Chromosomal disorders Questions in English

(D) In $Drosophila \text{ } melanogaster$, sex determination is based on the ratio of $X$ chromosomes to autosomes, where $XO$ results in a phenotypically normal male.
In humans, sex is determined by the presence of the $Y$ chromosome. The $XY$ genotype results in a male, while the $XX$ genotype results in a female.
A human individual with $44$ autosomes and a single $X$ chromosome $(45, XO)$ lacks the $Y$ chromosome required for male development and has only one $X$ chromosome instead of two. This condition is known as Turner's syndrome, which results in sterile females with underdeveloped secondary sexual characteristics.

(A) The condition where sex chromosomes fail to separate during meiosis is known as non-disjunction.
$(a)$ The genotype $22$ pairs $+ XXY$ represents Klinefelter syndrome,which occurs in males.
This condition arises due to the fertilization of an abnormal ovum containing $22 + XX$ chromosomes by a normal sperm containing $22 + Y$ chromosomes,or by the fertilization of a normal ovum by an abnormal sperm containing $22 + XY$ chromosomes.
Therefore,$22$ pairs $+ XXY$ is a correct genotype for a male phenotype.

(A) Euploidy is a condition in which an organism has one or more complete sets of chromosomes.
It is defined as the presence of chromosome numbers that are exact multiples of the basic haploid set ($n$ or $x$).
For example,organisms can be monoploid $(x)$,diploid $(2x)$,triploid $(3x)$,tetraploid $(4x)$,etc.
In contrast,aneuploidy involves the gain or loss of individual chromosomes rather than entire sets.

(A) The chromosomal abnormalities are matched as follows:
$(1)$ Patau's syndrome is caused by trisomy of the $13^{th}$ chromosome ($46 + 1 = 47$,Chromosome $13^{th}$),which corresponds to $(C)$.
$(2)$ Klinefelter's syndrome is characterized by an extra $X$ chromosome in males $(44 + XXY = 47)$,which corresponds to $(A)$.
$(3)$ Down's syndrome is caused by trisomy of the $21^{st}$ chromosome ($46 + 1 = 47$,Chromosome $21^{st}$),which corresponds to $(D)$.
$(4)$ Turner's syndrome is characterized by the loss of an $X$ chromosome in females $(44 + X = 45)$,which corresponds to $(B)$.
Therefore,the correct matching is $1-C, 2-A, 3-D, 4-B$.

(A) The $XYY$ syndrome,also known as Jacob's syndrome,is a chromosomal condition where a male has an extra $Y$ chromosome.
Individuals with this syndrome are typically characterized by a tall stature,potential learning disabilities,and a tendency toward impulsive or antisocial behavior.
The extra $Y$ chromosome is strongly male-determining and leads to an increased production of male hormones (androgens),which is often associated with increased aggressiveness and a predisposition toward violent or antisocial behavior.

(C) The normal human somatic cell contains $46$ chromosomes.
These $46$ chromosomes are arranged in $23$ pairs.
In $1956$,Joe Hin Tjio and Albert Levan established that the correct diploid chromosome number in humans is $46$ ($23$ pairs).

(C) An $Idiogram$ is a diagrammatic representation or a composite photograph of the chromosomes of a cell,typically at the $metaphase$ stage. In this arrangement,the chromosomes are organized in a series based on their decreasing length,thickness,position of the $centromere$,and overall shape. Therefore,the correct term for this arrangement is $Idiogram$.

(D) The correct answer is $D$.
A $Karyotype$ is the complete set of chromosomes in an individual, arranged in a standard format based on their size, shape, and morphology.
It provides a visual representation of the chromosomal complement, which helps in identifying chromosomal abnormalities or determining the species-specific chromosomal arrangement.

(A) Aneuploidy is the condition where there is an abnormal number of chromosomes in a cell.
When one chromosome is missing from a diploid set,the condition is represented as $2n-1$,which is known as Monosomy.
Nullosomy is represented as $2n-2$,where a pair of homologous chromosomes is missing.
Trisomy is represented as $2n+1$,where an extra chromosome is present.
Tetrasomy is represented as $2n+2$,where an extra pair of chromosomes is present.

(C) When a segment of a chromosome is lost from the middle,it is known as an intercalary deficiency or interstitial deletion.
This type of chromosomal aberration occurs when two breaks happen in the chromosome,and the segment between the breaks is lost,causing the chromosome to become shorter than its normal length.

(D) Human chromosomes are classified into $7$ groups ($A$ to $G$) based on the position of the centromere and the relative length of the chromosome arms.
Group $A$ includes chromosomes $1-3$.
Group $B$ includes chromosomes $4-5$,which are submetacentric.
Group $C$ includes chromosomes $6-12$.
Group $D$ includes chromosomes $13-15$.
Therefore,$B$-chromosomes correspond to $4-5$.

(D) The presence of a $Barr$ body is determined by the formula $N - 1$,where $N$ is the number of $X$ chromosomes. $A$ normal male $(XY)$ has $0$ $Barr$ bodies,and a normal female $(XX)$ has $1$ $Barr$ body.
An extra $Barr$ body in an embryo indicates the presence of an additional $X$ chromosome.
$Klinefelter$ syndrome is characterized by the genotype $XXY$.
In this case,the number of $Barr$ bodies is $2 - 1 = 1$. However,if the question implies an 'extra' $Barr$ body compared to a normal male embryo (which has $0$),or if it refers to a condition like $XXX$ (Triple $X$ syndrome) or $XXY$ ($Klinefelter$ syndrome),$Klinefelter$ syndrome is the most standard clinical association for an extra $X$ chromosome in a male-developing embryo.

(C) The condition in which a human male develops breast tissue similar to that of a female is known as $Gynecomastia$.
This often occurs due to a hormonal imbalance,specifically an increase in estrogen relative to testosterone.
In the context of genetic disorders,it is a characteristic feature of $Klinefelter$ syndrome,where males have an extra $X$ chromosome $(XXY)$.

(A) Down syndrome is a chromosomal disorder caused by trisomy of chromosome $21$.
An affected mother has the genotype $21, 21, 21$ (trisomy) in her germ cells,while a normal father has $21, 21$ (disomy).
During meiosis,the mother produces two types of gametes: $50\%$ with two copies of chromosome $21$ $(21, 21)$ and $50\%$ with one copy of chromosome $21$ $(21)$.
The father produces gametes with one copy of chromosome $21$ $(21)$.
When these gametes fuse:
$1$. $50\%$ of the zygotes will have three copies of chromosome $21$ $(21, 21 + 21 = 21, 21, 21)$,resulting in Down syndrome.
$2$. $50\%$ of the zygotes will have two copies of chromosome $21$ $(21 + 21 = 21, 21)$,resulting in normal offspring.
Therefore,$50\%$ of the offspring will be affected.

(B) Klinefelter's syndrome is a genetic disorder caused by the presence of an extra $X$ chromosome in males.
This results in a chromosomal constitution of $47, XXY$.
Individuals with this syndrome have a male phenotype but may exhibit some female characteristics,such as breast development (gynecomastia),and are typically sterile.

(C) The inheritance pattern described is sex-linked inheritance in Drosophila,first studied by Calvin Bridges.
Normally,a white-eyed female $(X^wX^w)$ crossed with a red-eyed male $(X^WY)$ produces red-eyed females $(X^WX^w)$ and white-eyed males $(X^wY)$.
However,the appearance of white-eyed females and red-eyed males is an exception to the standard Mendelian inheritance pattern.
This occurs due to the failure of the two $X$ chromosomes to separate during meiosis in the female,a phenomenon known as non-disjunction.
When non-disjunction occurs,the female produces gametes with two $X$ chromosomes $(XX)$ and gametes with no $X$ chromosome $(O)$.
When an $XX$ egg is fertilized by a $Y$ sperm,it results in an $XXY$ female (white-eyed),and when an $O$ egg is fertilized by an $X^W$ sperm,it results in an $X^WO$ male (red-eyed).

(B) The number of Barr bodies in a cell is calculated using the formula $N = n - 1$,where $N$ is the number of Barr bodies and $n$ is the total number of $X$ chromosomes.
Given that the male has one Barr body $(N = 1)$,we can substitute this into the formula: $1 = n - 1$,which gives $n = 2$.
This means the individual has two $X$ chromosomes.
Since the individual is male,the presence of a $Y$ chromosome is mandatory.
Therefore,the genetic constitution is $XXY$,which is characteristic of Klinefelter syndrome.

(B) The presence of $XX$ males or $XY$ females is a rare genetic condition often caused by the translocation of the $SRY$ gene (Sex-determining Region $Y$) from the $Y$ chromosome to the $X$ chromosome. During meiosis,an unequal crossing over event can lead to the $SRY$ gene being transferred to the $X$ chromosome. Consequently,an individual with an $XX$ genotype can develop as a male if the $X$ chromosome carries the $SRY$ gene,while an individual with an $XY$ genotype can develop as a female if the $Y$ chromosome lacks the $SRY$ gene.

(A) The $Cri-du-chat$ syndrome (also known as $5p-$ syndrome) is a genetic condition caused by the deletion of a portion of the short arm ($p$ arm) of chromosome $5$. This structural chromosomal abnormality leads to characteristic symptoms such as a high-pitched cry that sounds like a cat,intellectual disability,and delayed development. Therefore,the correct answer is deletion.

(B) The exchange of chromosomal segments between two non-homologous chromosomes is called $Translocation$.
$Crossing over$ involves the exchange of segments between homologous chromosomes.
$Inversion$ occurs when a segment of a chromosome breaks and reattaches in the reverse orientation.
$Transition$ is a type of point mutation where a purine is replaced by another purine or a pyrimidine by another pyrimidine.

(A) change in the number of chromosomes is referred to as chromosomal aberration or chromosomal mutation.
Chromosomal aberrations include changes in the number of chromosomes (aneuploidy or polyploidy) or changes in the structure of chromosomes.
Gene mutation refers to changes in the $DNA$ sequence of a single gene.
Somatic mutation occurs in non-reproductive cells and is not inherited.
Polyploidy is a specific type of chromosomal aberration where an organism has more than two complete sets of chromosomes,but the broader term for any change in chromosome number is chromosomal aberration.

(D) The condition $2n-1$ represents the loss of a single chromosome from a diploid set,which is known as monosomy.
Trisomy is represented as $2n+1$,where an extra chromosome is added.
Euploidy refers to the condition where the chromosome number is an exact multiple of the haploid set $(n)$.
Polyploidy refers to the condition where an organism has more than two complete sets of chromosomes (e.g.,$3n, 4n$).

(A) Chromosomal aberrations or mutations are structural changes in chromosomes that can lead to various syndromes.
Deletion is a type of chromosomal mutation where a segment of the chromosome is lost.
For example,the Cri-du-chat syndrome is caused by the deletion of a portion of the short arm of chromosome $5$.
Therefore,deletion is a primary event in the chromosomal structure that leads to the formation of specific syndromes.

(C) The number of $Barr$ bodies in a cell is calculated using the formula: $N = n - 1$,where $n$ is the total number of $X$ chromosomes present in the cell.
In the given genotype $XXXXY$,the total number of $X$ chromosomes $(n)$ is $4$.
Therefore,the number of $Barr$ bodies = $4 - 1 = 3$.
Thus,there are $3$ $Barr$ bodies in $XXXXY$.

(D) : Down's syndrome is an autosomal aneuploidy caused by the presence of an extra chromosome number $21$. During oogenesis,non-disjunction occurs,leading to both chromosomes of the $21$st pair passing into a single egg. This results in a trisomy of chromosome $21$ in the zygote.

(C) $(C) :$ Translocation is a chromosomal abnormality caused by the rearrangement of segments between non-homologous chromosomes.
This process can result in a gene moving from one linkage group to another,as the chromosomal segment containing the gene is transferred to a different chromosome.

(A) : Translocation is a chromosomal abnormality caused by the rearrangement of parts between non-homologous chromosomes.
It results in the transfer of a segment of $DNA$ from one chromosome to another,which may cause a gene to move from one linkage group to another.

(D) : The abnormal baby has an extra $X$ chromosome,thus it must have been produced by the fusion of an abnormal $XX$ ovum with a normal $X$ sperm.
Abnormal $XX$ sperm is not possible because males have an $XY$ genotype; if they produce abnormal sperms,then $XY$ sperms and $O$ sperms would be produced.
If the fusion of multiple gametes had occurred (either two ova with one sperm or two sperms with one ovum),the human baby would have a triploid genotype,not the trisomy of sex chromosomes.

(A) Turner's syndrome is a chromosomal disorder caused by the absence of one of the $X$ chromosomes,resulting in a $45, XO$ genotype.
This condition occurs in human females who have only one sex chromosome,i.e.,$44 + XO = 45$ chromosomes.
Such females are sterile and possess rudimentary ovaries.
Other associated phenotypes include short stature,webbed neck,broad chest,and a lack of secondary sexual characteristics.
Therefore,any imbalance in the number of sex chromosomes disrupts the genetic information necessary for normal sexual development.

(B) : Down's syndrome is the trisomy of the $21$st chromosome in humans.
Down's syndrome is characterized by short stature,warty skin,protruding tongue,slanting eyes,and folded eyelids.
The affected person's face presents a typical mongoloid look; hence,it is also called mongoloid idiocy.
It occurs due to the phenomenon of nondisjunction.
Nondisjunction occurs when a pair of homologous chromosomes does not separate during meiosis but migrates to the same pole of the cell,resulting in an uneven number of chromosomes in the daughter cells ($45$ in one and $47$ in the other).
This numerical abnormality results in trisomy $(2n + 1)$ and monosomy $(2n - 1)$.
Nondisjunction is more common in sex chromosomes.

(C) : Klinefelter's syndrome is a genetic disorder in which there are three sex chromosomes,$XXY$,rather than the normal $XX$ or $XY$.
The number of autosomes is normal,$i.e., 44$.
Affected individuals are phenotypically male but are tall and thin,with small testes,failure of normal sperm production (azoospermia),enlargement of the breasts (gynaecomastia),and absence of facial and body hairs.

(C) Hexaploid wheat $(Triticum \text{ } aestivum)$ has a total chromosome number of $2n = 42$.
The haploid number $(n)$ is half of the diploid number, so $n = 42 / 2 = 21$.
The basic chromosome number $(x)$ represents the number of chromosomes in a single genome set. Since hexaploid wheat is $6x = 42$, the basic number is $x = 42 / 6 = 7$.
Therefore, $n = 21$ and $x = 7$.

(C) : Cri-du-chat syndrome, also known as $5p$ deletion syndrome (or $5p$ minus), is a rare genetic disorder.
Cri-du-chat syndrome is caused by a partial deletion of the short arm ($p$ arm) of chromosome number $5$.
The name of this syndrome is French for "cry of the cat," referring to the distinctive high-pitched cry of infants with this disorder.
The cry is caused by abnormal larynx development, which typically improves within a few weeks of birth.
Infants with cri-du-chat often have low birth weight and may experience respiratory or feeding difficulties.

(B) The $XXY$ karyotype is characteristic of Klinefelter's syndrome.
This condition occurs due to the presence of an additional copy of the $X$ chromosome,resulting in a total of $47$ chromosomes $(44 + XXY)$.
Individuals with this syndrome are males who exhibit feminine characteristics such as breast development (gynecomastia) and are typically sterile.

(C) Turner's syndrome is a genetic disorder caused by the absence of one of the $X$ chromosomes in females.
This condition results in a $45, XO$ karyotype,meaning the individual has $45$ chromosomes instead of the normal $46$.
Individuals with Turner's syndrome are phenotypically female but are sterile and have underdeveloped secondary sexual characteristics.

(D) Aneuploidy is a condition where there is an abnormal number of chromosomes in a cell.
When a single chromosome is lost from a diploid set,the condition is represented as $(2n - 1)$.
This specific condition is known as Monosomy.
In contrast,Nullisomy is the loss of a pair of homologous chromosomes $(2n - 2)$,and Trisomy is the addition of an extra chromosome $(2n + 1)$.

(A) The symptoms described are characteristic of Down's syndrome.
Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
The clinical features include:
$(1)$ Short stature with a small round head.
$(2)$ Furrowed tongue and partially open mouth.
$(3)$ Broad palm with a characteristic palm crease (simian crease).
$(4)$ Physical,psychomotor,and mental development is retarded.
Therefore,the correct option is $A$.

(C) The symptoms described, such as retarded physical, psychomotor, and mental development, along with a broad palm and a characteristic palm crease (simian crease), are classic clinical features of $Down's syndrome$.
$Down's syndrome$ is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
Therefore, the correct option is $C$.

(B) Klinefelter's syndrome is a genetic disorder caused by the presence of an additional $X$ chromosome in males.
In humans,the normal diploid chromosome number is $46$ ($44$ autosomes + $2$ sex chromosomes).
In Klinefelter's syndrome,the individual has a $47, XXY$ karyotype.
This means there are $44$ autosomes and $3$ sex chromosomes $(XXY)$,totaling $47$ chromosomes.
Therefore,the correct constitution is $44$ autosomes and $XXY$ chromosomes.

(D) Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome number $21$ (trisomy of $21$).
It is a chromosomal disorder,not a gene mutation caused by crossing over.
Key features include mental retardation,short stature,small round head,furrowed tongue,and partially open mouth.
Therefore,the statement that it occurs due to crossing over is incorrect.

(B) Turner's syndrome is a genetic disorder caused by the absence of one $X$ chromosome in females.
Individuals with this condition have a $45, XO$ genotype instead of the normal $46, XX$ genotype.
This chromosomal abnormality leads to sterile females with rudimentary ovaries and other physical characteristics such as short stature and webbed neck.

(B) Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome number $21$.
This condition is known as trisomy of chromosome $21$.
It occurs due to the failure of segregation of homologous chromosomes during meiosis, leading to an individual having $47$ chromosomes instead of the normal $46$.

(D) The disorder described by Langdon Down is Down's syndrome,which is caused by the presence of an additional copy of chromosome number $21$ (Trisomy $21$).
Key characteristics of Down's syndrome include:
$1$. Mental retardation.
$2$. Short stature with small round head.
$3$. Furrowed tongue and partially open mouth.
$4$. Palm is broad with characteristic palm crease.
$5$. Physical,psychomotor,and mental development is retarded.
Gynaecomastia is a characteristic of Klinefelter's syndrome,not Down's syndrome.
Therefore,both $A$ and $C$ are correct characteristics of Down's syndrome.

(B) $1$. Klinefelter's syndrome is characterized by the presence of an additional copy of the $X$ chromosome $(47, XXY)$,so option $A$ is incorrect.
$2$. Turner's syndrome is caused by the absence of one $X$ chromosome,resulting in a $45, X0$ genotype,meaning only one $X$ sex chromosome is present. Thus,option $B$ is correct.
$3$. Down's syndrome is caused by trisomy of the $21^{st}$ chromosome,not monosomy. Thus,option $C$ is incorrect.
$4$. Beta thalassemia is caused by mutations in the $HBB$ gene located on chromosome $11$,not the $5^{th}$ pair. Thus,option $D$ is incorrect.

(D) Statement $(i)$ is False. Sickle-cell anaemia is an autosome-linked recessive trait that is controlled by a single pair of alleles,$Hb^A$ and $Hb^S$.
Statement $(ii)$ is True. Trisomy is a type of chromosomal disorder where an extra chromosome is present (e.g.,Down's syndrome,$2n+1$).
Statement $(iii)$ is False. Turner's syndrome is caused by the absence of one of the $X$ chromosomes,resulting in a $45$ chromosome count $(44 + XO)$.
Therefore,the correct sequence is $FTF$.

(B) Down syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This condition affects the autosomes,not the sex chromosomes.
Therefore,the sex chromosome complement in a male child with Down syndrome remains normal,which is $XY$.

(D) $Down's$ syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This condition arises due to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis,a phenomenon known as non-disjunction.
As a result,one gamete receives an extra chromosome,and upon fertilization,the zygote develops with $47$ chromosomes instead of the normal $46$.