18. Heredity

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.

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

(a) Segregation
(b) Crossing over
(c) Independent assortment
(d) Recessiveness

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

(a) Chromomere
(b) Centrosome
(c) Centromere
(d) Chromonema.

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

(a) Telocentric
(b) Metacentric
(c) Sub-metacentric
(d) Acrocentric

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

(a) 5 carbon sugar
(b) Phosphate
(c) Nitrogenous bases
(d) Sugar phosphate.

6. Okasaki fragments are joined together by:

(a) Helicase
(b) DNA polymerase
(c) RNA primer
(d) 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 of autosomes and 1 pair of allosomes.

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

(a) Tetraploidy
(b) Aneuploidy
(c) Euploidy
(d) polyploidy

II. Fill in the blanks.

1. The pairs of contrasting character (traits) of Mendel are called Alleles or allelomorphs.

2. The physical expression of a gene is called Phenotype.

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

4. DNA consists of two Polynucleotide chains.

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

III. Identify whether the statement is True or False. Correct the False Statement.

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

3. Each gamete has only one allele of a gene.
Answer:           True.

4. hybrid is an offspring from a cross between genetically different parent.
Answer:           True.

5. Some of the chromosomes have an elongated knob-like appendage known as a telomere.
Answer:           False.
Correct statement: Some of the chromosomes have an elongated knob – like appendage known as the satellite.

6. New nucleotides are added and a new complementary strand of DNA is formed with the help of enzyme DNA polymerase.
Answer:           True.

7. Down’s syndrome is a genetic condition with 45 chromosomes.
Answer:           False.
Correct statement: Down’s syndrome is the genetic condition with 21 chromosomes.

IV. Match the following.

Question 1.

Column A

Column B

1. Autosomes

(c) 22 pair of chromosome

2. Diploid condition

(d) 2n

3. Allosome

(e) 23rd pair of chromosome

4. Down’s syndrome

(a) Trisomy 21

5. Dihybrid ratio

(b) 9 : 3 : 3 : 1

V. Answer in a Sentence.

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

Dihybrid cross.

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

Homozygous

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

·         Axial position – a dominant trait

·         White flowers – a recessive trait

·         Terminal position – a recessive trait

·         Violet flower – dominant trait.

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

Genes

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

The hydrogen bonds bind the nucleotides in a DNA.

VI. Short Answer Questions.

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

·         The pea plant is self-pollinating and so it is very easy to raise pure breeding individuals.

·         It has a short life span, as it is an annual.

·         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 is the outward appearance or morphological character of an organism. The expression of gene or the genetic make up of an individual for a particular trait is called genotype.

3. What are allosomes?

Out of 23 pairs of chromosomes, 22 pairs are autosomes and the 23rd pair is the allosome or sex chromosome.

4. What are Okazaki fragments?

For the synthesis of new DNA, two things are required one is RNA primer and the enzyme primase. The DNA polymerase moves along the newly formed RNA primer nucleotides, which leads to the elongation of DNA. In the other strand, DNA is synthesis in small fragments called okazaki fragments. These fragments are linked by the enzyme called ligase.

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

In euploidy condition, the individual bears more than the usual number of diploid chromosomes. The triploid plants and animals are sterile. The tetraploid plants, often 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.

When a pure tall plant (TT) is crossed with pure dwarf plant (tt), In F1 generation all the plants will be tall. In F2 generation by selling the F1 monohybrid the tall and dwarf plant will be in the ratio of 3 : 1. In F2 generation phenotypic ratio will be 3 : 1. genotypic ratio will be 1 : 2 : 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 a centromere. Each chromatid is made up of a spirally coiled thin structure called chromonema. The bead-like structures along the length are called chromomeres. The chromosomes are made up of DNA, RNA, chromosomal proteins (histones and non-histones) and certain metallic ions. These proteins give structural support to the chromosome.

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

DNA is the hereditary material as it contains the genetic information. DNA is a large molecule, consisting of millions of nucleotides. Each nucleotide consists of three compounds.




(a) A sugar molecule – Deoxy Ribose sugar

(b) A nitrogenous base [Purines and Pyrimidines]

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

(c) A phosphate group Nucleoside and Nucleotide:
Nucleoside = Nitrogen base + sugar
Nucleotide = Nucleoside + Phosphate
The nucleotides are formed according to the purines and pyrimidines present in them.

VII. Long Answer Questions.

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

The dihybrid cross involves the inheritance of two pairs of contrast characteristics, round – yellow seeds and wrinkled – green seeds. When pea plants having round – yellow seeds cross – bred with pea plants having wrinkled – green seeds, in the first generation (F1), only round yellow seeds were produced.
No wrinkled – green seeds were obtained. Round yellow colour seeds were dominant and wrinkled-green seeds were recessive.

When round – yellow seeds were cross-bred by self-pollination, four types of seeds having different combinations of shape and colour were obtained in the F2 generation. They were round- yellow, round-green, wrinkled-yellow and wrinkled – green seeds.



A dihybrid cross produced four types of F2 offsprings in the ratio of 9 with two dominant traits, 3 with one dominant trait and one recessive trait, 3 with another dominant trait and another recessive trait and one with two recessive traits. The new combinations of traits with round green and wrinkled yellow had appeared in the dihybrid cross (F2 generation). The ratio of each phenotype of seeds in the F2 generation is 9 : 3 : 3 : 1. This is known as the Dihybrid ratio.

Difference between a monohybrid cross and dihybrid cross:
Monohybrid cross:
Monohybrid cross is a genetic cross, that involves a single pair of genes, which is responsible for one trait.
Parents differ by a single trait.
Monohybrid ratio in F2 generation is 3 : 1.

Dihybrid cross:
Dihybrid cross is a genetic cross, that involves two pairs of genes, which are responsible for two traits,
The parents have two different independent traits.
The dihybrid ratio in the F2 generation is 9 : 3 : 3 : 1.

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

DNA is the hereditary material, as it contains the genetic information. It is a large molecule consisting of millions of nucleotides, so it is called a polynucleotide. Each nucleotide consists of three components.
(a) A sugar molecule – Deoxyribose sugar

(b) A nitrogenous base – There are two types of the nitrogenous base in DNA they are

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

(c) A phosphate group – The polynucleotide chains from a double helix. Nitrogenous bases in the centre are linked to 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.

Adenine (A) links Thymine (T) with two hydrogen bonds [A=T]. Cytosine (C) links Guanine (G) with three hydrogen bonds (C = G). The hydrogen bonds between the nitrogenous bases make the DNA molecule stable. The nucleotides in a helix are joined together by phosphodiester bonds.



The biological significance of DNA:

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

3. The sex of the newborn 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?

Out of 23 pairs of chromosomes, 22 pairs are autosomes and one pair (23rd pair) is the sex chromosome. The female gametes or the eggs formed are similar in their chromosome type [22 + XX], So human females are homogametic. The male gametes or sperms produced are of two types. They are produced in equal proportions. The sperm bearing [22 + X] chromosomes and the sperm bearing (22 + Y) chromosomes. So human males are called heterogametic.

It is a chance, 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. If the egg [X] is fused by the Y – bearing sperm an [XY] individual (male) is produced. The sperm produced by the father determines the sex of the child. The mother is not responsible for determining the sex of the child.


VIII. Higher Order Thinking Skills (HOTS) Questions

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?

As the garden pea is self-pollinating plant the parent plant were emasculated to prevent self pollination.
The anthers were collected from male parent and dusted on the female parent and the stigma was bagged.

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

  1. What do the plants of the F1 generation look like?
  2. What is the ratio of tall plants to dwarf plants in the F2 generation?
  3. Which type of plants was missing in F1, generation but reappeared in the F2 generation?

Answer:

  1. Tall
  2. 1 : 2 : 1
  3. Dwarf plants.

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.

No, the statement is not true.
Sex determination is a chance of probability as to which category of sperm fuses with the eggs. If the egg(x) is fused by the x-bearing sperm, then
individual is female. If the egg (x) is fused by the y-bearing sperm then the individual is male. The sperm produced by the father only determines the sex of the child.

IX. Value-Based Questions

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

Mendel gave this law based on his dihybrid cross experiment. Here the total number of individuals is F2 will be sixteen which occur in a ratio of 9 : 3 : 3 : 1 where two parental classes and two new combination will be produced.