Mix Examples- Principles of Inheritance and Variation Questions in English

(D) Mendel's laws of inheritance,specifically the Law of Dominance and the Law of Independent Assortment,are not universally applicable.
$1$. Linkage is an exception to the Law of Independent Assortment,as genes located on the same chromosome tend to be inherited together.
$2$. Incomplete dominance is an exception to the Law of Dominance,where the $F_1$ hybrid shows an intermediate phenotype rather than the dominant trait.
$3$. Co-dominance is also an exception to the Law of Dominance,where both alleles are expressed equally in the heterozygote.
Therefore,all the given options are exceptions to Mendelism.

(C) $Drosophila$ $melanogaster$ (fruit fly) is considered an ideal organism for genetic studies due to several reasons:
$1$. They can be grown on simple synthetic medium in the laboratory.
$2$. They complete their life cycle in about two weeks,not seven weeks.
$3$. $A$ single mating produces a large number of progeny.
$4$. They show clear sexual dimorphism (male and female are morphologically distinct).
Therefore,the statement that they complete their life cycle in seven weeks is incorrect.

(D) $1$. In $\text{Dominance}$, the $F_1$ generation phenotype resembles one of the two parents, as seen in Mendel's monohybrid crosses.
$2$. In $\text{Incomplete dominance}$, the $F_1$ generation shows a phenotype that is intermediate between the two parents, such as in $Mirabilis \text{ } jalapa$ (flower color).
$3$. In $\text{Co-dominance}$, the $F_1$ generation expresses both parental traits simultaneously, such as in $ABO$ blood grouping in humans.
$4$. Since all the given statements correctly describe the respective genetic phenomena, the correct option is $D$.

(D) The $Rh$ factor (Rhesus factor) is an inherited protein found on the surface of red blood cells. It is inherited as an autosomal dominant trait.
Color blindness,albinism,and hemophilia are all examples of recessive genetic disorders or traits.

(B) To determine the parents of a child with $O^+$ blood group,we consider the inheritance of $ABO$ blood types and the $Rh$ factor.
$1$. For the $ABO$ blood group: $A$ child with $O$ blood group must have the genotype $ii$. This means both parents must contribute an $i$ allele. Parents with $A$ $(I^A i)$,$B$ $(I^B i)$,or $O$ $(ii)$ blood groups can produce an $O$ child. Parents with $AB$ $(I^A I^B)$ blood group cannot produce an $O$ child.
$2$. For the $Rh$ factor: $A$ child with $Rh^+$ $(+)$ blood group must have at least one dominant $Rh^+$ allele. If the child is $Rh^+$,at least one parent must be $Rh^+$.
$3$. Evaluating the options:
- Option $A$: $AB^-$ and $A^+$. $AB$ parent cannot produce an $O$ child.
- Option $B$: $A^+$ and $O^-$. $A$ parent $(I^A i)$ and $O$ parent $(ii)$ can produce an $O$ child $(ii)$. $A^+$ parent ($Rh^+ Rh^-$ or $Rh^+ Rh^+$) and $O^-$ parent $(Rh^- Rh^-)$ can produce an $Rh^+$ child. This is a possible combination.
- Option $C$: $O^+$ and $AB^+$. $AB$ parent cannot produce an $O$ child.
- Option $D$: $B^-$ and $O^-$. Both parents are $Rh^-$,so they cannot produce an $Rh^+$ child.
Therefore,the correct pair is $A^+$ and $O^-$.

(D) $(i)$ $ABO$ blood grouping is controlled by gene $I$. This is correct.
(ii) Gene $I$ has three alleles $(I^A, I^B, i)$,not four. This is incorrect.
(iii) $I^A$ produces $N$-acetylgalactosamine,while $I^B$ produces galactose. They produce different sugars. This is incorrect.
(iv) The allele $i$ does not produce any sugar,whereas $I^A$ and $I^B$ produce specific sugars. This is incorrect.
$(v)$ $I^A$ and $I^B$ are codominant,not incompletely dominant. This is incorrect.
Therefore,statements $(ii), (iii), (iv),$ and $(v)$ are incorrect.

(A) Color blindness is an $X$-linked recessive disorder. Let $X^C$ be the allele for normal vision and $X^c$ be the allele for color blindness.
The genotype of a color-blind husband is $X^cY$ (hemizygous).
The genotype of a carrier wife is $X^CX^c$ (heterozygous).
Crossing $X^cY \times X^CX^c$ results in the following offspring:
$1$. $X^CX^c$ (Carrier daughter - heterozygous)
$2$. $X^cX^c$ (Color-blind daughter - homozygous recessive)
$3$. $X^CY$ (Normal son - hemizygous)
$4$. $X^cY$ (Color-blind son - hemizygous)
Summary of genotypes:
- Heterozygous: $1$ $(X^CX^c)$
- Homozygous: $1$ $(X^cX^c)$
- Hemizygous: $2$ ($X^CY$ and $X^cY$)
The ratio is $1:1:2$.

(B) The phenomenon where the offspring of genetically diverse parents (hybrids) exhibit greater growth,productivity,and fitness than either of the parents is known as $Heterosis$ or $Hybrid$ $Vigor$.
This phenomenon is primarily attributed to $Heterozygosity$,where the masking of deleterious recessive alleles by dominant alleles and the accumulation of favorable dominant genes occur in the hybrid state.

(D) Sex-linked traits are those controlled by genes located on the sex chromosomes ($X$ or $Y$).
$1$. Hemophilia is an $X$-linked recessive disorder.
$2$. Color blindness is an $X$-linked recessive disorder.
$3$. Hypertrichosis (hairy pinna) is a $Y$-linked trait.
$4$. Baldness (pattern baldness) is a sex-influenced trait,not a sex-linked trait. It is controlled by autosomal genes whose expression is influenced by the presence of sex hormones (specifically testosterone).
Therefore,the correct answer is $D$.

(A) $1$. Baldness is a sex-influenced trait,not a sex-limited trait. Sex-influenced traits are expressed in both sexes but with different frequencies or intensities (e.g.,pattern baldness is more common in males). Sex-limited traits are expressed in only one sex.
$2$. Linkage refers to the physical association of genes on the same chromosome,which prevents them from assorting independently,thus acting as an exception to Mendel's Law of Independent Assortment.
$3$. Galactosemia is a genetic disorder caused by a deficiency of an enzyme (galactose$-1-$phosphate uridylyltransferase),making it an inborn error of metabolism.
$4$. Genetic drift is the change in the frequency of an existing gene variant in a population due to random sampling of organisms. It is more pronounced in small populations.

(D) The phenomenon of dominance is significant for several reasons:
$1$. Dominant genes often mask the expression of recessive,potentially harmful mutations,thereby protecting the organism from deleterious effects.
$2$. Dominant alleles contribute to the overall fitness and vigor of an organism,often leading to phenomena like heterosis (hybrid vigor) in cross-breeding.
$3$. Because dominant alleles are expressed in both homozygous and heterozygous conditions,they play a crucial role in the survival and adaptation of species.
Therefore,all the given statements correctly describe the significance of dominance.

(D) back-cross is defined as a cross between an $F_1$ hybrid and one of its parents.
Since the $F_1$ generation can be crossed with either the dominant parent or the recessive parent,the definition encompasses both possibilities.
Therefore,a back-cross is a cross between an $F_1$ individual and any of its parents.

(D) Male pattern baldness is a classic example of a sex-influenced trait.
It is controlled by an autosomal gene,meaning it is located on an autosome,not a sex chromosome.
However,the expression of this gene is influenced by the presence of sex hormones,specifically androgens.
In males,the trait acts as dominant,while in females,it acts as recessive,which is why it is classified as a sex-influenced trait.
Therefore,it is both an autosomal trait and a sex-influenced trait.

(D) In maize $(Zea \, mays)$, albinism is a classic example of a lethal gene effect.
When two heterozygous plants $(Aa)$ are crossed, the offspring follow a $1:2:1$ genotypic ratio.
The homozygous recessive genotype $(aa)$ results in the complete absence of chlorophyll, leading to the albino phenotype.
Since these plants cannot perform photosynthesis, they die at the seedling stage, making the gene lethal.

(C) In a diploid organism,if an individual possesses only one allele for a specific gene instead of the usual pair,the condition is known as $Hemizygous$. This typically occurs in males for genes located on the $X$ chromosome,as they have only one $X$ chromosome $(XY)$. In contrast,$Homozygous$ refers to having two identical alleles,and $Heterozygous$ refers to having two different alleles for a gene.

(B) Sex-limited traits are autosomal traits that are expressed in only one sex due to anatomical or physiological differences.
In humans,the development of a beard and mustache is a secondary sexual characteristic in males,controlled by autosomal genes that are expressed only in the presence of male sex hormones (androgens).
Therefore,these are classified as sex-limited traits.

(A) Variation refers to the differences in characteristics between individuals of the same species. In a population of starfish,while the standard number of arms is five,the occurrence of an individual with six arms due to genetic or developmental differences is a classic example of phenotypic variation.

(D) Hexaploid wheat $(Triticum \text{ } aestivum)$ has a total of $42$ chromosomes $(2n = 42)$.
Since $2n = 42$, the haploid number $(n)$ is $42 / 2 = 21$.
The basic chromosome number $(x)$ for wheat is $7$, as it is a polyploid derived from a genome with $7$ chromosomes.
Therefore, $n = 21$ and $x = 7$.

(A) Bread wheat ($Triticum$ $aestivum$) is a hexaploid organism.
It has a basic chromosome number of $x = 7$.
Since it is hexaploid $(6x)$,the total number of chromosomes in its somatic cells is $6 \times 7 = 42$.
Therefore,$Triticum$ $aestivum$ is classified as a hexaploid species.

(B) Variations can be classified as continuous or discontinuous.
Continuous variations are small,quantitative differences that show a range of phenotypes (e.g.,height in humans).
Discontinuous variations are distinct,qualitative differences that do not show a range of phenotypes,often controlled by a single gene or a small set of genes.
The presence of four horns instead of two is a clear,distinct,and qualitative trait that does not show intermediate forms,thus it is an example of discontinuous variation.

(C) Sickle-cell anemia is caused by a mutation in the $Hb^S$ gene. Individuals who are heterozygous $(Hb^A Hb^S)$ for the sickle-cell trait possess a survival advantage in regions where malaria is endemic. The presence of some sickled red blood cells inhibits the growth and reproduction of the malaria parasite $(Plasmodium)$. This phenomenon is known as balanced polymorphism or heterozygote advantage,which maintains the $Hb^S$ allele in the population despite the disease being lethal in the homozygous recessive $(Hb^S Hb^S)$ condition.

(B) Sickle-cell anemia is caused by a point mutation in the $\beta$-globin chain of hemoglobin,where glutamic acid is replaced by valine at the $6^{th}$ position. This substitution changes the electrical charge of the hemoglobin molecule. Because electrophoresis separates molecules based on their charge-to-mass ratio,normal hemoglobin $(HbA)$ and sickle-cell hemoglobin $(HbS)$ exhibit different electrophoretic mobilities. Therefore,they will show different mobility in an electrophoretic field.

(A) The correct matches are as follows:
$(i)$ William Bateson coined the term 'Genetics' $(i-c)$.
$(ii)$ $T$.$H$. Morgan prepared the first genetic map using Drosophila $(ii-d)$.
$(iii)$ $O$.$T$. Avery (along with MacLeod and McCarty) proved that the transforming principle is $DNA$ $(iii-b)$.
$(iv)$ Hugo de Vries was one of the three scientists who rediscovered Mendel's laws of inheritance $(iv-a)$.
Therefore, the correct sequence is $i-c, ii-d, iii-b, iv-a$.

(B) The genetic makeup of an animal species varies due to the processes of breeding and the presence of heterozygosity.
Breeding (specifically sexual reproduction) introduces genetic recombination,which shuffles alleles.
Heterozygosity refers to the presence of different alleles at a specific locus,which increases genetic variation within a population.
Therefore,the combination of breeding and heterozygosity is responsible for the diversity in genetic makeup among individuals of a species.

(B) Statement $A$ is true because every species possesses a distinct set of genes (genome) that defines its characteristics.
Statement $R$ is also true because sexual reproduction introduces genetic variation through processes like recombination,independent assortment,and mutation,which is a fundamental aspect of evolution and adaptation in animals.
However,Statement $R$ is not the direct explanation for why a species has a unique genetic makeup; rather,the unique genetic makeup is a result of evolutionary history and species-specific $DNA$ sequences. Therefore,$R$ is a true statement but not the correct explanation for $A$.

(A) Organisms reproduce to create new individuals that are similar to their parents but not identical.
This phenomenon is known as variation.
Variations arise due to changes in genetic material during reproduction (such as $DNA$ replication errors,recombination during meiosis,or mutations).
When these variations accumulate over generations,they lead to differences in the characteristics of the offspring compared to their parents.
Therefore,the degree of variation determines how distinct an organism is from its parents.

(B) Assertion $A$ is true because genes are the functional units of heredity that carry genetic information from parents to offspring.
Reason $R$ is also true because the properties of tissues arise as a result of interactions among the constituent cells,which is a fundamental principle of biological organization.
However,Reason $R$ does not explain why a gene is the unit of inheritance. Therefore,both statements are true,but $R$ is not the correct explanation of $A$.

(A) Speciation is the evolutionary process by which populations evolve to become distinct species. Among the given options,$Hybridization$ is the only process that can lead to the formation of a new species,particularly in plants,through the process of polyploidy or recombination of genetic material from two different species. $Fernery$ is a place for growing ferns,$Floriculture$ is the cultivation of flowers,and $Fumigation$ is a pest control method. Therefore,$Hybridization$ is the correct answer.

(D) The correct matches are as follows:
$(A)$ Dominance: In a heterozygous organism,only one allele expresses itself,masking the effect of the other allele. Thus,$(A)-(ii)$.
$(B)$ Codominance: In a heterozygous organism,both alleles express themselves fully,such as in $AB$ blood group inheritance. Thus,$(B)-(iii)$.
$(C)$ Pleiotropy: This occurs when a single gene influences multiple phenotypic characters. Thus,$(C)-(iv)$.
$(D)$ Polygenic inheritance: This occurs when many genes govern a single character,such as human skin color or height. Thus,$(D)-(i)$.
Therefore,the correct sequence is $A-(ii), B-(iii), C-(iv), D-(i)$,which corresponds to option $(D)$.

(C) The diagram shows a criss-cross inheritance pattern,where a trait is passed from the mother to her son and from the father to his daughter. This is characteristic of $X$-linked recessive inheritance.
$1$. Phenylketonuria,Sickle cell anaemia,and Thalassemia are autosomal recessive disorders.
$2$. Haemophilia is an $X$-linked recessive disorder that follows a criss-cross pattern of inheritance.
Therefore,the correct option is $C$.

(A) The correct answer is $(A)$. In human pedigree analysis,standard symbols are used to represent family relationships and genetic traits:
$1$. $A$ square represents a male,and a circle represents a female.
$2$. $A$ horizontal line connecting a square and a circle represents mating.
$3$. $A$ double horizontal line between a square and a circle represents mating between relatives (consanguineous mating).
$4$. An open (unshaded) symbol represents an unaffected individual.
$5$. $A$ solid (shaded) symbol represents an affected individual.
Based on the provided options:
- Option $(A)$ correctly shows a double horizontal line,which represents mating between relatives.
- Option $(B)$ shows a circle,which represents a female,not a male.
- Option $(C)$ shows a square,which represents a male,not a female.
- Option $(D)$ shows a diamond,which represents an individual of unspecified sex,not an affected male.

(C) is the incorrect statement. Baldness is a sex-influenced trait,not a sex-limited trait.
Sex-influenced traits are autosomal traits that are expressed differently in males and females due to the influence of sex hormones.
Sex-limited traits are expressed in only one sex (e.g.,milk production in females).
Galactosemia is indeed an inborn error of metabolism.
Genetic drift is more pronounced in small populations.
Linkage violates the law of independent assortment because linked genes do not assort independently during gamete formation.

(D) : Sickle cell anaemia is an autosomal hereditary disorder in which the erythrocytes become sickle-shaped. The disorder is caused by the formation of an abnormal haemoglobin called haemoglobin-$S$. As found out by Ingram $(1958)$,haemoglobin-$S$ differs from normal haemoglobin-$A$ in only one amino acid: the $6^{th}$ amino acid of the $\beta$-chain,glutamic acid,is replaced by valine.
Carriers of the sickle cell anaemia gene are protected against malaria because of this particular haemoglobin mutation. This explains why sickle cell anaemia is particularly common among people of African origin. The malarial parasite has a complex life cycle and spends part of it in red blood cells,feeding on haemoglobin. Both sickle-cell anaemia and thalassemia are more common in malaria-prone areas because these mutations convey some protection against the parasite. In a carrier,the presence of the malaria parasite causes the red blood cell to rupture,making the $Plasmodium$ unable to reproduce. Further,the polymerisation of $Hb$ affects the ability of the parasite to digest $Hb$. Therefore,in areas where malaria is a problem,people's chances of survival actually increase if they carry the sickle cell anaemia trait. Thus,sickle-cell anaemia acts as a potential saviour from malaria.

(A) The phenotype of an organism refers to its observable physical,morphological,or physiological characteristics.
It is determined by the interaction between the organism's genetic makeup $(genotype)$ and the environmental conditions in which it develops.
While the $genotype$ provides the potential for specific traits,the environment can influence how these traits are expressed.
Therefore,the phenotype is the result of the interaction between the $genotype$ and the environment.

(C) : Parasitism is a relationship between two living organisms of different species in which one organism,called the parasite,obtains its food directly from another living organism,called the host. The parasite spends a part or whole of its life either on or inside the body of the host.
The general parasitic adaptations are $(i)$ anaerobic respiration in internal parasites,$(ii)$ loss of certain unnecessary organs,$(iii)$ presence of adhesive organs,$(iv)$ excessive multiplication,$(v)$ resistant cysts and eggs for safe transfer of their progeny to new hosts,and $(vi)$ well-developed and complicated reproductive organs.
Therefore,the loss of reproductive capacity is not a parasitic adaptation; in fact,parasites typically exhibit a high reproductive capacity to ensure the survival of their species.

(D) The diploid chromosome number $(2n)$ of maize $(Zea \ mays)$ is $20$.
Since the question asks for the number of pairs of chromosomes,we divide the diploid number by $2$.
Number of pairs = $2n / 2 = 20 / 2 = 10$.
Therefore,maize has $10$ pairs of chromosomes.

(D) The incorrect pair is $Alfred \text{ } Sturtevant - \text{ } Law \text{ } of \text{ } segregation$.
$Alfred \text{ } Sturtevant$ is known for mapping the position of genes on chromosomes based on recombination frequency, not for the $Law \text{ } of \text{ } segregation$.
The $Law \text{ } of \text{ } segregation$ was proposed by $Gregor \text{ } Mendel$.
$Henking$ discovered the $X$ body in $1891$.
$Morgan$ is known for his work on $Linkage \text{ } and \text{ } recombination$ in $Drosophila$.
$Sutton$ and $Boveri$ proposed the $Chromosomal \text{ } theory \text{ } of \text{ } Inheritance$.

(A) $(1)$ Inheritance of two genes follows the dihybrid ratio of $9:3:3:1$ $(1-c)$.
$(2)$ Incomplete dominance (e.g.,in Snapdragon) results in a phenotypic and genotypic ratio of $1:2:1$ $(2-b)$.
$(3)$ Codominance is observed in $ABO$ blood grouping in humans where both alleles $I^A$ and $I^B$ express themselves simultaneously $(3-a)$.
$(4)$ Inheritance of one gene (monohybrid cross) follows the phenotypic ratio of $3:1$ $(4-d)$.
Therefore,the correct match is $(1-c), (2-b), (3-a), (4-d)$.

(C) $(1)$ Failure of cytokinesis leads to an increase in the whole set of chromosomes in an organism,a condition known as Polyploidy $(q)$.
$(2)$ Trisomy of chromosome $21$ is the genetic cause of Down's syndrome $(r)$.
$(3)$ Phenylketonuria is an inborn error of metabolism caused by the deficiency of the enzyme phenylalanine hydroxylase $(s)$.
$(4)$ Pedigree analysis is a tool used to study the inheritance of particular traits in families over several generations $(p)$.
Therefore,the correct matching is $(1-q), (2-r), (3-s), (4-p)$.

(A) The correct matching is as follows:
$1$. $(w)$ Blood group (specifically $ABO$ blood grouping) exhibits Co-dominance $(e)$.
$2$. $(x)$ Skin colour in humans is an example of Polygenic inheritance $(a)$.
$3$. $(y)$ Sickle-cell anaemia is a classic example of a Mendelian disorder $(d)$.
$4$. $(z)$ Chromosomal disorders,such as Down's syndrome,are caused by Aneuploidy $(b)$.
Therefore,the correct sequence is $w-e, x-a, y-d, z-b$.

(B) $(i)$ This statement is correct. $A$ gene is a segment of $DNA$ that codes for a specific protein or trait.
$(ii)$ This statement is incorrect. The gene responsible for $\beta$-thalassemia is located on the $11^{th}$ chromosome,not the $21^{st}$ chromosome.
$(iii)$ This statement is correct. Mendel's use of statistical analysis and mathematical logic to explain inheritance was ahead of its time and was not readily accepted by his contemporaries.
Therefore,there are $2$ correct sentences.

(B) The table represents different mechanisms of sex determination:
$1$. The $x$ row shows the $XY-XX$ type of sex determination,where males are heterogametic $(XY)$ and females are homogametic $(XX)$. This is observed in humans and Drosophila.
$2$. The $y$ row shows the $ZZ-ZW$ type of sex determination,where males are homogametic $(ZZ)$ and females are heterogametic $(ZW)$. This is observed in birds (e.g.,hen).
Therefore,$x$ corresponds to humans or Drosophila,and $y$ corresponds to birds like hens. Comparing this with the options,option $B$ is the most appropriate match.

(D) $(i)$ Queen Elizabeth was a carrier of haemophilia,and many of her descendants were haemophilic. This statement is True $(T)$.
$(ii)$ Variation in blood group prevalences is due to multiple allelism and co-dominance,not incomplete dominance. This statement is False $(F)$.
$(iii)$ Drosophila has $4$ pairs of chromosomes,of which $3$ pairs are autosomes and $1$ pair is sex chromosomes. This statement is True $(T)$.
$(iv)$ In birds,the sex determination system is $ZW-ZZ$ type,where the female is heterogametic $(ZW)$ and the male is homogametic $(ZZ)$. This statement is True $(T)$.
Therefore,the sequence is $T, F, T, T$.

(D) $(i)$ Haemophilia is an $X$-linked recessive disorder where heterozygous females (carriers) transmit the disease to their sons.
$(ii)$ Sickle cell anaemia is caused by a point mutation in the $\beta$-globin chain of haemoglobin,leading to the change in $RBC$ shape from biconcave to sickle-like.
$(iii)$ Phenylketonuria $(PKU)$ is an inborn error of metabolism that results in mental retardation due to the accumulation of phenylalanine.
$(iv)$ Thalassaemia is a quantitative disorder where there is a reduced synthesis of one of the globin chains of haemoglobin,leading to anaemia.

(B) The application of the principles of genetics to improve the hereditary qualities of the human race is known as $Eugenics$.
$Euthenics$ refers to the improvement of human well-being by improving environmental conditions.
$Euphenics$ refers to the improvement of the human condition by modifying the expression of genes (phenotype) through medical or environmental interventions.
$Ethnology$ is the branch of anthropology that compares and analyzes the characteristics of different peoples and the relationships between them.
Therefore,the correct option is $B$.

(C) Mr. Kapoor is a male,so his genotype for the sex chromosomes is $XY$. The sex-linked factor $d$ is located on the $X$ chromosome. Thus,his genotype is $BbXdY$.
During meiosis,the autosomal genes $B$ and $b$ segregate independently.
The possible combinations of autosomal genes and sex chromosomes in the sperm are:
$1$. $B$ with $X^d$
$2$. $B$ with $Y$
$3$. $b$ with $X^d$
$4$. $b$ with $Y$
Each of these four combinations has an equal probability of $1/4$.
Therefore,the proportion of sperm carrying $Bd$ (where $d$ is the sex-linked factor) is $1/4$.

(A) The $cis-trans$ test (or complementation test) is used to determine if two mutations are in the same gene or different genes.
If two mutations are in the same gene,they will show a mutant phenotype in the $trans$ configuration (complementation does not occur).
If two mutations are in different genes,they will show a wild-type phenotype in the $trans$ configuration (complementation occurs).
However,if two genetic loci produce identical phenotypes in both $cis$ and $trans$ configurations,it indicates that the mutations are not affecting the same functional unit in a way that prevents complementation,or they are effectively acting as independent units. In the context of classical genetics,when mutations at two closely linked loci show identical phenotypes and do not exhibit the expected $cis-trans$ effect,they are often referred to as Pseudoalleles.

(C) Genetic disorders are broadly grouped into two categories:
$1$. Mendelian disorders: These are primarily determined by alteration or mutation in the single gene. Examples include Hemophilia,Cystic fibrosis,Sickle-cell anemia,etc.
$2$. Chromosomal disorders: These are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Examples include Down's syndrome,Klinefelter's syndrome,and Turner's syndrome.
Therefore,both $A$ and $B$ are correct categories.

(D) Homeotic genes are a group of genes that control the pattern of body formation during early embryonic development of organisms.
These genes encode transcription factors that determine the identity of body segments along the anterior-posterior axis.
In $Drosophila$,mutations in homeotic genes can lead to homeotic transformations,where one body part is replaced by another,such as the development of legs in place of wings (e.g.,the $Antennapedia$ mutation).
Therefore,the correct answer is $D$.