10th Science Guide Unit 18 Genetics

10th Standard Science Biology Lesson 18 Genetics Book Back Answers | Samacheer Kalvi Guide

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10thh Science Guide Unit 18 Genetics

I. Choose the best answer

1. According to Mendel alleles have the following character

1. Pair of genes
2. Responsible for character
3. Production of gametes
4. Recessive factors

Ans ; Responsible for character

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

1. Segregation
2. Crossing over
3. Independent assortment
4. Recessiveness

Ans ; Independent assortment

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

1. Chromomere
2. Centrosome
3. Centromere
4. Chromonema

Ans ; Centromere

4. The centromere is found at the centre of the __________ chromosome.

1. Telocentric
2. Metacentric
3. Sub-metacentric
4. Acrocentric

Ans ; Metacentric

5. The __________ units form the backbone of the DNA.

1. 5 carbon sugar
2. Phosphate
3. Nitrogenous bases
4. Sugar phosphate

Ans ; Sugar phosphate

6. Okasaki fragments are joined together by __________.

1. Helicase
2. DNA polymerase
3. RNA primer
4. DNA ligase

Ans ; DNA ligase

7. The number of chromosomes found in human beings are __________.

1. 22 pairs of autosomes and 1 pair of allosomes
2. 22 autosomes and 1 allosome
3. 46 autosomes
4. 46 pairs autosomes and 1 pair of allosomes

Ans ; 22 pairs of autosomes and 1 pair of allosomes.

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

1. Tetraploidy
2. neuploidy
3. Euploidy
4. polyploidy

Ans ; Aneuploidy

II. Fill in the blanks

1. The pairs of contrasting character (traits) of Mendel are called __________. Ans ; Allele
2. Physical expression of a gene is called __________ Ans ; Phenotype
3. The thin thread like structures found in the nucleus of each cell are called __________. Ans ; Chromosomes

4. DNA consists of two __________ chains.Ans ; Polynucleotide
5. An inheritable change in the amount or the structure of a gene or a chromosome is called __________.Ans ; Mutation

III. State whether the following statements are True or False. Correct the false statement

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

• 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. ( False )

• A recessive factor is masked by the presence of a dominant factor.

3. Each gamete has only one allele of a gene. ( True )
4. Hybrid is an offspring from a cross between genetically different parent. ( True )
5. Some of the chromosomes have an elongated knob-like appendages known as telomere. ( False )

• Some of the chromosomes have an elongated knob-like appendages known as Satellite .

6. New nucleotides are added and new complementary strand of DNA is formed with the help of enzyme DNA polymerase. ( True )
7. Down’s syndrome is the genetic condition with 45 chromosomes. ( False )

• Down’s syndrome is the genetic condition with 47 chromosomes.

IV. Match the following

1. Autosomes –  Trisomy 21
2. Diploid condition – 9:3:3:1
3. Allosome – 22 pair of chromosome
4. Down’s syndrome – 2n
5. Dihybrid ratio – 23rd pair of chromosomet.

Ans ; 1 – C, 2 – D, 3 – E, 4 – A, 5 – B

V. Answer in a sentence

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

• A cross in which inheritance of two pairs of contrasting characters are studied is called Dihybrid cross.

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

• The conditions when both the alleles are identical (TT or tt) is known as Homozygous.

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

• The dominant trait is axial white flower.

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

• The segments of DNA, which are responsible for the inheritance of a particular character is gene.

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

• Hydrogen bond binds the nucleotides in a DNA.

VII. Short answer question

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

• It is naturally self–pollinating and is very easy to raise pure breeding individuals.
• It has a short life span so it is an annual and so it was possible to follow several generations.
• It is easy to cross–pollinate.
• It has deeply defined contrasting characters.
• The flowers are bisexual.

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

• Phenotype : External expression of of a particular trait.
• Genotype : Genetic expression of an organism.

3. What are allosomes?

Allosomes are chromosomes which are responsible for determining the sex of an individual. They are also called as sex chromosomes or hetero-chromosomes.

There are two types of sex chromosomes, X and Y- chromosomes.

• A male has XY chromosomes
• A female has XX Chromosomes

4. What are Okazaki fragments?

• Okazaki fragments are short sequences of DNA nucleotides which are synthesized discontinuously and later linked together by enzyme DNA ligase to create the lagging strand during DNA replication.

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

Organisms with multiples of the basic chromosome set are called euploid.

• Plants with euploidy condition have increased fruit and flower size.
• Plants and animals with euploidy condition are typically sterile.

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

Explain.

Mendel selected tall (TT) and dwarf (tt) garden pea plants, Pisum sativum, for the Monohybrid cross.

When a pure breeding tall plant (TT) was crossed with a pure breeding dwarf plant(tt), all plants were tall in the first filial generation (F1)

When such an F1 tall plant (Tt) was allowed to self-pollination, both the tall and dwarf plants appeared in second filial generation (F2) in the ratio of 3:1.

7. Explain the structure of a chromosome.

• The chromosomes are thin, long and thread like structures consisting of two identical strands called sister chromatids.
• They are held together by the centromere.
• Each chromatid is made up of spirally coiled thin structure called chromonema.
• The chromonema has number of bead-like structures along its length which are called chromomeres.
• The chromosomes are made up of DNA, RNA, chromosomal proteins (Histones and non–histones) and certain metallic ions.
• These proteins provide structural support to the chromosome.
• Some of the chromosomes have an elongated knob-like appendage at one end of the chromosome known as satellite.
• The chromosomes with satellites are called as the sat-chromosomes.

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

DNA is a large molecule consisting of millions of nucleotides. Each nucleotide consists of three components.

1. i) A sugar molecules – Deoxyribose sugar.
2. ii) A nitrogenous base. There are two types of nitrogenous bases in DNA. They are;

• Purines (Adenine and Guanine).
• Pyrimidines (Cytosine and Thymine).

iii) A phosphate group.

10th Science Guide Unit 18 Genetics

VII. Long answer question

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

• Dihybrid cross involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time.
• Mendel first crossed pure breeding pea plants having round-yellow (RRYY) seeds with pure breeding pea plants having wrinkled-green (rryy) seeds and found that only round-yellow (RrYy) seeds were produced in the first generation (F1).
• When the hybrids of F1 generation pea plants having round-yellow (RrYy) seeds were cross-bred by self pollination, then four types of seeds having different combinations of shape and color were obtained in second generation or F2 generation. They were round yellow, round-green, wrinkled yellow and wrinkled-green seeds.
• The ratio of each phenotype (or appearance) of seeds in the F2 generation is 9:3:3:1. This is known as the Dihybrid ratio.

o

Differences between Monohybrid and Dihybrid cross.

Monohybrid cross	Dihybrid cross
1 The inheritance of one pair of contrasting characteristics	The inheritance of two pairs of contrasting characteristics
2 The phenotypic ratio is 3:1	The phenotypic ratio is 9:3:3:1

2. How is the structure of DNA organised? What is the biological significance of DNA?

The structure of DNA

• DNA molecule consists of two polynucleotide chains.
• These chains form a double helix structure with two strands which run anti-parallel to one another.
• Nitrogenous bases in the centre is linked by sugar-phosphate units which form the backbone of the DNA.
• Pairing between the nitrogenous bases is very specific and is always between purine and pyrimidine linked by hydrogen bonds.

1. Adenine (A) links Thymine (T) with two hydrogen bonds (A = T).
2. Cytosine (C) links Guanine (G) with three hydrogen bonds( C = G).

This is called complementary base pairing.

• Hydrogen bonds between the nitrogenous bases make the DNA molecule stable.
• Each turn of the double helix is 34 A° (3.4 nm). There are ten base pairs in a complete turn.
• The nucleotides in a helix are joined together by phosphodiester bonds

The biological significance of DNA

• It is responsible for the transmission of hereditary information from one generation to next generation.
• It contains information required for the formation of proteins.
• It controls the developmental process and life activities of an organism.

3. The sex of the new born child is a matter of chance and neither of the parents may be considered responsible for it. What would be the possible fusion of gametes to determine the sex of the child?

The sex of the new born child is a chance of probability as to which category of sperm fuses with the egg.

If the egg (X) is fused by the X-bearing sperm an XX individual (female) is produced.

Egg (22+X) + Sperm (22+X) = Female child (44+XX)

If the egg (X) is fused by the Y-bearing sperm an XY individual (male) is produced.

Egg (22+X) + Sperm (22+Y) = Male child (44+XY)

Thus the sperm, produced by the father, determines the sex of the child.

The mother is not responsible in determining the sex of the child.

10th Science Guide Unit 18 Genetics

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?

In pea plants, cross pollination can be easily achieved by emasculation in which the stamen of the flower is removed without affecting the pistil. The emasculated flower is immediately enclosed in a bag to prevent pollination by unwanted pollen. Then, the specific , mature and viable pollen grains are collected from the male parent, the bag is opened and the pollen grains are dusted on 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 cross-bred to produce F2 generation of pea plants.

1. What do the plants of F1 generation look like?

All the plants of F1 generation are tall ( Tt ).

1. What is the ratio of tall plants to dwarf plants in F2 generation?

The ratio of tall plants to dwarf plants in F2 generation is 3:1.

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

The trait dwarf is missing in F1 generation but reappeared in F2 generation.

3. Kavitha gave birth to a female baby. Her family members say that she can give birth to only female babies because of her family history. Is the statement given by her family members true. Justify your answer.

The statement given by her family members is not true. Because, the sex of the new born child is a chance of probability as to which category of sperm fuses with the egg.

 

If the egg (X) is fused by the X-bearing sperm an XX individual (female) is produced.

Egg (22+X) + Sperm (22+X) = Female child (44+XX)

If the egg (X) is fused by the Y-bearing sperm an XY individual (male) is produced.

Egg (22+X) + Sperm (22+Y) = Male child (44+XY)

Thus the sperm, produced by the father, determines the sex of the child. The mother or her family history is not responsible in determining the sex of the child.

VIII. Higher Order Thinking Skills (HOTS)

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

• The factors for each character or trait remain independent and maintain their identity in the gametes.
• The factors are independent to each other and pass to the offspring (through gametes).
• If the law of independent assortment did not happen, all the genes have been locked with each other and not a single gene can be able to express independently.
• Independent assortment of genes is important to produce new genetic combinations that increase genetic variations within a population.