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Botany : Chromosomal Basis of Inheritance
Answer the following questions
13. When two different genes came from same parent they tend to remain together.
i) What is the name of this phenomenon?
ii) Draw the cross with suitable example.
iii) Write the observed phenotypic ratio.
Answer: (i) The phenomenon is called linkage.
(ii) William Bateson and Reginald C. Punnet crossed one homozygous strain of sweet peas having purple flowers and long pollen grains with another homozygous strain having red flowers and round pollen grains. Results of Test Cross showed that a greater number of F2 plants had purple flowers and long pollen or red flowers and round pollen. So they concluded that genes for purple colour and long pollen grain and the genes for red colour and round pollen grain were found close together in the same homologous pair of chromosomes. This type of tendency of genes to stay together during separation of chromosomes is called Linkage.
Alleles in coupling or cis configuration
(iii) Phenotypic ratio 7 : 1 : 1 : 7
14. If you cross dominant genotype PV/PV male Drosophila with double recessive female and obtain F1 hybrid. Now you cross F1 male with double recessive female.
i) What type of linkage is seen?
ii) Draw the cross with correct genotype.
iii) What is the possible genotype in F2 generation?
Answer: (i) Complete linkage
(The tendency of genes to stay together during separation of chromosomes)
(iii) PV/pv and pv/pv in the ratio 1:1.
i) What is the name of this test cross?
ii) How will you construct gene mapping from the above given data?
iii) Find out the correct order of genes.
Answer: (i) Three point test cross - gene mapping.
between A and B:
RF = [ Total no. of recombinants / Total no. of progenies ] x 100
= (239 / 1200) x 100 = 19.9 %
Recombinant frequency between A and C
RF = (482 / 1200) x 100 = 40.1%
Recombinant frequency between B and C
RF = (261 / 1200) x 100 = 21.7%
RF is higher between A and C, so they must be farthest apart. The locus B must lie between them.
(iii) ABC / abc
16. What is the difference between missense and nonsense mutation?
Answer: The mutation where the codon for one amino acid is changed into a codon for another amino acid is called Missense or non-synonymous mutations. The mutations where codon for one amino acid is changed into a termination or stop codon is called Nonsense mutation.
From the above figure identify the type of mutation and explain it.
Answer: The figure shows structural chemical aberration- Duplication-Reverse tandem type.
Reverse tandem duplication:
The duplicated segment is located immediately after the normal segment but the gene sequence order will be reversed.
18. Write the salient features of Sutton and Boveri concept.
Answer: Sutton and Boveri (1903) independently proposed the chromosome theory of inheritance. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it chromosomal theory of inheritance.
Salient features of the Chromosomal theory of inheritance:
Answer: (i) Somatic cells of organisms are derived from the zygote by repeated cell division (mitosis). These consist of two identical sets of chromosomes. One set is received from female parent (maternal) and the other from male parent (paternal). These two chromosomes constitute the homologous pair.
(ii) Chromosomes retain their structural uniqueness and individuality throughout the life cycle of an organism.
(iii) Each chromosome carries specific determiners or Mendelian factors which are now termed as genes.
(iv) The behaviour of chromosomes during the gamete formation (meiosis) provides evidence to the fact that genes or factors are located on chromosomes.
19. Explain the mechanism of crossing over.
Answer: Mechanism of Crossing Over
Crossing over is a precise process that includes stages like synapsis, tetrad formation, cross over and terminalization.
Intimate pairing between two homologous chromosomes is initiated during zygotene stage of prophase I of meiosis I. Homologous chromosomes are aligned side by side resulting in a pair of homologous chromosomes called bivalents. This pairing phenomenon is called synapsis or syndesis. It is of three types,
1. Procentric synapsis: Pairing starts from middle of the chromosome.
2. Proterminal synapsis: Pairing starts from the telomeres.
3. Random synapsis: Pairing may start from anywhere.
(ii) Tetrad Formation:
Each homologous chromosome of a bivalent begin to form two identical sister chromatids, which remain held together by a centromere. At this stage each bivalent has four chromatids. This stage is called tetrad stage.
(iii) Cross Over:
After tetrad formation, crossing over occurs in pachytene stage. The non-sister chromatids of homologous pair make a contact at one or more points. These points of contact between non-sister chromatids of homologous chromosomes are called Chiasmata (singular-Chiasma). At chiasma, cross-shaped or X-shaped structures are formed, where breaking and rejoining of two chromatids occur. This results in reciprocal exchange of equal and corresponding segments between them. Synapsis and chiasma formation are facilitated by a highly organised structure of filaments called Synaptonemal Complex (SC). This synaptonemal complex formation is absent in some species of male Drosophila. Hence crossing over does not takes place.
20. Write the steps involved in molecular mechanism of DNA recombination with diagram.
The widely accepted model of DNA recombination during crossing over is Holliday’s hybrid DNA model. It was first proposed by Robin Holliday in 1964. It involves several steps. ( Figure )
(i) Homologous DNA molecules are paired side by side with their duplicated copies of DNAs.
(ii) One strand of both DNAs cut in one place by the enzyme endonuclease.
(iii) The cut strands cross and join the homologous strands forming the Holliday strcture or Holliday junction.
(iv) The Holliday junction migrates away from the original site, a process called branch migration, as a result heteroduplex region is formed.
(v) DNA strands may cut along through the vertical (V) line or horizontal (H) line.
(vi) The vertical cut will result in heteroduplexes with recombinants.
(vii) The horizontal cut will result in heteroduplex with non recombinants.
21. How is Nicotiana exhibit self-incompatibility. Explain its mechanism.
Answer: (i) In plants, multiple alleles have been reported in association with self-sterility or self-incompatibility. Self - sterility means that the pollen from a plant is unable to germinate on its own stigma and will not be able to bring about fertilization in the ovules of the same plant.
The self-incompatibility in relation to its genotype in tobacco
(ii) East (1925) observed multiple alleles in Nicotiana which are responsible for self-incompatibility or self-sterility. The gene for self-incompatibility can be designated as S, which has allelic series S1, S2, S3, S4 and S5 .
(iii) The cross-fertilizing tobacco plants were not always homozygous as S1S1 or S2S2, but all plants were heterozygous as S1S2, S3S4, S5S6.
(iv) When crosses were made between different S1S2 plants, the pollen tube did not develop normally. But effective pollen tube development was observed when crossing was made with other than S1S2 for example S3S4.
of progeny in self-incompatibility
22. How sex is determined in monoecious plants. write their genes involved in it.
Answer: (i) Zea mays (maize) is an example for monoecious, which means male and female flowers are present on the same plant.
(ii) There are two types of inflorescence. The terminal inflorescence which bears staminate florets develops from shoot apical meristem called tassel. The lateral inflorescence which develop pistillate florets develops from axillary bud and is called ear or cob.
Sex determination in Maize (Superscript (+) denotes dominant character)
(iii) Unisexuality in maize occurs through the selective abortion of stamens in ear florets and pistils in tassel florets.
(iv) A substitution of two single gene pairs 'ba' for barren plant and 'ts' for tassel seed makes the difference between monoecious and dioecious (rare) maize plants. The allele for barren plant (ba) when homozygous makes the stalk staminate by eliminating silk and ears. The allele for tassel seed (ts) transforms tassel into a pistillate structure that produce no pollen. Most of these mutations are shown to be defects in gibberellin biosynthesis. Gibberellins play an important role in the suppression of stamens in florets on the ears.
23. What is gene mapping? Write its uses.
Answer: The diagrammatic representation of position of genes and related distances between the adjacent genes is called genetic mapping. It is directly proportional to the frequency of recombination between them. It is also called as linkage map. The concept of gene mapping was first developed Alfred H Sturtevant.
Uses of genetic mapping:
(i) It is used to determine gene order, identify the locus of a gene and calculate the distances between genes.
(ii) They are useful in predicting results of dihybrid and trihybrid crosses.
(iii) It allows the geneticists to understand the overall genetic complexity of particular organism.
24. Draw the diagram of different types of aneuploidy.
Answer: It is a condition in which diploid number is altered either by addition or deletion of one or more chromosomes. Different types of Aneuploidy is show below. It is classified into two main types - Hyperploidy and Hypoploidy.
Types of Aneuploidy
25. Mention the name of man-made cereal. How it is formed?
Answer: Triticale is the successful first man made cereal. Depending on the ploidy level Triticale can be divided into three main groups.
(i) Tetraploidy: Crosses between diploid wheat and rye.
(ii) Hexaploidy: Crosses between tetraploid wheat Triticum durum (macaroni wheat) and rye
(iii) Octoploidy: Crosses between hexaploid wheat T. aestivum (bread wheat) and rye.
Hexaploidy: Triticale hybrid plants demonstrate characteristics of both macaroni wheat and rye. For example, they combine the high-protein content of wheat with rye’s high content of the amino acid lysine, which is low in wheat.
Colchicine, an alkaloid when applied in low concentration to the growing tips of the plants induce polyploidy.
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