Chromosomal Inheritance Chapter 12. 2 Drosophila Chromosomes

Chromosomal Inheritance

Chapter 12

2Drosophila Chromosomes

3ChromosomalChromosomal Inheritance Inheritance

OutlineOutline

X-Linked AllelesX-Linked Alleles

Human X-Linked DisordersHuman X-Linked Disorders

Gene LinkageGene Linkage

Crossing-OverCrossing-Over

Chromosome MapChromosome Map

Changes in Chromosome NumberChanges in Chromosome Number

Changes in Chromosome StructureChanges in Chromosome Structure

Human SyndromesHuman Syndromes


Chromosomal InheritanceChromosomal Inheritance

Humans are diploid (2 chromosomes of each type)Humans are diploid (2 chromosomes of each type)

Humans have 23 different kinds of chromosomesHumans have 23 different kinds of chromosomes

Arranged in 23 pairs of homologous chromosomesArranged in 23 pairs of homologous chromosomes

Total of 46 chromosomes (23 pairs) per cellTotal of 46 chromosomes (23 pairs) per cell

One of the chromosome pairs determines the sex of One of the chromosome pairs determines the sex of an individual (The sex chromosomes)an individual (The sex chromosomes)

The other 22 pairs of chromosomes are autosomesThe other 22 pairs of chromosomes are autosomes

Autosomal chromosomes are numbered from Autosomal chromosomes are numbered from smallest (#1) to largest (#22)smallest (#1) to largest (#22)

The sex chromosomes are numbered as the 23The sex chromosomes are numbered as the 23rdrd pair pair


Sex Determination in HumansSex Determination in Humans

Sex is determined in humans by allocation of Sex is determined in humans by allocation of chromosomes at fertilizationchromosomes at fertilization

Both sperm and egg carry one of each of the 22 Both sperm and egg carry one of each of the 22 autosomesautosomes

The egg always carries the X chromosome as The egg always carries the X chromosome as number 23number 23

The sperm may carry either and X or YThe sperm may carry either and X or Y If the sperm donates an X in fertilization, the zygote If the sperm donates an X in fertilization, the zygote

will be femalewill be female If the sperm donates a Y in fertilization, the zygote If the sperm donates a Y in fertilization, the zygote

will be malewill be male Therefore, the sex of all humans is determined by the Therefore, the sex of all humans is determined by the

sperm donated by their fathersperm donated by their father



Genes carried on autosomes are said to be Genes carried on autosomes are said to be autosomally linkedautosomally linked

Genes carried on the female sex chromosome Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked)(X) are said to be X-linked (or sex-linked)

X-linked genes have a different pattern of X-linked genes have a different pattern of inheritance than autosomal genes haveinheritance than autosomal genes haveThe Y chromosome is blank for these genesThe Y chromosome is blank for these genesRecessive alleles on X chromosome:Recessive alleles on X chromosome: Follow familiar dominant/recessive rules in Follow familiar dominant/recessive rules in females (XX)females (XX) Are always expressed in males (XY), whether Are always expressed in males (XY), whether dominant or recessivedominant or recessiveMales said to be monozygous for X-linked Males said to be monozygous for X-linked genesgenes


Eye Color in Fruit FliesEye Color in Fruit Flies

Fruit flies (Fruit flies (Drosophila melanogasterDrosophila melanogaster) are common ) are common subjects for genetics researchsubjects for genetics research

They normally (wild-type) have red eyesThey normally (wild-type) have red eyes

A mutant recessive allele of a gene on the X A mutant recessive allele of a gene on the X chromosome can cause white eyeschromosome can cause white eyes

Possible combinations of genotype and phenotype:Possible combinations of genotype and phenotype:GenotypeGenotype PhenotypePhenotype

XXRRXXRR Homozygous DominantHomozygous Dominant FemaleFemale Red-eyedRed-eyed

XXRRXXrr HeterozygousHeterozygous FemaleFemale Red-eyedRed-eyed

XXrrXXrr Homozygous RecessiveHomozygous Recessive FemaleFemale White-eyedWhite-eyed

XXRRYY Monozygous DominantMonozygous Dominant MaleMale Red-eyedRed-eyed

XXrrYY Monozygous RecessiveMonozygous Recessive MaleMale White-eyedWhite-eyed

8X-Linked Inheritance

9ChromosomalChromosomal Inheritance InheritanceHuman X-Linked Disorders:Human X-Linked Disorders:

Red-Green Color BlindnessRed-Green Color Blindness

Color vision In humans:Color vision In humans:Depends three different classes of cone cells Depends three different classes of cone cells in the retinain the retina

Only one type of pigment is present in each Only one type of pigment is present in each class of cone cellclass of cone cell The gene for blue-sensitive is autosomalThe gene for blue-sensitive is autosomal The red-sensitive and green-sensitive genes are The red-sensitive and green-sensitive genes are on the X chromosomeon the X chromosomeMutations in X-linked genes cause RG color Mutations in X-linked genes cause RG color blindness:blindness: All males with mutation (XAll males with mutation (XbbY) are colorblindY) are colorblind Only homozygous mutant females (XOnly homozygous mutant females (XbbXXbb) are ) are

colorblindcolorblind Heterozygous females (XHeterozygous females (XBBXXbb) are asymptomatic ) are asymptomatic

carrierscarriers

10Red-Green Colorblindness Chart

11X-Linked Recessive Pedigree


Muscular DystrophyMuscular Dystrophy

Muscle cells operate by release and rapid Muscle cells operate by release and rapid sequestering of calciumsequestering of calcium

Protein dystrophin required to keep calcium Protein dystrophin required to keep calcium sequesteredsequestered

Dystrophin production depends on X-linked geneDystrophin production depends on X-linked geneA defective allele (when unopposed) causes A defective allele (when unopposed) causes

absence of dystrophinabsence of dystrophin Allows calcium to leak into muscle cellsAllows calcium to leak into muscle cells Causes muscular dystrophyCauses muscular dystrophy

All sufferers maleAll sufferers male Defective gene always unopposed in malesDefective gene always unopposed in males Males die before fathering potentially homozygous Males die before fathering potentially homozygous

recessive daughtersrecessive daughters


HemophiliaHemophilia

““Bleeder’s Disease”Bleeder’s Disease”

Blood of affected person either refuses to clot Blood of affected person either refuses to clot or clots too slowlyor clots too slowly

Hemophilia A – due to lack of clotting factor IXHemophilia A – due to lack of clotting factor IX

Hemophilia B – due to lack of clotting factor Hemophilia B – due to lack of clotting factor VIIIVIII

Most victims male, receiving the defective Most victims male, receiving the defective allele from carrier motherallele from carrier mother

Bleed to death from simple bruises, etc.Bleed to death from simple bruises, etc.

Factor VIII now available via biotechnologyFactor VIII now available via biotechnology

14Hemophilia Pedigree


Fragile X SyndromeFragile X Syndrome

Due to base-triplet repeats in a gene on the X Due to base-triplet repeats in a gene on the X chromosomechromosome

CGG repeated many timesCGG repeated many times

6-50 repeats – asymptomatic6-50 repeats – asymptomatic

230-2,000 repeats – growth distortions and 230-2,000 repeats – growth distortions and mental retardationmental retardation

Inheritance pattern is complex and Inheritance pattern is complex and unpredictableunpredictable

16Gene Linkage



When several genes of interest exist on the When several genes of interest exist on the same chromosomesame chromosome

Such genes form a linkage groupSuch genes form a linkage groupTend to be inherited as a blockTend to be inherited as a block If all genes on same chromosome:If all genes on same chromosome: Gametes of parent likely to have exact allele Gametes of parent likely to have exact allele combination as gamete of either grandparentcombination as gamete of either grandparent Independent assortment does not applyIndependent assortment does not apply

If all genes on separate chromosomes:If all genes on separate chromosomes: Allele combinations of grandparent gametes Allele combinations of grandparent gametes will be shuffled in parental gameteswill be shuffled in parental gametes Independent assortment workingIndependent assortment working

18Linkage Groups


Constructing a Chromosome MapConstructing a Chromosome MapCrossing-over can disrupt a blocked allele pattern on a Crossing-over can disrupt a blocked allele pattern on a

chromosomechromosomeAffected by distance between genetic lociAffected by distance between genetic lociConsider three genes on one chromosome:Consider three genes on one chromosome:

If one at one end, a second at the other and the third in the middleIf one at one end, a second at the other and the third in the middle Crossing over very likely to occur between lociCrossing over very likely to occur between loci Allelic patterns of grandparents will likely to be disrupted in Allelic patterns of grandparents will likely to be disrupted in

parental gametes with all allelic combinations possibleparental gametes with all allelic combinations possible If the three genetic loci occur in close sequence on the If the three genetic loci occur in close sequence on the

chromosomechromosome Crossing over very UNlikely to occur between lociCrossing over very UNlikely to occur between loci Allelic patterns of grandparents will likely to be preserved in Allelic patterns of grandparents will likely to be preserved in

parental gametesparental gametes

Rate at which allelic patterns are disrupted by crossing over:Rate at which allelic patterns are disrupted by crossing over: Indicates distance between lociIndicates distance between loci Can be used to develop linkage map or genetic map of Can be used to develop linkage map or genetic map of

chromosomechromosome

20Crossing Over

21Complete vs. Incomplete Linkage

22ChromosomalChromosomal Inheritance InheritanceChromosome Number:Chromosome Number:

PolyploidyPolyploidy

PolyploidyPolyploidy Occurs when eukaryotes have more than 2Occurs when eukaryotes have more than 2nn

chromosomeschromosomes

Named according to number of complete sets of Named according to number of complete sets of chromosomeschromosomes

Major method of speciation in plantsMajor method of speciation in plants

Diploid egg of one species joins with diploid pollen of Diploid egg of one species joins with diploid pollen of another speciesanother species

Result is new tetraploid species that is self-fertile but Result is new tetraploid species that is self-fertile but isolated from both “parent” speciesisolated from both “parent” species

Some estimate 47% of flowering plants are polyploidsSome estimate 47% of flowering plants are polyploids

Often lethal in higher animalsOften lethal in higher animals


AneuploidyAneuploidy

Monosomy (2Monosomy (2nn - 1) - 1) Diploid individual has only one of a particular Diploid individual has only one of a particular

chromosomechromosome

Caused by failure of synapsed chromosomes to Caused by failure of synapsed chromosomes to separate at Anaphase I (nondisjunction)separate at Anaphase I (nondisjunction)

Trisomy (2Trisomy (2nn + 1) occurs when an individual has + 1) occurs when an individual has three of a particular type of chromosomethree of a particular type of chromosome Diploid individual has three of a particular chromosomeDiploid individual has three of a particular chromosome

Also caused by nondisjunctionAlso caused by nondisjunction

This usually produces one monosomic daughter cell This usually produces one monosomic daughter cell and one trisomic daughter cell in meiosis Iand one trisomic daughter cell in meiosis I

Down syndrome is trisomy 21Down syndrome is trisomy 21

24Nondisjunction

25Trisomy 21


Abnormal Sex Chromosome NumberAbnormal Sex Chromosome Number

Result of inheriting too many or too few X or Result of inheriting too many or too few X or Y chromosomesY chromosomes

Caused by nondisjunction during oogenesis Caused by nondisjunction during oogenesis or spermatogenesisor spermatogenesis

Turner Syndrome (XO)Turner Syndrome (XO)

Female with single X chromosomeFemale with single X chromosome

Short, with broad chest and widely spaced Short, with broad chest and widely spaced nipplesnipples

Can be of normal intelligence and function Can be of normal intelligence and function with hormone therapywith hormone therapy



Klinefelter Syndrome (XXY)Klinefelter Syndrome (XXY)

Male with underdeveloped testes and Male with underdeveloped testes and prostate; some breast overdevelopmentprostate; some breast overdevelopment

Long arms and legs; large handsLong arms and legs; large hands

Near normal intelligence unless XXXY, XXXXY, Near normal intelligence unless XXXY, XXXXY, etc.etc.

No matter how many X chromosomes, No matter how many X chromosomes, presence of Y renders individual malepresence of Y renders individual male

28Turner and Klinefelter Syndromes



Ploy-X femalesPloy-X females

XXX simply taller & thinner than usualXXX simply taller & thinner than usual

Some learning difficultiesSome learning difficulties

Many menstruate regularly and are fertileMany menstruate regularly and are fertile

More than 3 Xs renders severe mental More than 3 Xs renders severe mental retardationretardation

Jacob’s syndrome (XYY)Jacob’s syndrome (XYY)

Tall, persistent acne, speech & reading Tall, persistent acne, speech & reading problemsproblems


Abnormal Chromosome StructureAbnormal Chromosome Structure

DeletionDeletion

Missing segment of chromosomeMissing segment of chromosome

Lost during breakageLost during breakage

TranslocationTranslocation

A segment from one chromosome moves to a A segment from one chromosome moves to a non-homologous chromosomenon-homologous chromosome

Follows breakage of two nonhomologous Follows breakage of two nonhomologous chromosomes and improper re-assemblychromosomes and improper re-assembly

31Deletion, Translocation,Duplication, and

Inversion



DuplicationDuplication

A segment of a chromosome is repeated in A segment of a chromosome is repeated in the same chromosomethe same chromosome

InversionInversion

Occurs as a result of two breaks in a Occurs as a result of two breaks in a chromosomechromosome

The internal segment is reversed before re-The internal segment is reversed before re-insertioninsertion

Genes occur in reverse order in inverted Genes occur in reverse order in inverted segmentsegment

33Inversion Leading toDuplication and

Deletion



Deletion SyndromesDeletion Syndromes

Williams syndrome - Loss of segment of Williams syndrome - Loss of segment of chromosome 7chromosome 7

Cri du chat syndrome (cat’s cry) - Loss of Cri du chat syndrome (cat’s cry) - Loss of segment of chromosome 5segment of chromosome 5

TranslocationsTranslocations

Alagille syndromeAlagille syndrome

Some cancersSome cancers

35Williams Syndrome

36Alagille Syndrome


ReviewReview


Human X-Linked DisordersHuman X-Linked Disorders


Crossing-OverCrossing-Over

Chromosome MapChromosome Map

Changes in Chromosome NumberChanges in Chromosome Number

Changes in Chromosome StructureChanges in Chromosome Structure

Human SyndromesHuman Syndromes

Chromosomal Inheritance

Ending Slide Chapter 12

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Chromosomal Inheritance Chapter 12. 2 Drosophila Chromosomes