Principles Of Inheritance And Variation

Step 1: Definition.
Incomplete dominance is a type of inheritance where the phenotype of the heterozygote is an intermediate form between the two homozygotes. Neither allele is completely dominant over the other.
Step 2: Example.
In Mirabilis jalapa (4 o'clock plant), when red-flowered (RR) and white-flowered (rr) plants are crossed, the F\textsubscript{1} hybrid (Rr) has pink flowers, which is intermediate.
Step 3: Conclusion.
Thus, incomplete dominance shows blending of traits in heterozygotes.

Step 1: Parental genotypes.
Mother’s blood group = AB → genotype =
Father’s blood group = O → genotype =
Step 2: Cross.
Step 3: Blood groups of offspring.
- → Blood group A
- → Blood group B
Step 4: Conclusion.
The offspring will have either blood group A or B, but not AB or O.

Step 1: Pedigree symbols.
- Male → Square (□)
- Female → Circle (○)
- Mating → Horizontal line connecting male and female
- Consanguineous mating → Double horizontal line connecting male and female
Step 2: Importance.
Pedigree analysis helps in tracing inheritance of traits and genetic disorders across generations.

Step 1: Concept of incomplete dominance.
In incomplete dominance, neither allele is completely dominant. The heterozygous condition shows an intermediate phenotype.
Step 2: Example in Antirrhinum.
Crossing red (RR) and white (rr) flowers produces pink (Rr) in F\textsubscript{1}, showing incomplete dominance.
Step 3: Conclusion.
Thus, the phenomenon is an example of Incomplete dominance.

Step 1: Mendel’s principles.
Mendel proposed that characters are controlled by discrete units called factors (now known as genes). These occur in pairs in organisms.
Step 2: Blending inheritance concept.
Mendel disproved blending theory. Alleles do not mix permanently but segregate and retain their identity.
Step 3: Analyzing options.
(A) Correct — Characters are controlled by factors.
(B) Correct — Factors are discrete.
(C) Correct — In peas, factors are in pairs.
(D) Incorrect — Alleles do not blend but segregate independently.
Step 4: Conclusion.
Thus, the incorrect statement is Alleles blend with each other.

Step 1: Understanding law of independent assortment.
This law states that alleles of different gene pairs assort independently of one another during gamete formation.
Step 2: Cytological explanation.
During metaphase I of meiosis, homologous chromosomes align at the equatorial plate randomly. This independent alignment results in independent assortment of alleles.
Step 3: Analyzing options.
(A) Incorrect — No blending of alleles is true but not the cytological basis.
(B) Incorrect — Separation of alleles explains segregation, not independent assortment.
(C) Incorrect — Dominance/recessiveness is unrelated to independent assortment.
(D) Correct — Random orientation of homologous chromosomes in metaphase I is the true basis.
Step 4: Conclusion.
Thus, the cytological basis of law of independent assortment is independent alignment of chromosomes in metaphase I.

Step 1: Background.
Mendel proposed the basic principles of inheritance, but the chromosomal basis was established later.
Step 2: Sutton and Boveri’s contribution.
In 1902–1903, Sutton and Boveri independently proposed that chromosomes carry Mendel’s factors (genes) and thus form the basis of inheritance.
Step 3: Conclusion.
Hence, the chromosomal theory of inheritance was propounded by Sutton and Boveri.

Step 1: Chromosomal disorders.
Disorders caused due to numerical or structural abnormalities in chromosomes include Down’s syndrome (trisomy 21), Klinefelter’s syndrome (XXY), and Turner’s syndrome (XO).
Step 2: Haemophilia.
Haemophilia is a genetic disorder caused by a mutation in a single gene located on the X-chromosome, not due to chromosomal number abnormalities.
Step 3: Conclusion.
Thus, the disorder that is not chromosomal is Haemophilia.