Punnett square

A Punnett square, also Rekombinationsquadrat (Latin: re = " back ", " back ", " new") or shorter combination square is a tool that has been developed by British geneticist Reginald Punnett. In biology, it is used to determine the frequency of the different genotypes in the offspring.

In the example above, 50 % of the offspring will have the dominant homozygous genotype RR, the other 50% of the heterozygous genotype Rr The dominant allele R is present in all four possible genotypes and will be marked in the phenotype.

Interpretation

• As an example of the use of the Punnett square, two rats are crossed with different coat color. With respect to the color coat it is a gene which exists in the form of two alleles: B for black coat color and b for the white coat color. A homozygous black rat with the alleles BB ( pink background ) is crossed with a homozygous white rat with the genotype bb (blue background). Homozygous means that both alleles are equal. Heterozygous means that the two alleles are different. Would a rat heterozygous genotype Bb then you were you would have an allele for black and one allele for white.
• The two possible alleles of the genotype of the male rat are in the first column (blue), the alleles of the female rat in the top line ( pink ) are shown. The allele combinations of the offspring are listed right below it.

Dominant alleles receive uppercase letters, lowercase letters recessive alleles.

• Now you writes the genotype letters of the parents in the field in which intersect the column and the row. The dominant allele is normally writes first.

• The remaining fields for the offspring to be filled in the same way:

• If the result of the BB genotype shows the dominant allele B is marked in the phenotype, i.e. the rat with the BB genotype has a black coat color. The recessive allele b is suppressed. At this intersection all the offspring show in the phenotype of the black coat color. You are heterozygous. The dominant allele has prevailed.

Intersection results

When crossing a homozygous black rat with a homozygous white rat ( as shown above), the offspring with a probability of 100 % have the genotype Bb and be phenotypically black.

If you cross the heterozygous offspring from the previous example ( called the F1 generation ) with each other, then their children in genotype and in phenotype are different. With a probability of 25% of the children are white (with bb as genotype). There is a 50 % chance for the genotype Bb and a 25 % probability for the genotype BB. The children with one or two dominant alleles B are black. The ratio of phenotypes is 3:1. This is a mono hybrid cross.

If you cross a heterozygous rat (Bb) with a white rat ( bb ), the offspring are likely to be 50 % white and 50 % black:

The probability of the inheritance of some diseases can be explained by the Punnett square. Wear both parents a sickening recessive allele, then 25% of children are likely to be ill, 50% in genotype have a disease-causing recessive allele, but his phenotypically healthy, and 25 % will be perfectly healthy. It also follows that an average of two thirds of the clinically healthy full siblings of an affected individual are carriers of the disease.

Complicated intersections

The example above used only one feature, with four possible outcomes. In reality, the intersections are complicated because usually more than one feature is crossed. In a dihybrid cross, for example, two pea plants are crossed that differ in two traits. As an example, the characteristics of seed shape and seed color are listed below. The allele R for the round shape is dominant, while the allele r is recessive for wrinkled shape. The allele Y for yellow color is dominant, the allele y green color is recessive. The characteristics of the underlying genes, respectively exist in the form of two alleles, which are responsible for the characteristic variations described above. If the original races have the following genotypes: RRYY x rryy ( possible also RRyy rrYY x ), then the heterozygotes of the first filial generation the genotype and the phenotype RrYy have uniform round and yellow. The germ cells of each heterozygous pea plant then have the combinations RY, Ry, rY and ry.

Then in the second filial generation, there is a splitting of the phenotypes in a ratio of 9: round and yellow, 3: wrinkled and yellow, 3: round and green, 1: wrinkled and green.

The result is a splitting of the phenotype in the ratio 9:3:3:1.

Note that this gap ratio yields only at a dihybrid cross if both genes have a dominant- recessive inheritance.

( 6: 3: 1: 2: If one of the two genes a intermediary inheritance to 6 different phenotypes in a ratio of 3 would result. 1 The gap sum number in both cases is 16, which is an evidence of a dihybrid cross )