Chapter 9: Heredity

Class X Science

Introduction to Genetics

Genetics deals with the study of Heredity and Variation.

• Heredity: The transmission of characters/traits from one generation to the next generation.
• Variation: The differences in the characters/traits between the parent and offspring.

Types of Variation

Somatic Variation	Gametic Variation
Takes place in the body cells.	Takes place in the gametes/reproductive cells.
Neither inherited nor transmitted.	Inherited as well as transmitted.
Also known as acquired traits.	Also known as inherited traits.
Example: Boring of pinna, cutting of tails in dogs.	Example: Human height, skin colour.

Accumulation of Variation during Reproduction

• Asexual Reproduction: Variations are fewer. Occurs due to small inaccuracies in DNA copying (Mutation).
• Sexual Reproduction: Variations are large. Occurs due to crossing over during separation of chromosomes and mutation.

Importance of Variations

1. Depending upon the nature of variation, different individuals would have different kinds of advantages. Example: Bacteria that can withstand heat will survive better in a heat wave.
2. The main advantage of variation to species is that it increases the chances of its survival in a changing environment.

Key Genetic Terms

• Traits: Characteristic features of an organism, manifested in a physical form that is visible or in a physiological aspect of the organism. Sexually reproducing individuals have two copies of genes for the same trait.
• Dominant Traits: The traits that express themselves in an organism in every possible combination and can be seen.
• Recessive Traits: A trait which is not expressed in the presence of a dominant allele is known as recessive.
• Gene: It is the basic unit of inheritance. It consists of a sequence of DNA, which is the genetic material. Genes can mutate and can take two or more alternative forms.
• Alleles: The alternative forms of genes. They affect the same characteristics or traits in alternate forms. They are located on the same place of the chromosome.
• Homozygous: Each organism has two alleles for every trait. In homozygous, both the alleles are same. Example: \( TT \) is the homozygous expression for tallness trait.
• Heterozygous: If the two alleles are different from each other, then they are heterozygous in nature. Example: \( Tt \) is the heterozygous expression for tallness trait.
• Chromosomes: Thread-like structures made up of nucleic acids (DNA) and proteins found in the nucleus. They carry the hereditary information.
• Genotype: It is the complete heritable genetic identity of an organism. It is the set of alleles that are carried by the organism.
• Phenotype: It is the description of the actual physical characteristics of an organism or the expressed form of the genotype.

Mendel and His Work on Inheritance

Gregor Johann Mendel (1822–1884): Known as the Father of Genetics. He proposed the laws of inheritance in living organisms.

Plant Selected: Pisum sativum (garden pea). Mendel used a number of contrasting characters for the garden pea.

Dihybrid Cross

A cross between two plants having two pairs of contrasting characters is called a dihybrid cross.

Example: Cross between Round Green seeds (\( RRyy \)) and Wrinkled Yellow seeds (\( rrYY \)).

• F1 Generation: Produces plants with Round & Yellow seeds (\( RrYy \)).
• F2 Phenotypic Ratio:
  Round Yellow : 9
  Round Green : 3
  Wrinkled Yellow : 3
  Wrinkled Green : 1

[DIAGRAM: Punnett Square representing the Dihybrid Cross showing the 9:3:3:1 ratio]

Sex Determination

Determination of sex of an offspring involves different factors:

• Environmental: In some animals, the temperature at which fertilized eggs are kept decides the gender. Example: Turtle.
• Genetic: In some animals like humans, the gender of the individual is determined by the pair of sex chromosomes.
  • \( XX \) – Female
  • \( XY \) – Male

Sex Determination in Human Beings

In human beings, there are 23 pairs of chromosomes. 22 pairs are called autosomes, and the last pair is the sex chromosome.

• Father: Has \( XY \) chromosomes. Produces 50% sperms with \( X \) and 50% with \( Y \).
• Mother: Has \( XX \) chromosomes. Produces all eggs with \( X \).
• If a sperm with \( X \) fertilizes the egg, the child is Female (\( XX \)).
• If a sperm with \( Y \) fertilizes the egg, the child is Male (\( XY \)).
• Therefore, the sex of the children is determined by what they inherit from their father.

[DIAGRAM: Flow chart showing Sex Determination in Humans: Parents (XY, XX) -> Gametes -> Offspring (Girl XX, Boy XY)]

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Question Bank

Multiple Choice Questions

1. The process where characteristics are transmitted from parent to offspring is called:
   Ans: (b) Heredity
2. Exchange of genetic material takes place in:
   Ans: (c) Sexual reproduction
3. The following results were obtained by a scientist who crossed the F1 generation of pure-breeding parents for round and wrinkled seeds. Dominant trait (Round): 7524. Recessive trait (Wrinkled): 2508. The actual number of round seeds obtained was?
   Ans: (d) 5643 (Note: Source answer key indicates (d))
4. In peas, a pure tall plant (TT) is crossed with a pure short plant (tt). The ratio of pure tall plants to pure short plants in F2 generation will be:
   Ans: (c) 1 : 1 (Ratio of TT : tt in F2 is 1:1)
5. What will be the number of chromosomes present in each gamete produced by the plants if the palisade cells of a species of plant contain 28 chromosomes in all?
   Ans: (c) 14

Assertion-Reason Questions

Options: (a) Both A & R true, R is correct explanation. (b) Both A & R true, R is not correct explanation. (c) A true, R false. (d) A false, R true.

1. Assertion: Variations are seen in offspring produced by sexual reproduction.
   Reason: DNA molecule generated by replication is not exactly identical to original DNA.
   Ans: (a)
2. Assertion: When pea plants (pure line) having round yellow seeds are crossed with pure line plants having wrinkled green seeds, then all pea plants obtained in F1 generation bear wrinkled green seeds.
   Reason: Round and yellow seeds are dominant to wrinkled and green seeds.
   Ans: (d) Assertion is false (F1 will be Round Yellow), Reason is true.
3. Assertion: Selfing of a plant for several generations helps plant breeders to obtain pure breeding varieties.
   Reason: Pure breeding plants are heterozygous for many traits.
   Ans: (c) Assertion is true, Reason is false (Pure breeding plants are homozygous).
4. Assertion: The sex of a child is determined by the mother.
   Reason: Humans have two types of sex chromosomes: XX and XY.
   Ans: (d) Assertion is false (determined by father), Reason is true.
5. Assertion: Mendel chose a number of varieties of garden pea as plant material for his experiments.
   Reason: Garden pea has well defined characters and is bisexual.
   Ans: (a)

Case Study Questions

Context: Pea plants can have smooth seeds or wrinkled seeds. One of the phenotypes is completely dominant over the other. A farmer pollinates one flower of a plant with smooth seeds using pollen from a plant with wrinkled seeds. The resulting pea pod has all smooth seeds.

1. Which conclusion can be drawn? (1) Smooth is dominant. (2) Smooth parent is heterozygous. (3) Wrinkled parent is homozygous.
   Ans: (c) 1 and 3 only.
2. Which cross will give smooth and wrinkled seeds in same proportion (1:1)?
   Ans: (b) \( Rr \times rr \) (Test cross).
3. Which cross determines genotype of a plant with dominant phenotype?
   Ans: (d) \( RR/Rr \times rr \) (Test cross).
4. On crossing two heterozygous smooth plants (Rr), total 1000 F1 plants obtained. Number of smooth vs wrinkled?
   Ans: (a) 750 : 250 (3:1 ratio).

Short Answer Questions

Q1. How do Mendel’s experiments show that traits may be dominant or recessive?
Ans: Mendel crossed true breeding tall (TT) and dwarf (tt) pea plants. All F1 hybrids were tall. The trait expressing itself in the hybrid (Tall) is the dominant one, while the one inhibited (Dwarf) is recessive. This confirmed that traits can be dominant or recessive.

Q2. How do Mendel’s experiments show that traits are inherited independently?
Ans: In a dihybrid cross (Tall Round x Short Wrinkled), the F1 generation was all Tall and Round. In F2, new combinations appeared: Tall Wrinkled and Short Round, along with the parental types. This showed that the trait for height and the trait for seed shape were inherited independently (Law of Independent Assortment).

Q4. "The chromosome number of the sexually reproducing parents and their offspring is same." Justify.
Ans: Gametes (reproductive cells) produced by parents contain half the number of chromosomes (haploid) compared to body cells. During fertilization, the fusion of male and female gametes restores the original (diploid) number of chromosomes in the zygote.

Q6. Green stemmed rose (GG) x Brown stemmed rose (gg).
Ans:
(i) F1 Colour: All Green.
(ii) Percentage of brown stems in F2 (after selfing F1): 25%.
(iii) Ratio of GG : Gg in F2: 1:2.
Conclusion: Green is dominant, brown is recessive.

Long Answer Questions

Q1. (a) Why did Mendel choose garden pea? (b) Strategies for sex determination.
Ans:
(a) Reasons: 1. Easy to grow and short life cycle. 2. Large number of contrasting characters available. 3. Self and cross-pollination possible.
(b) Strategies:
1. Environmental Cue: In turtles, incubation temperature determines sex.
2. Genetic Cue: In humans, sex chromosomes (XX/XY) determine sex.

Q2. Cross pure tall (TT) with pure dwarf (tt). Self cross F1.
Ans:
(a) F2 plants: Mixture of Tall and Dwarf.
(b) Ratio: 3 Tall : 1 Dwarf.
(c) Dwarf plants appeared in F2 but not F1 because the recessive trait (dwarfness) was suppressed by the dominant trait in the F1 hybrid, but reappeared when homozygous recessive (tt) in F2.