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 ' $B$ '.
What is Down's syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down's syndrome increases if the age of the mother exceeds forty years?
Down's syndrome is a human genetic disorder caused due to trisomy of chromosome number 21 . Such individuals are aneuploid and have 41 chromosomes, i.e., $(2 n+1)$
Symptoms of down's syndrome are
(i) Mental retardation
(ii) Growth abnormalities
(iii) Constantly open mouth
(iv) Dwarfness, etc., gonads and genitalia under developed
The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosome (a pair 21 during meiotic division. The chances of having a child with Down's syndrome increases with the age of the mother ( +40 ) because age adversely affects meiotic chromosome behaviour.
Meiosis in the egg cells is not completed, until after fertilisation. During this long gap (till meiosis is not completed) egg cells are arrested in prophase I and chromosomes are unpaired. The greater the time they remain upaired greater the chance for unpairing and chromosome non-disjunction.
How was it concluded that genes are located on chromosomes?
Chromosome theory of inheritance was proposed by Sutton and Boveri independently in 1902. The theory believes that chromosomes are vehicles of heriditary information, possess Mendelian factors or genes and it is the chromosomes which segregate and assort independently during transmission from one generation to the next.
A plant with red flowers was crossed with another plant with yellow flowers. If F1 showed all flowers orange in colour, explain the inheritance.
Incomplete dominance is the phenomenon where neither of the two alleles shows dominance thus producing intermediate hybrid between the expression of two alleles in homozygous state. In this case, a new phenotype in between the two original phenotype appears.
