Q. 30

(a) How is sex de

Answer :

(a)


• In the case of humans, the sex-determining mechanism is XY- type.


• In both males and females, a total of 23-pairs of chromosomes are present and from which 22-pairs are autosomes and one pair is of sex-chromosomes.


• In females, a pair of X-chromosomes is present whereas X and Y chromosomes are determinant of male.


• In males, two types of gametes are produced in which 50 % of the total sperm carries the X-chromosome and the rest 50 % has Y-chromosome along with 22-autosomes.


• Females produce only one type of ovum with an X-chromosome along with 22-autosomes.


• So, there is an equal possibility of an ovum (X-chromosome) that can fertilize with any one type of sperm (X- or Y-chromosome).


• At the time of fertilization, if ovum fertilizes with sperm carrying X-chromosome then zygote will develop as female (XX) whereas the fertilization of ovum with a Y-chromosome carrying sperm results in a male (XY) offspring.


• It proves that it is the genetic makeup of the sperm that determines the sex of the child. Thus in each pregnancy, there is always a 50 % probability of either a male or a female child.


(b)


• In many birds, a female has a pair of dissimilar chromosomes ZW and male two similar ZZ chromosomes.


• In the case of honeybee, a haploid-diploid sex-determination system determines the sex of the offspring.


Explanation:


I) Sex determination in human



Note:


Female:


46 chromosomes= 22-pair (Autosomes)+ XX chromosomes


Male:


46 chromosomes= 22-pair (Autosomes)+ XY chromosomes


II) Sex determination of birds



III) Sex determination of honeybee



OR


(a)


Genetic code- A codon is defined as a sequence of three nucleotides on the mRNA strand. Each codon codes for a specific amino acid.


(b)


Degenerate code-there are a few amino acids that are coded by more than one codon.


Unambiguous code-one codon codes for only one amino acid.


Universal code-codes are nearly universal from bacteria to humans for all amino acids.


Initiator code-codes for amino acids as well as act as initiator codon.


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