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Whilst these notes have started with some of the molecular biology of the gene that is known now, the discovery of the gene and its role in inheritance of characteristics came well before the structure and function of DNA was discovered.
Since it would be almost impossible to find a student course of genetics which does not mention the scientific history behind this area of biology, we should not let this be ignored here.
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Gregor Mendel, an Austrian monk who experimented with breeding experiments in the garden pea, Pisum sativum, is credited with being the "father" of the science of genetics - heredity. He did not know about chromosomes or DNA, nor did he know about genes as we do today. However, he performed many experiments in which he controlled the breeding of his pea plants. By observing the number and types of phenotypes in the offspring plants he developed the rules of inheritance known as Mendel's laws today.
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For each of the hundreds of experiments that Mendel performed, using a variety of traits, he found that the F1 generation always had offspring displaying only one of the parental traits. However, the F2 generation was seen to display both of the parental traits in a ratio, which was calculated over many thousands of plants, of 3:1 . Mendel described the trait which was always seen in the F1 generation as dominant. This was also the most frequently observed phenotype in the F2 generation. The trait which disappeared in the F1 generation, but appeared in 25% of the F2 generation, he called recessive.
Putting together these observations of Mendel's and what we now know about chromosomes, genes, alleles and meiosis, it becomes possible to understand these basic genetic experiments.
It is important to remember that predicting the outcomes of genetics experiments is based on probability, and that if enough repeats are performed the theoretical outcome becomes more likely. However, chance is just that, and the theory and result do not always match very well!A simple monohybrid cross is one in which data about one gene with two alleles is obtained. A diagram called a Punnett square (named after Reginald Punnett!) can be used to predict the outcomes of such a cross.
For a simple Mendel's pea experiment, using pure breeding tall (genotype TT) and short (tt) plants as the P generation, the theoretical outcomes for the F1 and F2 generations can be calculated using Punnett squares:
P generation: |
T | T |
|---|---|---|
| t | ||
| t |
Using members of the F1 generation to breed the F2 generation, the same process is used to predict the outcome:
F1 generation:| |
T | t |
|---|---|---|
| T | ||
| t |
Occasionally, "tricks" can appear in apparently straightforward monohybrid cross questions. The most common of these is that the expected ratios in the F2 generation are not observed. This is almost always explained by one of the homozygous combinations being lethal. This means that individuals with the lethal genotype fail to develop and they die as embryos or perhaps they die immediately after birth.
Let's consider this possibility in the genotype and phenotype ratios of the F2 generation be in the previous example if :
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A dihybrid cross involves two pairs of contrasting traits. In the simplest dihybrid crosses, it is assumed that the genes for the traits are located on different chromosomes and so Mendel's laws are true for them.
Predicting the results of a dihybrid cross is rather more complicated than a monohybrid cross because one needs to consider all the possible combinations of the two alleles for the two traits in each parent's gametes and then all the possible gamete combinations at fertilization.
A Punnett square is again used and if students recognise that the most complex dihybrid cross requires a 4x4 (16 cell) Punnett square for the offspring, with plenty of practice all possible dihybrid cross questions become easy. The following example illustrates this.
In guinea pigs the allele for short hair (S) is dominant over the allele for long hair (s). (Note that although short hair is "little hair" the allele symbol is "big S" because it's dominant.) The allele for black hair (B) is dominant over the allele for brown hair (b).
Suppose two guinea pigs, both heterozygous for both traits, mate. What are the theoretical types and ratios of the phenotypes and genotypes of the offspring? (This is the most complex possible dihybrid cross!)
Parents:| |
SB | Sb | sB | sb |
|---|---|---|---|---|
| SB | |
|||
| Sb | |
|||
| sB | |
|||
| sb | |
Now comes the (tedious) task of counting all the genotypes and assigning phenotypes!
Offspring:After doing several of these problems, students will immediately recognise the phenotype ratios of 9:3:3:1 as the theoretical outcome of a heterozygous dihybrid cross.
Please do lots of examples of dihybrid cross problems. These questions on Biology CAT 3 exams have been answered poorly in the past, so here is a chance to pick up some marks that others may not get! But you'll need plenty of practice to become an expert at these!The relationships between genes and their alleles are not always as simple as the dominant/recessive independently assorted traits described so far. Most of the time, genes show more complex patterns of heredity.
In some organisms, heterozygous individuals display a phenotype somewhere between the two homozygous phenotypes. An example of this is in snapdragon flowers:
| GENOTYPE | PHENOTYPE |
|---|---|
| RR (or better, R'R') | Red flowers |
| Rr (or better, R'R) | Pink flowers |
| rr (or better, RR) | White flowers |
In other cases, two dominant alleles are expressed at the same time. This also results in a mid-way phenotype in the heterozygous individual but for slightly different reasons. The roan coat of horses illustrates this:
| GENOTYPE | PHENOTYPE |
|---|---|
| R'R' | Red hairs - red coat |
| R'R | Red hairs AND white hairs - roan coat |
| RR | White hairs - white coat |
Sometimes it is difficult to distinguish between incomplete dominance and codominance as the heterozygote in both cases can appear to be somewhere between the two homozygote extremes. But if the heterozygote is "blended", (as in pink flowers) the inheritance is usually described as incomplete. But if the heterozygote clearly shows both of the homozygotes features (as in red-and-white striped carnations with genotype R+R-, or in the blood group phenotype AB which corresponds to the genotype IA IB), then it is codominance.
This concludes the basic genetic theory for Area of Study 1. But, as in all areas of biology, genetics can get far more complicated than this. The final section in Area of Study 1 is covered in the next section, which I've called Real-Life Genetics.
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