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Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
• 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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Updated April 2006 Created by C. IppolitoApril 2006
Mendel’s Laws
• GregorGregor MendelMendel – Austrian monk – first to explain how characteristics
are passed– Experimented with pea plants
Updated April 2006 Created by C. IppolitoApril 2006
Law of Segregation• Each trait is determined by twotwo
factorsfactors.– In sex cells factorsfactors separateseparate.– Zygote gets one factor from each
parent.
Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
Phenotype• the physicalphysical appearanceappearance
caused by the alleles.– Both TTTT and TtTt have samesame
appearanceappearance - tallness
Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
•Linked Traits–Inherited together because on same chromosome pair•Exception Law of Independent Assortment
Gene Linkage
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Updated April 2006 Created by C. IppolitoApril 2006
• During synapsis of meiosis chromosomes exchange pieces
• Breaks linkage
Crossing Over
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Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
• chromosome pairs do not separate properly – NondisjunctionNondisjunction is the failurefailure of
chromosome pairs to separateseparate properlyproperly.
Chromosome Abnormalities
Updated April 2006 Created by C. IppolitoApril 2006
• The failure to separate results in:– sex cells extraextra chromosomechromosome– or– sex cells missingmissing chromosomechromosome
Nondisjunction
Updated April 2006 Created by C. IppolitoApril 2006
Down Syndrome• TrisomyTrisomy 2121
– 1:800 < 40 ♀ 1:90 > 40 ♀– short stature; enlarged
tongue– rounded head; eyefold– mentalmental retardationretardation
Updated April 2006 Created by C. IppolitoApril 2006
Cri du Chat Syndrome• MonosomyMonosomy 55 (piece
deleted)– moon face as infant– malformed larynx– eyefold– misshapen, low ears– severesevere mentalmental
retardationretardation
Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
• 4545, XOXO 1:6000 ♀– short with broad chest– webbed neck– ovaries, oviduct, uterus
small– nono pubertypuberty; nono
menstruationmenstruation– most normal mentally
Turner Syndrome
Updated April 2006 Created by C. IppolitoApril 2006
• 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
Updated April 2006 Created by C. IppolitoApril 2006
• 4747, XYYXYY 1:1000 ♂ – taller than normal– persistent acne– speech & reading problems– more aggressive due to size– “criminalcriminal” syndromesyndrome
Jacob Syndrome
Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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
Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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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Updated April 2006 Created by C. IppolitoApril 2006
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)