Law of segregation
On the basis of the monohybrid cross (a cross involving only one trait), Mendel formulated the law of segregation. This law states that each individual possesses two factors (later termed as genes) for a particular character. At the time of formation of gametes each member of the pair of genes separates from each other so that each gamete carries only one factor (gene)i.e. gametes are always pure (law of purity of gametes). It also explains that hereditary factors are discrete and don't blend when present together. Law of segregation applies only to diploid organisms that form haploid gamete to reproduce sexually.
Explanation:
Let's use Mendel's cross of tall and dwarf pea plants as an example. The letters T and t are used to represent the alleles of the gene that determine plant height; by conventions the uppercase letter represents the dominant allele and the recessive allele is represented by the same letter in lowercase. For the P cross, both parents are true breeding plants; the tall plant is homozygous for the tall allele "T", while the dwarf plant is homozygous for the dwarf allele 't'. Mendel tracked each trait through two generations. P generation is the parental generation in a breeding experiment. When true breeding plants were crossed to each other, this is called a P cross and offspring comprise the first filial or F1 generation. When the members of the F1 generation were crossed, this produced the F2 generation or second filial generation. A cross between true breeding tall and dwarf plants of the P generation yield phenotypically tall plants. Now understand the reason why all the plants were tall in F1 generation which were obtained by crossing of pure tall with pure short plants. To determine the kind and frequencies of various types of offsprings expected, we usually use Punnett squares (used to predict the outcome of simple genetic crosses, proposed by R. Punnett). The genetic constitution of gametes of one sex is kept on top of the squares and those of other sex on one side. The genetic constitution of all possible zygote is then entered in squares of the grid.
At the time of formation of gametes, members of the pair of genes separate from each other. The number of chromosomes reduces to half in gametes. Thus the pair of gene presents at homologous loci separates from each other so that each gamete carries only one gene, never both as depicted in figure 1.2.
When plants with these genetic constitutions are crossed (mated), the offspring gets "T" gene from one parent and 't' gene from the other. All the hybrid plants of F1 generation are genetically heterozygous with the genotype "Tt.
Though all the hybrid plants of F1 generation are genetically Tt but all these plants were tall similar to the pure bred tail plants with genetic constitution TT. Thus, we see that though their physical appearance is same (all tall) but their genetic constitution has different TT and Te
When plants of FI generation were allowed to self-pollinate, both tall and short plants appeared in the second generation (F2). From figure 1.3, it is evident that all the plants of F1 generation (hybrid) were tall, but in F2 generation both tall and short plants appeared. This indicates that the character of tallness dominates or conceals the character of shortness in first generation so that the character of shortness could not appear at all. Thus, the character of tallness which expressed itself in heterozygous condition (Tt) in F1 generation is considered as a dominant character and gene "T' is considered as dominant gene. On the other hand, the character for shortness, which could not express itself in the F1 generation, is considered as recessive and gene 't' is considered as recessive gene. Thus, a recessive gene 't' cannot express itself when it is in a single dose. However, the character of shortness reappeared in F2 generation, where genes were in double dose (tt) and the dominant gene (T) was absent. Genotypic ratio 1:2:1 or a phenotypic ratio of 3:1 (classical Mendelian ratio) provides the best test of Mendel's law of segregation.
Mendel's 3:1 phenotypic ratio of monohybrid cross states that
a. F1 hybrids contained two factors for each trait, one dominant and one recessive;
b. Factors separated when gametes were formed; a gamete carried one copy of each factor;
c. The random fusion of all possible gametes occurred upon fertilization.
0 Comments