Foothill High School Science Department The Human Genome Chromosomes & Phenotype Genetic Disorders.

Foothill High School Science DepartmentFoothill High School Science Department

The Human GenomeThe Human Genome

Chromosomes & PhenotypeGenetic Disorders

Chromosomes & PhenotypeGenetic Disorders


Autosomal vs Sex Linked TraitsAutosomal vs Sex Linked Traits

• Autosomes– All the chromosomes that do NOT play a role in

sex determination• In humans, chromosomes 1 – 22

– Two copies» One from Mom» One from Dad» Same genes» Same location on chromosome» BUT may be different alleles (recipes)


Autosomal vs Sex Linked TraitsAutosomal vs Sex Linked Traits

• Sex Linked Genes– Genes on the X or Y chromosome– X Chromosome• Large• Contains many genes, some essential for survival

– Y Chromosome• Small• Few genes except those necessary to change fetal

development from female to male


Autosomal Disorders caused byRecessive Alleles


• Recessive Alleles cause disorder– Heterozygotes are Carriers• Can pass damaged allele to offspring BUT don’t have

the disease/disorder

– Homozygous Recessive individuals have the disease/disorder




Cystic FibrosisCystic Fibrosis• Mutation of CFTR gene

on chromosome 7• Causes thick mucus in

lungs & blockage of gastrointestinal tract

• Damages the pancreas• Fatal• Both parents must be

heterozygous to have a child with the disease


Autosomal Disorders caused byDominant Alleles


Dominant allele disorders far less common

Huntington’s Disease• Mutation of HTT gene on chromosome 4• Appears during adulthood – usually 35-44 yrs. old

– Allow time for person with disease to reproduce

• Nervous system disintegrates slowly over years• Fatal




NeurofibromatosisNeurofibromatosis

Autosomal Dominant Autosomal Dominant DisorderDisorder

Chromosome 22Chromosome 22


Sex-Linked GenesSex-Linked Genes

Sex Linked Genes Sex Linked Genes Are Genes Located Are Genes Located

On The X & Y ChromosomeOn The X & Y ChromosomeXX – Contains > 100 Genes

YY – Contains Only A Few Genes


Sex-Linked GenesSex-Linked Genes


ColorblindnessColorblindness

The X ChromosomeThe X ChromosomeContains Three (3) Genes Contains Three (3) Genes

Associated With Color VisionAssociated With Color Vision

The Y Chromosome Is Missing Them AllThe Y Chromosome Is Missing Them All

Therefore, Males Are Particularly Therefore, Males Are Particularly Vulnerable To ColorblindnessVulnerable To Colorblindness


ColorblindnessThe Images Below are NOT Diagnostic

ColorblindnessThe Images Below are NOT Diagnostic



RedRed-GreenGreen ColorblindnessMost Common Form

1:10 Males1:100 In Females

Why The Difference?



Colorblind Males Only PassColorblind Males Only PassThe Recessive Allele On To Their DaughtersThe Recessive Allele On To Their DaughtersIt is not on the Y Chromosome They Pass To It is not on the Y Chromosome They Pass To

Their Sons. Their Sons.

(only daughters will receive the fathers X Chromosome)

Thus - The Gene Appears To Skip A Generation And Thus - The Gene Appears To Skip A Generation And Will Only Reappear In His GrandchildrenWill Only Reappear In His Grandchildren


HemophiliaHemophilia

Two Important Blood Clotting Genes Are On Two Important Blood Clotting Genes Are On The X ChromosomeThe X Chromosome

Occurrence Rate = 1:10,000 Males



MildMildPost-InjectionPost-Injection

BleedingBleedingIn AIn A

PatientPatientWithWith



Duchenne Muscular DystrophyDuchenne Muscular Dystrophy

Sex-LinkedSex-LinkedAbnormal Muscle ProteinAbnormal Muscle Protein

Patients Rarely Live Beyond Patients Rarely Live Beyond Early AdulthoodEarly Adulthood

Occurrence Rate =Occurrence Rate =1:3000 Males1:3000 Males


In MammalsFemale X-Chromosome Inactivation


FemalesHave Two (2) X Chromosomes

MalesMalesOnly Have (1) X ChromosomeOnly Have (1) X Chromosome

Do Female ExpressDo Female ExpressBoth X Chromosomes?Both X Chromosomes?

NONO




One X Chromosome Is Randomly Switched OffOne X Chromosome Is Randomly Switched Off– That Chromosome Forms A Dense Region In The

Nucleus Known As A Barr Body




Arrows Indicate Barr BodiesArrows Indicate Barr Bodies


Chromosome DisordersChromosome Disorders

• Most Common Is NondisjunctionNondisjunctionDuring Meiosis, Homologous Chromosomes During Meiosis, Homologous Chromosomes

Fail To SeparateFail To Separate

One Gamete Will Have An Extra Chromosome (TrisomyTrisomy)

The Other Will Have One Less


Down SyndromeDown Syndrome• NondisjunctionNondisjunction of Chromosome 21 (trisomy)• Occurs 1:800 Births• Mental Retardation May Be Mild To Severe• Increased Susceptibility To Disease


Down SyndromeDown Syndrome

Why Would 3 Copies Cause ProblemsWhy Would 3 Copies Cause ProblemsWhen 2 Copies Don’t?When 2 Copies Don’t?

Don’t Know YetDon’t Know YetWorking Hard To Find OutWorking Hard To Find Out


Sex Chromosome DisordersSex Chromosome Disorders

• Turner’s SyndromeTurner’s Syndrome– Females– Nondisjunction

Results In AXO FemaleXO Female

– Sterile– Sex Organs Do Not

Develop During Puberty



• Klinefelter’s SyndromeKlinefelter’s Syndrome– Males– Nondisjunction Results In A XXY Male– The Extra X Interferes With Meiosis – Generally

Sterile– Also Found XXXY & XXXXY– Exhibit Female Body Patterns In Hips, Partial

Breast Development, Female Pubic Hair Pattern



No Babies Are Born Without At Least No Babies Are Born Without At Least One X ChromosomeOne X Chromosome

Indicates The X Chromosome Carries Indicates The X Chromosome Carries At Least One Gene Required For LifeAt Least One Gene Required For Life


Y ChromosomeY Chromosome

• If Present, Individual Is MaleIf Present, Individual Is Male– Even When Multiple X

Chromosomes Are Present– Contains At Least One Gene

Required For Male Sexual Development

No “Y” Present – Embryo Develops As A Female

Foothill High School Science Department The Human Genome Chromosomes & Phenotype Genetic Disorders.

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