Samacheer Kalvi 10th Science Solutions Chapter 18 Pdf

Tamilnadu Board Samacheer Kalvi 10th Science Solutions Chapter 18: Tamilnadu State Board Solution Class 10 Science Chapter 18 – Genetics.

Samacheer Kalvi 10th Science Solutions Chapter 18: Overview

Board	Samacheer Kalvi
Class	10
Subject	Science
Chapter	18
Chapter Name	Genetics

Samacheer Kalvi 10th Science Solutions Chapter 18 Genetics

I.) Choose the correct answer

1.) According to Mendel alleles have the following character

a) Pair of genes

b) Responsible for character

c) Production of gametes

d) Recessive factors

Ans: a) Pair of genes

Explanation: Because, the pairs of contrasting character (traits) of Mendel are called alleles having pairs of genes.

2.) 9:3:3: 1 ratio is due to

a) Segregation

b) Crossing over

c) Independent assortment

d) Recessiveness

Ans:c) Independent assortment

Explanation: Because, according to this law, in case of inheritance of two or more pairs of characters simultaneously, the genes of one pair assort out independently of the other pair.

The total number of individuals produced in F2 generation are 16 and which occurs in the dihybrid ratio 9:3:3:1.

3.) The region of the chromosome where the spindle fibres get attached during cell division

a) Chromomere

b) Centrosome

c) Centromere

d) Chromonema

Ans:c) Centromere

Explanation: Because, during cell division, at the centromere the spindle fibers attaches to the chromosomes.

4.) The centromere is found at the centre of the ________________ chromosome.

a) Telocentric

b) Metacentric

c) Sub-metacentric

d) Acrocentric

Ans:b) Metacentric

Explanation: Because, the centromere is at the centre of metacentric chromosome.

5.) The _____________ units form the backbone of the DNA.

a) 5 carbon sugar

b) Phosphate

c) Nitrogenous bases

d) Sugar phosphate

Ans:c) Nitrogenous bases

Explanation: Because, the centrally situated nitrogenous base are linked to sugar phosphate units forming the backbone of DNA.

6.) Okasaki fragments are joined together by ___________________.

a) Helicase

b) DNA polymerase

c) RNA primer

d) DNA ligase

Ans: d) DNA ligase

Explanation: Because, Okasaki fragments are joined together by DNA ligase.

7.) The number of chromosomes found in human beings are __________________.

a) 22 pairs of autosomes and 1 pair of allosomes.

b) 22 autosomes and 1 allosome

c) 46 autosomes

d) 46 pairs autosomes and 1 pair of allosomes.

Ans:a) 22 pairs of autosomes and 1 pair of allosomes.

Explanation: Because, in human each cell is normally contains 23 pairs of chromosomes, out of which 22 pairs are autosomes and 23rd pair is called allosome or sex chromosome.

8.) The loss of one or more chromosome in a ploidy is called ___________.

a) Tetraploidy

b) Aneuploidy

c) Euploidy

d) Polyploidy

Ans: b) Aneuploidy

Explanation: Because, aneuploidy is the loss of one or more chromosomes in a ploidy.

II.) Fill in the blanks

1) The pairs of contrasting character (traits) of Mendel are called _________.

Ans:The pairs of contrasting character (traits) of Mendel are called alleles.

2) Physical expression of a gene is called _________

Ans: Physical expression of a gene is called phenotype.

3) The thin thread like structures found in the nucleus of each cell are called _________.

Ans:The thin thread like structures found in the nucleus of each cell are called chromosomes.

4) DNA consists of two ____________ chains

Ans: DNA consists of two polynucleotide chains.

5) An inheritable change in the amount or the structure of a gene or a chromosome is called _______________.

Ans: An inheritable change in the amount or the structure of a gene or a chromosome is called mutation.

III.) Identify whether the statement are True or False. Correct the false statement

1) A typical Mendelian dihybrid ratio of F2 generation is 3:1.

Ans: False

Correct statement: A typical Mendelian dihybrid ratio of F2 generation is 9:3:3:1.

2) A recessive factor is altered by the presence of a dominant factor.

Ans:True.

3) Each gamete has only one allele of a gene.

Ans:True.

4) Hybrid is an offspring from a cross between genetically different parents.

Ans:True.

5) Some of the chromosomes have an elongated knob-like appendages known as telomere.

Ans:False

Correct statement: Some of the chromosomes have an elongated knob like appendage known as the satellite.

6) New nucleotides are added and new complementary strand of DNA is formed with the help of enzyme DNA polymerase.

Ans: True

7) Down’s syndrome is the genetic condition with 45 chromosomes.

Ans: False

Correct statement: Down’s syndrome is the genetic condition with 21 chromosomes.

IV.) Match the following

Ans:

Autosomes: 22 pairs of chromosomes
Diploid condition: 2n
Allosome: 23^rd pair of chromosome
Down’s syndrome: Trisomy 21
Dihybrid ratio: 9: 3: 3: 1

V.) Answer in a sentence

1.) What is a cross in which inheritance of two pairs of contrasting characters are studied?

Ans: Dihybrid cross involves the inheritance of two pairs of contrasting characteristics at the same time.

2) Name the conditions when both the alleles are identical?

Ans: The condition when both the alleles are same is called as homozygous.

3) A garden pea plant produces axial white flowers. Another of the same species produced terminal violet flowers. Identify the dominant trait?

Ans: As a garden pea plant produces axial white flowers and another plant of same species produces terminal violet flowers.

Then according to Mendel,

Axial position: Dominant character
White flowers: Recessive character
Terminal position: Recessive character
Violet flowers: Dominant character

4) What is the name given to the segments of DNA, which are responsible for the inheritance of a particular character?

Ans: The segments of DNA which are responsible for the inheritance of a particular phenotype character are called as genes.

5) Name the bond which binds the nucleotides in a DNA.

Ans: The hydrogen bonding binds the nucleotides in DNA.

VI.) Short answers questions

1.) Why did Mendel select pea plant for his experiments?

Ans: Mendel selected pea plant for his experiment because of following reasons.

In pea plants self-pollination occurs naturally and it is easier to grow pure breeding individuals of pea plant.
Pea plant has short life span as it is annual and hence we can observe several generations in short time.
Cross pollination will be easily occurred in pea plants.
Pea plant flowers are bisexual and it has deeply defined contrasting characters.
Above all the reasons were needed for Mendel’s experiment and hence he selected pea plant for it.

2) What do you understand by the term phenotype and genotype?

Ans:

The external expression of a trait is known as phenotype.
Genetic expression of an organism is called as genotype.

3) What are allosomes?

Ans: In human each cell is normally contains 23 pairs of chromosomes, out of which 22 pairs are autosomes and 23rd pair is called allosome or sex chromosome.

4) What are Okazaki fragments?

Ans:

Nucleotides are added by using an enzyme DNA polymerase after formation of RNA primer, and new complementary strand of DNA is formed from the parent strand.
One strand is called leading strand on which daughter strand is synthesized continuously.
While on the other strand short segments of DNA are synthesized and it is called as lagging strand. The short segments of DNA are called as Okazaki fragments.

5) Why is euploidy considered to be advantageous to both plants and animals?

Ans:

Euploidy is the condition in which the individual has more than the usual number of diploid chromosome. And triploid plants and animals are typically sterile and if having four haploid sets of chromosomes then it is called as tetraploidy. Tetraploidy plants are advantageous because they result in increased fruit and flower size.

6) A pure tall plant (TT) is crossed with pure dwarf plant (tt), what would be the F1 and F2 generations? Explain.

Ans:

When a pure tall plant TT is crossed with a pure dwarf tt plant, then F1 generation contains all the plants which are tall.
For F2 generation, selfing of the F1 monohybrids resulted in tall and dwarf plant respectively in the ratio 3:1.
F2 generation has phenotypic ratio as 3:1 and genotypic ratio as 1:2:1.

7) Explain the structure of a chromosome.

Ans:

• The following figure shows the structure of chromosome.

• The chromosomes are long, thin and thread like structures having two identical strands called as chromatids which are joined together by centromere. Each chromatid is formed from the spiral coiled thin structure called as chromonema which is having large number of bead like structures along its length called as chromomeres.

• The chromosomes are constructed from DNA, RNA, chromosomal proteins and some metallic ions. Structural support is provided by proteins in chromosomes. The chromosome has following regions which are explained below

Fig. Structure of chromosome

1.) Primary constriction:

The point at which two arms of chromosome meet is called as primary constriction or centromere. During cell division, at the centromere the spindle fibers attaches to the chromosomes.

2.) Secondary constriction:

At any point of chromosome, some chromosomes has secondary constriction and they are called as nucleolar organizer.

3.) Telomere:

Telomere is the end of the chromosome. It maintains and provides stability to the chromosome.

4.) Satellite:

At one end of the chromosomes, some elongated knob like appendage is present called as satellite. The chromosomes having satellite are called as sat chromosomes.

8) Label the parts of the DNA in the diagram given below. Explain the structure briefly.

Ans:

DNA is a polynucleotide which is a large molecule having millions of nucleotides. And each nucleotide has three components

1.) A sugar molecules: It is a deoxyribose sugar.

2.) A nitrogen base: DNA has two types of nitrogen bases a) Purines which includes Adenine and Guanine.

b) Pyrimidines which includes Cytosine and Thymine.

3.) A phosphate group

A: Adenine, T: Thymine, G: Guanine, C: Cytosine

Fig. Nucleotides in DNA

According to Watson and Crick model of DNA,

DNA molecule is having polynucleotide chains.
The double helix structure with the two strands running antiparallel to each other is formed by this chains.
The centrally situated nitrogenous base are linked to sugar phosphate units forming the backbone of DNA.
Adenine A is linked to Thymine T with two hydrogen bonds i.e. A=T.
Cytosine C is linked to Guanine G by three hydrogen bonds i.e. C≡G.
This pairing is named as complementary base pairing. Nitrogenous bases having hydrogen bonds makes the DNA stable.
Each turn of double helix is having 34A° with 10 base pairs in a complete turn.
The nucleotides in a helix are linked together by the phosphodiester bonds.

VII. Long answer questions

1) Explain with an example the inheritance of dihybrid cross. How is it different from monohybrid cross?

Phenotypic ratio of F2 generation- 9: 3: 3: 1

Round yellow-9, wrinkled yellow-9

Round green-3, wrinkled green-1

Fig. Dihybrid cross

Dihybrid cross involves the inheritance of two pairs of contrasting characteristics at the same time. The pairs of contrasting characteristics chosen are shape and colour of seeds: round yellow seeds and green wrinkled seeds.
Mendel first crossed pure breeding pea plant having yellow round seeds with the pure breeding pea plant having green wrinkled seeds and observed that the first generation F1 produced will have only round yellow seeds and no other seeds are observed in F1 generation. And from this observation Mendel concludes that, yellow colour and round shape is dominant over the green colour and wrinkled shape.
When round yellow seeds of hybrid F1 generation of pea plants were cross bred by self-pollination, then four types of seeds having different types of shape and colour are obtained in next generation F2.
They were round yellow, round green, wrinkled yellow and wrinkled green seeds.
For the F2 generation the phenotypic ratio was observed is 9:3:3:1 and it is called as dihybrid ratio.

Monohybrid cross:

The genetic cross which involves the single pair of genes which is responsible for one trait is called as monohybrid cross.
In this cross, parents may differ by a single trait.
The monohybrid ratio of F2 generation is 3:1.

Dihybrid cross:

The genetic cross which involves the two pairs of genes which is responsible for two traits is called as dihybrid cross.
In this cross, parents may have two different independent traits.
The dihybrid ratio of F2 generation is 9:3:3:1.

2) How is the structure of DNA organized? What is the biological significance of DNA?

Ans:

DNA is a polynucleotide which is a large molecule having millions of nucleotides. And each nucleotide has three components

1.) A sugar molecules: It is a deoxyribose sugar.

2.) A nitrogen base: DNA has two types of nitrogen bases a) Purines which includes Adenine and Guanine.

b) Pyrimidines which includes Cytosine and Thymine.

3.) A phosphate group

According to Watson and Crick model of DNA,

DNA molecule is having polynucleotide chains.
The double helix structure with the two strands running antiparallel to each other is formed by this chains.
The centrally situated nitrogenous base are linked to sugar phosphate units forming the backbone of DNA.
Adenine A is linked to Thymine T with two hydrogen bonds i.e. A=T.
Cytosine C is linked to Guanine G by three hydrogen bonds i.e. C=G.
This pairing is named as complementary base pairing. Nitrogenous bases having hydrogen bonds makes the DNA stable.
Each turn of double helix is having 34A° with 10 base pairs in a complete turn.
The nucleotides in a helix are linked together by the phosphodiester bonds.

A: Adenine T: Thymine G: Guanine C: Cytosine

Fig. Nucleotides in DNA

The biological significance of DNA is as follows.

DNA is responsible for transmitting hereditary information from one generation to the next generation.
The information required for formation of proteins is contained in DNA.
It helps in controlling the developmental processes and life activities of an organism.

VIII.) Higher Order Thinking Skills (HOTS)

1) Flowers of the garden pea are bisexual and self-pollinated. Therefore, it is difficult to perform hybridization experiment by crossing a particular pistil with the specific pollen grains. How Mendel made it possible in his monohybrid and dihybrid crosses?

Ans:

The garden pea plant is self-pollinating plant which is made to prevent self-pollination by collecting anthers from male parent and dusted on female parent by bagging the stigma.

2) Pure-bred tall pea plants are first crossed with pure-bred dwarf pea plants. The pea plants obtained in F1 generation are then selfed to produce F2 generation of pea plants.

Ans:

a) What do the plants of F1 generation look like?

When pure bred tall pea plants are crossed with pure bred dwarf pea plants, the F₁ generation look like tall because tall pea plants are dominant over dwarf pea plants.

b) What is the ratio of tall plants to dwarf plants in F2 generation?

The ratio of tall plants to dwarf plants in in F2 generation is 1:2:1.

c.) Which type of plants were missing in F1 generation but reappeared in F2 generation?

The dwarf plants are missing in F1 generation but are reappeared in F2 generation.

IX.) Value based question

1) Under which conditions does the law of independent assortment hold good and why?

Ans:

Mendel gives the law of independent assortment on the basis of his dihybrid cross experiment. According to this law, in case of inheritance of two or more pairs of characters simultaneously, the genes of one pair assort out independently of the other pair.
The total number of individuals produced in F2 generation are 16 and which occurs in the dihybrid ratio 9:3:3:1and two parental classes with two new combinations will be produced.