle Cell Anemia

– Recessive - mainly affects African Americans• abnormal hemoglobin • Mutation substituted a valine for

glutamic acid• originated in areas where malaria was

endemic– HbN = normal Hbn = sickle– HbNHbN - normal but can get malaria– HbNHbn - normal but immune to malaria– HbnHbn - sickle cell anemia

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•Linked Traits–Inherited together because on same chromosome pair•Exception Law of Independent Assortment

Gene Linkage

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• During synapsis of meiosis chromosomes exchange pieces

• Breaks linkage

Crossing Over

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Sex-Linked Genes

• genes on the “X” with no equivalent part on the “Y”– appear more often in males -

inherit only one allele• hemophilia• Duchenne Muscular Dystrophy• colorblindness


• chromosome pairs do not separate properly – NondisjunctionNondisjunction is the failurefailure of

chromosome pairs to separateseparate properlyproperly.

Chromosome Abnormalities


• The failure to separate results in:– sex cells extraextra chromosomechromosome– or– sex cells missingmissing chromosomechromosome

Nondisjunction


Down Syndrome• TrisomyTrisomy 2121

– 1:800 < 40 ♀ 1:90 > 40 ♀– short stature; enlarged

tongue– rounded head; eyefold– mentalmental retardationretardation


Down Syndrome Karyotype


Cri du Chat Syndrome• MonosomyMonosomy 55 (piece

deleted)– moon face as infant– malformed larynx– eyefold– misshapen, low ears– severesevere mentalmental

retardationretardation


Fragile X Syndrome• 1:1000 ♂ 1:2500♀• children

– normal but hyperactivehyperactive or autistic

• adults:– short stature; long face– prominent jaw; large ears– stubby hands– ♂’s more likely to be

mentally deficient


• 4545, XOXO 1:6000 ♀– short with broad chest– webbed neck– ovaries, oviduct, uterus

small– nono pubertypuberty; nono

menstruationmenstruation– most normal mentally

Turner Syndrome


Turner Syndrome Karyotype


• 4747, XXYXXY 1:1500 ♂ – genitalia, testes, prostate

small– no 2o sex characteristics– some breast development– large hands, feet; long

limbs– slow but no deficiency

(XXXY)

Klinefelter Syndrome


Klinefelter Karyotype


• 4747, XYYXYY 1:1000 ♂ – taller than normal– persistent acne– speech & reading problems– more aggressive due to size– “criminalcriminal” syndromesyndrome

Jacob Syndrome


Human Genetic

Disorders


Achondroplasia• Autosomal dominant allele• Gene controlling fibroblast function

– short stature, legs and arms short compared to trunk (body), prominent forehead, a protruding jaw, bowed legs

• Diagnosis– X-ray and clinical interview– Molecular testing can identify mutated

gene (FGFR3)• Treatment

– monitor growth, check for apnea, surgical correction if walking difficult

• Genetic Counseling


Huntington’s Chorea• Autosomal dominant allele

– 1:20,000 births• Chronic neurological disease

– slow destruction of brain cells. – uncontrolled motions– appears in middle age (40’s)– death within 10 to 20 years of onset

• Woody Guthrie - hobo poet and folk singer - “This Land is Your Land”

• No Cure; No treatment• Gene has been identified and presence can be

determined by blood test

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http://www.woodyguthrie.org/biography.htm

http://www.arlo.net/lyrics/this-land.shtml


Albinism• Autosomal recessive allele• Missing pigments

– in skin, hair, and eyes– low vision due to abnormal retina

• Treatment– sunscreen protection and opaque

clothing– vision aids

• Type I – missing enzyme to form pigment

• Type II– some pigment formed “P” gene

defective


Cystic Fibrosis• Autosomal recessive allele

– 1:20 Caucasians are carriers– occurs in 1:2,500 births

• bronchial mucus becomes very thick and viscous– Mucus sits in alveoli and bronchi– Daily manual message known as

“cupping and clapping” used to loosen mucus.

– Regular visits to hospital needed to “suction” lungs

• usually die before their 18th birthday – current treatment may extend

life into 28• No cure but gene has been identified • Genetic screening possible

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Phenylketonuria (PKU)

• Autosomal recessive – 1:5,000

• No enzyme – to break down phenylalanine.– phenylketone build up which can harm developing

brain cells.– mental retardation.

• Diagnosis - blood test at birth.• Treatment

– special diet low in phenylalanine until age 7.

• Adults – excess phenylalanine causes headaches

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Tay Sachs Disease

• Autosomal recessive– high occurrence in American Jews - 1:1,000 births– higher occurrence in Ashkenazy Jews - 1:40 births

• Lipid buildup around nerve cells.– Normal until 4 months of age.– 5th month - lethargy– 8th month reactions and movements not normal– child becomes blind, deaf, has seizures, paralysis,

and dies before 5th year.• No cure, no treatment, no definite test for

presence of gene• Genetic Counseling - Carriers should not

marry

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Human Traits

• Pedigree Chart– a diagram that shows the relationships within

a family• a circle represents a female• a square represents a male• horizontal line connecting a circle to a square

represents a marriage• a filled-in circle or square shows that the

individual has the trait • vertical line(s) and brackets below that line

show their child(ren)

Updated April 2006Created by C. Ippolito April 2006 Chapter 12 Patterns of Inheritance.

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Transcript of Updated April 2006Created by C. Ippolito April 2006 Chapter 12 Patterns of Inheritance.