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Chromosomal InheritanceChromosomal
Inheritance
Chapter 12Chapter 12
Drosophila ChromosomesDrosophila Chromosomes
Chromosomal InheritanceChromosomal Inheritance
Humans are diploid (2 chromosomes of each type) Humans have 23 different kinds of chromosomes Arranged in 23 pairs of homologous chromosomes Total of 46 chromosomes (23 pairs) per cell
One of the chromosome pairs determines the sex of an individual (the sex chromosomes)
The other 22 pairs of chromosomes are autosomes.
Autosomal chromosomes are numbered from smallest (#1) to largest (#22)
The sex chromosomes are numbered as the 23rd pair
Humans are diploid (2 chromosomes of each type) Humans have 23 different kinds of chromosomes Arranged in 23 pairs of homologous chromosomes Total of 46 chromosomes (23 pairs) per cell
One of the chromosome pairs determines the sex of an individual (the sex chromosomes)
The other 22 pairs of chromosomes are autosomes.
Autosomal chromosomes are numbered from smallest (#1) to largest (#22)
The sex chromosomes are numbered as the 23rd pair
Sex Determination in Humans
Sex Determination in Humans Sex is determined in humans by
allocation of chromosomes at fertilization.
Both sperm and egg carry one of each of the 22 autosomes
The egg always carries the X chromosomes as number 23
The sperm may carry either and X or Y If the sperm donates an X in fertilization, the zygote will be female.
If the sperm donates a Y in fertilization, the zygote will be male.
Therefore, the sex of all humans is determined by the sperm donated by their father.
Sex is determined in humans by allocation of chromosomes at fertilization.
Both sperm and egg carry one of each of the 22 autosomes
The egg always carries the X chromosomes as number 23
The sperm may carry either and X or Y If the sperm donates an X in fertilization, the zygote will be female.
If the sperm donates a Y in fertilization, the zygote will be male.
Therefore, the sex of all humans is determined by the sperm donated by their father.
X-linked AllelesX-linked Alleles Genes carried on autosomes are said to be autosomally linked
Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked)
X-linked genes have a different pattern of inheritance than autosomal genes have The Y chromosome is blank for these genes Recessive alleles on X chromosome:
Follow familiar dominant/recessive rules in females (XX)
Are always expressed in males (XY), whether dominant or recessive
Males said to be monozygous for X-linked genes
Genes carried on autosomes are said to be autosomally linked
Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked)
X-linked genes have a different pattern of inheritance than autosomal genes have The Y chromosome is blank for these genes Recessive alleles on X chromosome:
Follow familiar dominant/recessive rules in females (XX)
Are always expressed in males (XY), whether dominant or recessive
Males said to be monozygous for X-linked genes
Eye Color in Fruit Flies
Eye Color in Fruit Flies Fruit flies (Drosophila melanogaster)
are common subjects for genetics research
They normally (wild-type) have red eyes A mutant recessive allele of a gene on the X chromosome can cause white eyes
Possible combinations of genotypes and phenotypes
Fruit flies (Drosophila melanogaster) are common subjects for genetics research
They normally (wild-type) have red eyes A mutant recessive allele of a gene on the X chromosome can cause white eyes
Possible combinations of genotypes and phenotypes
Genotype Phenotype
XRXR Homozygous Dominant Female Red-eyed
XRXr Heterozygous Female Red-eyed
XrXr Homozygous Recessive Female White-eyed
XRY Monozygous Dominant Male Red-eyed
XrY Monozygous Recessive Male White-eyed
X-linked InheritanceX-linked Inheritance
Human X-linked Disorders: Red-Green
Color Blindness
Human X-linked Disorders: Red-Green
Color Blindness Color vision in humans: Depends three different classes of cone cells in the retina
Only one type of pigment is present in each class of cone cell The gene for blue-sensitive is autosomal The red-sensitive and green-sensitive genes are on the X chromosome
Mutations in X-linked genes cause RG color blindness
All males with mutation (XbY) are colorblind Only homozygous mutant females (XbXb) are colorblind
Heterozygous females (XBXb) are asymptomatic carriers
Color vision in humans: Depends three different classes of cone cells in the retina
Only one type of pigment is present in each class of cone cell The gene for blue-sensitive is autosomal The red-sensitive and green-sensitive genes are on the X chromosome
Mutations in X-linked genes cause RG color blindness
All males with mutation (XbY) are colorblind Only homozygous mutant females (XbXb) are colorblind
Heterozygous females (XBXb) are asymptomatic carriers
Red-Green Colorblindness Chart
Red-Green Colorblindness Chart
X-linked Recessive Pedigree
X-linked Recessive Pedigree
Human X-linked Disorders:Muscular Dystrophy
Human X-linked Disorders:Muscular Dystrophy
Muscle cells operate by release and rapid sequestering of calcium
Protein dystrophin required to keep calcium sequestered
Dystrophin production depends on X-linked gene
A defective allele (when unopposed) causes absence of dystrophin Allows calcium to leak into muscle cells Causes muscular dystrophy
All sufferers are male Defective gene always unopposed in males Males die before fathering potentially homozygous recessive daughters
Muscle cells operate by release and rapid sequestering of calcium
Protein dystrophin required to keep calcium sequestered
Dystrophin production depends on X-linked gene
A defective allele (when unopposed) causes absence of dystrophin Allows calcium to leak into muscle cells Causes muscular dystrophy
All sufferers are male Defective gene always unopposed in males Males die before fathering potentially homozygous recessive daughters
Human X-linked Disorders:Hemophilia
Human X-linked Disorders:Hemophilia
“Bleeder’s Disease” Blood of affected person either refuses to clot or clots too slowly Hemophilia A -due to lack of clotting factor X Hemophilia B - due to lack of clotting factor VIII
Most victims male, receiving the defective allele from carrier mother
Bleed to death from simple bruises, etc. Factor VIII now available via biotechnology
“Bleeder’s Disease” Blood of affected person either refuses to clot or clots too slowly Hemophilia A -due to lack of clotting factor X Hemophilia B - due to lack of clotting factor VIII
Most victims male, receiving the defective allele from carrier mother
Bleed to death from simple bruises, etc. Factor VIII now available via biotechnology
Hemophilia PedigreeHemophilia Pedigree
Human X-linked Disorders: Fragile X
Syndrome
Human X-linked Disorders: Fragile X
Syndrome Due to base-triplet repeats in a gene on the X chromosome
CGG repeated many times 6-50 repeats -- asymptomatic 230-2,000 repeats -- growth distortions and mental retardation
Inheritance pattern is complex and unpredictable
Due to base-triplet repeats in a gene on the X chromosome
CGG repeated many times 6-50 repeats -- asymptomatic 230-2,000 repeats -- growth distortions and mental retardation
Inheritance pattern is complex and unpredictable
Gene LinkageGene Linkage
Gene LinkageGene Linkage When several genes of interest exist on the same chromosome
Such genes form a linkage group Tend to be inherited on same chromosome: Gametes of parent likely to have exact allele combination as gamete of either grandparent
Independent assortment does not apply If all genes on separate chromosomes:
Allele combinations of grandparent gametes will be shuffled in parental gametes
Independent assortment working
When several genes of interest exist on the same chromosome
Such genes form a linkage group Tend to be inherited on same chromosome: Gametes of parent likely to have exact allele combination as gamete of either grandparent
Independent assortment does not apply If all genes on separate chromosomes:
Allele combinations of grandparent gametes will be shuffled in parental gametes
Independent assortment working
Linkage GroupsLinkage Groups
Constructing a Chromosome MapConstructing a Chromosome Map Crossing over can disrupt a blocked allele pattern on a
chromosome Affected by distance between genetic loci Consider three genes on one chromosome:
If one at one end, a second at the other and the third in the middle
Allelic patterns of grandparents will likely to be disrupted in parental gametes with all allelic combinations possible
If the three genetic loci occur in close sequence on the chromosome Crossing over very Unlikely to occur between loci Allelic patterns of grandparents will likely to be preserved in parental gametes
Rate at which allelic patterns are disrupted by crossing over: Indicates distance between loci Can be used to develop linkage map or genetic map of chromosome
Crossing over can disrupt a blocked allele pattern on a chromosome
Affected by distance between genetic loci Consider three genes on one chromosome:
If one at one end, a second at the other and the third in the middle
Allelic patterns of grandparents will likely to be disrupted in parental gametes with all allelic combinations possible
If the three genetic loci occur in close sequence on the chromosome Crossing over very Unlikely to occur between loci Allelic patterns of grandparents will likely to be preserved in parental gametes
Rate at which allelic patterns are disrupted by crossing over: Indicates distance between loci Can be used to develop linkage map or genetic map of chromosome
Crossing-OverCrossing-Over
Complete vs. Incomplete Linkage
Complete vs. Incomplete Linkage
Chromosome Number: Polyploidy
Chromosome Number: Polyploidy Polyploidy
Occurs when eukaryotes have more than 2n chromosomes
Named according to number of complete sets of chromosomes
Major method of speciation in plants Diploid egg of one species joins with diploid pollen of another species
Result in tetraploid species that is self-fertile but isolated from both “parent” species
Ofen lethal in higher animals
Polyploidy Occurs when eukaryotes have more than 2n chromosomes
Named according to number of complete sets of chromosomes
Major method of speciation in plants Diploid egg of one species joins with diploid pollen of another species
Result in tetraploid species that is self-fertile but isolated from both “parent” species
Ofen lethal in higher animals
Chromosome Number: Aneuploidy
Chromosome Number: Aneuploidy Monosomy (2n-1)
Diploid individual has only one of a particular chromosome
Caused by failure of synapsed chromosome to separate at Anaphase (nondisjunction)
Trisomy (2n+1) occurs when an individual has three of a particular type of chromosome Diploid individual has three of a particular chromosome
Also caused by nondisjunction This usually produces one nonsomic daughter cell and one trisomic daughter cell in meiosis I
Down syndrome is trisomy 21
Monosomy (2n-1) Diploid individual has only one of a particular chromosome
Caused by failure of synapsed chromosome to separate at Anaphase (nondisjunction)
Trisomy (2n+1) occurs when an individual has three of a particular type of chromosome Diploid individual has three of a particular chromosome
Also caused by nondisjunction This usually produces one nonsomic daughter cell and one trisomic daughter cell in meiosis I
Down syndrome is trisomy 21
NondisjunctionNondisjunction
Trisomy 21Trisomy 21
Chromosome Number: Abnormal Sex Chromosome
Number
Chromosome Number: Abnormal Sex Chromosome
Number Result of inheriting too many or too few X or Y chromosomes
Caused by nondisjunction during oogenesis or spermatogenesis
Turner Syndrome (XO) Female with single X chromosome Short, with broad chest and widely spaced nipples
Cam be of normal intelligence and function with hormone therapy
Result of inheriting too many or too few X or Y chromosomes
Caused by nondisjunction during oogenesis or spermatogenesis
Turner Syndrome (XO) Female with single X chromosome Short, with broad chest and widely spaced nipples
Cam be of normal intelligence and function with hormone therapy
Chromosome Number: Abnormal Sex Chromosome
Number
Chromosome Number: Abnormal Sex Chromosome
Number Klinefelter Syndrome (XXY)
Male with underdeveloped testes and prostate; some breast overdevelopment
Long arms and legs; large hands Near normal intelligence unless XXXY, XXXXY, etc.
No matter how many X chromosomes, presence of Y renders individual male
Klinefelter Syndrome (XXY) Male with underdeveloped testes and prostate; some breast overdevelopment
Long arms and legs; large hands Near normal intelligence unless XXXY, XXXXY, etc.
No matter how many X chromosomes, presence of Y renders individual male
Turner & Klinefelter Syndromes
Turner & Klinefelter Syndromes
Chromosome Number: Abnormal Sex Chromosome
Number
Chromosome Number: Abnormal Sex Chromosome
Number
Ploy-X females XXX simply taller and thinner than normal
Some learning difficulties Many menstruate regularly and are fertile
More than 3 Xs renders severe mental retardation
Jacob’s syndrome (XYY) Tall, persistent acne, speech and reading problems
Ploy-X females XXX simply taller and thinner than normal
Some learning difficulties Many menstruate regularly and are fertile
More than 3 Xs renders severe mental retardation
Jacob’s syndrome (XYY) Tall, persistent acne, speech and reading problems
Abnormal Chromosome Structure
Abnormal Chromosome Structure
Deletion Missing segment of chromosme Lost during breakage
Translocation A segment from one chromosome moves to a non-homologous chromosome
Follows breakage of two homologous chromosomes and improper re-assembly
Deletion Missing segment of chromosme Lost during breakage
Translocation A segment from one chromosome moves to a non-homologous chromosome
Follows breakage of two homologous chromosomes and improper re-assembly
Deletion, Translocation, Duplication, & InversionDeletion, Translocation, Duplication, & Inversion
Abnormal Chromosome Structure
Abnormal Chromosome Structure
Duplication A segment of a chromosome is repeated in the same chromosome
Inversion Occurs as a result of two breaks in a chromosome The internal segment is reversed before reinsertion
Genes occur in reverse order in inverted segment
Duplication A segment of a chromosome is repeated in the same chromosome
Inversion Occurs as a result of two breaks in a chromosome The internal segment is reversed before reinsertion
Genes occur in reverse order in inverted segment
Inversion Leading to Duplication & DeletionInversion Leading to
Duplication & Deletion
Abnormal Chromosome Structure
Abnormal Chromosome Structure
Deletion Syndromes Williams Syndrome -- loss of segment of chromosome 7
Cri du chat syndrome (cat’s cry) -- loss of segment of chromosome 5
Translocation Alagille syndrome Some cancers
Deletion Syndromes Williams Syndrome -- loss of segment of chromosome 7
Cri du chat syndrome (cat’s cry) -- loss of segment of chromosome 5
Translocation Alagille syndrome Some cancers
Williams SyndromeWilliams Syndrome
Alagille SyndromeAlagille Syndrome
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