Biology SEGREGATION OF ALLELES AND PUNETTE SQUARE

### KEY TOPICS

star Segregation of Allele
star Punette Square
star Test Cross

### SEGREGATION OF ALLELES

● From the observation that the color{violet}"recessive parental trait" is expressed without any color{violet}"blending" in the F_2 generation, it can be inferred that:

● When the tall and dwarf plant produce gametes, by the process of meiosis,
star the alleles of thecolor{violet}" parental pair separate" or segregate from each other

star and only color{violet}"one allele is transmitted" to a gamete.

● This segregation of alleles is a color{violet}"random process" and so there is a color{violet}"50 per cent chance" of a gamete containing either allele, as has been verified by the results of the crossings.

● In this way the gametes of the color{violet}"tall TT" plants have the color{violet}"allele T" and the gametes of the color{violet}"dwarf tt plants" have the color{violet}"allele t".

● During fertilisation the two alleles, color{violet}"T from one parent" say, through the pollen, and t from the other parent, then through the egg, are united to produce zygotes that have one color{violet}"T allele and one t allele".

● In other words the color{violet}"hybrids have Tt".

● Since these hybrids contain alleles which express color{violet}"contrasting traits", the plants are color{violet}"heterozygous".

### PUNNETT SQUARE

● The production of color{violet}"gametes" by the parents, the formation of the color{violet}"zygotes", the F_1and F_2 plants can be understood from a diagram called color{violet}"Punnett Square".

● It was developed by a British geneticist, color{violet}"Reginald C. Punnett".

● It is a color{violet}"graphical representation" to calculate the probability of all color{violet}"possible genotypes" of offspring in a genetic cross.

● The possible gametes are written on color{violet}"two sides", usually the top row and left columns.

● All color{violet}"possible combinations" are represented in boxes below in the squares, which generates a color{violet}"square output form".

● The Punnett Square shows the color{violet}"parental tall TT" (color{violet}"male") and color{violet}"dwarf tt" (color{violet}"female") plants, the gametes produced by them and, the F_1 color{violet}"Tt progeny".

● The F_1 plants of genotype Tt are color{violet}"self-pollinated".

● The symbols color{violet}"& and %" are used to denote the color{violet}"female (eggs)" and color{violet}"male (pollen)" of the F_1 generation, respectively.

● The F_1 plant of the color{violet}"genotype Tt" when color{violet}"self-pollinated", produces gametes of the genotype color{violet}"T and t" in equal proportion.

● When fertilisation takes place, the pollen grains of genotype color{violet}"T have a 50 per cent" chance to pollinate eggs of the genotype T, as well as of genotype t.

● Also pollen grains of genotype t have a color{violet}"50 per cent chance" of pollinating eggs of genotype T, as well as of genotype t.

● As a result of color{violet}"random fertilisation", the resultant zygotes can be of the genotypes color{violet}"TT, Tt or tt".

● From the Punnett square it is easily seen that color{violet}"1/4th of the random fertilisations" lead to TT, 1/2 lead to Tt and 1/4th to tt.

● Though the F_1 have a genotype of color{violet}"Tt", but the phenotypic character seen is color{violet}"tall".

● At F_2, color{violet}"3/4th" of the plants are color{violet}"tall", where some of them are TT while others are Tt.

● Externally it is color{violet}"not possible" to distinguish between the plants with the color{violet}"genotypes TT and Tt".

● Hence, within the color{violet}"genopytic pair" Tt only one character color{violet}"tall" is expressed.

● Hence the character T or ‘tall’ is said to color{violet}"dominate" over the other allele t or ‘dwarf’ character.

● It is thus due to this color{violet}"dominance of one character" over the other that all the F_1 are color{violet}"tall" (though the genotype is Tt) and in the F_2 color{violet}"3/4th of the plants are tall" (though genotypically 1/2 are Tt and only 1/4th are TT).

● This leads to a color{violet}"phenotypic ratio" of 3/4th tall : (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a color{violet}"3:1 ratio", but a color{violet}"genotypic ratio" of color{violet}"1:2:1".

● The color{violet}"1/4 : 1/2 : 1/4" ratio of TT: Tt: tt is color{violet}"mathematically condensable" to the form of the color{violet}"binomial expression" (ax +by)^2, that has the gametes bearing genes T or t in equal frequency of ½.

● The expression is expanded as given below :
color{brown}"(1/2T + 1/2 t)2" = color{brown}"(1/2T + 1/2t) X (1/2T + 1/2t)" = color{brown}"1/4 TT + 1/2Tt + 1/4 tt"

● Mendel color{violet}"self-pollinated" the F_2 plants and found that dwarf F_2 plants continued to generate color{violet}"dwarf plants" in F_3 and F_4 generations.

● He concluded that the genotype of the color{violet}"dwarfs" was color{violet}"homozygous – tt".

### TEST CROSS

● From the preceeding paragraphs it is clear that though the genotypic ratios can be calculated using color{Violet}"mathematical probability", but simply looking at the color{violet}"phenotype of a dominant" trait, it is color{violet}"not possible" to know the color{violet}"genotypic composition".

● That is, for example, whether a tall plant from F_1or F_2 has color{violet}"TT or Tt" composition, cannot be predicted.

● Therefore, to determine the color{violet}"genotype of a tall plant" at F_2, Mendel crossed the tall plant from F_2 with a dwarf plant.

● This he called a color{violet}"test cross".

● In a typical test cross an organism (pea plants here) showing a color{violet}"dominant phenotype" (and whose genotype is to be determined) is crossed with the color{violet}"recessive parent" instead of self-crossing.

● The progenies of such a cross can easily be color{violet}"analysed" to color{violet}"predict the genotype" of the test organism.

● Figure shows the results of color{violet}"typical test cross" where violet colour flower (color{violet}"W") is color{violet}"dominant" over white colour flower (color{violet}"w").