$\mathrm{A}, \mathrm{B}, \mathrm{D}$ are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross was made between individuals of Aa bb DD genotype with aa bb dd. Find out the type of genotypes of the offspring produced.
The given cross $\mathrm{Aa} \mathrm{bb} \mathrm{DD} \times \mathrm{aa} \mathrm{bb} \mathrm{dd}$, is a trihybrid cross, Accordingly the type of offspring produced would be,
In our society a woman is often blamed for not bearing male child. Do you think it is right? Justify.
It is unfortunate that in our society women are blamed for giving birth to female children and have been ostracised and ill-treated because of this false notion. Out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females, these are the autosomes.
A pair of X-chromosomes are present in the female, whereas the presence of an X and Y-chromosome are determinant of the male characteristic. During spermatogenesis among males, two types of gametes are produced.
50 per cent of the total sperm produced carry the X-chromosome and the rest $50 \%$ has Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome. There is an equal probability of fertilisation of the ovum with the sperm carrying either X or Y -chromosome.
In case when the ovum fertilises with a sperm carrying X-chromosome the zygote develops into a female ( XX ) and the fertilisation of ovum with Y -chromosome carrying sperm results into a male offspring. Thus, it is evident that it is the genetic makeup of the sperm (male) that determines the sex of the child.
It is also evident that in each pregnancy there is always $50 \%$ probability of having either a male or a female child.
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.