🌸 Chapter 4
Principles of Inheritance and Variation
(2 Marks)
1. Who is known as the Father of
Genetics and why?
Answer:
Gregor Johann Mendel is known as the Father of Genetics because he discovered
the basic principles of heredity through experiments on pea plants (Pisum
sativum).
2. What are alleles?
Answer:
Alleles are alternative forms of the same gene that occupy the same position
(locus) on homologous chromosomes and determine variations in a character.
3. What is meant by genotype and
phenotype?
Answer:
- Genotype: Genetic constitution of an
organism (e.g., TT, Tt).
- Phenotype: Observable physical or
physiological trait (e.g., tall or dwarf plant).
4. State Mendel’s Law of Dominance.
Answer:
When two contrasting alleles are present in an organism, only one (dominant)
allele expresses itself, while the other (recessive) remains unexpressed.
5. Define Mendel’s Law of
Segregation.
Answer:
Allele pairs segregate during gamete formation, and each gamete receives only
one allele of a gene.
6. What is the Law of Independent
Assortment?
Answer:
When two or more pairs of contrasting traits are considered, they segregate
independently of each other during gamete formation.
7. What is a monohybrid cross?
Answer:
A cross between two individuals differing in one trait (e.g., tall × dwarf).
The F2 generation shows a 3:1 phenotypic ratio.
8. What is a dihybrid cross?
Answer:
A cross between two individuals differing in two traits (e.g., seed color and
seed shape). The F2 generation shows a 9:3:3:1 ratio.
9. What is test cross and why is it
performed?
Answer:
It is a cross between an F1 hybrid and a homozygous recessive parent to
determine the unknown genotype of the hybrid.
10. What is back cross?
Answer:
Crossing an F1 hybrid with any of its parents (dominant or recessive) is called
a back cross.
11. What is incomplete dominance?
Give one example.
Answer:
When neither allele is completely dominant, and the hybrid shows an
intermediate phenotype.
Example: Red × White flowers of Mirabilis jalapa produce pink
flowers.
12. What is co-dominance? Give an
example.
Answer:
When both alleles express themselves equally in the heterozygote.
Example: Blood group AB (IAIB).
13. What is multiple allelism? Give
an example.
Answer:
Presence of more than two allelic forms of a gene.
Example: Human ABO blood group system (IA, IB, i).
14. What is pleiotropy? Give one
example.
Answer:
When a single gene influences more than one trait.
Example: The gene causing sickle cell anemia also affects oxygen
transport and RBC shape.
15. Define polygenic inheritance.
Answer:
Inheritance controlled by multiple genes, each contributing additively to a
single trait (e.g., human skin color).
16. What is linkage?
Answer:
Tendency of genes located close together on the same chromosome to be inherited
together.
17. Who discovered linkage?
Answer:
Bateson and Punnett discovered linkage in Lathyrus odoratus (sweet pea).
18. What is recombination?
Answer:
Formation of new gene combinations due to crossing over between homologous
chromosomes during meiosis.
19. Who gave the Chromosomal Theory
of Inheritance?
Answer:
Sutton and Boveri proposed it, relating Mendel’s factors (genes) to
chromosomes.
20. Define crossing over.
Answer:
Physical exchange of chromosomal segments between non-sister chromatids of
homologous chromosomes during meiosis.
21. What is a linkage group?
Answer:
All genes located on a single chromosome form one linkage group.
22. Name the organism in which T.H.
Morgan studied linkage.
Answer:
Drosophila melanogaster (fruit fly).
23. What are sex-linked traits?
Answer:
Traits controlled by genes present on the sex chromosomes (e.g., color
blindness, haemophilia).
24. Why are sex-linked diseases more
common in males?
Answer:
Because males have only one X chromosome; a single recessive allele is enough
to express the trait.
25. What is color blindness?
Answer:
It is an X-linked recessive disorder in which a person cannot distinguish
between red and green colors.
26. What is haemophilia?
Answer:
X-linked recessive disorder where blood fails to clot properly due to lack of
clotting factors.
27. Define mutation.
Answer:
Sudden heritable change in DNA sequence or chromosome structure.
28. What type of mutation causes
sickle-cell anemia?
Answer:
Point mutation — substitution of valine for glutamic acid in the β-globin gene.
29. What are chromosomal disorders?
Answer:
Disorders caused by abnormalities in chromosome number or structure (e.g.,
Down’s, Turner’s, Klinefelter’s syndromes).
30. What is Down’s syndrome?
Answer:
A trisomy of chromosome 21, characterized by mental retardation and short
stature.
31. What is Turner’s syndrome?
Answer:
Condition in females with only one X chromosome (XO). Such females are sterile
and short.
32. What is Klinefelter’s syndrome?
Answer:
Genetic disorder in males with XXY condition. Males show underdeveloped testes
and gynecomastia.
33. What is the chromosomal
composition of a normal male and female in humans?
Answer:
Male: 44 autosomes + XY
Female: 44 autosomes + XX
34. What is a pedigree chart?
Answer:
A diagram showing the inheritance pattern of traits in a family over generations.
35. What is the purpose of a
pedigree analysis?
Answer:
To study inheritance of a trait, determine carriers, and predict genetic
disorders.
36. Define autosomes.
Answer:
Chromosomes other than sex chromosomes that carry genes for general body
traits.
37. What are sex chromosomes?
Answer:
Chromosomes responsible for determining the sex of an individual (X and Y in
humans).
38. What is homozygous and
heterozygous condition?
Answer:
- Homozygous: Same alleles for a gene (TT or
tt).
- Heterozygous: Different alleles for a gene
(Tt).
39. What is the function of Punnett
square?
Answer:
It predicts the genotypic and phenotypic ratios of offspring from genetic
crosses.
40. Define dominant and recessive
traits.
Answer:
- Dominant: Expressed in both homozygous
and heterozygous conditions.
- Recessive: Expressed only in homozygous
condition.
41. What is genetic recombination
frequency?
Answer:
Percentage of recombinant offspring; used to estimate distance between genes
(1% = 1 map unit).
42. What is gene mapping?
Answer:
Determining the relative positions of genes on a chromosome using recombination
frequencies.
43. What are the possible genotypes
of blood group A?
Answer:
IAIA or IAi.
44. What are the possible genotypes
of blood group B?
Answer:
IBIB or IBi.
45. What is the genotype of a person
with O blood group?
Answer:
ii (both alleles recessive).
46. Which blood group is a universal
donor and why?
Answer:
Blood group O; it lacks A and B antigens on RBCs.
47. Which blood group is a universal
recipient and why?
Answer:
Blood group AB; it has both A and B antigens and no antibodies.
48. What is aneuploidy?
Answer:
Abnormal number of chromosomes due to non-disjunction during meiosis (e.g.,
2n+1 or 2n–1).
49. What is the chromosomal
condition in Down’s syndrome?
Answer:
Trisomy of 21st chromosome (47 chromosomes in total).
50. Define genetic disorders and
give two examples.
Answer:
Disorders caused by abnormalities in genes or chromosomes.
Examples: Sickle-cell anemia (gene mutation), Down’s syndrome
(chromosomal aberration).
