How are alleles of particular gene differ from each other? Explain its significance.
Alleles are polymorphs that differ in their nucleotide sequence resulting in contrasting phenotype expression. Alleles are the alternative forms of a same gene for, e.g., genes for height have two allele, one for dwarfness ( t ) and one for tallness ( T ).
Significance
(i) A character may have two or more contrasting phenotypic expression, thus resulting variation in the population.
(ii) These are used in the studies of inheritance and in understanding their behaviour.
In a monohybrid cross of plants with red and white flowered plants. Mendel got only red flowered plants. On self-pollinating these $F_1$ plants got both red and white flowered plants in 3:1 ratio. Explain the basis of using RR and rr symbols to represent the genotype of plants of parental generation.
Phenotypic ratio Red flower : White flower $3: 1$
Genotype ratio RR : Rr : rr 1:2:1
Generally, upper case letters are used as symbols for dominant and lower case for recessive traits of the same gene (alleles). Experiment shows that it is a monohybrid cross with $3: 1$ ratio in $\mathrm{F}_2$-generation.
This shows parents must be true-breeds. As parents are diploid and homologous chromosomes carry alleles with similar type they are represented with RR and rr.
For the expression of traits genes provide only the potentiality and the environment provides the opportunity. Comment on the veracity of the statement.
Obviously, genes are not the only factors that determine phenotype. Environment also plays an important role in the expression of traits. Genes are actually quite active throughout our lives, switching their expression on and off in response to the environment.
Besides the effect of internal factors like hormones and metabolism on gene expression, external factors like temperature, light, nutrition, etc., also affect the gene expression and ultimately exhibiting phenotypic changes.
So, we can say that genes provide only the potentiality and the environment provides the opportunity for the expression of traits.
$\mathrm{A}, \mathrm{B}, \mathrm{D}$ 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.