Discuss the genetic basis of wrinkled phenotype of pea seed.
Seed shape is determined by a single gene, with the allele (R) for round peas dominant over the allele ( $r$ ) for wrinkled peas (recessive trait).
It the alleles for the gene controlling the seed shape are homozygous in a plant, it will show the character or phenotype of same alleles, i.e.,-RR- round seed, rr-wrinkled seed.
On the other hand, if the alleles of gene are heterozygous. They will express the phenotype of dominant allele.
Rr - Round seed ( r - wrinkled is recessive)
This is the genetic basis of wrinkled phenotype of pea seed.
Even if a character shows multiple allelism, an individual will only have two alleles for that character. Why?
Multiple alleles are the multiple forms of a gene which occur on the same gene locus, but distributed in different organisms in the gene pool with an organism, which carry only two alleles and the gamete have only one allele.
Despite multiple allelism, an individual will have only two alleles because an individual develops from a zygote which is the result of fusion of sperm (carrying father set of ( $n$ )haploid chromosomes) and an egg (carrying mother set of haploid chromosomes).
Sperm and an egg have only one gene (allele) for each trait. A zygote when becomes diploid, have two alleles for each trait. It is the maximum number of alleles an individual can have. e.g., genes of blood groups.
How does a mutagen induce mutation? Explain with example.
Mutagens may be physical, i.e., ionising radiations X-ray,UV rays, gamma rays, DNA reactive chemicals, i.e., hydroxyl radicals, $\mathrm{H}_2 \mathrm{O}_2$, etc., or biological such as virus.
A mutagen can induce mutation by inducing, a change in the base sequence by insertion, deletion or substitution.
e.g., a single base sequence substitution at the sixth codon of the $\beta$-globin gene changes the codon from GAG to GUG. This results in the substitution of glutamic acid (Glu) by valine (Val) at the sixth position of the $\beta$-globin chain of the haemoglobin molecule.
The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to the elongated sickle, i.e., like structure which is not functional.
In a Mendelian monohybrid cross, the $\mathrm{F}_2$-generation shows identical genotypic and phenotypic ratios. What does it tell us about the nature of alleles involved? Justify your answer.
$$ \text { In case of incomplete dominance, a monohybrid cross shows the result as follows } $$
Here, the phenotypic and genotypic both ratios are the same. So, we can conclude that when genotypic and phenotypic ratios are the same, alleles show incomplete dominance. i.e., none of the two alleles shows dominance thus producing hybrid intermediate from the expression of two homozygous alleles.
Can a child have blood group ' 0 ' if his parents have blood group ' $A$ ' and ' $B$ ' Explain.
A child have blood group O in the following two cases Case I When father is $\mathrm{I}^{\mathrm{A}} \mathrm{i}$ and mother is $\mathrm{I}^{\mathrm{B}} \mathrm{i}$.
The offsprings will have the above possible blood groups. i.e., $\mathrm{AB}, \mathrm{A}, \mathrm{B}$ and O Case II When father is $\mathrm{I}^{\mathrm{B}} \mathrm{i}$ and mother is $\mathrm{I}^{\mathrm{A}} \mathrm{i}$.
The offsprings will have the above possible blood groups, i.e., $\mathrm{AB}, \mathrm{A}, \mathrm{B}$ and O . Thus, a child can have blood group ' $O$ ' if parents have heterozygous alleles for group ' $A$ ' and ' $B$ '.