Chromosomal disorders Questions in English

(D) In $Down's$ syndrome,the chromosomal abnormality is an autosomal aneuploidy,specifically $trisomy$ of chromosome $21$. The total number of chromosomes is $47$ ($45 + XY$ or $45 + XX$). Since the sex chromosomes ($XX$ or $XY$) remain in their normal pair configuration,the number of sex chromosomes is normal. In contrast,$Turner's$ syndrome $(45, XO)$ has one sex chromosome,$Klinefelter's$ syndrome $(47, XXY)$ has three sex chromosomes,and $Super$ female $(47, XXX)$ has three sex chromosomes.

(C) Mongolism,also known as Down's syndrome,is caused by the nondisjunction of the $21^{st}$ chromosome pair during anaphase $I$ of meiosis.
As a result,gametes with $(n + 1)$ chromosomes are formed.
Fertilization of such an abnormal gamete with a normal gamete $(n)$ results in a zygote with trisomy $(2n + 1)$,specifically trisomy $21$.

(C) The symptoms described,such as short neck,stubby hands,slanting eyes,permanently open mouth,and short stature,are characteristic of Down syndrome.
Down syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This results in a total of $47$ chromosomes in the affected individual instead of the normal $46$.

(A) The reduction of a chromosome pair in humans,such as the fusion of two ancestral chromosomes to form chromosome $2$,is a structural chromosomal aberration. This process involves the fusion of two acrocentric chromosomes into one metacentric chromosome,which explains why humans have $23$ pairs of chromosomes while other great apes have $24$ pairs.

(B) Mongolism is also known as $Down$ syndrome.
It is a chromosomal disorder caused by the presence of an extra copy of chromosome number $21$.
This condition is known as trisomy of chromosome $21$,where the individual has $47$ chromosomes instead of the normal $46$.
Therefore,the correct answer is trisomy.

(C) Klinefelter's syndrome is a genetic disorder caused by the presence of an extra $X$ chromosome in males.
This condition results from the nondisjunction of sex chromosomes during meiosis.
Therefore,the sex chromosome complement in individuals with Klinefelter's syndrome is typically $XXY$.

(B) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This condition was first described by Langdon Down in $1866$.
The affected individual is short-statured with a small round head,furrowed tongue,and partially open mouth.
Physical,psychomotor,and mental development is retarded.

(D) The monosomic condition in human beings represented as $XO$ is known as Turner's syndrome.
Individuals with Turner's syndrome have a $45$ chromosome count,specifically $44 + XO$ genotype.
This condition arises due to the absence of one $X$ chromosome,which is caused by the non-disjunction of sex chromosomes during gametogenesis in the parents.

(A) Turner's syndrome is a chromosomal disorder caused by the absence of one of the $X$ chromosomes in females.
In this condition,the individual has a $45, XO$ genotype.
Therefore,the total number of chromosomes is $44 + 1 = 45$.

(A) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This condition results in a total of $47$ chromosomes instead of the normal $46$.
For a female with Down's syndrome,the genotype is $45 + XX$,and for a male,it is $45 + XY$.
Among the given options,$45 + XX$ represents the genotype of a female with Down's syndrome.

(A) Turner's syndrome is a genetic disorder caused by the absence of one of the $X$ chromosomes in females.
This condition results in a $45$ chromosome count,represented as $45, XO$.
Individuals with this syndrome are phenotypically female but have underdeveloped reproductive organs 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 $21$ (trisomy $21$).
This results in a total chromosome count of $46 + 1 = 47$ chromosomes.
Therefore,the correct option is $B$.

(B) Klinefelter's syndrome is a chromosomal disorder caused by the presence of an additional copy of the $X$ chromosome,resulting in a karyotype of $47, XXY$.
Phenotypically,these individuals are males.
However,they exhibit some female secondary sexual characteristics,such as breast development (gynecomastia).
They are usually sterile and have underdeveloped testes.

(C) Edward's syndrome (trisomy $18$),Patau's syndrome (trisomy $13$),and Down's syndrome (trisomy $21$) are all examples of autosomal aneuploidy.
These conditions occur due to the presence of an extra chromosome in the autosomal pairs,which is a numerical change in the autosomes rather than the sex chromosomes.

(B) $Syndrome$ is defined as a set of medical signs and symptoms that are correlated with each other and often associated with a particular disease or disorder.
In the context of genetics,chromosomal abnormalities (such as $Down$ syndrome,$Turner$ syndrome,or $Klinefelter$ syndrome) result in a specific collection of phenotypic symptoms caused by the presence of an abnormal number of chromosomes or structural changes in chromosomes.
Therefore,the term used to describe this collection of symptoms is a $Syndrome$.

(D) Klinefelter's syndrome is a chromosomal disorder characterized by an extra $X$ chromosome,resulting in a genotype of $47, XXY$.
In a normal human,the diploid number of chromosomes is $46$ ($44 + XX$ or $44 + XY$).
$A$ normal sperm provides $23$ chromosomes ($22 + X$ or $22 + Y$).
To result in a $47, XXY$ zygote,the ovum must have contributed $24$ chromosomes ($22 + XX$ due to non-disjunction of $X$ chromosomes during meiosis).
Therefore,the ovum contained $24$ chromosomes.

(A) Down's 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.
Since the individual is a male child,the sex chromosomes remain normal,which is $XY$.
Therefore,the karyotype of a male with Down's syndrome is $47, XY, +21$.

(C) $Down's$ syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ ($trisomy$ $21$).
This condition occurs due to the failure of homologous chromosomes to separate during meiosis,a process known as non-disjunction.
As a result,one gamete receives an extra chromosome,and upon fertilization,the zygote develops the $trisomic$ condition.

(D) The condition involving an extra $X$ chromosome in a human male,resulting in a genotype of $47, XXY$,is known as Klinefelter syndrome.
This chromosomal disorder occurs due to the presence of an additional copy of the $X$ chromosome in males.
Individuals with this syndrome exhibit overall masculine development,but they also show feminine characteristics such as the development of breast tissue (gynecomastia).
Such individuals are sterile.

(A) The phenomenon where the number of chromosomes increases or decreases is known as aneuploidy.
This occurs primarily due to the failure of segregation of chromatids during cell division,a process called non-disjunction.
While mutation is a broad term that can include chromosomal changes,non-disjunction is the specific mechanism responsible for the gain or loss of chromosomes (aneuploidy).
Therefore,non-disjunction is the most direct cause of changes in chromosome number.

(B) The correct answer is $B$.
Individuals with Klinefelter's syndrome possess $47$ chromosomes due to the presence of an additional $X$ chromosome,resulting in an $XXY$ genotype.
This condition occurs in males.
It is generally observed that as the number of $X$ chromosomes increases,the severity of mental defects and physical abnormalities also tends to increase.

(B) Edward syndrome is a chromosomal disorder caused by trisomy of chromosome $18$.
This means there are three copies of the $18^{th}$ chromosome instead of the usual pair.
The karyotype is represented as $47, XX, +18$ or $47, XY, +18$,which is denoted as $2n + 1 = 47$.

(D) Turner's syndrome is a genetic disorder caused by the absence of one $X$ chromosome in human females. The genotype of an individual with Turner's syndrome is $45, XO$ (or $44 + XO$). This condition is classified as a sex-chromosomal abnormality because it involves the loss of an entire sex chromosome,leading to a total of $45$ chromosomes instead of the normal $46$.

(D) Down's syndrome is a chromosomal disorder caused by the presence of an extra copy of chromosome number $21$ (trisomy of $21$).
In a normal human cell,there are $46$ chromosomes ($23$ pairs).
Due to the presence of an additional chromosome $21$,the total number of chromosomes in an individual with Down's syndrome becomes $46 + 1 = 47$.
Therefore,the correct option is $47$.

(D) Gynaecomastia refers to the development of breast tissue in males. This condition is a characteristic clinical feature of $Klinefelter's$ syndrome. In this genetic disorder,the individual has an extra $X$ chromosome,resulting in a $47, XXY$ karyotype. The presence of the additional $X$ chromosome leads to feminization,including the development of breast tissue,sparse body hair,and reduced muscle mass.

(D) The correct answer is $D$.
Multivalents are associations of more than two homologous chromosomes during meiosis.
They typically occur in organisms that are heterozygous for reciprocal translocations.
In these individuals,segments of non-homologous chromosomes are exchanged,leading to a cross-shaped configuration (quadrivalent) during the pachytene stage of prophase-$I$ of meiosis,as the homologous regions attempt to pair.

(B) Mental retardation associated with sex chromosomal abnormalities is most commonly observed in conditions like Klinefelter syndrome $(47, XXY)$ or Triple $X$ syndrome $(47, XXX)$.
In these conditions,there is an increase in the number of $X$ chromosomes (aneuploidy).
Specifically,the presence of extra $X$ chromosomes interferes with normal development,often leading to cognitive impairment or mental retardation.
Therefore,an increase in the $X$ complement is the primary cause associated with these chromosomal disorders.

(D) The haploid chromosome number $(n)$ is given as $12$.
Therefore,the diploid chromosome number $(2n)$ is $2 \times 12 = 24$.
Monosomy is a type of aneuploidy where one chromosome is missing from the diploid set,represented as $(2n - 1)$.
Substituting the value of $2n$,we get $24 - 1 = 23$.
Thus,the monosomic number is $23$.

(A) Down's syndrome was first described by Langdon Down in $1866$.
Later,in $1959$,Lejeune and his colleagues discovered that this condition is caused by the presence of an additional copy of chromosome number $21$,a condition known as trisomy of chromosome $21$.

(D) . Epicanthus is a characteristic symptom of Down's syndrome. It refers to the presence of a vertical fold of skin on either side of the nose that covers the inner corner of the eye.

(A) Klinefelter's syndrome is a genetic disorder caused by the presence of an extra $X$ chromosome in males.
This condition results in a total of $47$ chromosomes, with the sex chromosome complement being $XXY$.
Therefore, the chromosomal constitution is $44 + XXY$.

(B) The correct answer is $B$. The Philadelphia chromosome is an abnormal chromosome found in the cells of patients with chronic myeloid leukemia $(CML)$.
It was discovered in $1960$ by researchers in Philadelphia,who observed that it resulted from a reciprocal translocation between chromosome $9$ and chromosome $22$,specifically denoted as $t(9;22)(q34;q11)$.
This translocation creates a fusion gene,$BCR-ABL1$,which leads to the production of a constitutively active tyrosine kinase,driving the uncontrolled proliferation of white blood cells.

(C) 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 the $21^{st}$ chromosome.
Therefore,the correct pair of chromosomes associated with Down's syndrome is the $21^{st}$ pair.

(C) $Polyploidy$ is the phenomenon of having more than two genomes or sets of chromosomes,for example,$3n, 4n, 5n$,etc. This condition often arises due to the failure of chromosomes to separate during cell division.

(D) Down's syndrome is a chromosomal disorder caused by trisomy of chromosome $21$.
An affected mother has $47$ chromosomes (including an extra $21$st chromosome),while a normal father has $46$ chromosomes.
During meiosis in the mother,the gametes produced will have either $23$ or $24$ chromosomes. Specifically,$50\%$ of the eggs will carry the extra $21$st chromosome.
When these eggs are fertilized by normal sperm ($23$ chromosomes) from the father,$50\%$ of the resulting zygotes will have $47$ chromosomes (trisomy $21$),leading to Down's syndrome.
Therefore,$50\%$ of the offspring are expected to be affected.

(D) Monosomy is a condition where there is a loss of one chromosome from a diploid set,represented as $(2n - 1)$.
Trisomy is a condition where there is an addition of one extra chromosome to a diploid set,represented as $(2n + 1)$.

(A) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
The frequency of this syndrome is known to be correlated with maternal age.
For women aged $25$ years or under,the incidence is approximately $1/700$ births.
As maternal age increases,the risk of non-disjunction during oogenesis increases,leading to a higher probability of producing aneuploid gametes.
The frequency increases significantly to about $1/100$ for women of age $40$ and up to $1/10$ for women of age $45$. Therefore,the risk is notably higher when the maternal age is above $35$ years.

(A) Turner's syndrome is a chromosomal disorder caused by the absence of one of the $X$ chromosomes in females.
This condition results in a $45, XO$ genotype,meaning the individual has only $45$ chromosomes instead of the normal $46$.
This loss of a single chromosome from a pair is known as monosomy $(2n-1)$.
Therefore,Turner's syndrome is a classic example of monosomy.

(A) Aneuploidy is a condition characterized by the gain or loss of one or more chromosomes due to failure of segregation of chromatids during cell division.
In humans,Klinefelter's syndrome is a classic example of aneuploidy involving sex chromosomes,where the genotype is $44 + XXY$ ($22$ pairs of autosomes + $XXY$ sex chromosomes). This results in a male phenotype with some feminine characteristics.
Option $A$ correctly represents the genotype and phenotype associated with Klinefelter's syndrome.

(A) The "cri-du-chat" syndrome is a genetic disorder caused by a partial deletion of the short arm ($p$ arm) of chromosome $5$.
This structural chromosomal abnormality leads to characteristic symptoms, including a high-pitched cry in infants that sounds like a cat, which is why it is named "cri-du-chat" (French for "cry of the cat").
Therefore, the correct mechanism is deletion.

(B) Turner's syndrome is a chromosomal disorder caused by the absence of one of the $X$ chromosomes,resulting in a $45, XO$ karyotype.
This condition is a type of sex chromosome aneuploidy,specifically monosomy,where the individual has only $45$ chromosomes instead of the normal $46$.
Therefore,the correct option is $B$.

(C) The number of Barr bodies in a cell is calculated using the formula: $\text{Number of Barr bodies} = n - 1$, where $n$ is the total number of $X$ chromosomes present in the cell.
In this case, the female has $XXXX$ chromosomes, so $n = 4$.
Therefore, the number of Barr bodies = $4 - 1 = 3$.

(B) The Barr body is a condensed,inactive $X$ chromosome found in the nuclei of somatic cells of female mammals. In human blood smears,it appears as a small,drumstick-shaped appendage attached to the nucleus of neutrophils in females. It is observed in approximately $3$ to $5\%$ of neutrophils in females,but it is absent in males.

(B) The genotype $XXY$ is associated with Klinefelter syndrome.
Individuals with this syndrome are phenotypically male but may exhibit some female secondary sexual characteristics,such as breast development (gynecomastia).
These individuals are typically sterile and have an additional $X$ chromosome.

(B) Barr bodies (found in somatic cells of females) and drumsticks (found in polymorphonuclear leukocytes) represent the inactivated $X$ chromosome,known as facultative heterochromatin. In individuals with more than one $X$ chromosome,the number of Barr bodies is equal to $n-1$,where $n$ is the total number of $X$ chromosomes. Therefore,their presence indicates the existence of more than one $X$ chromosome in the cell.

(C) $Gynandromorph$ is an organism that contains both male and female tissues. In $Drosophila$,sex is determined by the ratio of $X$ chromosomes to autosomes. If a $XX$ (female) zygote loses one $X$ chromosome in one of its cells during early embryonic development,that cell becomes $XO$ (male). As the organism develops,the descendants of the $XO$ cell will exhibit male characteristics,while the descendants of the original $XX$ cells will exhibit female characteristics. This mosaic of male and female tissues in a single individual is known as a $Gynandromorph$.

(B) The number of Barr bodies in a cell is calculated by the formula $n - 1$,where $n$ is the number of $X$ chromosomes.
In a normal human male $(46, XY)$,there is only one $X$ chromosome,so the number of Barr bodies is $1 - 1 = 0$.
If a male has one Barr body,the number of $X$ chromosomes must be $n = 1 + 1 = 2$.
Therefore,the individual has two $X$ chromosomes and one $Y$ chromosome,resulting in the genotype $XXY$ (Klinefelter syndrome).

(B) Lyon's hypothesis states that the number of Barr bodies in a cell is equal to $(n - 1)$,where $n$ is the number of $X$ chromosomes.
Down's syndrome is a chromosomal disorder caused by trisomy of chromosome $21$.
The total number of chromosomes in a female with Down's syndrome is $47$,with the sex chromosome complement being $XX$.
Using the formula: $\text{Number of Barr bodies} = (\text{Number of } X \text{ chromosomes} - 1) = (2 - 1) = 1$.
Therefore,a human female suffering from Down's syndrome has $1$ Barr body.