NON-MENDELIAN GENETICS.

Mendelian genetics is the foundation of genetics, true but non-Mendelian genetics is the roof. Minor exceptions were noticed after detailed studies of Gregor Mendel’s experiments. Let’s study a few. For the sake of understanding and simplicity, we will be focusing on monohybrid inheritance at this level (with the exception of sex linkage).

It is important we remember a few things as far as Mendelian genetics is concerned. Let’s recall that a test cross with a homozygous dominant individual gives a unique phenotype which is that of the dominant allele. This is in accordance with the law of dominance studied in part 3 (Mendelian Genetics). When these offspring are crossbred or interbreed, their offspring have a phenotypic ratio of 3:1. Consider the figure below. Figure A is the test cross with the homozygous dominant individual while figure B is the cross that occurs between offspring with the genotypes of the first filial (F1) generation to produce the second filial (F2) generation.

Incomplete or Partial dominance:

Incomplete dominance is that phenomenon in genetics whereby a given trait is not completely dominant or recessive over or to another in a given organism. Simply said, there is no master between both traits. A perfect example is the snapdragon flower shown.. In this flower, we see red, white, pink and yellow flowers. The red and white exhibit incomplete dominance. Experiments have shown that a plant homozygous for red appears RED and a plant homozygous for white will produce WHITE flowers but when the plant is heterozygous for both, a PINK colour is noticed. Notice that a new colour is formed in the F1 generation. This goes against the very first of Mendel’s laws. This is because both traits try to dominate over each other without success. The detailed genetics is out of the scope of this note (restricted to high school information).

One thing we notice is that, the first filial generation has the exception from Mendel’s law that it introduces a new colour.

EXERCISE 3:

In four o’clock plants, red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of

1. The F1 generation?
2. The F1 generation crossed with its red parent?
3. The F1 generation crossed with its white parent?

Codominance:

This is the phenomenon whereby 2 or more expressions of a given trait, express themselves phenotypically on a given organism at the same time. In this case, both expressions are dominant. For example, pure black cats and pure white cats are well known but we also know and recognise cats with both black and white fur colourations. This is due to codominance. Let’s actually consider the case of a black cat (BB) and a white cat (WW). Knowing that they are both dominant, the organisms that present only the black or white colour are therefore homozygous for the colour. Also, there aren’t any new colours in this cross.

Sex-linkage:

In the study of certain traits, it was noticed that some characteristics where more prevalent in some genders (mostly men). Men where noticed to be more viable to present traits, mostly diseases like colour blindness than females. This raised thought as to the fact that these genes could be linked to the sex chromosomes. The male gender is determined by the presence of a Y-chromosome amongst the total chromosomes, leading to an XY pair of sex chromosomes while females have a double X-chromosome (XX). The Y-chromosome is a small chromosome and hence lacks space for so many genes. This influences trait inheritance.

EXAMPLE 4:

Can a man with haemophilia pass on the disease to his son and grandson?

Answer:

It is important to note that here that haemophilia is a disease expressed only in the double recessive state. To properly answer this question we have to consider 2 cases:

• The man and his wife: The wife can be healthy, a carrier, or a sick individual. Whatever be the case, the man’s son can never get the disease from him because the gene for the disease is on the X – chromosome as shown below. The cross below is that of a haemophilic man and a healthy wife. You can execute that of the carrier and sick wife as exercise.

For the sick wife, you’ll have: 2 sick boys and 2 sick girls.

For the carrier wife, you’ll have: 1 sick boy, 1 sick girl, 1 carrier girl and 1 healthy boy.

• The man’s son and wife: The man’s grandson can get haemophilia even if the son was perfectly healthy. This is true if the son marries either a carrier or a sick woman. Below is a situation where the woman is haemophilic. You can execute the cross for a situation where the woman is a carrier. The results of your cross should be; 1 sick boy, 1 healthy boy, 1 carrier girl and 1 healthy girl.

Crossing over:

Crossing over refers to the exchange of genes between homologous chromosomes. As observed in the diagram (above), this takes place between non-sister chromatids. Recall that the point where sister chromatids intersect or join is called the Chiasmata, the point where the exchange of genes takes place during crossing over is called the Chiasma. Crossing over is not obliged or forced to occur but may occur during prophase one, of meiosis. Crossing over increases diversity amongst organisms and thus increases adaptation. Nonetheless, it is still unclear on what exactly causes crossing over to occur or what even initiates it.

There equally exist a concept called crossover value (COV). Crossover value refers to the rate at which parental genotypes repeat amongst the offspring. It is one the parameters considered during gene mapping. This is because, if genes are too distant from each other in a chromosome, the tendency for them to be included in the crossover process is less likely.

Multiple Alleles:

According to Mendel’s papers, every trait is coded by two alleles on the same locus of sister chromatids. Nonetheless, it’s been noticed that his claims are true for diploid organisms in most cases but slight changes are observable in such a way that two different populations of the same organism may have different alleles on that same locus. Like in population A, the alleles are AA while in population B, it is rather BB but are all for the same characteristic. Such a trait is found in humans. It’s their blood groups. The most studied and common blood group system in humans is the ABO blood system. We realise that it is coded by three different co-dominant alleles, A, B and O (only two are expressed per individual).

The ABO blood system is studied in greater detail in the cardiovascular system. Below is an example of multiple alleles in rabbits.

Lethal genes

These are genes that cause death when found in an individual. Generally, this happens when the alleles are in a homozygous condition. It is important to note that lethal genes must not kill the individual immediately the mutation occurs (e.g achondroplasia, Epiloia), it can equally kill the individual a while after birth (e.g sickle cell anaemia). It is important to note that it must not be recessive but could also be as a result of dominant alleles.