Complementary genes , Lethal genes and Effect of environmental conditions on action of some genes

Complementary genes

They are genes that work together to emerge a specific trait where the inheritance of this trait is controlled by two pairs of genes and the emergence of dominant character depends on the presence of a dominant gene at least in each pair .

Whereas absence of any pair of dominant genes or both will lead to disappearance of the dominant character and the recessive character appears .

The ratio in the first generation ( F1 ) is 100 % dominant and in the second generation ( F2 ) is 9 ( dominant ) : 7 ( recessive ) .

Example : Inheritance of flower colour in pea flower plant , where pink colour is the dominant  character , while the white colour is the recessive one .

When crossing a white-flowered plant ( AAbb ) with a white- flowered plant ( aaBB ) , the plants of F1 have pink flowers ( AaBb ) in ratio 100 % , where they gather a dominant gene from each pair .

When F1 plants were self-pollinated and their seeds were cultivated , the F2 plants have pink flowers and white flowers in ratio 9 : 7 , respectively .

In scientific references , it is mentioned that crossing a white-flowered plant ( AAbb ) with a white-flowered one ( aaBB ) will produce a generation of purple-flowered plants not pink .

Explanation : The appearance of the dominant character requires the gathering of a dominant gene or more from both pairs because both dominant genes participate to produce the dominant character where each of them controls the production of a specific enzyme that affects the formation of the pigment of colour .

This indicates the complement of genes action , where in this case the dominant character can be obtained from two parents , each carries the recessive character .

It is noticed that , The ratio of second generation ( F2 ) in case of complementary genes ( non-mendelian characters ) is 9 : 7 , whereas the ratio of second generation in case of law of independent assortment of genes ( mendelian characters ) is 9 : 3 : 3 : 1 .

Lethal genes

Lethal genes

Lethal genes

They are genes when found in pure state ( dominant or recessive ) cause harms to the living organism resulting in disruption of some vital processes and leading to the death of organism in different stages of life .

Types of lethal genes are Dominant lethal genes ( such as Gene of yellow fur colour in mice and Gene of bulldog strain in cattles ) and Recessive lethal genes ( Gene of chlorophyll absence in corn plant and Gene of infantile dementia in humans ) .

Dominant lethal genes

Example : Inheritance of fur colour in mice :

The gene of yellow fur colour in mice ( Y ) dominates over the gene of gray fur colour ( y ) , The presence of a pair of pure ( homozygous ) dominant genes of yellow fur colour causes death of yellow mice inside their mother’s uterus .

The dead mice represent about ¼ of the resulted generation ( 25 % ) , The inheritance of this character takes place through heterozygous parents in their genotype .

Example : what is the percentage of loss of mice when a yellow male mice is mated with a gray female mice ?

There is no loss in mice due to the lack of a pair of dominant pure ( homozygous ) lethal genes converted together ( there are no pure yellow mice among the individuals of the resulted generation ) .

Recessive lethal genes

Example : Inheritance of chlorophyll absence character in corn plant :

The gene of chlorophyll presence ( C ) in corn plants is dominant over the gene of chlorophyll absence ( c ) .

When some corn plants were self-pollinated and their seeds were cultivated , some seedlings free of chlorophyll ( white coloured ) were shortly grown , then wilt and die due to the presence of a recessive lethal gene , This gene is the chlorophyll absence gene .

The convergence of a pair of chlorophyll absence recessive genes ( in pure state ) in some seedlings leads to the prevention of formation of chlorophyll substance that acquires plants their green colour and responsible for absorbing light energy needed for photosynthesis .

The seedlings that wilt and die represent ¼ of the resulted generation ( 25 % ) , The inheritance of this character takes place through heterozygous parents in their genotype .

Effect of environmental conditions on action of some genes

Modern researches have proven that the action of some genes is affected by the factors surrounding the living organism , Studying these factors helps in avoiding risks that may arise from them .

such as Air pollutants , Oxygen deficiency , Exposure to rays , Some environmental factors as light and temperature .

Effect of light on appearance of chlorophyll character in green plants :

The gene responsible for chlorophyll formation in green plants needs the light factor to show its effect .

In absence of the gene causing the appearance of chlorophyll , the plant can not form chlorophyll pigment even if it was placed in light .

The internal cabbage leaves are coloured white because they are not exposed to light needed for appearing the effect of the gene of green chlorophyll pigment , on contrast the external leaves are characterized by the green colour due to their continuous exposure to light which helps the formation of chlorophyll .

When exposing internal cabbage leaves to light , they are changed to the green due to the appearance of the effect of chlorophyll gene .

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