Initially using examples of the investigations of Mendel into the inheritance of characteristics by plants - tables of possibilities of outcomes, Punnett square analysis and genetic cross diagrams.

A good example is to consider some of the results of Mendel’s work which preceded the work by other scientists which links Mendel’s ‘inherited factors’ with the chromosomes of the humble pea.

Mendel was an Austrian monk, educated in mathematics and natural history at the University of Vienna.

Gregor Mendel, working in a humble garden plot of his monastery in the mid 19th century, made notes that provided good experimental evidence on how characteristics of pea plants were passed on from generation to the next.

Mendel conducted many experiments to investigate how characteristics of plants (particularly pea plants) were passed on from one generation to the next.

His 'classic' investigations included looking at the height and colour of pea plants.

His research results were published in 1866 and eventually became an important work and foundation of the relatively modern study of genetics.

We are now able to explain why Mendel proposed the idea of separately inherited factors. The importance of his discoveries were not recognised until after his death because there was no knowledge of chromosomes, genes and how DNA functions.

The principles used by Mendel in investigating monohybrid inheritance in peas were ...

His worked involved (as far as he could tell) crossing different pure bred pea plants of a particular characteristic eg a particular colour or tall or short plants and then cross-breeding the offspring e.g.

(a) Mendel's experiment on height - first cross

Mendel crossed a tall pea plant (in modern notation, genotype TT) with a dwarf pea plant (genotype tt) and found all the offspring were tall.

Note that he used pure bred tall or dwarf (short), which we know recognise as homozygous genotypes.

TT = homozygous alleles (dominant)

Tt = heterozygous alleles

tt = homozygous alleles (recessive)

Genetic diagram for TT x tt

Above and below are the 'modern' genetic diagram and Punnett square for crossing the tall pea with a dwarf pea (1st cross to give F1)

This is crossing a homozygous dominant 'parent' TT with a homozygous recessive 'parent' tt.

The TT and tt allele plants were pure bred, i.e. no heterozygous allele pairs i.e. no Tt pairings.

The diagrams above and below give a modern genetic interpretation of Mendel's results from initially crossing a pure line of tall pea plants with a pure line of dwarf pea plants (F1 hybrids)

From the  using a Punnett square, gives 100% tall plants (genotype Tt), but, in terms of modern genetics, they all carry the allele t for dwarf pea plants.

(b) Mendel's second cross of two of the tall plants from the first set of offspring

(from the F1 hybrids above)

Genetic diagram for Tt x Tt

This is crossing a heterozygous 'parent' Tt with another heterozygous 'parent' Tt.

The 'modern' genetic diagram and Punnett square for crossing two plants from the 1st cross (2nd cross to give F2 hybrids)

The modern interpretation is shown by the Punnett square and inheritance diagram analyses below.

This gave approximately 75% tall plants (genotype TT or Tt) and 25% dwarf pea plants (genotype tt)

Mendel found that the second cross produced tall : dwarf pea plants in the approximate ratio of 3 : 1.

He therefore showed that the tall pea plant trait was dominant over the dwarf pea plant trait.

The genetic diagrams and Punnett squares shows why you statistically expect these results.

The ratio of tall plants to dwarf plants (3 : 1) showed that the dominant factor was 'tall' over the 'dwarf factor'.

BUT, he also showed that under the right circumstances, dwarf pea plants were formed and we now know this is due to the double recessive gene combination.

From these humble, but carefully done experiments, Mendel deduced that the height characteristics (and other characteristics) were determined by what he called 'separate inherited factors' passed on from each parent plant.

We now know that these 'separate inherited units' in modern genetic theory are genes.

 

(c) He did similar experiments with the colour of pea plants.

He did similar experiments with pea plants with purple and white coloured flowers.

The PP (purple) and pp (white) allele plants were pure bred, i.e. no heterozygous allele pairs i.e. no Pp pairings.

Again, the modern interpretation is shown by the Punnett square analyses.

Genetic table for crossing purple pea with white pea
Parent genotypes: PP x pp
Gametes: P, P, p and p (alleles)
Genotypes of plants - gametes - alleles	P	P
p	Pp	Pp
p	Pp	Pp

PP = homozygous alleles (dominant)

Pp = heterozygous alleles

pp = homozygous alleles (recessive)

The diagrams above and below give a modern genetic interpretation of Mendel's results from initially crossing a pure line of purple pea plants with a pure line of white pea plants (Punnett square of F1 hybrids)

This gives 100% purple plants (genotype Pp), but, in terms of modern genetics, they all carry the allele dominant P for purple flowers and recessive allele p for white pea plants.

The first resulting offspring (F1) were all purple pea plants, and these were then crossed with each other, to give the second set of offspring (F2) shown above.

(d) The outcome and importance of Mendel's experiments

and why wasn't Mendel's brilliant work recognised at the time?

From his experiments Mendel concluded the following:

(i) Characteristics in plants are determined by some kind of 'hereditary units' (we now know as genes).

(ii) These hereditary units are passed from one generation to their offspring unchanged from both parents AND one 'unit' from each parent (plant).

(iii) These 'hereditary units' can be 'dominant' or 'recessive' -if a plant has both the 'dominant unit' and 'recessive unit', the dominant characteristic would be expressed (the observed phenotype).

Mendel's work was so new and revolutionary that most scientists just didn't appreciate the results of his experiments - his results didn't fit in with any current theory of the time!

Few, if any? other scientists seem to doing the same sort of experiments as Mendel and then publishing their results, so there was no independent verification of his results.

Mid 19th century scientists had no knowledge of modern genetics e.g. DNA, genes, chromosomes etc.

Fortunately, after his death, scientists e.g. biologists, began to realise the significant of his work after it was published in 1866 and linking inherited factors with genes and chromosomes.

Using Mendel's experiments as a guide, many experiments have been done to confirm his ideas and further contribute to our understanding of genetics - the fundamental theory of inheritance at the molecular level e.g.

From the late 19th century the structures we call chromosomes were recognised and microscopes were good enough to see how they behaved during cell division.

But, it was only early in the 20th century that scientists realised the similarity between the way chromosomes behaved and Mendel's 'inheritance units'

Therefore it was proposed that these Mendelian 'units' were part of the structure of chromosomes - these, as we now know, are genes/alleles.

Finally (sort of), in 1953, through the work of Crick, Watson and others, the double helix structure of DNA was worked out.

The science of genetics has advanced so much that we now the sequence of the nucleotides (and their bases) in the complete genome of an organism - known as genome sequencing.

With this knowledge we can now understand how genes work at the molecular level e.g. from DNA, via RNA, codes for proteins and many other functions of an organism.

Scientists can use genome sequencing to identify which parts (genes) control particular characteristics of an organism.

This can get very complicated because (i) most characteristics are controlled by several genes and (ii) genetic variants interact with each other.

Footnote on (sort of): The chemistry of genetics is developing all the time and is turning out to be far more complicated than could ever have been envisaged back in 1953.

We are now able to test whether people are susceptible to a particular disease or inherited disorder. See genetic screening.

We can modify organisms to introduce a specific gene into their genome.

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