Chapter 8: Heredity
Class X Science
Key Concepts: Heredity and Variation
Genetics deals with the study of Heredity and Variation.
| Heredity | Variation |
|---|---|
| The transmission of characters/traits from one generation to the next generation. | 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
- 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.
- 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 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 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 (\( RRyy \)) and Wrinkled Yellow (\( rrYY \)) seeds.
- 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 showing Dihybrid Cross results with 9:3:3:1 ratio]
Sex Determination
Determination of sex of an offspring.
Factors Responsible:
- Environmental: In some animals, the temperature at which fertilized eggs are kept decides the gender (e.g., Turtle).
- Genetic: In humans, gender 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 autosomes, and the last pair is the sex chromosome.
- Fathers have \( XY \) chromosomes. Mothers have \( XX \) chromosomes.
- Gametes (Sperms) produced by father: 50% have \( X \) and 50% have \( Y \).
- Gametes (Eggs) produced by mother: All have \( X \).
- If a sperm with \( X \) fertilizes the egg (\( X \)), the child is Female (\( XX \)).
- If a sperm with \( Y \) fertilizes the egg (\( X \)), the child is Male (\( XY \)).
- Thus, the sex of children is determined by what they inherit from their father.
[DIAGRAM: Flow chart of Sex Determination showing Father (XY) and Mother (XX) crossing to produce XX (Girl) and XY (Boy) offspring with 50% probability]
Question Bank
Multiple Choice Questions
- The process where characteristics are transmitted from parent to offspring is called:
Ans: (b) Heredity - Exchange of genetic material takes place in:
Ans: (c) Sexual reproduction - 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: The source answer key says (d), but mathematically 7524 is the number of round seeds given in the table itself. The question phrasing suggests predicting F2 from a total. If total is ~7500+2500, ratio 3:1 implies Round is 3/4. The source key likely refers to a specific calculation not fully detailed in the snippet, but lists 5643). - 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 (Genotypic ratio is 1 TT : 2 Tt : 1 tt. Ratio of pure tall to pure short is 1:1). - 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 (Gametes are haploid).
Assertion-Reason Questions
(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.
- 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) - 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. - 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 are homozygous). - 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. - 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 or wrinkled seeds. One phenotype is dominant. A farmer crosses smooth x wrinkled and gets all smooth seeds.
- Which conclusion is drawn? (1) Smooth is dominant. (2) Smooth parent is heterozygous. (3) Wrinkled parent is homozygous.
Ans: (c) 1 and 3 only. (If all F1 are smooth, the smooth parent must be homozygous dominant RR). - Which cross gives smooth and wrinkled seeds in same proportion (1:1)?
Ans: (b) \( Rr \times rr \) (Test cross). - Which cross determines genotype of a plant with dominant phenotype?
Ans: (d) \( RR/Rr \times rr \) (Test Cross). - 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 pure tall (TT) and pure dwarf (tt) plants. All F1 hybrids were tall. This showed that the trait for tallness inhibited the trait for dwarfness. The trait that appeared (Tall) is dominant, and the one that remained suppressed (Dwarf) is recessive.
Q2. How do Mendel’s experiments show that traits are inherited independently?
Ans: In a dihybrid cross (Round Yellow x Wrinkled Green), the F1 generation was Round Yellow. In the F2 generation, new combinations like Round Green and Wrinkled Yellow appeared besides parental types. This showed that the inheritance of seed shape was independent of seed colour (Law of Independent Assortment).
Q4. "The chromosome number of the sexually reproducing parents and their offspring is same." Justify.
Ans: Gametes produced by parents contain half the number of chromosomes (haploid). During fertilization, the fusion of male and female gametes restores the original (diploid) number of chromosomes in the zygote/offspring.
Q6. Green stemmed rose (GG) x Brown stemmed rose (gg).
Ans:
(i) F1 Colour: All Green.
(ii) % of brown stem in F2: 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. 3. Self and cross-pollination possible.
(b) Environmental cue: In turtles, incubation temperature determines sex. 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) Type not found in F1 but in F2: Dwarf. Reason: The recessive trait (dwarfness) was suppressed in F1 by the dominant trait but reappeared in F2 when homozygous recessive condition (tt) occurred.