Punnett square

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

R. R.
R. RR RR
r Rr Rr

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

interpretation

  • As an example for the use of the Punnett square, two rats with different fur colors are to be crossed. Regarding coat color, there is a gene that exists in the form of two alleles: B for black coat color and b for 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 the same. Heterozygous means that the two alleles are different. If a rat were heterozygous, its genotype would be Bb . It would have an allele for black and an allele for white.
  • The two possible alleles of the genotype of the male rat are shown in the first column (blue), the alleles of the female rat in the top row (pink). The allele combinations of the offspring are listed below on the right.

Dominant alleles are given upper case letters, recessive alleles are given lower case letters.

B. B.
b _ _ _ _
b _ _ _ _
  • Now write the genotype letters of the parents in the field where the column and the row cross. The dominant allele is usually written first.
B. B.
b Port _ _
b _ _ _ _
  • The remaining fields for the descendants are completed in the same way:
B. B.
b Port Port
b Port Port
  • If the result for the genotype is Bb , the dominant B allele is pronounced for the phenotype, i.e. H. the rat with the genotype Bb has a black coat color. The recessive allele b is suppressed. In this cross all offspring show the black coat color in the phenotype. You are heterozygous. The dominant allele has prevailed.

Intersection results

When a homozygous black rat is crossed with a homozygous white rat (as shown above), there is a 100% probability that the offspring will be genotype Bb and phenotypically black.

B. b
B. BB Port
b Port bb

If the heterozygous offspring from the previous example (referred to as the F 1 generation) are crossed with each other, then their children are different in genotype and phenotype. There is a 25% probability that the children are white (with bb as the genotype). There is a 50% chance for genotype Bb and a 25% chance for genotype BB . The children with one or two dominant B alleles are black. The phenotype ratio is 3: 1 . This is a monohybrid cross.

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

b b
B. Port Port
b bb bb

The probability of some diseases being inherited can be explained by Punnett's square. If both parents have a disease-causing recessive allele, then 25% of the children will likely be sick, 50% will have a disease-causing recessive allele in the genotype but be phenotypically healthy, and 25% will be perfectly healthy. It also follows that on average two thirds of the clinically healthy full siblings of a sick individual are carriers of the disease.

Complicated intersections

The above example used only one feature, with 4 possible outcomes. In reality, the crossings are more complicated as more than one feature is usually crossed. In a dihybrid cross, for example, two pea plants are crossed that differ in two characteristics. The characteristics of seed shape and seed color are given as an example. The R allele for the round shape is dominant, while the r allele for the wrinkled shape is recessive. The allele Y for yellow color is dominant, allele y for green color is recessive. The genes on which the traits are based each exist in the form of two alleles that are responsible for the trait variants described above. If the original breeds have the following genotypes: RRYY x rryy (RRyy x rrYY is also possible), then the heterozygotes of the 1st branch generation have the genotype RrYy and the phenotype round and yellow. The germ cells of every heterozygous pea plant then have the combinations RY , Ry , rY or ry .

RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy

In the 2nd generation of branches, the phenotypes are split up in the ratio 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 .

It should be noted that this gap number ratio only results in a dihybrid cross if both genes have a dominant-recessive inheritance.

(If one of the two genes had an intermediate inheritance, there would be 6 different phenotypes in a ratio of 3: 6: 3: 1: 2: 1. The total number of cleavages is 16 in both cases, which is evidence of a dihybrid cross.)

See also

Web links

Commons : Punnett Square  - collection of images, videos, and audio files