Dihybrid cross
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A dihybrid cross is a cross between two F1 offspring of two individuals that differ in two traits. For example: RRyy/rrYY or RRYY/rryy parents result in F1 offspring that are heterozygous for both R & Y.
A dihybrid cross is often used to test for dominant and recessive genes in two separate characteristics. Such a cross has a variety of uses in Mendelian genetics.
Meiosis is the cellular process of gamete creation, it is where sperm and eggs get the unique set of genetic information that will be used in the development and growth of the offspring of the mating. The rules of meiosis as they apply to the dihybrid are codified in Mendel's First Law and Mendel's Second Law also called the Law of Segregation and the Law of Independent Assortment.
For genes on separate chromosomes each allele pair shows independent segregation. If the first filial generation (F1 generation) produces four offspring, the second filial generation, which occurs by crossing the members of the first filial generation, shows a phenotypic (appearance) ratio of 9:3:3:1.
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[edit] Inventor of the Punnett Square:Reginald Punnett
Reginald Punnett was born in England. As a young boy, Punnett suffered from appendicitis. During one of his recuperative periods, he started reading a series of books - Naturalist's Library. His father had bought the books because of the elegant binding; Punnett was fascinated by the subject. Although he went to Cambridge University as a medical student, Punnett graduated with a zoology degree in 1898. After graduation, Punnett continued at Cambridge as a researcher. He did work on the morphology of nemertine (ribbon) worms. Punnett has two species of marine worms named after him, Cerbratulus punnetti, Punnettia splendia.
While at Cambridge Punnett became interested in the experimental process, and wrote to William Bateson who was doing Mendelian experimentation on plants and animals. This began a scientific collaboration which helped establish "genetics" at Cambridge. Bateson and Punnett published the first account of gene linkage in sweet peas and Punnett developed the "Punnett Square" to depict the number and variety of genetic combinations.
Punnett had a role in connecting Mendelism with statistics. In 1908, Punnett was asked at a lecture to explain why recessive phenotypes still persist — if brown eyes were dominant, then why wasn't the whole country becoming brown-eyed? Punnett couldn't answer the question to his own satisfaction. He in turn asked his friend the mathematician, G. H. Hardy. Out of this conversation came the Hardy-Weinberg Law which calculates how population affects genetic inheritance.
In 1912, when William Bateson decided not to return to Cambridge, Punnett became the first Arthur Balfour Professor of Genetics at the university. He worked on the genetics of sweet pea, maize and poultry, developing many breeds. He even used linkage as a way to sex type baby chicks. Punnett continued to do experiments even after his retirement in 1940.
Punnett was a quiet, tolerant, cultured man who was excellent at all sport involving a small fast ball. At 80, he was still an active member of the Savile Club in London where he played snooker. He died at his home in Somerset, England at the age of 92.
[edit] Mendel's Dihybrid Cross
Mendel crossed pure breeding plants with round seeds and yellow albumen to pure breeding plants with wrinkled seeds and green albumen. The F1 plants all had found seeds and yellow albumen and Mendel predicted that they would be heterozygous for both traits (RrYy). [[Image: http://www.mediafire.com/imgbnc.php/9bab09e058de9d37bd91bb10f6de4f5d2g.jpg]]
He then did a self-cross of the F1 plants to produce the results shown as the F2. He predicted that the plants with round seeds and yellow albumen would be of four genotypes in the specific ratio of 1 RRYY : 2 RRYy : 2 RrYY : 4 RrYy. And he made similar predictions for the plants with round seeds and green albumen and the ones with wrinkled seeds and yellow albumen. In order to test his prediction of more than one genotype within a phenotypic category in the F2, he allowed the F2 plants to self-cross and he recorded how many of the plants bred true or showed segregation for one or both of the characters. This is shown in the F3 data. The observed numbers of phentoypic types matches the predicted ratios.
[edit] Punnett square for a Dihybrid Cross
In the pea plant, two characteristics for the peas, shape and color, will be used to demonstrate an example of a dihybrid cross in a punnett square. R is the dominant gene for roundness for shape, with lower-case r to stand for the recessive wrinkled shape. Y stands for the dominant yellow pea, and lower-case y stands for the recessive green color. By using a punnett square (parents RYry x RYry; making the gametes are RY, Ry, rY, and 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 |
The result in this cross is a 9:3:3:1 phenotypic ratio, as shown by the colors, where yellow represents a round yellow (both dominant genes) phenotype, green representing a round green phenotype, red representing a wrinkled yellow phenotype, and blue representing a wrinkled green phenotype (both recessive genes).
