- Email: [email protected]
First and Second Division Segregation
698
F i l ia l G e ne r a t io ns
Jiang SC, Matte M, Matte G, Huq A and Colwell RR (2000) Genetic diversity of clinical and environmental isolates of Vibrio cholerae determined by amplified fragment length polymorphism fingerprinting. Applied and Environmental Microbiology 66: 148±153. Kimsey HH and Waldor MK (1998) Vibrio cholerae hemagglutinin/protease inactivates CTXphi. Infection and Immunity 66: 4025±4029. Miao EA and Miller SI (1999) Bacteriophages in the evolution of pathogen±host interactions. Proceedings of the National Academy of Sciences, USA 96: 9452±9454. Muniesa M and Jofre J (1998) Abundance in sewage of bacteriophages that infect Escherichia coli O157:H7 and that carry the shiga toxin 2 gene. Applied and Environmental Microbiology 64: 2443±2448. Murley YM, Carroll PA, Skorupski K, Taylor RK and Calderwood SB (1999) Differential transcription of the tcpPH operon confers biotype-specific control of the Vibrio cholerae ToxR virulence regulon. Infection and Immunity 67: 5117±5123. Nasu H, Iida T, Sugahara T et al. (2000) A filamentous phage associated with recent pandemic Vibrio parahaemolyticus O3:K6 strains. Journal of Clinical Microbiology 38: 2156± 2161. SchloÈr S, Riedl S, Blass J and Reidl J (2000) Genetic rearrangements of the regions adjacent to genes encoding heat-labile enterotoxins (eltAB) of enterotoxigenic Escherichia coli strains. Applied and Environmental Microbiology 66: 352± 358. Sharma C, Thungapathra M, Ghosh A et al. (1998) Molecular analysis of non-O1, non-O139, Vibrio cholerae associated with an unusual upsurge in the incidence of cholera-like disease in Calcutta, India. Journal of Clinical Microbiology 36: 756±763.
See also: Bacteriophages; Capsid; Gene Rearrangements, Prokaryotic; Plasmids
generations of brother±sister matings numbered with integer increments. See also: F1 Hybrid; Inbred Strain
Filter Hybridization Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1842
Filter hybridization is a technique for in situ solidphase hybridization whereby denatured DNA is immobilized on a nitrocellulose filter and incubated with a solution of radioactively labeled RNA or DNA. See also: In situ Hybridization
Fingerprinting J H Miller Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0458
The chromatographic pattern of spots produced by proteolytic digestion of a protein followed by electrophoresis. See also: Proteins and Protein Structure
First and Second Division Segregation J R S Fincham
Filial Generations
Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1428
L Silver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0456
Filial generation is the term pertaining to a particular generation in a sequence of brother±sister matings that can be carried out to form an inbred strain. The first filial generation, symbolized as F1, refers to the offspring of a cross between animals having nonidentical genomes. When F1 siblings are crossed to each other, their offspring are considered to be members of the second filial generation or F2, with subsequent
During the first division of meiosis in a diploid cell the chromosomes are each divided into chromatids, but sister chromatids remain attached together at the centromere. At first anaphase, the centromeres do not split, as in anaphase of mitosis; instead the centromeres of homologous chromosomes separate (segregate) from each other toward the two poles of the division spindle as wholes, each taking two chromatids with it. Centromeres always segregate at the first division of meiosis and do not split to allow their two halves to separate into different meiotic products until the second division.
First and Seco nd Division Segregation 699
Diplotene / metaphase I
anaphase I
anaphase II
(A)
A A A a a
A A a a a
(B)
A A a a
A
A A
a
A
a
A
a
a a
Figure 1 First division (A) or second division (B) segregation of alleles A and a depending on whether or not a crossover occurs between the A/a locus and the centromere (vertical bar in the left panel), the point at which sister chromatids remain connected until the second division of meiosis. When two homologous chromosomes are distinguished by a genetic marker, say with allele A on one chromosome and allele a on the other, the A±a difference will segregate at the first division provided that the alleles remain attached to their original centromeres. However, when a single crossover, which always involves just one chromatid of each chromosome, occurs between the A/a locus and the centromere, different alleles become joined to the same centromere, and the anaphase separation at first anaphase will not be between A-A and a-a but rather between A-a and A-a (Figure 1). Then segregation of A from a will be delayed to the the second division. The effect of two crossovers in the locus±centromere interval depends on whether the same chromatids are involved in the second crossover as in the first. If the same two cross over twice (a two-strand double), or if the second crossover involves the two chromatids not involved in the first crossover (a four-strand double), the effect in either case is to restore first division segregation. If one chromatid crosses over twice, two cross over once each, and the other not at all (threestrand double), the effect is second division segregation. So double crossovers give, on average, 50% second division segregation. An indefinitely large number of crossovers will give, on average, two-thirds second division segregation, a result most easily understood by imagining the four alleles as totally uncoupled from their centromeres and distributed two-and-two to the first division spindle poles at
random. An A allele will then be twice as likely to be accompanied by an a allele as by the other A allele. First and second division segregation can be distinguished by tetrad analysis (see Tetrad Analysis). When the marker is a gross chromosomal feature such as a large terminal deletion, first and second division segregation can also sometimes be seen under the microscope (Figure 2). The second division frequency of a genetic marker is a measure of the frequency of its crossing-over with the centromere, and hence of the map length of the marker±centromere interval. To make second division segregation percentages equivalent to recombination percentages, on which map units (centimorgans, cM) are conventionally based, they need to be divided by two. This is because a single crossover in a marker± centromere interval will always give second division segregation, whereas a single crossover in a marker± marker interval will recombine only two out of the four chromatids. In fact, second division segregation and recombination frequencies relate linearly to true map distance (total average number of crossovers per chromosome pair 50) only when there is never more than one crossover in the interval concerned. Both measures approach a maximum value as the number of crossovers in the interval becomes large, and this maximum value is different in the two cases: 50% recombination and 67% second division segregation, which, without correction, would convert to 50 and 33.3 cM. Thus, as distance increases, both measures
700
F I S H ( F l u o re s c en t i n s i t u H y b r i d i z a t i o n)
No chiasma between centromere and the deletion in one homolog
Chiasma formed between centromere and deletion
Interpretation Bivalents at first metaphase Actual appearance
Anaphase II
Fisher, R.A. A W F Edwards Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0459
Arm shortened by deletion
Anaphase I
Segregation of length difference at 1st division
Anaphase II
Segregation of length difference at 2nd division
Figure 2 First and second division segregation made visible in a lily heterozygous for a chromosome length difference. (Reproduced with permission from Fincham JRS (1983) Genetics after Brown and Zohary (1955) Genetics 40: 850.) increasingly underestimate true map distance, but second division segregation does so to a greater extent. See also: Centimorgan (cM); Centromere; Crossing-Over; Map Distance, Unit; Meiosis; Tetrad Analysis
FISH (Fluorescent in situ Hybridization) J Read and S Brenner Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.2090
Fluorescent in situ hybridization (FISH) is a technique used to identify the chromosomal location of a particular DNA sequence. A DNA probe is fluorescently labeled and hybridized to denatured metaphase chromosomes spread out on glass slides. See also: Physical Mapping
Sir Ronald Fisher (1890±1962), the father of modern statistics, was for most of his life a professor of genetics, first in London and then at Cambridge. He made lasting contributions to mathematical and evolutionary genetics as well as to statistical theory applied to genetics, and experimented widely, studying especially linkage in the mouse and in polysomic plants and natural selection in the wild. Fisher was born in London on 17 February 1890, the son of a fine-art auctioneer. His twin brother was stillborn. At Harrow School he distinguished himself in mathematics despite being handicapped by poor eyesight which prevented him working by artificial light. His teachers used to instruct him by ear, and Fisher developed a remarkable capacity for pursuing complex mathematical arguments in his head. This manifested itself later in life in an ability to reach a conclusion whilst forgetting the argument, to handle complex geometrical trains of thought, and to develop and report essentially mathematical arguments in English (only for students to have to reconstruct the mathematics later). Fisher's interest in natural history was reflected in the books chosen for special school prizes at Harrow, culminating in his last year in the choice of the complete works of Charles Darwin in 13 volumes. Fisher entered Gonville and Caius College, Cambridge, as a scholar in 1909, graduating BA in mathematics in 1912. At college he instigated the formation of a Cambridge University Eugenics Society through which he met Major Leonard Darwin, Charles's fourth son and president of the Eugenics Education Society of London, who was to become his mentor and friend. Prevented from entering war service in 1914 by his poor eyesight, Fisher taught in schools for the duration of the war and in 1919 was appointed Statistician to Rothamsted Experimental Station, an agricultural station at Harpenden north of London. In 1933 he was elected to succeed Karl Pearson as Galton Professor of Eugenics (i.e., of Human Genetics, as it later became) at University College, London, and in 1943 he was elected Arthur Balfour Professor of Genetics at Cambridge and a Fellow of Gonville and Caius College. He retired in 1957 and spent his last few years in Adelaide, Australia, where he died of a postoperative embolism on 29 July 1962. His ashes lie under a plaque in the nave of Adelaide Cathedral.
F i l ia l G e ne r a t io ns
Jiang SC, Matte M, Matte G, Huq A and Colwell RR (2000) Genetic diversity of clinical and environmental isolates of Vibrio cholerae determined by amplified fragment length polymorphism fingerprinting. Applied and Environmental Microbiology 66: 148±153. Kimsey HH and Waldor MK (1998) Vibrio cholerae hemagglutinin/protease inactivates CTXphi. Infection and Immunity 66: 4025±4029. Miao EA and Miller SI (1999) Bacteriophages in the evolution of pathogen±host interactions. Proceedings of the National Academy of Sciences, USA 96: 9452±9454. Muniesa M and Jofre J (1998) Abundance in sewage of bacteriophages that infect Escherichia coli O157:H7 and that carry the shiga toxin 2 gene. Applied and Environmental Microbiology 64: 2443±2448. Murley YM, Carroll PA, Skorupski K, Taylor RK and Calderwood SB (1999) Differential transcription of the tcpPH operon confers biotype-specific control of the Vibrio cholerae ToxR virulence regulon. Infection and Immunity 67: 5117±5123. Nasu H, Iida T, Sugahara T et al. (2000) A filamentous phage associated with recent pandemic Vibrio parahaemolyticus O3:K6 strains. Journal of Clinical Microbiology 38: 2156± 2161. SchloÈr S, Riedl S, Blass J and Reidl J (2000) Genetic rearrangements of the regions adjacent to genes encoding heat-labile enterotoxins (eltAB) of enterotoxigenic Escherichia coli strains. Applied and Environmental Microbiology 66: 352± 358. Sharma C, Thungapathra M, Ghosh A et al. (1998) Molecular analysis of non-O1, non-O139, Vibrio cholerae associated with an unusual upsurge in the incidence of cholera-like disease in Calcutta, India. Journal of Clinical Microbiology 36: 756±763.
See also: Bacteriophages; Capsid; Gene Rearrangements, Prokaryotic; Plasmids
generations of brother±sister matings numbered with integer increments. See also: F1 Hybrid; Inbred Strain
Filter Hybridization Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1842
Filter hybridization is a technique for in situ solidphase hybridization whereby denatured DNA is immobilized on a nitrocellulose filter and incubated with a solution of radioactively labeled RNA or DNA. See also: In situ Hybridization
Fingerprinting J H Miller Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0458
The chromatographic pattern of spots produced by proteolytic digestion of a protein followed by electrophoresis. See also: Proteins and Protein Structure
First and Second Division Segregation J R S Fincham
Filial Generations
Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1428
L Silver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0456
Filial generation is the term pertaining to a particular generation in a sequence of brother±sister matings that can be carried out to form an inbred strain. The first filial generation, symbolized as F1, refers to the offspring of a cross between animals having nonidentical genomes. When F1 siblings are crossed to each other, their offspring are considered to be members of the second filial generation or F2, with subsequent
During the first division of meiosis in a diploid cell the chromosomes are each divided into chromatids, but sister chromatids remain attached together at the centromere. At first anaphase, the centromeres do not split, as in anaphase of mitosis; instead the centromeres of homologous chromosomes separate (segregate) from each other toward the two poles of the division spindle as wholes, each taking two chromatids with it. Centromeres always segregate at the first division of meiosis and do not split to allow their two halves to separate into different meiotic products until the second division.
First and Seco nd Division Segregation 699
Diplotene / metaphase I
anaphase I
anaphase II
(A)
A A A a a
A A a a a
(B)
A A a a
A
A A
a
A
a
A
a
a a
Figure 1 First division (A) or second division (B) segregation of alleles A and a depending on whether or not a crossover occurs between the A/a locus and the centromere (vertical bar in the left panel), the point at which sister chromatids remain connected until the second division of meiosis. When two homologous chromosomes are distinguished by a genetic marker, say with allele A on one chromosome and allele a on the other, the A±a difference will segregate at the first division provided that the alleles remain attached to their original centromeres. However, when a single crossover, which always involves just one chromatid of each chromosome, occurs between the A/a locus and the centromere, different alleles become joined to the same centromere, and the anaphase separation at first anaphase will not be between A-A and a-a but rather between A-a and A-a (Figure 1). Then segregation of A from a will be delayed to the the second division. The effect of two crossovers in the locus±centromere interval depends on whether the same chromatids are involved in the second crossover as in the first. If the same two cross over twice (a two-strand double), or if the second crossover involves the two chromatids not involved in the first crossover (a four-strand double), the effect in either case is to restore first division segregation. If one chromatid crosses over twice, two cross over once each, and the other not at all (threestrand double), the effect is second division segregation. So double crossovers give, on average, 50% second division segregation. An indefinitely large number of crossovers will give, on average, two-thirds second division segregation, a result most easily understood by imagining the four alleles as totally uncoupled from their centromeres and distributed two-and-two to the first division spindle poles at
random. An A allele will then be twice as likely to be accompanied by an a allele as by the other A allele. First and second division segregation can be distinguished by tetrad analysis (see Tetrad Analysis). When the marker is a gross chromosomal feature such as a large terminal deletion, first and second division segregation can also sometimes be seen under the microscope (Figure 2). The second division frequency of a genetic marker is a measure of the frequency of its crossing-over with the centromere, and hence of the map length of the marker±centromere interval. To make second division segregation percentages equivalent to recombination percentages, on which map units (centimorgans, cM) are conventionally based, they need to be divided by two. This is because a single crossover in a marker± centromere interval will always give second division segregation, whereas a single crossover in a marker± marker interval will recombine only two out of the four chromatids. In fact, second division segregation and recombination frequencies relate linearly to true map distance (total average number of crossovers per chromosome pair 50) only when there is never more than one crossover in the interval concerned. Both measures approach a maximum value as the number of crossovers in the interval becomes large, and this maximum value is different in the two cases: 50% recombination and 67% second division segregation, which, without correction, would convert to 50 and 33.3 cM. Thus, as distance increases, both measures
700
F I S H ( F l u o re s c en t i n s i t u H y b r i d i z a t i o n)
No chiasma between centromere and the deletion in one homolog
Chiasma formed between centromere and deletion
Interpretation Bivalents at first metaphase Actual appearance
Anaphase II
Fisher, R.A. A W F Edwards Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0459
Arm shortened by deletion
Anaphase I
Segregation of length difference at 1st division
Anaphase II
Segregation of length difference at 2nd division
Figure 2 First and second division segregation made visible in a lily heterozygous for a chromosome length difference. (Reproduced with permission from Fincham JRS (1983) Genetics after Brown and Zohary (1955) Genetics 40: 850.) increasingly underestimate true map distance, but second division segregation does so to a greater extent. See also: Centimorgan (cM); Centromere; Crossing-Over; Map Distance, Unit; Meiosis; Tetrad Analysis
FISH (Fluorescent in situ Hybridization) J Read and S Brenner Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.2090
Fluorescent in situ hybridization (FISH) is a technique used to identify the chromosomal location of a particular DNA sequence. A DNA probe is fluorescently labeled and hybridized to denatured metaphase chromosomes spread out on glass slides. See also: Physical Mapping
Sir Ronald Fisher (1890±1962), the father of modern statistics, was for most of his life a professor of genetics, first in London and then at Cambridge. He made lasting contributions to mathematical and evolutionary genetics as well as to statistical theory applied to genetics, and experimented widely, studying especially linkage in the mouse and in polysomic plants and natural selection in the wild. Fisher was born in London on 17 February 1890, the son of a fine-art auctioneer. His twin brother was stillborn. At Harrow School he distinguished himself in mathematics despite being handicapped by poor eyesight which prevented him working by artificial light. His teachers used to instruct him by ear, and Fisher developed a remarkable capacity for pursuing complex mathematical arguments in his head. This manifested itself later in life in an ability to reach a conclusion whilst forgetting the argument, to handle complex geometrical trains of thought, and to develop and report essentially mathematical arguments in English (only for students to have to reconstruct the mathematics later). Fisher's interest in natural history was reflected in the books chosen for special school prizes at Harrow, culminating in his last year in the choice of the complete works of Charles Darwin in 13 volumes. Fisher entered Gonville and Caius College, Cambridge, as a scholar in 1909, graduating BA in mathematics in 1912. At college he instigated the formation of a Cambridge University Eugenics Society through which he met Major Leonard Darwin, Charles's fourth son and president of the Eugenics Education Society of London, who was to become his mentor and friend. Prevented from entering war service in 1914 by his poor eyesight, Fisher taught in schools for the duration of the war and in 1919 was appointed Statistician to Rothamsted Experimental Station, an agricultural station at Harpenden north of London. In 1933 he was elected to succeed Karl Pearson as Galton Professor of Eugenics (i.e., of Human Genetics, as it later became) at University College, London, and in 1943 he was elected Arthur Balfour Professor of Genetics at Cambridge and a Fellow of Gonville and Caius College. He retired in 1957 and spent his last few years in Adelaide, Australia, where he died of a postoperative embolism on 29 July 1962. His ashes lie under a plaque in the nave of Adelaide Cathedral.