Post on 02-Jan-2016
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
4ChromosomalChromosomal Inheritance Inheritance
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
5ChromosomalChromosomal Inheritance Inheritance
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
6ChromosomalChromosomal Inheritance Inheritance
X-Linked AllelesX-Linked Alleles
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
7ChromosomalChromosomal Inheritance Inheritance
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
12ChromosomalChromosomal Inheritance InheritanceHuman X-Linked Disorders:Human X-Linked Disorders:
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
13ChromosomalChromosomal Inheritance InheritanceHuman X-Linked Disorders:Human X-Linked Disorders:
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
15ChromosomalChromosomal Inheritance InheritanceHuman X-Linked Disorders:Human X-Linked Disorders:
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
17ChromosomalChromosomal Inheritance Inheritance
Gene LinkageGene 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
19ChromosomalChromosomal Inheritance Inheritance
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
23ChromosomalChromosomal Inheritance InheritanceChromosome Number:Chromosome Number:
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
26ChromosomalChromosomal Inheritance InheritanceChromosome Number:Chromosome Number:
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
27ChromosomalChromosomal Inheritance InheritanceChromosome Number:Chromosome Number:
Abnormal Sex Chromosome NumberAbnormal Sex Chromosome Number
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
29ChromosomalChromosomal Inheritance InheritanceChromosome Number:Chromosome Number:
Abnormal Sex Chromosome NumberAbnormal Sex Chromosome Number
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
30ChromosomalChromosomal Inheritance Inheritance
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
32ChromosomalChromosomal Inheritance Inheritance
Abnormal Chromosome StructureAbnormal Chromosome Structure
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
34ChromosomalChromosomal Inheritance Inheritance
Abnormal Chromosome StructureAbnormal Chromosome Structure
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
37ChromosomalChromosomal Inheritance Inheritance
ReviewReview
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 Inheritance
Ending Slide Chapter 12