The Open Door Web Site
Introduction to Mendelian Genetics
Genetics is the study of hereditary variations
Organisms have characteristic appearances. These appearances may vary from one individual to another.
The characteristics shown by an organisms are called its phenotype (From the Greek phainein = to show and typos = type)
Variations between organisms may be:
It is intraspecific variations that concern us here.
Nature or nurture?
Variations may be influenced by what is inherited from the parents (the genotype) or by the environment the organism encounters as it grows and develops.
The way we grow and develop could be because of the genes we inherit from our parents or because of the environment we experience (e.g. diet, climate, education) as we grow.
It could be a mixture of the two.You could grow to 1m90 because you inherited genes for tallness from your parents but you will not reach 1m90 unless you are well nourished as you grow.
Mendel's breeding experiments
Taking one character only as an example, seed colour, Mendel observed the following:
The reciprocal cross
Mendel tried the cross the other way round (green seed female plant x yellow seed male plant) and got the same results.
Dominant and recessive traits
The green seed coloured trait had disappeared but it reappeared in later generations as though it were hidden.
Traits which disappear and reappear (e.g. green seed colour in peas) are called recessive. Those that hide them are called dominant traits (e.g. yellow seed colour in peas).
Mendel produced a second generation of plants using the first generation. He brushed the male pollen grains onto the female parts of the same flower (remember pea plants are hermaphrodites).
This is called self pollination and it leads to self fertilisation or selfing.
The particulate theory of inheritance
Mendel concluded from this and other similar experiments, that characters are controlled by factors (later called genes) which act like separate particles, passed on from generation to generation. They are not changed or diluted to give intermediates.
The gene is the unit of hereditary information.
A character is controlled by a gene which may come in different types called allelomorphs (meaning "other forms") or alleles. These different alleles produce the different traits in a character.
In the example above pea seed colour is controlled by the seed colour gene. There are two alleles of this gene, the yellow allele and the green allele. The yellow allele is dominant and the green allele is recessive.
It becomes a bit tedious referring to "the recessive green pea seed colour allele" so it is convenient to use symbols for alleles.
Dominant alleles are given CAPITAL CASE LETTERS and recessive alleles are given small case letters.
It does not matter what letter you choose but it is useful if it has some connection with the name of the allele. It is also very useful if the alleles look different when written as small case and capital case (e.g. avoid C, O, P, S, U)
Thus: Yellow seed colour allele = Y and green seed colour allele = y (Note not "g", use the same letter for the alleles of a gene).
There is another notation which tends to used by geneticists using animals such as fruit flies.
Both letters are in small case.
Thus: w+ and w would represent two alleles for a gene where w+ is the dominant wild type allele and w is the recessive mutant allele.
Another convention exists for what are called co-dominant alleles.
The Law of Segregation
Mendel observed that the behaviour of the alleles controlling different traits revealed patterns in the way they are inherited. These patters always seemed to be the same so they are considered as a scientific law.
If we only consider the inheritance of one character (eg seed colour) and ignore all the others (such as flower colour, seed shape etc) this is called monohybrid inheritance.
Mendel worked this out in 1866. Though he no doubt understood fertilisation, meiosis was not observed until 30 years later
Homozygotes and heterozygotes
The zygote is the fertilised egg cell. So homozygous indicates that this individual received two identical alleles from their parents at fertilisation.
In our example:
In human genetics heterozygotes who have a dominant and a recessive alleles are called carriers. They are carrying a recessive allele without expressing it. Many genetic diseases are caused by recessive alleles.
When explaining a cross between two individuals we use a genetic diagram. There are certain conventions which you must use.
The Open Door Web Site is non-profit making. Your donations help towards the cost of maintaining this free service on-line.
Donate to the Open Door Web Site using PayPal