Q. Write a note on sex determination in Man.

    Ans.: 1) The chromosomal mechanism of sex determination in human is
XX - XY type.

     2) The nucleus of each cell in humans shows presence of 46 chromosomes OF WHICH 44 chromosomes are autosomes and last two are sex chromosomes. (22 pairs - autosome and 23rd pair is sex chromosomes)

3)  Autosomes are responsible for determination of body characters. While a pair of sex chromosomes decides sex of an individual.

4)  Human female is homomorphic i.e. with two X - chromosomes as sex chromosomes whereas male is heteromorphic with one X - chromosome and other Y - chromosome as sex chromosome.

5)  Thus, the genotype of female is 44 + XX and that of male is 44 + XY.

6)  Mechanism of sex determination:

a) During gametogenesis, both the parents produce haploid gametes containing one set. The gametes are produced by meiosis.

At this time, the homologous chromosomes separate and enter into different gametes.

b) Human male is heterogametic and produces two types of gametes such as - (22 + X) and (22 + Y)

c)   Human female is homogametic as she produces only one type of gamete as (22 + X).

d) During fertilization if X containing sperm fertilizes the egg with X - chromosome, then, the resulting offspring will show XX chromosome and will be a female child.

e) If Y - containing sperm fertilizes ovum with X - chromosome then offspring will show XY - chromosomes and will be a male child.

7) It can be represented in the form of chart as follows:

     Parents : -               Father                          Mother

     Genotype : -            44 + XY                 44 + XX

 

Gametes :-      (22 + X)    (22 + Y)               (22 + X)

    

 

Progeny : -      44 + X  X                      44 + XY

                     Female Child                 Male Child

8) This indicates that the sex of child depends on the type of sperm fertilizing the egg and thus father is responsible for sex of child and not the mother.

Q.: In human beings the sex of child is determined by male parent explain?

Ans.:  Human female is homomorphic i.e. with two X - chromosomes as sex chromosomes whereas male is heteromorphic with one X - chromosome and other Y - chromosome as sex chromosome. Thus, the genotype of female is 44 + XX and that of male is 44 + XY.

     Mechanism of sex determination:

a)   During gametogenesis, both the parents produce haploid gametes containing    one set. The gametes are produced by meiosis.

At this time, the homologous chromosomes separate and enter into different gametes.

b)  Human male is heterogametic and produces two types of gametes such as - (22 + X) and (22 + Y)

c)   Human female is homogametic as she produces only one type of gamete    as (22 + X).

d) During fertilization if X - containing sperm fertilizes the egg with X - chromosome, then, the resulting offspring will show XX chromosome and will be a female child.

     e) If Y - containing sperm fertilizes ovum with X - chromosome then offspring will       show XY - chromosomes and will be a male child.

This indicates that the sex of child depends on the type of sperm fertilizing the egg   and thus father is responsible for sex of child and not the mother.

Q.: Explain the mechanism of sex determination in Honey bees.

    Ans. 1) The mechanism of sex determination in honey bees is Haplo - Diploid sex determination.

2) In honey bees, sex is determined by the number of sets of chromosomes received by an individual.

3) The fertilized egg develops as female offspring may be worker of queen. It is diploid (2n = 32).

4) An unfertilized egg develops as a male / drone by parthenogenesis. The drones have haploid i.e. n = 16 number of chromosomes.

5) The sperms are produced by drones through mitosis.

6) It can be represented as follows -

        

        Parents: -               Female      X      Male

                        (2n = 32)           (n = 16)

                        Diploid              Haploid

 

                        Meiosis             Mitosis

 

                     n = 16           n = 16               n = 16

                     (Haploid)      (Haploid)   (Haploid)

 

                     Without                                       Fertilization

                     Fertilization         2 n = 32

                     (n = 1 6)                Diploid female

                        Male or Drone            (Worker or Queen)

Q. Explain the mechanism of sex determination in birds.

OR

Explain the mechanism of sex determination in which female is heterogametic.

Ans.: 1) The mechanism of sex determination in birds is ZW - ZZ type.

2)  The male is homogametic with ZZ chromosomes and female shows heterogametic nature with ZW chromosomes.

3) It can be represented by following chart:

Parent  : -      Male                     X      Female

Genotype: -      44 + ZZ             X      44 + ZW

Gametes :- (22 + Z) (22 + Z)              (22 + z) (22 + w)

 

     F1:           44 + ZW                       44 + ZZ            

                  Female                    Male

(REFER TABLSE FORM TEXTBOOK,PAGE NO. 65 AND 66)