SEX DETERMINATION

 SEX DETERMINATION: Sex determination deals with the occurrence of sex in the next generation, or it can also be spoken as- occurrence of  sexes of organisms. Earlier it was believed that women plays a great role in determining the sexuality of an organism but later on scientifically it has been found that it is not the female but Male who are the determiner of sex for the next generation. 

Higher Organisms  are 2n, means they are diploid. We have 23 pairs of chromosomes ie. 46 number of chromosomes. Out of 23 pairs or 46 number of chromosomes, 22 pairs or 44 number of chromosomes are called autosomes and one pair or 2 chromosomes are only sex chromosomes.

23 pairs of chromosomes an organism has 50% come from the mother side and other 50% come from the Father side.

Sexually reproducing organism produces gametes and the gametes are produced after Meiosis division or reduction division. Male gametes are called Sperms and female are called egg or ova.

Due to sexual reproduction both the both the gametes  ie. Sperm and an egg unite together to form Zygote. Zygote leads to the formation of an embryo or the future organism.

Dihybrid Cross

 Dihybrid Cross: For conducting Dihybrid cross, Mendel selected 2 pairs of contrasting characters. Earlier Mendel took one pair of contrasting character for conducting his monohybrid experiment. For the dihybrid experiment, Mendel completed his experiment in the following way -

1st pair - Height             contrasting characters          Tall( TT )         and      Dwarf( tt )

2nd pair Seed shape        contrasting characters         Round( RR )    and      Wrinkle( rr )

For dihybrid experiment Mendel took dominant characters in one side and crossed it with the recessive characters in the following way-

                                       Tall Round     x    Dwarf Wrinkle

                                             TT RR                  tt rr

Ratio that he obtained from the dihybrid cross is 9:3:3:1

While doing this experiment Mendel formulated his another most important Law, "which is Law of  Independent Assortment".

Mendel found that the parental characters of Dihybrid experiment assort out independently in F2 generation.

That means -Tall Round(TT RR )  when crossed with Dwarf Wrinkle ( tt rr ) the F1 generation became Tall Round with the genotype (Tt Rr ), but in the F2 generation parental characters ie. TTRR and ttrr reappear.

Some Important Terminology:

Phenotype: Visible expression means can be seen, like tall or dwarf.

Genotype: Genetic constitution of individual, which cannot be seen. Like tall may be TT OR Tt. These  TT or Tt which are found through genetic study, known as Genotype.

Homozygous :      The organism carrying similar alleles for a trait, eg TT or tt.

Heterozygous :     The organism carrying different  alleles for a trait eg Tt

F1 Generation  :    The first Filial generation

Gametes         :    Reproductive unit, ( Sperm and Egg in case of human beings)

Dominant allele :   The parental character that occurs in the next generation.

Recessive allele:   The character which is unable to be expressed and remain suppressed is called Recessive allele 

MONOHYBRID EXPERIMENT AND LAWS

For doing monohybrid experiment Mendel took one pair of contrasting characters from his selected seven pairs of contrasting characters.As Maendel took one pair so it is known as Mono and crossing as hybrid.From the crossing the ratio that is obtained, he named it as monohybrid ratio and it is 3:1

   7 pairs of characters that Mendel took

First experiment in monohybrid cross he took the pair Height. Height has two contrasting characters one is tall and other is dwarf. For tall he gave the symbol TT which is dominant and dwarf is recessive so he used the small letters of dominant ie. tt.

From the monohybrid cross Mendel formulated two laws ie. Law of Dominance  and Law of segregation.

Law of Dominance- Mendel stated the law in this way- One factor in a pair may express itself and prevent the expression of other. The factor that  expressed itself was described by Mendel as dominant factor and unexpressed one is the recessive factor.

That means that character from father and mother come through gametes. Gametes unit to form zygote and later on leads to the formation of an organism. Sometimes character of father is dominant and sometimes mother's character. The dominant character is expressed in the F1 generation. F1 means first filial generation ie son or daughter.

Law of Segregation:  Segregate means  to separate. In the F1 generation we found that the character is tall and it comes from Tt . But these two allele never get mixed up or are contaminated, they simply remain together and has a tendency to get separated. And this separation occur in the F2 generation, where we find the parental characters to  reappear ie. TT and  tt. This is segregation.

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