Chapter Summary
Genetics explains how traits are transmitted from parents to offspring. Mendel proposed the Law of Segregation and Law of Independent Assortment using pea plants. Key ideas include genes & alleles, dominance, monohybrid and dihybrid crosses, incomplete dominance, codominance, multiple alleles (ABO blood groups), pleiotropy, epistasis, linkage and crossing over, gene mapping, sex‑linked inheritance (haemophilia, colour blindness), cytoplasmic inheritance, mutations, and interpretation of pedigrees. Variations arise through recombination, independent assortment, and mutation.
Mendelian Laws
Monohybrid & Dihybrid
Epistasis
Linkage & Mapping
Sex‑linked Traits
Mutations
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50 Questions & Answers
1) Define genetics.
Study of heredity and variation.
2) Who is the father of genetics?
Gregor Johann Mendel.
3) What is a gene?
A heritable unit of information located at a specific locus on DNA that influences a trait.
4) Define allele.
Alternative form of a gene at the same locus on homologous chromosomes.
5) State Mendel’s Law of Segregation.
Allele pairs separate during gamete formation so each gamete carries only one allele of a gene.
6) State Mendel’s Law of Independent Assortment.
Allele pairs of different genes assort independently during gamete formation (for genes on different chromosomes or far apart).
7) What is a monohybrid cross?
Cross involving a single contrasting pair of traits.
8) Give the F2 ratios for a monohybrid cross under complete dominance.
Phenotypic 3:1; Genotypic 1:2:1.
9) What is a dihybrid cross and its F2 ratio?
Cross tracking two traits simultaneously; phenotypic ratio 9:3:3:1 under independent assortment.
10) Define phenotype and genotype.
Phenotype: observable traits; Genotype: genetic makeup (allele combination).
11) What is a test cross?
Cross of an individual with a homozygous recessive to reveal its genotype.
12) What is a back cross?
Cross of F1 with either of the parents (dominant or recessive).
13) What is reciprocal cross?
Two crosses where the sexes of the parents are interchanged to test role of sex in inheritance.
14) State the addition rule of probability in genetics.
Probability of either of mutually exclusive events = sum of individual probabilities.
15) State the multiplication rule of probability.
Probability of independent events occurring together = product of their probabilities.
16) Why did Mendel choose pea plant?
Clear contrasting traits, short generation time, self and cross‑pollination possible, large progeny.
17) What is a pure line?
Population homozygous for a trait; breeds true.
18) Define dominant and recessive traits.
Dominant allele expresses in heterozygote; recessive expressed only when homozygous.
19) What is incomplete dominance?
Heterozygote shows intermediate phenotype (e.g., red × white snapdragon → pink).
20) What is codominance?
Both alleles express fully in heterozygote (e.g., AB blood group).
21) What are multiple alleles?
More than two alternative forms of a gene in a population (e.g., IA, IB, i in ABO).
22) Distinguish incomplete dominance and codominance.
Incomplete: blended intermediate; Codominance: both parental phenotypes appear side‑by‑side.
23) Define pleiotropy with an example.
Single gene affects multiple traits; e.g., sickle‑cell allele affects RBC shape, anaemia, resistance to malaria.
24) What is epistasis?
Interaction where one gene masks/modifies expression of another non‑allelic gene.
25) Give classic epistasis ratios.
Recessive epistasis 9:3:4; dominant epistasis 12:3:1; duplicate recessive (complementary) 9:7; duplicate dominant 15:1.
26) What is complementary gene action?
Two non‑allelic dominant genes required together to express a trait (9:7).
27) What is duplicate gene action?
Either of two dominant genes can produce the phenotype (15:1).
28) Define penetrance.
Proportion of individuals with a genotype that actually express the phenotype.
29) Define expressivity.
Degree/intensity with which a genotype is expressed in individuals.
30) What are lethal alleles?
Alleles that cause death when present in certain genotypes (often homozygous).
31) What is linkage?
Tendency of genes on the same chromosome to be inherited together.
32) What is crossing over?
Reciprocal exchange of chromosome segments between non‑sister chromatids of homologues during prophase I (pachytene).
33) Define chiasma.
Visible site of crossing over between homologous chromosomes.
34) What is a linkage map?
Linear arrangement of genes on a chromosome based on recombination frequencies.
35) Unit of map distance?
1 map unit or centiMorgan (cM) = 1% recombination frequency.
36) What reduces independent assortment?
Strong linkage (genes close together on the same chromosome).
37) Define sex‑linked inheritance.
Transmission of traits determined by genes located on sex chromosomes (X or Y).
38) Name two X‑linked recessive disorders.
Haemophilia A and red‑green colour blindness.
39) What is a holandric trait?
Y‑linked trait transmitted from father to all sons (e.g., hypertrichosis pinna).
40) What is criss‑cross inheritance?
X‑linked traits passing from father to daughter to grandson.
41) State the inheritance of ABO blood groups.
Multiple alleles with codominance (IA and IB) and recessive i; genotypes determine phenotypes A, B, AB, O.
42) What is the Rh factor and erythroblastosis fetalis?
Rh is antigen D on RBCs. Rh− mother carrying Rh+ fetus may develop anti‑D; second Rh+ pregnancy risk of haemolytic disease of newborn.
43) What is a pedigree chart?
Diagram showing occurrence of a trait across generations to infer inheritance pattern.
44) How do you identify autosomal recessive pattern in pedigree?
Trait skips generations; affected offspring often from unaffected carrier parents; both sexes equally affected.
45) Define mutation and its types.
Heritable change in DNA. Gene (point) mutations and chromosomal mutations (structural/number).
46) What is a point mutation?
Change in a single nucleotide (substitution, insertion, deletion). Substitution may cause missense, nonsense, or silent changes.
47) Name numerical chromosomal disorders.
Aneuploidies such as Down syndrome (Trisomy 21), Turner (45,X), Klinefelter (47,XXY).
48) What is non‑disjunction?
Failure of homologous chromosomes/sister chromatids to separate during meiosis causing aneuploidy.
49) What is polygenic inheritance?
Trait controlled by many genes with additive effects (e.g., human skin colour, height); shows continuous variation.
50) What is the chi‑square (χ²) test used for in genetics?
To test goodness‑of‑fit between observed and expected ratios and assess if deviations are due to chance.