Standard 9 Science and Technology - 16. Heredity and Variation

Question 1:

Complete the following sentences by choosing the appropriate words from the brackets.

(Inheritance, sexual reproduction, asexual reproduction, chromosomes, DNA, RNA, gene)

a. Hereditary characters are transferred from parents to offsprings by ............, hence they are said to be structural and functional units of heredity.

b. Organisms produced by ............ show minor variations.

c. The component which is in the nuclei of cells and carries the hereditary characteristics is called .............

d. Chromosomes are mainly made up of .............

e. Organisms produced through ............ show major variations.

Answer 1:

a. Hereditary characters are transferred from parents to offsprings by gene, hence they are said to be structural and functional units of heredity.
b. Organisms produced by asexual reproduction show minor variations.
c. The component which is in the nuclei of cells and carries the hereditary characteristics is called chromosome.
d. Chromosomes are mainly made up of DNA.
e. Organisms produced through sexual reproduction show major variations.

Question 2:

Explain following.

a. Explain Mendel’s monohybrid progeny with the help of any one cross.

b. Explain Mendel’s dihybrid ratio with the help of any one cross.

c. Distinguish between monohybrid and dihybrid cross.

d. Is it right to avoid living with a person suffering from a genetic disorder?

Answer 2:

a. Monohybrid cross is a cross between two parents that have one pair of contrasting characters; for example, if pea plant with yellow seed coat is crossed with pea plant having green seed coat then in the F1 generation all the plants produce yellow seeds.

b. Dihybrid cross is a cross between two parents that have two pairs of contrasting characters, for example, a plant having round and yellow seeds is crossed with a plant having green and wrinkled seeds.

Monohybrid cross	Dihybrid cross
The cross in which only one pair of contrasting characters is involved is known as monohybrid cross.	The cross in which only two pairs of contrasting characters are involved is known as dihybrid cross.
A monohybrid cross is useful in determining the dominance of genes.	A dihybrid cross is useful in studying the assortment of the offspring.
example- a cross between tall and dwarf plant	example- a cross between tall plant having red flower and a dwarf plant having white flower

d. No, it is not right to avoid living with a person suffering from a genetic disorder. Genetic disorders are not communicable diseases that would be transmitted to people who come in contact with people with genetic disoders. Genetic disorders are caused by changes in DNA sequences which can only be passed from one generation to another under specific circumstances. Rather, we sholud support and accept people with such disorders, so that they can live a normal life.

Question 3:

Answers the following questions in your own words.

a. What is meant by ‘chromosome’. Explain its types.

b. Describe the structure of the DNA molecule.

c. Express your opinion about the use of DNA fingerprinting.

d. Explain the structure, function and types of RNA.

e. Why is it necessary for people to have their blood examined before marriage?

Answer 3:

a. Chromosomes are thread-like structures found in the nucleus of all living cells. Chromosomes are divided into four types based on the position of the centromere.

(1) Metacentric chromosomes: In these chromosomes, the centromere is present in the middle, which gives rise to two equal arms.

(2) Sub-metacentric chromosomes: In sub-metacentric chromosomes, the centromere lies slightly away from the middle region. As a result, it has one arm slightly longer than the other.

(3) Acrocentric chromosomes: In acrocentric chromosomes, the centromere is located close to the end of the chromosome. As a result, it has one arm, which is extremely long and the other, which is extremely short.

(4) Telocentric chromosomes: In telocentric chromosomes, the centromere is present at the terminal end. As a result, the chromosome has only one arm.

b. DNA is a very large single molecule also called as macromolecule. It has a double helix structure, similar to a ladder, which is twisted at both ends. The DNA molecule is made up of basic materials called nucleotides and each nucleotide is made up of three components :

Sugar
Phosphate groups

Sugar and phosphate are arranged lengthwise.

Nitrogen bases are attached sugar from inwards that extends to join hydrogen bond and the complimentary nitrogenous base from other strand.

The nitrogen bases are :

Adenine(A)
Guanine(G)
Cytosine(C)
Thymine(T)

Adinine pairs with Thymine with two hydrogen bonds. Guanine pairs with Cytosine with three hydrogen bonds.

During the interphase of cell cycle, DNA molecules duplicates for their equal distribution in the daughter cells during mitosis. The double helix opens at one end, makes the two strands free and from there new strands are formed.

c. DNA fingerprinting is a method for comparing the DNA sequences of any two individuals. 99.9% of the base sequences in all human beings are identical. It is the remaining 0.1% that makes every individual unique.

The applications of DNA fingerprinting are as follows:

DNA fingerprinting is widely used in forensics since DNA of every tissue from an individual has the same degree of polymorphism.
DNA fingerprinting forms the basis of paternity testing since a child inherits polymorphism from both its parents.
It can be used for studying evolution and genetic diversity in a population.

d. A RNA nucleotide has three main components − a nitrogenous base, a ribose sugar and a phosphate group.

• The ribose sugar and the phosphates form the backbone of a polynucleotide chain with nitrogenous bases linked to sugar moiety and projecting from the backbone.

• Two types of nitrogenous bases are present i.e. Purines (Adenine and Guanine) and Pyrimidines (Cytosine and Uracil).

• A nitrogenous base is linked to the ribose sugar through N-glycosidic linkages to form a nucleoside (like adenosine, guanosine or cytidine and uridine).

• A phosphate group is linked to 5'-OH of a nucleoside through phosphoester linkage to form a corresponding nucleotide.

• Every nucleotide residue has an additional −OH group present at 2' -position in the ribose.

• Many nucleotides are linked through 3'-5' phosphodiester linkages to each other to form the polynucleotide chain.

• The end of the chain which has a free phosphate moiety at 5'-end of ribose sugar is referred to as 5’-end and the other end of the chain having a free 3'-OH group at the ribose sugar is referred to as 3' -end of the polynucleotide chain.

There are 3 types of RNA:

1. mRNA (messenger RNA) − It serves as a template for protein synthesis. DNA is transcribed to form an mRNA, which in turn is translated to form protein.

2. tRNA (transfer RNA) − It brings amino acids during translation and reads the genetic code.
3. rRNA (ribosomal RNA) − These are the work benches of translation. They play a structural and catalytic role during translation.

e. It is necessary for people to have their blood examined before marriage because the genetic disorders are transmitted only by reproduction. If a carrier/sufferer of a genetic disorder marries a person who is also a carrier/ sufferer of the disorder, then there are chances that disorder will be passed on to the offsprings. In order to prevent this transmission, people should get their blood examined before marriage to know if they are a carrier of any genetic disorder.

Question 4:

Write a brief note on each.

a. Down syndrome

b. Monogenic disorders

c. Sickle cell anaemia: symptoms and treatment.

Answer 4:

a. Down’s Syndrome: Down's syndrome is caused due to the presence of an additional copy of chromosome 21 (Trisomy of 21). It was the first discovered and described chromosomal disorder in humans. The total number of chromosomes in people affected with Down's syndrome becomes 47. Affected individual has short stature, small, round head, furrowed tongue, partially opened mouth, palm crease, congenital heart disease and mental retardation.

b. Monogenic disorders: Monogenic disorders are genetic disorders which are caused by a mutation in a single gene. This mutation may be present on one or both the chromosomes. Some of the examples of monogenic disorders are sickle cell anemia, cystic fibrosis, polycystic kidney etc.

c. Sickle cell anaemia: Sickle-cell anaemia is an autosome-linked recessive trait exhibiting change in shape of the red blood cells from biconcave disk to sickle shape under low oxygen tension. It is a result of replacement of GAG by GUG leading to the substitution of Glu by Val at sixth position of beta globin chain of haemoglobin. It is characterised by low haemoglobin count and other symptoms of anaemia such as fatigue and irritability, swelling on hands and legs, pain in joints, constant low grade fever etc.
There is no particular treatment for sickle cell anemia, the treatments which are available provide symptomatic relief from the symptoms associated with this disorder.

Question 5:

How are the items in groups A, B and C inter-releated?

A	B	C
Leber hereditary optic neuropathy	44+XXY	Pale skin,white hairs
Diabetes	45+X	Men are sterile
Albinism	Mitochondrial disorder	Women are sterile
Turner syndrome	Polygenic disorder	This disorder arises during development of zygote.
Klinefelter syndrome	Monogenic disorder	Effect on blood-glucose level

Answer 5:

A	B	C
Leber hereditary optic neuropathy	Mitochondrial disorder	This disorder arises during development of zygote.
Diabetes	Polygenic disorder	Effect on blood-glucose level
Albinism	Monogenic disorder	Pale skin,white hairs
Turner syndrome	45+X	Women are sterile
Klinefelter syndrome	44+XXY	Men are sterile