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Turner's syndrome chromosome pattern is:

  • XO

  • XXX

  • XYY

  • XXY



Turner's Syndrome is a chromosomal condition that mainly affects development in females. It is a monosomic condition with 44 + XO genotype. It occurs by the union of an abnormal egg and normal sperm or vice versa. These are sterile females with rudimentary ovaries, underdeveloped breasts, small uterus, short stature and mental retardation.


One of the genes present exclusively on the X-chromosome in humans is concerned with

  • baldness

  • red-green colour blindness

  • facial hair/moustaches in males

  • night blindness


red-green colour blindness

The most common form of red-green colour blindness is an 'X chromosome linked recessive' disorder. The 'red' and 'green' genes are known to reside at the tip of the long arm of the X chromosome. Women have two copies of the X-chromosome, and so they may have normal colour vision, even if they carry one copy of the defective gene. Men have only one X-chromosome, and so will be colour blind.


Pure line breed refers to

  • heterozygosity only

  • heterozygosity and linkage

  • homozygosity only

  • homozygosity and self assortment


homozygosity only

Pure line is true breeding genotype. A line in which homozygous individuals produce only homozygous offspring. Thus pure line breed refers to homozygosity only.


Frameshift mutation occurs when

  • base is subsituted

  • base is deleted or added

  • anticodons are absent

  • none of these


base is deleted or added

Frameshift mutation occurs when one or more nucleotides are either added r deleted from DNA. Eg.

Original (wild type) message or reading frame


After Deletion


Message out of frame.


Sex determination in humans takes place by

  • sex chromosomes of father

  • measurement of sperm

  • measurement of ovum

  • sex chromosomes of mother


sex chromosomes of father

In humans, sex of offspring is determined at the time of fertilization. Man is heterogametic i.e. having two types of gametes 22 + X and 22 + Y and is responsible for sex determination whereas female is homogametic i.e. having single type of gametes 22 + XX.


If a homozygous red flowered plant is crossed with a homozygous white flowered plant, the offspring would be

  • half red-flowered

  • all red-flowered

  • half white-flowered

  • half pink-flowered.


all red-flowered

When a homozygous red flowered plant is crossed with homozygous white flowered plant in F1 generation, then all red flowered are because red colour is dominant on white colour of flower.


The main cause of Edward syndrome, Patau syndrome and Down syndrome is :

  • mutation of gene

  • change in both autosomes and heterosomes

  • change in autosomes

  • change in heterosomes


change in autosomes

Down's syndrome is due to trisomy of 21st chromosome. Patau's syndrome is trisomy of 13th pair of chromosome and Edward's syndrome is due to trisomy of 18th chromosome. Therefore, these all syndromes are caused due to change in autosomes.


When paternal and maternal chromosomes change their materials with each other in cell division this event is called

  • synapsis

  • crossing over

  • bivalent forming

  • dyad- forming


crossing over

Crossing over is the mutual exchange of segments of homologous chromatids during first meiotic prophase.

Synapsis is the fusion of chromosome pairs at the start of meiosis.

DNA replication precedes the start of Meiosis I. During Prophase I, homologous chromosomes pair and form synapses. The paired chromosomes are called bivalents and the formation of chiasmata caused by genetic recombination becomes apparent.

Dyad formation is a double chromosome resulting from the splitting of a tetrad during meiosis (germ cell formation).


2n - 1 condition is called

  • trisomy

  • monosomy

  • nullisomy

  • tetrasomy



2n - 1 condition represents diploid set of chromosomes having loss of one chromosome, the presence of one unpaired chromosomes along with diploid set is called monosomy.

Trisomy is a chromosomal disorder characterized by an additional chromosome. Eg. Down syndrome.

 Nullisomy is a genetic condition involving the lack of both the normal chromosomal pairs for a species. This condition is represented by 2n - 2.

Tetrasomy is a form of aneuploidy with the presence of four copies, instead of normal two, of particular chromosomes.


The ratio in a dihybrid test cross between two individuals is given by

  • 2 : 1

  • 1 : 2 : 1

  • 3 : 1

  • 1 : 1 : 1 :1


1 : 1 : 1 :1

Test cross is a cross to know the genotype of the individual.

For eg, Tall white and dwarf black can be assumed as TtWw and ttww respectively.

Gametes of TtWw = TW, Tw, tW, tw

Gametes of ttww == tW

 or  tw
Tw Ttww
tW ttWw
tw ttww

Thus the ratio of TtWw : Ttww : ttWw : ttww is 1 : 1 : 1 :1.