Maharashtra Board Class 12 Biology Chapter 3- Inheritance and Variation


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Q. 1 Multiple choice questions. 
1. Phenotypic ratio of incomplete 
dominance in Mirabilis jalapa.
a. 2 : 1 : 1
b. 1 : 2 : 1 
c. 3 : 1
d. 2 : 2

2. In dihybrid cross, F2 generation offsprings show four different phenotypes while the genotypes are ................
a. six
b. nine
c. eight
d. sixteen

3. A cross between an individual with 
unknown genotype for a trait with 
recessive plant for that trait is .............. 
a. back cross
b. reciprocal cross
c. monohybrid cross
d. test cross

4. When phenotypic and genotypic ratios 
are the same, then it is an example of 
............ 
 a. incomplete dominance
 b. complete dominance
 c. Multiple alleles
 d. cytoplasmic inheritance

5. If the centromere is situated near the end of the chromosome, the chromosome is called ................
a. Metacentric
b. Acrocentric
c. Sub-Metacentric
d. Telocentric

6. Chromosomal theory of inheritance was proposed by ................
 a. Sutton and Boveri
 b. Watson and Crick
 c. Miller and Urey
 d. Oparin and Halden

7. If the genes are located in a chromosome as p-q-r-s-t, which of the following gene pairs will have least probability of being inherited together?
a. p and q
b. r and s
c. s and t
d. p and s
Answer - ------------

8. Find the mis match pair :- 
 a. Down’s syndrome = 44 + XY
 b. Turner’s syndrome = 44 + XO
 c. Klinefelter syndrome = 44 + XXY
 d. Super female = 44 + XXX

9. A colourblind man marries a woman, 
who is homozygous for normal colour 
vision ,the probability of their son being 
colourblind is – 
a. 0%
b. 25%
c. 50%
d. 100%


Q. 2 Very Short Answer Questions.

1. Explain the statements :
a. Test cross is back cross but back cross is not necessarily a test cross.
Answer : Testcross is a crossing of a F1 hybrid with one of its recessive parent or individual with similar genetic makeup.

- Backcross is a crossing of a hybrid with one of its parents or individual with similar genetic makeup.
- If back cross involves crossing of F1 generation with recessive parent, it's also a test cross.
- But if back cross involves crossing of F1 generation with dominant parent, it's not a test cross.
- Therefore, test cross is a backcross but back cross is not necessarily a test cross.

b. Law of dominance is not universal.
Answer :  The allele that expresses itself in offspring is called dominant allele while the allele that is suppressed by dominant allele is called recessive allele. However, this law is not universally applicable because there are some cases where neither of the two alleles is completely dominant over the other.

2. Define the following terms :

a. Dihybrid cross : Dihybrid cross is the cross between two different genes that differ in two observed traits.

b. Homozygous : An individual possessing identical allels for a particular trait, is called homozygous or pure for that trait.

c. Heterozygous : An individual possessing contrasting alleles for a particular trait, is called heterozygous.

d. Test cross : The cross of F1 hybrid with the homozygous recessive parent is known as a test cross.


3. What is allosome?
Answer : Sex chromosomes, also called allosomes, are the chromosomes that contain the genes that determine biological sex.


4. What is crossing over?
Answer : Crossing over is a process that produces new combinations (recombinations) of genes by interchanging and exhanging of corresponding segments between non-sister chromatids of homologous chromosomes.


5. Give one example of autosomal recessive disorder.
Answer : Examples of autosomal recessive disorders include cystic fibrosis, sickle cell anemia, and Tay Sachs disease.

6. What are X-linked genes?
Answer : The inheritance of X sex linked genes from parents to their offsprings, is called X linked inheritance. The X linked genes are located on non homologous region of X chromosome and these gene do not have corresponding alleles on Y chromosome.

7. What are holandric traits?
Answer : The genes that are carried on the Y chromosome are called holandric genes. Holandric genes can only be passed by males onto their sons; they code for 'maleness' but sometimes cause rare conditions like hypertrichosis pinnae and color blindness.


8. Give an example of chromosomal disorder caused due to non-disjunction of autosomes.
Answer : Down's syndrome is caused due to the trisomy of 21 chromosome which is an autosome. It is mainly caused due to the non-disjunction of the autosomes.

9. Give Examples of Complete sex linkage.
Answer : An example of sex linkage is the trait of red-green colorblindness. Colorblindness is carried as a mutation on the X chromosome.


Q. 3 Short Answer Questions.


1. Enlist seven traits of pea plant selected/ studied by Mendel.
Answer : Mendel selected seven pairs of contrasting characters of pea plants for his experiments which are as follows:

Pea shape: Round or Wrinkled.
Pea color: Green or Yellow.
Pod shape: Constricted or Inflated.
Pod color: Green or Yellow.
Flower color: Purple or White.
Plant size: Tall or Dwarf.
Position of flowers: Axial or Terminal


2. Why law of segregation is also called the law of purity of gametes.
Answer : When a pair of contrasting allele is brought together in a hybrid, the two members of the allelic pair remain together without any mixing and when gametes are formed from the hybrid, the two separate and segregate out from each other and moves to different gametes that's why law of segregation is called as law of purity of gametes.


5. “Father is responsible for determination of sex of child and not the mother”. Justify.
Answer : This can be justified as follows -

1. Male will have 2 sex chromosomes ie XY and female will have XX as their sex chromosomes.

2. Thus male can donate either X or Y and female will donate only X at the time of gamete formation.

3. Now if, X chromosome is given by male and X from female - it will form female and if male donates Y chromosome and female gives X then it will be male (as XY is male and XX is female).

4. Thus we can say that sex determining chromosome is given by males hence they are responsible for sex of the children.


6. What is linkage? How many linkage groups do occur in human being?
Answer : The tendency of two or more genes present on the same chromosomes that are inherited together is known as linkage. Humans can have 2 linkage groups based on gender. Male-> 22 pairs autosomes + 1X +1Y = 24 linkage groups. Female -> 22 pairs autosomes+ 1X pair = 23 linkage groups.


7. Write note on –PKU
Answer : It is an inborn metabolic disorder caused due to recessive autosomal genes. When recessive genes are present in homozygous condition, phenylalanine hydroxylase enzyme is not produced. This enzyme is essential for conversion of amino acid phenylalanine into tyrosine. Due to absence of this enzyme, phenylalanine is not converted into tyrosine. Hence, phenylalanine and its derivatives are accumulated in blood and cerebrospinal fluid (CSF). It affects development of brain and causes mental retardation. Excess phenylalanine is excreted in urine, hence this disease is called phenylketonuria.



8. Compare - X-chromosome and Y- chromosome.
Answer : X chromosome is straight, rod like and longer than Y chromosome. X chromosome is metacentric, while Y chromosome is acrocentric. X chromosome has large amount of euchromatin (extended region) and small amount of heterochromatin (highly condensed region). Euchromatin has large amount of DNA material, hence genetically active. Y chromosome has small amount of euchromatin and large amount of heterochromatin, hence it is genetically less active or inert. Both X and Y chromosome show homologous and non- homologous regions. Homologous regions show similar genes while non-homologous regions show dissimilar genes. non-homologous region of Y chromosome. X-linked genes are present on non-homologous region of X-chromosome while Y linked genes are present on non-homologous region of Y-chromosome.


9. Explain the chromosomal theory of inheritance.
Answer : In 1902, Theodor Boveri observed that proper embryonic development of sea urchins does not occur unless chromosomes are present. That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis. Together, these observations led to the development of the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.
The chromosomal theory of inheritance was given by Boveri and Sutton in the early 1900s. It is the fundamental theory of genetics. According to this theory the chromosomes are present in pairs in somatic cells. During gamete formation homologous chromosomes pair, segregate and assort independently during meiosis. Thus, each gamete contains only one chromosome from a pair. The Chromosomal Theory of Inheritance was consistent with Mendel’s laws and was supported by the following observations:

During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs. The sorting of chromosomes from each homologous pair into pre-gametes appears to be random. Each parent synthesizes gametes that contain only half of their chromosomal complement. Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent. The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.



Q. 5 Long answer type questions.


1. What is dihybrid cross? Explain with suitable example and checker board method.
Answer :

Fig : Dihybrid Cross

A cross between parents differing in two heritable traits, is called dihybrid cross e.g. cross of pure tall, round seeded plant with dwarf, wrinkled seeded plant. Mendel also performed the dihybrid cross between pea plants that differed in two pairs of contrasting characters. In a dihybrid cross, the parents carry different pair of alleles for each trait. One parent carries homozygous dominant allele, while the other one carries homozygous recessive allele. The offsprings produced after the crosses in the F1 generation are all heterozygous for specific traits.

Dihybrid Cross Examples
Mendel took a pair of contradicting traits together for crossing, for example colour and the shape of seeds at a time. He picked the wrinkled-green seed and round-yellow seed and crossed them. He obtained only round-yellow seeds in the F1 generation. This indicated that round shape and yellow colour of seeds are dominant in nature.


2. Explain with suitable example an independent assotrment.
Answer :
Image Source: biology-forums

Law of Independent Assortment :
This law is based on dihybrid cross. It is basic principle of genetics developed by a Mendel. It describes how different genes or alleles present on separate chromosomes independently separate from each other, during formation of gametes. These alleles are then randomly united in fertilization. In dihybrid cross, F2 phenotypic ratio 9:3:3:1 indicates that the two pairs of characters behave independent of each other. It can be concluded that the two characters under consideration are assorted independently giving rise to different combinations.
Statement of Law of Independent Assortment: The law states that “When hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair”.


3. Define test cross and explain its significance.
Answer :

Fig : Graphical representation of test Cross

The cross of F1 hybrid with the homozygous recessive parent is known as a test cross. It is used to test whether an individual is homozygous (pure) or heterozygous (hybrid). Test cross is easy, simple, repeatable and predictable.

Significance of Test Cross :
Test cross can be used to find out genotype of any plant with dominant expression. But it is not known whether it is homozygous (pure) or heterozygous for that trait. For example, A pea plant having violet (purple) flowers is crossed with a pea plant with white flowers. If all flowers produced are violet, we can conclude that plant is pure or homozygous and if we get violet and white flowers in 1:1 ratio. We can conclude that plant is heterozygous. Test cross is also used to introduce useful recessive traits in the hybrids of self pollinated plants during rapid crop improvement programs.
Following is the graphic representation of test cross. Recessive parent is crossed to find out unknown genotype.


4. What is parthenogenesis? Explain the haplo-diploid method of sex determination in Honey bee.
Answer : Parthenogenesis is a form of asexual reproduction in which an unfertilized egg develops into a new individual. It is a method in which a new individual developed without fertilization. Examples of plants showing parthenogenesis include honey bees, ants, birds.

Sex Determination in honey bees :

Fig : Sex determination in honey bees

In honey bees, chromosomal mechanism of sex determination is haplo-diploid type. In this type, sex of individual is determined by the number of set of chromosomes received. Females are diploid (2n=32) and males are haploid (n=16). The female produces haploid eggs (n=16) by meiosis and male produces haplaid sperms (n=16) by mitosis. If the egg is fertilized by sperm, the zygote develops into a diploid female (2n=32) (queen and worker) and unfertilised egg develops into haploid male (n=16) (Drone) by way of parthenogenesis.
The diploid female gets differentiated into either worker or queen depending on the food they consume during their development. Diploid larvae which get royal jelly as food develops into queen (fertile female) and other develops into workers (sterile females).


5. In the answer for inheritance of X-linked genes, Madhav had shown carrier male. His answer was marked incorrect. Madhav was wondering why his marks were cut. Explain the reason.
Answer : Male has only one X-chromosome. If X chromosome carries X-linked recessive gene for sex linked trait, then it is expressed phenotypically, because there is no dominant gene on Y chromosome to suppress its effect. Therefore, sex-linked / X-linked traits appear more frequently in males than in the females.



6. With the help of neat labelled diagram, describe the structure of chromosome.
Answer :

Fig : Structure of chromosome 

Chromosomes are filamentous bodies present in the eukaryotic nucleus. The term chromosomes (Gr., Chromo = colour, soma = body) was coined by W. Waldeyer (1888). The size of chromosome varies from species to species. Each metaphase chromosome varies from 0.1 to 33 mm in length and 0.2 to 2 mm in thickness. Chromosomes are visible during cell division. They are capable of self replication and play vital role in heredity, mutation, variation, and evolutionary devlopment of eukaryotic species. Chemically eukaryotic chromosomes are made of DNA, histone and non-histone proteins.

Structure of chromosome :
Chromosomes are best visible under microscope, when the cell is at metaphase stage. It is because at this stage chromosomes are highly condensed. Typical chromosome consists of two chromatides joind together at centromere or primary constriction. Primary constriction constists of a disk shape plate called kinetochore. It is at the kinetochore, spindle fibres get attached during cell division. Besides primary constriction, some few chromosomes possess additional one or two constrictions called secondary constriction.

At secondary constriction I, nucleolus becomes organized during interphase. A satellite body (SAT body) is attached at secondary constriction II, in very few chromosomes. Each chromatid in turn contains a long, unbranched, slender, highly coiled DNA thread, called Chromonema, extending through the length of chromatid. Chromatid consists a double stranded DNA molecule which extends from one end of chromosomes to other. Depending upon the position of centromere there are four types (shapes) of chromosomes viz. Acrocentric (j shaped), Telocentric (i shaped), Submetacentric (L shaped) and Metacentric (V shaped). The ends of chromosome (i.e. chromatids) are known as telomeres.


7. What is cris-cross inheritance? Explain with suitable example.
Answer : There are different type of inheritance patterns observed in different disorders. The criss-cross type of inheritance is the inheritance of sex-linked characters. These characters are transmitted from fathers to daughters or from mothers to sons. The common example is X-linked inheritance. Haemophilia C is an X-linked disorder showing this type of inheritance. There is passage of the trait from the father to daughter or mother to son. If the father is the affected, the trait is passed to the daughter and she is a carrier. If the mother is carrier, the trait is passed to the son who is affected.



8. Describe the different types of choromosomes.
Answer : Depending upon the position of centromere there are four types (shapes) of chromosomes viz. Acrocentric (j shaped), Telocentric (i shaped), Submetacentric (L shaped) and Metacentric (V shaped). The ends of chromosome (i.e. chromatids) are known as telomeres.

Metacentric Chromosomes :
Metacentric chromosomes have the centromere in the center, such that both sections are of equal length. Human chromosome 1 and 3 are metacentric.

Submetacentric Chromosomes :
Submetacentric chromosomes have the centromere slightly offset from the center leading to a slight asymmetry in the length of the two sections. Human chromosomes 4 through 12 are submetacentric.

Acrocentric Chromosomes : Acrocentric chromosomes have a centromere which is severely offset from the center leading to one very long and one very short section. Human chromosomes 13,15, 21, and 22 are acrocentric.

Telocentric Chromosomes : Telocentric chromosomes have the centromere at the very end of the chromosome. Humans do not possess telocentric chromosomes but they are found in other species such as mice.