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Frequently asked questions (FAQs)
Q.1. What is crossing over and what are its consequences?
Ans. Exchange of chromosomal segments between chromatids of
homologous chromosomes is called Crossing-over. It occurs at
pachytene stage of prophase I in meiosis; each event involves
two of the four chromatids. Any chromatids may be involved in
crossing over.The genetic consequence of crossing over is that
each chromosome that goes into a gamete is a combination of
maternal and paternal chromosomes.
Q.2. What is the significance of a test cross?
Ans. A testcross is a cross between F1 hybrid and homozygous
recessive parent. Test cross helps in detection of linkage
relationship between two genes.If the two genes are not
linked, during a test cross all four possible combinations of
the two traits will be present in equal numbers in the progeny.
A significant deviation in this ratio (more parental and fewer
recombinant types) indicates linkage.
Q.3. What is coupling and repulsion?
Ans. The condition of having the dominant alleles for both genes
on the same parental chromosome, with both recessives on
the other parental chromosome, is called “coupling”, e.g. P and
L genes when present on same chromosome are “in coupling
phase”. The opposite condition, having one dominant and one
recessive on each parental chromosome, is called “repulsion”.
e.g. if the original parents were P l x p L, their offspring would
have the genes in repulsion phase: Pl / pL.
Q.4. Differentiate between homozygous and heterozygous nature
of a gene.
Ans. In a diploid species, each individual carries two copies of
each gene (with some exceptions). The two copies of a gene
also called alleles are located on two members of a homologous
chromosome pair. If the two copies of the gene are identical
alleles, then the individual is homozygous for the gene. If
the two copies are different alleles, then the individual is
heterozygous for the gene
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Q.5. Differentiate between genotype and phenotype.
Ans. Genotype is the genetic makeup of an individual with
reference to one or more specific traits. A genotype is
designated by using symbols to represent the alleles of the
gene.
Phenotype is the external appearance of a trait in an individual.
Phenotypes can be determined by a combination of genetic
and environmental factors
Q.6. Differentiate between a gene and an allele.
Ans. A gene is a unit of inheritance or a factor transmitted during
reproduction and responsible for the appearance of a given
trait. At molecular level it is a segment of a DNA molecule
usually at a specific location (locus) on a chromosome and is
characterized by its nucleotide sequence. Genes play three
notable roles:
• To encode the amino acid sequences of proteins
• To encode the nucleotide sequences of tRNA or rRNA
• To regulate the expression of other genes
Allele is a variant form of a gene found within a population.
Alleles of a gene usually have small differences in their nucleotide
sequences. The differences can affect the trait for which the
gene is responsible. Most genes have more than one allele.
Q.7. What is recombination and what are its consequences?
Recombination means that meiosis generates gametes with
different allelic combinations than the original gametes the
organism inherited. If the organism was created by the fusion
of an egg bearing AB and a sperm bearing ab, recombination
generates gametes that are Ab and aB. Recombination
may be caused by loci on different chromosomes that sort
independently or by a physical crossing over between two
loci on the same chromosome, with breakage and exchange
of strands of homologous chromosomes paired in meiotic
prophase I.
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Q.8. In a testcross for two genes, what types of gametes
are produced with (a) complete linkage, (b) independent
assortment, and (c) incomplete linkage?
Ans. (a) Complete linkage of two genes means that only non
recombinant gametes will be
produced; the recombination frequency is zero.
(b) Independent assortment of two genes will result in
50% of the gametes being recombinants and 50% being
non recombinants, as would be observed for genes on two
different chromosomes. Independent assortment may also
be observed for genes on the same chromosome if they
are far enough apart that one or more crossovers occur
between them during meiosis.
(c) Incomplete linkage means that greater than 50% of
the gametes produced are non recombinants and less than
50% of the gametes are recombinants; the recombination
frequency is greater than 0 and less than 50%.
Q.9. What effect does crossing over have on linkage?
Ans. Crossing over generates recombination between genes
located on the same
chromosome, and thus renders linkage incomplete.
Q.10. Why is the frequency of recombinant gametes always
half the frequency of crossing over?
Ans. Crossing over occurs at the four-strand stage, when two
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homologous chromosomes,
each consisting of a pair of sister chromatids, are paired.
Each crossover involves just two of the four strands and
generates two recombinant strands. The remaining two
strands that were not involved in the crossover generate
two non recombinant strands. Therefore, the frequency
of recombinant gametes is always half the frequency of
crossovers.
Q.11. What is the difference between genes in coupling
configuration and genes in repulsion? What effect does the
arrangement of linked genes (whether they are in coupling
configuration or in repulsion) have on the results of a cross?
Ans. Genes in coupling configuration have two wild-type alleles
on the same chromosome
and the two mutant alleles on the homologous chromosome.
Genes in repulsion have
a wild-type allele of one gene together with the mutant allele
of the second gene on the
same chromosome, and vice versa on the homologous
chromosome. The two
arrangements have opposite effects on the results of a cross.
For genes in coupling configuration, most of the progeny
will be either wild type for both genes, or mutant for both
genes, with relatively few that are wild type for one gene
and mutant for the other. For genes in repulsion, most of
the progeny will be mutant for only one gene and wild-type
for the other, with relatively few recombinants that are wildtype for both or mutant for both.
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Q.12. How can one test whether two genes are linked?
Ans. One first obtains individuals that are heterozygous for
both genes. This may be achieved by crossing an individual
homozygous dominant for both genes to one homozygous
recessive for both genes, resulting in a heterozygote with
genes in coupling configuration. Alternatively, an individual
that is homozygous recessive for one gene may be crossed
to an individual homozygous recessive for the second gene,
resulting in a heterozygote with genes in repulsion. Then,
the heterozygote is mated to a homozygous recessive tester
and the progeny of each phenotypic class are tallied. If the
proportion of recombinant progeny is far less than 50%, the
genes are linked. If the results are not so clear-cut, then they
may be tested by chi-square, first for equal segregation at
each locus, then for independent assortment of the two loci.
Significant deviation from results expected for independent
assortment indicates linkage of the two genes.
Q.13. Explain briefly the law of segregation.
Ans. Mendel’s first principle of inheritance is called ‘the law of
segregation’. It states that “the
allelic genes in zygote do not blend or contaminate each
other but segregate and pass into different gametes”. The
segregation occurs at the meotic division.
Q.14. Explain briefly the law of dominance.
Ans. A second principle recognized by Mendel is called ‘law
of dominance’. The law emphasizes that the two alleles
of a particular gene may not be identical and may impart
different character to a particular trait e.g., one allele of a
particular gene, say R, may impart red colour to a flower
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while its allele, r, may be responsible for white colour to the
flower. When either is present in a homozygous condition, the
colour of the flower is either red (RR) or white (rr). On the
other hand, if they are present in a heterozygous condition
(Rr), the expression of the white colour is masked by the
dominant red colour and the flower emerges as red. In this
case, red colour allele (R) is said to be dominant and white
colour gene (r) is said to be recessive.
Q.15. Explain briefly the law of independent assortment.
Ans. Mendel’s third principle is known as the law of independent
assortment. It states that, “during meiosis one of the
chromosomes in the pair is contributed independently in the
gamete without being influenced by other chromosomes or
cytoplasmic factors”. To explain this law, it may be recalled
that the chromosomes and the genes always exist in pairs
(all allelomorph) and at the time of reduction division during
gamete formation, any one partner of the pair goes to
one gamete and the other to the other gamete. Thus, the
gametes formed may have similar number of chromosomes
and the genes but may not be identical. Thus, if there are
two pairs of allelic genes, four types of gametes with different
genetic composition are formed, provided both the pairs
are heterozygous. If the number of allelic pairs is n, the
possible number of gametes is 2n. This is due to uninhibited
combination of any one allele for each pair of gene and is
called independent assortment.
Q.16. What is a centimorgan (cM)?
Ans. Morgan is considered as the founder of drosophila genetics
and in his honor, a recombination map unit is called a
centimorgan. A map unit or centimorgan is equal to crossing
over between two genes in 1% of the gametes. or. One
centimorgam (1cM) is the distance between genes for which
the recombination frequency is 1%.