Concept


When Mendel proposed that each trait is determined by a pair of genes, it presented a potential problem. If parents pass on both copies of a gene pair, then offspring would end up with four genes for each trait. Mendel deduced that sex cells — sperm and eggs — contain only one parental gene of each pair. The half-sets of genes contributed by sperm and egg restore a whole set of genes in the offspring. Mendel found that different gene combinations in the parents resulted in specific ratios of dominant-to-recessive traits. The results of a cross between two hybrid parents — each carrying one dominant and one recessive gene — were key to his synthesis. For example, a cross between two yellow-seed hybrids produces three times as many yellow seeds as green seeds. This is Mendel's famous 3-to-1 ratio.

Animation


I'm Reginald Punnett. William Bateson and I were very keen on Mendel's laws of inheritance. Herr Mendel was correct about how parental genes are passed to offspring. He was also correct about the 3:1 ratio of dominant to recessive traits in a hybrid cross. Let me show you what happens in these crosses. Each trait is defined by a pair of genes. Each parent can produce two types of gametes: one allele is "randomly separated" into each gamete. This is Mendel's law of segregation. I invented the Punnett square to keep track of the alleles in the gametes. So in Herr Mendel's Yy heterozygous cross, the alleles of one parent are represented along the top, the other along the side of the square. See how they pair up in the empty boxes that represent the offspring. Offspring result from pairing of the parental gametes; one from each parent. The offspring genotypes are YY, Yy and yy. Of course, yy peas are green. And while YY and Yy peas have different genotypes, they have the same yellow phenotype. As you can see, there are 3 yellow peas to 1 green pea. This is Mendel's three to one ratio. My squares are especially useful when tracking the inheritance of more than one trait at a time. Try the problem in this section and you'll see what I mean.

Gallery


Reginald Punnett worked with William Bateson on genetics research. Punnett later became the first Professor of Genetics at Cambridge.
Group portrait of the Cambridge University Natural Science Club. R. C. Punnett is in the second row, far right. Ernest Rutherford, the physicist, is in the top row, far left.
First page of a 1905 letter written by William Bateson, first Director of the John Innes Institute, to Adam Sedgewick, Cambridge professor. Bateson coined the term "genetics" in this letter.
Second page of a 1905 letter written by William Bateson, first Director of the John Innes Institute, to Adam Sedgewick, Cambridge professor. Bateson coined the term "genetics" in this letter.
Signed picture of William Bateson, around 1920.
William Bateson on a visit to Cornell. Also in photo, A.H. Sturtevant, C. Bridges, and R.A. Emerson, 1921.
William Bateson (left) with R.A. Emerson, 1922.
William Bateson (right) in his garden in Merton, England with Wilhelm Johannsen, 1924.

Audio/Video


Audio Glossary

Diploid, Dominant, Genotype, Germ line, Heterozygous, Homozygous, Phenotype, Recessive, Sex chromosome

Video Interviews

Robert Olby

Professor of History Robert Olby teaches at the University of Pittsburgh, and has written several books on the history of genetics, including Origins of Mendelism and The Path to the Double Helix.

Clip 1 (1:10)
Were Mendel's results too perfect?: Addressing the lack of a Chi-Square test for significance.

Clip 2 (1:00)
Were Mendel's results too perfect?: Examining his numbers.

Clip 3 (0:35)
Were Mendel's results too perfect?: Summary and opinion.

Biography


 

Reginald Punnett and William Bateson were among the first English geneticists. Punnett devised the "Punnett Square" to depict the number and variety of genetic combinations, and had a role in shaping the Hardy-Weinberg law. Punnett and Bateson co-discovered "coupling" or gene linkage. William Bateson brought Mendel's laws to the attention of English scientists.

REGINALD CRUNDALL PUNNETT (1875-1967)

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.

WILLIAM BATESON (1861-1926)

William Bateson was born in Whitby, England. As a young boy, Bateson was asked what he wanted to be. He replied that he wanted to be a naturalist, but if he wasn't good enough then he would have to be a doctor. Bateson was not a star student - he didn't see the value of learning the "classics," and favored the natural sciences. In 1878, Bateson entered St. John's College at Cambridge University. His father was the master of the college at the time. As a student and later a researcher in Cambridge, Bateson was interested in species variation and heredity. He traveled to the Central Asian steppe and collected data on how environmental conditions relate to variation. In 1894, he published a book Materials for the Study of Variation based on his observations. In this book, he outlined the experimental approach that should be used to study inheritance. He was designing Mendelian experiments.

Around 1897, Bateson began doing some of these hybrid experiments with poultry and butterflies. When he read De Vries' and Mendel's papers, Bateson recognized the importance of "Mendelian Law," especially given his own experiments. By 1902, Bateson had translated Mendel's works into English and was a strong supporter of the Mendelian laws of inheritance. Bateson is credited with coining the terms "genetics," "allelomorphs" (later shortened to allele), "zygote," "heterozygote" and "homozygote." In 1908, as a Professor of Biology at Cambridge, Bateson helped establish the Cambridge School of Genetics.

Bateson left Cambridge in 1910 to accept the Directorship of the John Innes Horticultural Institute at Merton. He continued to have ties to Cambridge, collaborating with R. C. Punnett on genetic experiments and publications. Bateson and Punnett co-founded the Journal of Genetics in 1910.

Bateson's work and Bateson himself influenced other biologists and scientists such as Archibald Garrod, Thomas Hunt Morgan, and Charles Davenport. Bateson had a combative, forceful personality, well suited to his self-appointed role of Mendel advocate. However, Bateson was reluctant to believe in the chromosomal theory of inheritance. He was vocally antagonistic to the idea and it wasn't until 1922 after a visit to Thomas Hunt Morgan's fly lab that he publicly accepted chromosomes and their role in heredity.

Factoid

Links


 

Links

Mendel Web

This is a site where you can get a lot of information about Gregor Mendel, his life and scientific achievements. Read how Mendel derived his 3:1 ratio from first generation and second generation hybrid results. Read what happened when Mendel looked at more than one trait at a time.

Cambridge University

Reginald Punnett and William Bateson were both professors at Cambridge University.

Bibliography

  • Bateson, Beatrice, 1928, William Bateson, F.R.S., Naturalist, Cambridge University Press, Cambridge.

  • Bowler, Peter J., 1989, The Mendelian Revolution, The Athlone Press, London.

  • Crew, F. A. E., 1968, R. C. Punnett: a Biography, Genetics, 58: 1-7, Genetics Society of America.

  • Dunn, L. C., 1965, A Short History of Genetics, McGraw-Hill, Inc., New York.

  • Editor, 1927, William Bateson: a Biography, Genetics, 12: ix-x, Genetics Society of America.

  • Iltis, Hugo, 1932, Life of Mendel, W. W. Norton & Company, Inc., London.

  • Moore, John A., 1985, Science as a Way of Knowing, American Society of Zoologists.

  • Olby, Robert C., 1966, Origins of Mendelism, Constable and Company Ltd., London.

  • Orel, Vitezslav, 1996, Gregor Mendel, the First Geneticist, Oxford University Press, Oxford.

  • Portugal, Franklin H., and Cohen, Jack S., 1977, A Century of DNA: A History of the Structure and Function of the Genetic Substance, The Massachusetts Institute of Technology, Cambridge, Massachusetts.

  • Stern, Curt, and Sherwood, Eva R., ed., 1966, The Origin of Genetics: A Mendel Source Book, W. H. Freeman and Company, San Francisco.

  • Stubbe, Hans, 1972 (English Translation), History of Genetics, The Massachusetts Institute of Technology, Cambridge, Massachusetts.

  • Sturtevant, A. H., 1965. A History of Genetics, Harper & Row, Publishers, New York.

Glossary


Diploid - Refers to a cell that has two sets of chromosomes. In humans, almost all the cells, except the gametes are diploid. Because there are 23 chromosomes in a single human set, most human cells contain 46 chromosomes.
Dominant - A gene that almost always results in a specific physical characteristic, for example, a disease, even though the patient's genome possesses only one copy. With a dominant gene, the chance of passing on the gene (and therefore the disease) to children is 50-50 in each pregnancy.
Genotype - The genetic identity of an individual, which may not show as outward characteristics.
Germ line - A sequence of cells, each descended from earlier cells in the lineage, which will develop into new sperm and egg cells for the subsequent generation.
Heterozygous - Possessing two different forms of a particular gene, one inherited from each parent.
Homozygous - Possessing two identical forms of a particular gene, one inherited from each parent.
Phenotype - The observable traits or characteristics of an organism, for example hair color, weight, or the presence or absence of a disease. Phenotypic traits are not necessarily genetic.
Recessive - A genetic disorder that appears only in patients who have received two copies of a mutant gene, one from each parent.
Sex chromosome - One of the two chromosomes that specify an organism's genetic sex. Humans have two kinds of sex chromosomes, one called X and the other Y. Normal females possess two X chromosomes and normal males one X and one Y.

Children resemble their parents.
Genes come in pairs.
Genes don't blend.
Some genes are dominant.
Genes are real things.
All cells arise from pre-existing cells.
Sex cells have one set of chromosomes; body cells have two.
Specialized chromosomes determine gender.
Chromosomes carry genes.
Genes get shuffled when chromosomes exchange pieces.
Evolution begins with the inheritance of gene variation.
Mendelian laws apply to human beings.
Mendelian genetics cannot fully explain human health and behavior.
DNA and proteins are the molecules of the cell nucleus.
One gene makes one protein.
A gene is made of DNA.
Bacteria and viruses have DNA too.
The DNA molecule is shaped like a twisted ladder.
A half DNA ladder is a template for copying the whole.
RNA is an intermediary between DNA and protein.
DNA words are three letters long.
A gene is a discrete sequence of DNA nucleotides.
The RNA message is sometimes edited.
Some viruses store genetic information in RNA.
RNA was the first genetic molecule.
Mutations are changes in genetic information.
Some types of mutations are automatically repaired.
A chromosome is a package for DNA.
Higher cells incorporate an ancient chromosome.
Some DNA does not encode protein.
Some DNA can jump.
Genes can be turned on and off.
Genes can be moved between species.
DNA responds to signals from outside the cell.
Different genes are active in different kinds of cells.
Master genes control basic body plans.
Development balances cell growth and death.
A genome is an entire set of genes.
Living things share common genes.
DNA is only the starting point for understanding human biology.
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