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Punnet Square: Genetic Inheritance Basic Calculation

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Reginald Punnett

From Wikipedia, the free encyclopedia

Professor Reginald Crundall Punnett FRS (20 June 1875 – 3 January 1967) was a British geneticist who co-founded, with William Bateson, the Journal of Genetics in 1910. Punnett is probably best remembered today as the creator of the Punnett square, a tool still used by biologists to predict the probability of possible genotypes of offspring. His Mendelism (1905) is sometimes said to have been the first textbook on genetics; it was probably the first popular science book to introduce genetics to the public.

Life and work

Reginald Punnett was born in 1875 in the town of Tonbridge in Kent, England . While recovering from a childhood bout of appendicitis, Punnett became acquainted with Jardine’s Naturalist’s Library and developed an interest in natural history. Punnett was educated at Clifton College.

Attending Gonville and Caius College at the University of Cambridge, Punnett earned a degree in zoology in 1898, and a masters in 1901. Between these degrees he worked as a demonstrator and part-time lecturer at the University of St. Andrew’s Natural History Department. In October 1901, Punnett was back at Cambridge when he was elected to a Fellowship at Gonville and Caius College, working in zoology, primarily the study of worms, specifically nemerteans. It was during this time that he and William Bateson began a research collaboration, which lasted several years.

When Punnett was an undergraduate, Gregor Mendel’s work on inheritance was largely unknown and unappreciated by scientists. However, in 1900, Mendel’s work was rediscovered by Carl Correns, Erich Tschermak von Seysenegg and Hugo de Vries. William Bateson became a proponent of Mendelian genetics, and had Mendel’s work translated into English. It was with Bateson that Reginald Punnett helped established the new science of genetics at Cambridge. He, Bateson and Saunders co-discovered genetic linkage through experiments with chickens and sweet peas.

In 1908, unable to explain how a dominant gene would not become fixed and ubiquitous in a population, Punnett introduced one of his problems to the mathematician G. H. Hardy, with whom he played cricket. Hardy went on to formulate the Hardy-Weinberg principle, independently of the German Wilhelm Weinberg. He was Superintendent of the Cambridge University Museum of Zoology from 1908-1909.

In 1909 he went to Sri Lanka to meet Arthur Willey, FRS, then Director of the Colombo Museum and R H Lock, then Scientific Assistant at the Peradeniya Botanical Gardens and to catch butterflies. The following year, he published a monograph, ‘”Mimicry” in Ceylon Butterflies, with a suggestion as to the nature of Polymorphism’, in Spolia Zeylanica, the journal of the Colombo Museum, in which he voiced his opposition to gradualistic accounts of the evolution of mimicry which he later expanded on, in his 1915 book ‘Mimicry in Butterflies’.

In 1910 Punnett became professor of biology at Cambridge, and then the first Arthur Balfour Professor of Genetics when Bateson left in 1912. In the same year, Punnett was elected a Fellow of the Royal Society. He received the society’s Darwin Medal in 1922.

During World War I, Punnett successfully applied his expertise to the problem of the early determination of gender in chickens. Since only females were used for egg-production, early identification of male chicks, which were destroyed or separated for fattening, meant that limited animal-feed and other resources could be used more efficiently. Punnett’s work in this area was summarized in Heredity in Poultry (1923).

Reginald Punnett retired in 1940, and died at the age of 91 in 1967 in Bilbrook, Somerset.

Punnett square

From Wikipedia, the free encyclopedia

A Punnett square showing a typical test cross

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring’s having a particular genotype. The Punnett square is a tabular summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross. These tables give the correct probabilities for the genotype outcomes of independent crosses where the probability of inheriting copies of each parental allele is independent. The Punnett Square is a visual representation of Mendelian inheritance.

Monohybrid cross

Main article: Monohybrid cross

In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or the b allele. (It is conventional in genetics to use capital letters to indicate dominant alleles and lower-case letters to indicate recessive alleles.) The probability of an individual offspring’s having the genotype BB is 25%, Bb is 50%, and bb is 25%.

Paternal
B b
Maternal B BB Bb
b Bb bb

It is important to note that Punnett squares give probabilities only for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact (see Mendelian inheritance). For classical dominant/recessive genes, like that which determines whether a rat has black hair (B) or white hair (b), the dominant allele will mask the recessive one. Thus, in the example above, 75% of the offspring will be black (BB or Bb) while only 25% will be white (bb). The ratio of the phenotypes is 3:1, typical for a monohybrid cross.

Dihybrid cross

Main article: Dihybrid cross

More complicated crosses can be made by looking at two or more genes. The Punnett square works, however, only if the genes are independent of each other, which means that having a particular allele of gene A does not alter the probability of possessing an allele of gene B. This is equivalent to stating that the genes are not linked, so that the two genes do not tend to sort together during meiosis.

The following example illustrates a dihybrid cross between two heterozygous pea plants. R represents the dominant allele for shape (round), while r represents the recessive allele (wrinkled). A represents the dominant allele for color (yellow), while a represents the recessive allele (green). If each plant has the genotype RrAa, and since the alleles for shape and color genes are independent, then they can produce four types of gametes with all possible combinations: RARarA, and ra.

RA Ra rA ra
RA RRAA RRAa RrAA RrAa
Ra RRAa RRaa RrAa Rraa
rA RrAA RrAa rrAA rrAa
ra RrAa Rraa rrAa rraa

Since dominant traits mask recessive traits, there are nine combinations that have the phenotype round yellow, three that are round green, three that are wrinkled yellow, and one that is wrinkled green. The ratio 9:3:3:1 is typical for a dihybrid cross.

Tree method

The tree method (AKA forkline, branching system) can also solve dihybrid and multihybrid crosses. A problem is converted to a series of monohybrid crosses, and the results are combined in a tree. This method does not calculate the gamete genotypes. However, a tree produces the same result as a Punnett square in half the time.

Dihybrid Cross Tree Method.png

This method is particularly advantageous when crossing homozygous organisms.

Homozygous cross tree method.png

See also

Further reading

  • Campbell, Neil. Biology (7th ed.). Benjamin-Cummings Publishing Company. ISBN 978-0-8053-7146-8. OCLC 71890442.

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