Gene Lecture 7 Chromosome Mutations
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Fundamental Genetics Lecture 7
Chromosome Mutations John Donnie A. Ramos, Ph.D. Dept. of Biological Sciences College of Science University of Santo Tomas
Chromosome Mutations Also called chromosome aberrations Change in number of chromosomes, deletion or duplication of genes or segments of chromosome, or rearrangement of genetic material Inherited (can be passed from one generation to another) Results to new phenotypic variation or maybe lethal
1
Variation in Chromosome Number
Nondisjunction Failure of homologous chromosomes to segregate during anaphase of meiosis Primary nondisjunction – failure of a homolog to segregation during anaphase I Secondary nondisjunction – failure of a homolog to segregate during anaphase II Results to aneuploidy
2
Monosomy Loss of one chromosome (2n-1) Caused by either primary or secondary nondisjunction Human Example: Turner syndrome (2n=45, 44+X) Monosomy involving autosomes in humans is lethal Drosophila Example: Haplo IV (monosomic at chromosome no. 4) Slow development, reduced body size, impaired viability Common in plants (maize, tobacco, primrose) – less viable compare normal plants
Cri-du-Chat Syndrome
Cri-du-Chat syndrome (cry of cat) Deletion in part of chromosome 5 (46, 5p_) Gastrointestinal and cardiac complications Mentally retarded Abnormal development of glottis and larynx (cry like a cat) Incidence: 1 in 50,000 births Different cases have different degrees of truncations
3
Trisomy
Gain of chromosome (2n+1) Caused by either primary or secondary nondisjunction Affected are viable in humans, animals and plants Example: Trisomy 21 (Down Syndrome) Trisomy of chromosome 21 (2n=47, 21+) 1 in every 800 live births Flat faces, round heads, protruding and furrowed tongues, short and broad fingers Physical, psychomotor, and mental development is retarded Life expectancy: 50 yrs old 95 % of cases are nondisjunction in ovum (related to age of female)
Trisomy Patau Syndrome (Tisomy 13) 2n= 47, 13+
Edwards Syndrome (Trisomy 18) 2n = 47, 18+
4
Polyploids Presence of more than two sets of chromosomes Infrequent in many animals species but common in amphibians, lizards and fishes Very common in plant species
Same species
Different species (hybridization)
Autopolyploids Triploid (AAA), tetraploid (AAAA), pentaploid (AAAAA) if A represents haploid set Arise as a result of: (for triploids) Failure of all chromosomes to segragate Two sperm fertilizing an egg Experimentally induced (diploid x tetraploid)
Autotetraploids are more likely to occur in nature than autotriploids (because of genetically unbalanced gametes) Tetraploids result when chromosomes replicated but sister chromatids failed to divide Examples: potatoes, seedless watermelon, commercial bananas, apples, peanuts, coffee, strawberry (octaploid)
5
Allopolyploids Hybridization of two closely related species Allotretraploid – 4 sets of chromosomes (Two sets from species 1 and two sets from species 2) Amphidiploid – the resulting hybrid from2 species Ex. American cotton (2n=26); Raphanus brassica (2n=18) = Raphanus sativus (radish) + Brassica olaracea (cabbage) Triticale = rye (high lysine) and wheat (high protein)
Variation in Chromosome Number Structural changes in a particular chromosome Deletions, duplications or rearrangements of genes Caused by chromosomal breaks (mutations) – spontaneous or artificial (due to mutagenic agents) Heritable Can change the phenotype of an organism
6
Deletions Lost of a gene or a part of gene Either terminal or intercalary deletions When a chromosome has intercalary deletion, its homolog will undergo compensation loop during pairing of homologous chromosomes. Example: Cri-du-Chat Syndrome
Duplications A gene locus or apiece of chromosome is present more than once in a genome Result of unequal crossing-over during meiosis
Can form compensation loop in succeeding homologous pairing Causes gene redundancy and amplification (ex. Genes coding for rRNA – E. coli has 5-10 copies but oocyte has 400 copies) Plays a role in evolution of genes (ex. Trypsin and chymotrypsin genes; myoglobin and hemoglobin genes; myosin and paramyosin genes)
7
Duplications Can cause phenotypic variation Ex. Bar eye mutation in Drosophila (slit-like eyes) Identified in 1920s by Alfred Sturtevant and Thomas Morgan Duplication in region 16A of X chromosome
Normal Wild-type
Heterozygous
Homozygous recessive Double bar
Inversions Occurs when a region of chromosome is turned 180° Chromosome part is not lost but rearrangement of genes occur Two breaks occur
Types: Paracentric Inversion – centromers is not a part of the inverted sequence Pericentric Inversion – centromere is part of the inverted sequence
8
Paracentric Inversion Heterozygotes Only 1 homolog is inverted Forms inversion loop during meiosis If no crossing-over: results to 2 normal sequence and 2 inverted sequences If crossing occurs: results to 4 different sequence combinations Acentric chromosome may be lost during anaphase
Pericentric Inversion Heterozygotes
9
Translocations Transfer of chromosome segment to another location (different chromosome) Reciprocal translocation – exchange of segments between two non-homologous chromosomes Results to partial monosomy or trisomy
Translocations in Humans Familial Down Syndrome 14/21 D/G Robertsonian translocation (transfer of large segment of chrom. 21 to chrom. 14)
10
Fragile Sites in Humans
Regions in chromosomes susceptible to DNA breakage Result of short sequence duplications Examples: Fragile X Syndrome (Martin-Bell Syndrome)
Presence of Folate-sensitive site on X chromosome Most common form of mental retardation Affects 1 in 1250 males and 1 in 1500 females A dominant trait (but not fully expressed – only 30% and 80% of females and males with fragile X express the disease, respectively) Long, narrow faces; enlarge ears; increased testicular size Caused by FMR-1 gene – (trinucleotide repeats- CGG repeats) Normal persons = 6-54 repeats; carriers =55-200 repeats; fagile X= more than 200 repeats) Undergoes Genetic Anticipation –repeats increase in succeeding generations
Fragile Sites and Cancer Lung, stomach, esophagus, colon cancers Caused by FHIT gene (Fragile histidine triad) located in chromosome 3
11
Chromosome Mutations John Donnie A. Ramos, Ph.D. Dept. of Biological Sciences College of Science University of Santo Tomas
Chromosome Mutations Also called chromosome aberrations Change in number of chromosomes, deletion or duplication of genes or segments of chromosome, or rearrangement of genetic material Inherited (can be passed from one generation to another) Results to new phenotypic variation or maybe lethal
1
Variation in Chromosome Number
Nondisjunction Failure of homologous chromosomes to segregate during anaphase of meiosis Primary nondisjunction – failure of a homolog to segregation during anaphase I Secondary nondisjunction – failure of a homolog to segregate during anaphase II Results to aneuploidy
2
Monosomy Loss of one chromosome (2n-1) Caused by either primary or secondary nondisjunction Human Example: Turner syndrome (2n=45, 44+X) Monosomy involving autosomes in humans is lethal Drosophila Example: Haplo IV (monosomic at chromosome no. 4) Slow development, reduced body size, impaired viability Common in plants (maize, tobacco, primrose) – less viable compare normal plants
Cri-du-Chat Syndrome
Cri-du-Chat syndrome (cry of cat) Deletion in part of chromosome 5 (46, 5p_) Gastrointestinal and cardiac complications Mentally retarded Abnormal development of glottis and larynx (cry like a cat) Incidence: 1 in 50,000 births Different cases have different degrees of truncations
3
Trisomy
Gain of chromosome (2n+1) Caused by either primary or secondary nondisjunction Affected are viable in humans, animals and plants Example: Trisomy 21 (Down Syndrome) Trisomy of chromosome 21 (2n=47, 21+) 1 in every 800 live births Flat faces, round heads, protruding and furrowed tongues, short and broad fingers Physical, psychomotor, and mental development is retarded Life expectancy: 50 yrs old 95 % of cases are nondisjunction in ovum (related to age of female)
Trisomy Patau Syndrome (Tisomy 13) 2n= 47, 13+
Edwards Syndrome (Trisomy 18) 2n = 47, 18+
4
Polyploids Presence of more than two sets of chromosomes Infrequent in many animals species but common in amphibians, lizards and fishes Very common in plant species
Same species
Different species (hybridization)
Autopolyploids Triploid (AAA), tetraploid (AAAA), pentaploid (AAAAA) if A represents haploid set Arise as a result of: (for triploids) Failure of all chromosomes to segragate Two sperm fertilizing an egg Experimentally induced (diploid x tetraploid)
Autotetraploids are more likely to occur in nature than autotriploids (because of genetically unbalanced gametes) Tetraploids result when chromosomes replicated but sister chromatids failed to divide Examples: potatoes, seedless watermelon, commercial bananas, apples, peanuts, coffee, strawberry (octaploid)
5
Allopolyploids Hybridization of two closely related species Allotretraploid – 4 sets of chromosomes (Two sets from species 1 and two sets from species 2) Amphidiploid – the resulting hybrid from2 species Ex. American cotton (2n=26); Raphanus brassica (2n=18) = Raphanus sativus (radish) + Brassica olaracea (cabbage) Triticale = rye (high lysine) and wheat (high protein)
Variation in Chromosome Number Structural changes in a particular chromosome Deletions, duplications or rearrangements of genes Caused by chromosomal breaks (mutations) – spontaneous or artificial (due to mutagenic agents) Heritable Can change the phenotype of an organism
6
Deletions Lost of a gene or a part of gene Either terminal or intercalary deletions When a chromosome has intercalary deletion, its homolog will undergo compensation loop during pairing of homologous chromosomes. Example: Cri-du-Chat Syndrome
Duplications A gene locus or apiece of chromosome is present more than once in a genome Result of unequal crossing-over during meiosis
Can form compensation loop in succeeding homologous pairing Causes gene redundancy and amplification (ex. Genes coding for rRNA – E. coli has 5-10 copies but oocyte has 400 copies) Plays a role in evolution of genes (ex. Trypsin and chymotrypsin genes; myoglobin and hemoglobin genes; myosin and paramyosin genes)
7
Duplications Can cause phenotypic variation Ex. Bar eye mutation in Drosophila (slit-like eyes) Identified in 1920s by Alfred Sturtevant and Thomas Morgan Duplication in region 16A of X chromosome
Normal Wild-type
Heterozygous
Homozygous recessive Double bar
Inversions Occurs when a region of chromosome is turned 180° Chromosome part is not lost but rearrangement of genes occur Two breaks occur
Types: Paracentric Inversion – centromers is not a part of the inverted sequence Pericentric Inversion – centromere is part of the inverted sequence
8
Paracentric Inversion Heterozygotes Only 1 homolog is inverted Forms inversion loop during meiosis If no crossing-over: results to 2 normal sequence and 2 inverted sequences If crossing occurs: results to 4 different sequence combinations Acentric chromosome may be lost during anaphase
Pericentric Inversion Heterozygotes
9
Translocations Transfer of chromosome segment to another location (different chromosome) Reciprocal translocation – exchange of segments between two non-homologous chromosomes Results to partial monosomy or trisomy
Translocations in Humans Familial Down Syndrome 14/21 D/G Robertsonian translocation (transfer of large segment of chrom. 21 to chrom. 14)
10
Fragile Sites in Humans
Regions in chromosomes susceptible to DNA breakage Result of short sequence duplications Examples: Fragile X Syndrome (Martin-Bell Syndrome)
Presence of Folate-sensitive site on X chromosome Most common form of mental retardation Affects 1 in 1250 males and 1 in 1500 females A dominant trait (but not fully expressed – only 30% and 80% of females and males with fragile X express the disease, respectively) Long, narrow faces; enlarge ears; increased testicular size Caused by FMR-1 gene – (trinucleotide repeats- CGG repeats) Normal persons = 6-54 repeats; carriers =55-200 repeats; fagile X= more than 200 repeats) Undergoes Genetic Anticipation –repeats increase in succeeding generations
Fragile Sites and Cancer Lung, stomach, esophagus, colon cancers Caused by FHIT gene (Fragile histidine triad) located in chromosome 3
11
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