//M2//QN1//SUB//DL0

Describe the meaning of inheritance and variation.

//X

Inheritance : The phenomenon of parental traits being passed on to offspring is called inheritance.
The continuous transmission of traits from one generation to the next is carried out by genes.
(2) Variation : Variation means that offspring differ from their parents.
Differences that occur among individuals of the same species are called variation.
Through the ability to reproduce, every organism produces new generations of offspring that resemble their parents.
Inheritance and variation in offspring are the result of sexual reproduction.

//M3//QN2//SUB//DL0

Why is Gregor Mendel known as Father of genetics?

//X

The contribution of Mendel to genetics is called Mendelism.
Gregor Mendel conducted hybridisation experiments on garden peas during 1856–1863 for several contrasting traits such as seed colour, pod colour, flower portion, and other characteristics, and collected systematic data.
He postulated the laws of inheritance, which are also called Mendel’s laws.
These laws laid the foundation of heredity.
Hence, G. J. Mendel is called the father of genetics.

//M0//QN3//SUB//DL0//EQ

Mention the advantages of selecting pea plant for experiment by Mendel.

//X

The advantages of selecting pea plants for experiments are as follows:
The cultivation of pea plants is easy.
The plant shows clear contrasting traits.
The life span is short, about 2 to 4 months.
The bisexual flowers of pea plants normally undergo self-pollination, but cross-fertilisation can also be carried out easily.
Artificial hybridisation is highly successful.
The hybrids of pea plants are fertile.
A large number of offsprings are produced.

//M4//QN4//SUB//DL0//EQ

Explain the Punnett square used to understand a typical monohybrid cross conducted by Mendel between true breeding tall plant and true line dwarf plant.

OR
Explain one gene inheritance by Punnett square.

//X

Mendel selected the pea plant (Pisum sativum), in which the selected contrasting traits were tall and dwarf.
The monohybrid cross between the parental plants (tall and dwarf) resulted in hybrids in the F1 generation.
All the offspring in the F1 generation were tall.
When the F1 generation hybrids were crossed among themselves, the traits segregated in the F2 generation to give three tall and one dwarf plants (phenotypic ratio 3 : 1) and a genotypic ratio of 1 : 2 : 1.
The genotypic and phenotypic ratios of the offspring can be represented using a Punnett square.
Monohybrid Cross
Monohybrid cross

//M0//QN5//SUB//DL0//EQ

Differentiate between: homozygous and heterozygous.

//X

Homozygous

Heterozygous

1.

A zygote having identical genes / factors (alleles) is called homozygous.

A zygote having dissimilar genes / factors (alleles) is called heterozygous.

2.

It may possess either dominant alleles or recessive alleles.

Example: TT or tt — such a zygote is called pure (true).

It possesses one dominant and one recessive allele.

Example: Tt — such a zygote is called a hybrid.

//M0//QN6//SUB//DL0//EQ

Differentiate between: dominant gene and recessive gene.

//X

Dominant gene

Recessive gene

1.

A gene that is expressed in the presence or absence of its alternative gene is called a dominant gene.

A gene that is expressed only in the absence of its alternative gene is called a recessive gene.

2.

It dominates phenotypically and is expressed in the F1 generation.

It remains phenotypically suppressed in the F1 generation.

3.

It is represented by a capital letter.

Example: T for tall.

It is represented by a small letter.

Example: t for dwarf.

//M0//QN7//SUB//DL0//EQ

Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

//X

F1 generation: Punnett Square

Female

Gametes

A

a

a

Aa

aa

a

Aa

aa

Phenotype = 1 : 1 (Heterozygous 50%, Homozygous 50%)

//M0//QN8//SUB//DL0//EQ

A diploid organism is heterozygous for 4 loci; how many types of gametes can be produced?

//X

Here, number of heterozygous pair n = 4
Types of gametes = 2n; where n = 4

= 24

= 16

16 types of gametes will be formed.

//M3//QN9//SUB//DL0//EQ

Explain the Law of Dominance using a monohybrid cross.

OR
Describe the laws concluded from monohybrid cross.

//X

The study of the inheritance of one trait with contrast characteristics is called monohybrid cross.
Law of Dominance (Mendel’s First Law):

(i) Characters are controlled by discrete units called factors.

(ii) Factors occur in pairs.

(iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).

The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2.
Law of Segregation (Mendel’s Second Law):

(i) This law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage.

(ii) The phenotypic ratio is 3 tall and 1 dwarf and genotypic ratio is 1 : 2 : 1.

(iii) Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.

Of course, a homozygous parent produces all gametes that are similar while a heterozygous one produces two kinds of gametes each having one allele with equal proportion.

//M2//QN10//SUB//DL0//EQ

What is test cross? Give Punnett square presentation for the test cross for the colour of flower in the pea plant.

OR
Define and design a test-cross.[NCERT Exercise Q.5]

//X

To determine the genotype whether it is homozygous or heterozygous, test cross is conducted by crossing the unknown (hybrid) genotype and a homozygous recessive.
The genotype of dominant allele can be determined.
Example, Selected trait- flower colour.
Phenotype- violet flower (WW) and white (ww).
The F1 generation is a result of hybridisation, it contains violet flowers which can be homozygous (WW) or heterozygous (Ww).
For the test cross, F1 violet flower is crossed with the white flower which has two possible results:

(i) If the F1 generation is homozygous violet (WW), then all the offsprings will have violet flower.

(ii) If the F1 generation is heterozygous violet (Ww), then 50% flowers are white where as 50% flowers are violet, which means the ratio is 1 : 1.

.

//M3//QN11//SUB//DL0

Explain the concept of dominance with suitable example.

//X

Every gene, contains the information to express a particular trait.
In a diploid organism, there are two copies of each gene, i.e., a pair of alleles.
These two alleles are not always identical, as in a heterozygote.
One of them may be different due to some changes that it has undergone.

Let’s take an example :

A gene that contains the information for producing an enzyme.
Now, there are two copies of this gene, the two allelic forms.
Let us assume (as is more common) that the normal allele produces the normal enzyme that is needed for the transformation of a substrate S.
Theoretically, the modified allele could be responsible for production of –

(i) the normal/less efficient enzyme, or

(ii) a non-functional enzyme, or

(iii) no enzyme at all

In the first case, the modified allele is equivalent to the unmodified allele, i.e., it will produce the same phenotype / trait, i.e., result in the transformation of substrate S.
Such equivalent allele pairs are very common. But, if the allele produces a non-functional enzyme or no enzyme, the phenotype may be affected.
The phenotype / trait will only be dependent on the functioning of the unmodified allele.
The unmodified (functioning) allele, which represents the original phenotype is the dominant allele and the modified allele is generally the recessive allele.
Hence, in the example above the recessive trait is seen due to non-functional enzyme or because no enzyme is produced.

//M3//QN12//SUB//DL0//EQ//PYQ

Explain incomplete dominance in dog flower.[NCERT Exercise Q.13(b)]

OR
Explain with suitable example the monohybrid cross in which the phenotypic and genotypic ratio of F2 generation is equal.
(MARCH 2023)

//X

The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance.
In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F1 (Rr) was Pink.
When the F1 was self-pollinated, the F2
resulted in the following ratio: 1 (RR) Red: 2
(Rr) Pink: 1 (rr) White. Here, the genotype ratios were exactly as we would expect in any Mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio to 1 : 2 : 1.
Conclusion was that R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white).

//M4//QN13//SUB//DL0

Explain the basis of inheritance of the blood group in human population. OR Explain multiple allelism.[NCERT Exercise Q.13(a)](MARCH 2020)

OR
Explain regulation of ABO blood group OR Explain Co-dominance regarding ABO blood group in humans.(JULY 2022)

//X

When three or more then three alleles are responsible for any trait is called multiple allelism.
Co-dominance is a phenomenon in which both the parental trait is expressed in the F1 generation.
A good example is different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene I.
The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.
The gene (I) has three alleles IA, IB and i.
The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar.
Because, humans are diploid organisms, each person possesses any two of the three I gene alleles.
IA and IB are completely dominant over i, in other words when IA and i are present, only IA expresses (because i does not produce any sugar), and when IB and i are present, IB expresses.
But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance.
Hence red blood cells have both A and B types of sugars.
Since, there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes of the human ABO blood types.

Allele from Parent 1

Allele

from Parent 2

Genotype

of

offspring

Blood (group) types of offspring

I A

I A

I A I A

A

I A

I B

I A I B

AB

I A

i

I A i

A

I B

I A

I A I B

AB

I B

I B

I B I B

B

I B

i

I B i

B

i

i

i i

O

Genotype: Given in table
Phenotype: blood group A, B, AB and O
There are more than two, i.e., three alleles, governing the same character. Since, in an individual only two alleles can be present, multiple alleles can be found only when population studies are made.

//M3//QN14//SUB//DL0

A single gene product may produce more than one effect- explain this with starch synthesis in pea seeds. OR Explain pleiotropy in pea seeds.

//X

Starch synthesis in pea seeds is controlled by one gene. It has two alleles (B and b). Starch is synthesised effectively by BB homozygotes and therefore, large starch grains are produced. In contrast, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains.
After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled. Heterozygotes produce round seeds, and so B seems to be the dominant allele.
But, the starch grains produced are of intermediate size in Bb seeds.
So, if starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance.
Therefore, dominance is not an autonomous feature of a gene or the product that it has information for. It depends as much on the gene product and the production of a particular phenotype from this product as it does on the particular phenotype.

//M4//QN15//SUB//DL0//EQ

Explain Mendel’s monohybrid cross in detail.

OR
explain one gene inheritance experiment done by Mendel.

//X

The study of inheritance of only one trait is called monohybrid cross.
Mendel had selected two plants of Pisum sativum.
One is pure line tall and other is pure line dwarf plant and both are taken as parents.
Hybridisation experiment were carried out by Mendel where he crossed tall and dwarf pea plants to study the inheritance of one gene.
He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation.
This generation is also called the first Filial progeny or the F1.
Mendel observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf.
He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.
Mendel then self-pollinated the tall F1 plants and to his surprise found that in the second Filial F2 generation some of the offspring were dwarf ; the character that was not seen in the F1 generation was now expressed.
Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F1 generation while at the F2 stage both the traits were expressed in the proportion 3:1( 75% are tall and 25% dwarf).
The contrasting traits did not show any blending at either F1 or F2 stage.
Based on these observations, Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations.

//M3//QN16//SUB//DL0

A child has blood group O. If the father has blood group A and mother has blood group B, work out the genotypes of the parents and the possible genotypes of their offsprings.[NCERT Exercise Q.12]

//X

Child's blood group O, genotype = ii
Father's blood group A, genotype = IA IA or IA i
Mother's blood group B, genotype = IB IB or IB i
(i) If father's blood group is A with genotype IA IA and mother's blood is B with genotype IB IB then,
IA IA × IB IB

gamete

IA

IA

IB

IA IB

IA IB

IB

IA IB

IA IB
Possible blood group of child is AB.

(ii) If father's blood group is A with genotype IA IA and mother's blood is B with genotype IB i then,

IA IA × IB i

gamete

IA

IA

IB

IA IB

IA IB

i

IA i

IA i

Possible blood group of child is A or AB.

(iii) If father's blood group is A with genotype IA i and mother's blood is B with genotype IB IB then,

IA i × IB IB

gamete

IA

i

IB

IA IB

IB i

IB

IA IB

IB i

Possible blood group of child is B or AB.

(iv) If father's blood group is A with genotype IA i and mother's blood is B with genotype IB i then,

IA i × IB i

gamete

IA

i

IB

IA IB

IB i

i

IA i

ii

Possible blood group of child would be A, B, AB or O.

//M2//QN17//SUB//DL0

Drosophila (fruit fly) is used in experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan. Why?

//X

Following are the reasons behind choosing Drosophila in the study of inheritance by T. H. Morgan:
(1) Drosophila melanogaster which were found very suitable for such studies. They could be grown on simple synthetic medium in the laboratory.
(2) They complete their life cycle in about two weeks.
(3) A single mating could produce a large number of progeny flies.
(4) Also, there was a clear differentiation of the sexes – the male and female flies are easily distinguishable. And it has many types of hereditary variations that can be seen with low power microscopes.

//M2//QN18//SUB//DL0

Mendel published his work on inheritance of characters in 1865 but for several reasons, it remained unrecognised till 1900. Why?

//X

Because of following reasons, Mendel’s work remained unrecognised:
(1) Communication was not easy (as it is now) in those days and his work could not be widely publicised.
(2) His concept of genes (or factors, in Mendel’s words) as stable and discrete units that controlled the expression of traits and, of the pair of alleles which did not ‘blend’ with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature.
(3) Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
(4) Though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of.
The role of nucleus and chromosome was not known in the reproduction.

//M2//QN19//SUB//DL0

compare a chromosome and gene.

//X

Chromosome

Gene

1

Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete.

Segregate at the time of gamete formation and only one of each pair is transmitted to a gamete.

2

Independent pairs segregate independently of each other.

One pair segregates independently from another pair.

//M1//QN20//SUB//DL0//EQ

When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be(a) tall and green.(b) dwarf and green.

//X

Phenotype : Tall and yellow 3
Tall and green 3
Dwarf and yellow 1
Dwarf and green 1
(a) Tall plant and green seed - 3
(b) Dwarf plant and green seed - 1

//M0//QN21//SUB//DL0//EQ

Two heterozygous parents are crossed. If the two loci are linked, what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?

//X

Two or more genes located on the same chromosome shows linkage.
If genes are located on one chromosome and tightly linked, it can be inherited to offspring, these genes are called linked gane.

Only two types of gametes are formed because the genes are linked.

//M2//QN22//SUB//DL0//EQ

Explain dihybrid cross in detail. OR Explain the inheritance of two genes with chart.

OR Explain dihybrid cross with chart for given characteristics.

//X

The gametes RY and ry unite on fertilization to produce the F1 hybrid RrYy.
When Mendel self hybridised the F1 plants he found that 3/4th of F2 plants had yellow seeds and 1/4th had green.
The yellow and green colour segregated in a 3:1 ratio.
Round and wrinkled seed shape also segregated in a 3:1 ratio; just like in a monohybrid cross.
Gametogenesis in F1 generation:

The Punnett square can be effectively used to understand the independent segregation of the two pairs of genes during meiosis and the production of eggs and pollen in the F1 RrYy plant.

Consider the segregation of one pair of genes R and r. Fifty per cent of the gametes have the gene R and the other 50 per cent have r.
Now besides each gamete having either R or r, it should also have the allele Y or y.
The important thing to remember here is that segregation of 50 per cent R and 50 per cent r is independent from the segregation of 50 per cent Y and 50 per cent y.
Therefore, 50 per cent of the r bearing gametes
has Y and the other 50 per cent has y.
Similarly, 50 per cent of the R bearing gametes has Y and the other 50 per cent has y.
Thus, there are four genotypes of gametes (four types of pollen and four types of eggs).
The four types are RY, Ry, rY and ry each with a frequency of 25 per cent or 1/4th of the total gametes produced.

In dihybrid cross

genotype in F2

RRyy - 1 : Rryy - 2 : rryy - 1

RRyy - 2 : Rryy - 4 : rryy - 2

RRyy - 1 : Rryy - 2 : rryy - 1

Genotypic ratio: 1 : 2 : 1 : 2 : 4 : 1 : 2 : 1

//M0//QN23//SUB//DL0//EQ

Give difference between monohybrid and dihybrid cross.

//X

//M0//QN24//SUB//DL0//EQ

Explain chromosomal theory of inheritance.

//X

Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel’s laws.
Chromosomes and genes both are in a pair.
The two alleles of a gene pair are located on homologous sites on homologous chromosomes.
Segregation at the time of gamete formation such that only one of each pair is transmitted to a gamete.

Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
Following this synthesis of ideas, experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced.

//M0//QN25//SUB//DL0//EQ

Explain dihybrid crosses carried out by Morgan in Drosophila to study genes that were sex- linked. OR Briefly mention the contribution of T.H Morgan. OR Describe contribution of Morgan in genetics.OR Who had proposed the chromosomal theory of inheritance?

//X

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas.
For example Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny.
He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent).
Morgan and his group knew that the genes were located on the X-chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.
Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations.
Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) while others were loosely linked (showed higher recombination).
For example, he found that the genes white and yellow were very tightly linked and showed only 1.3 per cent recombination while white and miniature wing showed 37.2 per cent recombination.

//M3//QN26//SUB//DL0//EQ

Explain polygenic inheritance with example.

OR
Explain inheritance of human skin colour by polygenic inheritance.

//X

Mendel’s studies mainly described those traits that have distinct alternate forms such as pea flower colour which are either purple or white.
But, if we look around we will find that there are many traits which are not so distinct in their occurrence and are spread across a gradient.
For example, in humans we don’t just have tall or short people as two distinct alternatives but a whole range of possible heights. Such traits are generally controlled by three or more genes and are thus, called as polygenic traits.
Besides, the involvement of multiple genes polygenic inheritance also takes into account the influence of environment.
In a polygenic trait the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.
Human skin colour is another classic example for this.
The three genes A, B, C control skin colour in human with the dominant forms A, B and C responsible for dark skin colour and the recessive forms a, b and c for light skin colour.

(1) The genotype with all the dominant alleles (AABBCC) will have the darkest skin colour and that with all the recessive alleles (aabbcc) will have the lightest skin colour.

(2) As expected the genotype with three dominant alleles and three recessive alleles will have an intermediate skin colour.

In this manner, the number of each type of alleles in the genotype would determine the darkness or lightness of the skin in an individual.

//M0//QN27//SUB//DL0//EQ

Describe pleiotropism.

OR
What is pleiotropy? Explain with suitable example.

//X

A single gene can exhibit multiple phenotypic expression.
Such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes.
An example of this is the disease phenylketonuria, which occurs in humans.
The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation).
This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.

//M2//QN28//SUB//DL0

Explain the basis of sex determination in insects.

OR
Explain the contribution of Henking in sex determination.

//X

Henking (1891) could trace a specific nuclear structure throughout spermatogenesis in a few insects, and it was also observed by him that 50 per cent of the sperm received this structure after spermatogenesis, whereas the other 50 per cent sperm did not receive it.
Henking gave a name to this structure as the X body but he could not explain its significance.
Further investigations by other scientists led to the conclusion that the ‘X body’ of Henking was in fact a chromosome and that is why it was given the name X-chromosome.
It was also observed that in a large number of insects the mechanism of sex determination is of the XO type, i.e., all eggs bear an additional X-chromosome besides the other chromosomes (autosomes).
On the other hand, some of the sperms bear the X-chromosome whereas some do not.
Eggs fertilised by sperm having an X-chromosome become females and, those fertilised by sperms that do not have an X-chromosome become males.
Due to the involvement of the X-chromosome in the determination of sex, it was designated to be the sex chromosome, and the rest of the chromosomes were named as autosomes.

//M2//QN29//SUB//DL0//EQ

Explain sex determination in birds.

//X

In some other organisms, e.g., birds, a different mechanism of sex determination is observed.
In this case the total number of chromosome is same in both males and females. But two different types of gametes in terms of the sex chromosomes, are produced by females,
i.e., female heterogamety.
The two different sex chromosomes of a female bird has been designated to be the Z and W chromosomes.
In these organisms the females have one Z and one W chromosome, whereas males have a pair of Z-chromosomes besides the autosomes.

//M2//QN30//SUB//DL0//EQ

Explain sex determination in insects.

OR
Explain XX-XO type of sex determination.

//X

This type of sex determination is studied first time in squash bug (protonate).
In this type of sex determination, the number of chromosomes determine the sex of animal.
XO type of sex determination in which the males have only one X-chromosome besides the autosomes, whereas females have a pair of
X-chromosomes.
Following chart explains sex determination in bugs and grosshopper.

AA = Autosomal chromosome

XX, XO = Sex chromosome

//M0//QN31//SUB//DL0//EQ

Explain sex determination in Human.

//X

The sex determining mechanism in case of humans is XY type. Out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females; these are the autosomes. A pair of X-chromosomes are present in the female, whereas, the presence of an X and Y chromosome are determinant of the male characteristic.
During spermatogenesis among males, two types of gametes are produced.
50 per cent of the total sperm produced carry the X-chromosome and the rest 50 per cent has Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome.
There is an equal probability of fertilization of the ovum with the sperm carrying either X or Y chromosome. In case the ovum fertilises with a sperm carrying X-chromosome the zygote develops into a female (XX) and the fertilization of ovum with Y-chromosome carrying sperm results into a male offspring.
Thus, it is evident that it is the genetic makeup of the sperm that determines the sex of the child.
In each pregnancy there is always 50 per cent probability of either a male or a female child.

//M4//QN32//SUB//DL0//EQ

Explain haplo- diploid sex determination with suitable example. OR Explain sex determination in honey bee.

OR
Explain sex determination in honey bee with chart.(MARCH / APRIL 2022)

//X

The sex determination in honey bee is based on the number of sets of chromosomes an individual receives.
An offspring formed from the union of a sperm and an egg develops as a female (queen or worker), and an unfertilised egg develops as a male (drone) by means of parthenogenesis.
This means that the males have half the number of chromosomes than that of a female. The females are diploid having 32 chromosomes and males are haploid, i.e., having 16 chromosomes.
This is called as haplodiploid sex-determination system and has special characteristic features such as the males produce sperms by mitosis.

//M3//QN33//SUB//DL0

what is mutation? Explain in general.

//X

Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.
In addition to recombination, mutation is another phenomenon that leads to variation in DNA.
One DNA helix runs continuously from one end to the other in each chromatid, in a highly supercoiled form.
Therefore, loss (deletions) or gain (insertion/duplication) of a segment of DNA, result in alteration in chromosomes.
Since genes are known to be located on chromosomes, alteration in chromosomes results in abnormalities or aberrations. Chromosomal aberrations are commonly observed in cancer cells.
In addition to the above, mutation also arise due to change in a single base pair of DNA. This is known as point mutation.
A classical example of such a mutation is sickle cell anemia.
Deletions and insertions of base pairs of DNA, causes frame-shift mutations.
However, there are many chemical and physical factors that induce mutations.
These are referred to as mutagens. UV radiations can cause mutations in organisms – it is a mutagen.

//M2//QN34//SUB//DL0

A male contains extra X- chromosome in the 23 pair of chromosome. Identify the disorder and describe its characteristics.

OR
Explain : Klinefelter's Syndrome

//X

This genetic disorder is caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY. An individual has Klinefelter’s Syndrome.
Characteristics:

(1) Such an individual has overall masculine development.

(2) Testes are under developed.

(3) Less hair on the skin.

(4) The feminine development (development of breast, i.e., Gynaecomastia) is also expressed.

(5) Voice is also female like.

(6) Such individuals are sterile.

//M2//QN35//SUB//DL0//PYQ

Describe: Turner’s syndrome (June 2024)

OR
Explain X - Chromosomal monosomy seen in female of human.

//X

In Turner’s syndrome, these is monosomy of
X-chromosomes.
Here, chromosomes are found 45 instead of 46.
Characteristics :
  • Stunted growth, short, wrinkled neck females are infertile due to underdeveloped ovaries.
  • Absence or underdevelopment of secondary sexual characters.

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Differentiate between Down’s syndrome and Turner’s syndrome. (AUGUST 2020)

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Down’s syndrome

Turner’s syndrome

1.

It is disorder related to autosomal chromosome.

It is disorder related to sex chromosome.

2.

In this abnormality is seen in the 21 chromosome.

In this abnormality is seen in sek chromosome.

3.

It shows trisomy of 21st chromosome.

It shows monosomy in 23rd chromosome.

4.

Can occur to both male and female.

It can only occur in female.

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Describe phenylketonuria (PKU).

(MARCH 2024, June 2025)

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This inborn error of metabolism is also inherited as the autosomal recessive trait.
The affected individual lacks an enzyme phenyl alanine hydroxylase that converts the amino acid phenylalanine into tyrosine.
As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives.
Accumulation of these in cerebrospinal fluid in the brain results in mental retardation.
These are also excreted through urine because of their poor absorption by kidney.

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Describe Mendelian abnormality: Thalassemia.

(June 2025)

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This is an autosome-linked recessive blood disease transmitted from parents to the offspring when both the partners are unaffected carrier for the gene (or heterozygous).
The defect could be due to either mutation or deletion which ultimately results in reduced rate of synthesis of one of the globin chains (α and β chains) that make up haemoglobin.
This causes the formation of abnormal haemoglobin molecules resulting into anaemia which is characteristic of the disease.
Thalassemia can be classified according to which chain of the haemoglobin molecule is affected.

1. α Thalassemia:

  • In α Thalassemia, production of α globin chain is affected.
  • α Thalassemia is controlled by two closely linked genes HBA 1 and HBA 2 on chromosome 16 of each parent and it is observed due to mutation or deletion of one or more of the four genes. The more genes affected, the less alpha globin molecules produced.

2. β Thalassemia :

  • In β Thalassemia, production of β globin chain is affected. While β Thalassemia is controlled by a single gene HBB on chromosome 11 of each parent and occurs due to mutation of one or both the genes.
  • Thalassemia differs from sickle-cell anaemia in that the former is a quantitative problem of synthesising too few globin molecules.

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what is pedigree analysis? Give symbols used for pedigree analysis and draw pedigree chart for myotonic dystrophy.

OR
What is pedigree analysis? Suggest how such an analysis can be useful.

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An analysis of traits in a several generations of a family is called the pedigree analysis.
In the pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations. In human genetics, pedigree study provides a strong tool, which is utilised to trace the inheritance of a specific trait, abnormality or disease.
Some of the important standard symbols used in the pedigree analysis have been shown as follows.
Each and every feature in any organism is controlled by one or the other gene located on the DNA present in the chromosome. DNA is the carrier of genetic information. It is hence transmitted from one generation to the other without any change or alteration. However, changes or alteration do take place occasionally.
Such an alteration or change in the genetic material is referred to as mutation. A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes.
The pedigree analysis of Autosomal dominant trait Myotonic dystrophy is as follows:

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what is Mendelian disorders (genetic disorders)? Explain genetic abnormalities seen in haemophilia and colour blindness with its characteristics. OR Mention any two genetic disorders with their symptoms. OR Explain genetic disorders caused due to

X- chromosom linked recessive gene.

(JULY 2022)
OR
What are Mendelian disorders? Explain any two disorders related to sex linked chromosomal recessive genes.

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Mendelian disorders are mainly determined by alteration or mutation in the single gene. These disorders are transmitted to the offsprings on the same lines as we have studied in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. It is important to mention that such Mendelian disorders may be dominant or recessive.
1. Colour Blindness : (JUNE 2024)
It is a sex-linked recessive disorder due to defect in either red or green cone cell of eye resulting in failure to differentiate discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome (Sex chromosome).
It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X-chromosome.
Males have only one X-chromosome and females have two. The son of a woman who carries the gene has a 50 per cent chance of being colour blind.
The mother is not herself colour blind because the gene is recessive.
That means that its effect is suppressed by her matching dominant normal gene.
A daughter will not normally be colour blind, unless her mother is a carrier and her father is colour blind.
2. Haemophilia :
This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected.
Due to this, in an affected individual a simple cut will results in non-stop bleeding.
The heterozygous female (carrier) for haemophilia may transmit the disease to sons.
The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be atleast carrier and the father should be haemophilic (unviable in the later stage of life).
The family pedigree of Queen Victoria shows a number of haemophilic descendants as she was a carrier of the disease.

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what is point mutation? Explain with suitable example. OR Explain: sickle cell anemia.

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Mutation also arise due to change in a single base pair of DNA. This is known as point mutation.
A classical example of such a mutation is sickle cell anemia.

Sickle cell anemia :

This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele, HbA and HbS. Out of the three possible genotypes only homozygous individuals for HbS (HbS HbS) show the diseased phenotype. Heterozygous (HbA HbS) individuals appear apparently unaffected but they are carrier of the disease as there is 50 per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait.
The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG.
The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.

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what are chromosomal abnormalities? Give any two aneuploidy that occurs in human chromosomes, with characteristics. OR Explain the disorder occurs due to trisomy of Chromosome-21.

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The number of chromosomes are fixed in any species. The chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Such a change occurs due to loss or gain of one chromosome in a group. There are two types of chromosomal abnormalities.
Euploidy and Aneuploidy
Aneuploidy : Loss or gain of one or more chromosomes in a pair of chromosomes is called aneuploidy.
  • Normally, a pair of homozygous chromosomes consists of two chromosomes. but in such cases there is only one or none at all or three or four chromosomes.

(a) Hypoploidy : Monosomy (2n – 1), nullisomy
(2n – 2).

(b) Hyperploidy : Trisomy (2n + 1), Tetrasomy
(2n + 2)

  • Somatic chromosomal aneuploidy is associated with the somatic chromosomes.
  • Sex chromosomal aneuploidy is associated with the sex chromosomes.

(i) Down’s syndrome :

  • The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21).
  • This disorder was first described by Langdon Down (1866).

(1) The affected individual is short statured with small round head, furrowed tongue, partially open mouth.

(2) Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

(ii) Klinefelter’s Syndrome :

  • This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY.

(1) Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. Testes are underdeveloped.

(2) Such individuals are sterile.

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Who gave chromosomal theory of inheritance?

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In 1903, Sutton and Boveri gave chromosomal theory of inheritance.