Gregor Mendel formulated three fundamental laws of inheritance based on his experiments with pea plants.
Gregor Mendel formulated three fundamental laws of inheritance based on his experiments with pea plants.
Mendel was postulated basically two law
1. law of segregation
2. inheritance. these law known as law of heredity or law of inheritance or mendelian heredity.
1. Laws of segregation or law of purity of gametes –
According to this law, two alleles of a gene remain separate and do not contaminate each other in F1 or the hybrid.
At the time of gamete formation in F1, two allele separate and pass into different gametes.
Gamete have single copy (allele) of any gene while somatic cell of an individual have two copy (allele) of any gene.
As a result a somatic cell may be either pure (two identical copy of a gene e.g. RR) or hybrid (containing two dissimilar copies of a gene e.g. Rr) for a gene.
But for necessity , the gametes are always pure for every gene since they have only one copy of each gene (either R or r) therefore law of purity of gametes are used in context of law of segregation.
The main feature of this law are –
üWhen a dominant and recessive alleles a genes come together in a hybrid after crossing between two plants having contrasting characters, they don’t mix or blend together.
üThey remain together in pure form without affecting each other. For this reason law of segregation also known as law of purity of gametes.
üThey separate into different gametes in equal numbers. Each gametes have only one type of alleles (A or a).
üSeparation of two alleles of a genes during gamete formation takes place usually due to the separation of homologous chromosomes during meiosis at anaphase 1, because of alleles are located in chromosomes.
üWith complete dominance, segregation leads to pheynotypic ratio of 3:1 in F2 for character govern by single gene and 9:3:3:1 ratio for character govern by two genes
üIf crossing over doesn’t takes place, segregation of genes takes place during anaphase 1. If crossing over occurs, segregation of genes will takes place during anaphase 2.
Example –
2. Law of independence assortment
According to this law, the segregation of two or more characters in the same hybrids is independent of each other.
Thus any allele of a gene is equally likely to combine with any allele of other gene and pass into the same gamete.
Independent segregation of two gene produces four different types of gametes in equal proportion.
A random union between these gametes give rise to 16 possible zygotes.
üSimultaneous inheritance of two characters of plant
üIn f1 when two different genes controlling two different characters, come together, each other, exhibits independent dominant behaviour without affecting or modifying the effect of other genes.
üThese gene pair segregate independently during gamete formation.
üEach alleles of one gene has equal chance to combine with each alleles of another genes.
üFree assortment leads to formation of new gene combination called recombinant type which is not similar to parent one.
üIt gives 16 possible combination of zygotes in 9:3:3:1 form.
Illustration -
When round yellow (RRYY) seeds of pea crossed with green wrinkled (rryy).
In F1 round yellow (RrYy) plants produces due to dominance effect of round and yellow over wrinkled green character.
F1 forms four type of gametes like round yellow (RY), round green (Ry), wrinkled yellow (rY) and wrinkled green (ry).
Selfing of f1 produces 9 (round yellow): 3 (round green): 3 (wrinkled yellow): 1 (wrinkled green) = 9:3:3:1 ratio.
Reason for Mendel’s success -
Selection of materials -
Mendel selected a plant which have short duration crop span, self pollinated (helpful in avoidance of contamination by other plants pollen) and have identical character for clear identification of different one.
Maintenance of record -
A systematic record of various contrasting characters which recorded by mendel at the the of observation helped in understanding of principle for character transmission from one generation to another generation.
Reason of failure of previous workers -
Mendel recorded shortfall of earlier workers and he not repeated that again in his investigation.
Earlier worker taken plant as a whole mean selected a large variety of characters which was difficult in interpretation and conclusions. Sometime purity was compromised due to inefficient management of pollination process.
Selection of characters -
Mendel selected only one contrasting character at a time and also selected a variety of characters which identified easily in the filed like shape seed coat, colour of seed, height of plants etc.
Mathematics -
His knowledge of math helped him a lot in the interpretation of investigations and findings.
Dominance -
Mendel in his investigation found that in F1 heterozygotic condition only one allele of a gene able to express. Expressing allele called as dominant allele and which is unable to express in heterozygote called recessive allele. On the basis of allele expression there are certain types of dominance prevails in the organisms which are as follows -
Complete dominance -
Phenotype produced by heterozygote is similar to homozygote of concerned dominant allele.
In shape of pea seed round shape is dominant over wrinkled shape.
Incomplete dominance -
When produced phenotype intensity of heterozygote is less than the phenotype of concerned dominant allele and has medium phenotypic effect neither as dominant nor recessive one.
In Mirabilis jalapa plant when red flower (R) plant crossed with white flower (r) plant.
F1 produces Pink colour (Rr) flower which is neither red nor white but their intensity is in between the two colour flower plants.
Co-dominance -
When in heterozygote produces phenotype of both alleles.
In blood group of human called A, B and AB group based on the antigen presence in blood.
Gene i responsible for the production of antigen.
Dominant allele of gene i - IA produces - antigen A called blood group A.
Dominant allele of gene i - IB produces - antigen B called blood group B.
In heterozygote where both alleles IA and IB of gene i produces respective antigen and called blood group AB.
Over-dominance -
When heterozygote has character intensity more than both alleles of homozygotes concerned.
White eye gene of Drosophila.
Exceptions of Mendelian genetics -
When Mendel was studying the inheritance of character in plants, many concepts were not there or concepts not follow the Mendelian rules, these concepts known as exception of Mendelian genetics.
Incomplete dominance -
When produced phenotype intensity of heterozygote is less than the phenotype of concerned dominant allele and has medium phenotypic effect neither as dominant nor recessive one.
In Mirabilis jalapa plant when red flower (R) plant crossed with white flower (r) plant.
F1 produces Pink colour (Rr) flower which is neither red nor white but their intensity is in between the two colour flower plants.
Co-dominance -
When in heterozygote produces phenotype of both alleles.
In blood group of human called A, B and AB group based on the antigen presence in blood.
Gene i responsible for the production of antigen.
Dominant allele of gene i - IA produces - antigen A called blood group A.
Dominant allele of gene i - IB produces - antigen B called blood group B.
In heterozygote where both alleles IA and IB of gene i produces respective antigen and called blood group AB.
Lethal gene -
Mendel got generally 3:1 ration in F2 generation which shows all genotype survival but in case of lethal gene, in which all individual carries lethal gene in appropriate manner dies and modified the typical monohybrid ratio of 3:1 to 2:1 or 1:1.
Multiple alleles -
Mendel identified on two form of a gene (factor) which govern the single character of an individuals. Now a days example exists where single character governs more than two form of a gene. Multiple allele are the condition in which more than two alleles govern the single character. Blood group is the best example of multiple alleles.
Pleiotropic gene -
When one gene express more than one one character or one gene controls more than one character such gene are called pleiotropic genes and phenomenon called pleiotropism.
At the time of mendels discovery one gene express only one character. Best example are awns control in wheat by a single gene.
Polygenes -
Nelsson and Ehle observed that some character are affected by several genes.
In polygenes each gene have additive effect on the character development. Mendel observed that one single character affected by only one gene not more than one gene.
Gene interaction -
When the expression of one gene depends on the presence of another allele of a gene is called as gene interaction.
It modifies the typical dihybrid ratio of Mendel’s observation which is 9:3:3:1.
There are many types of interactions we will peacefully learn in gene interaction section.
Cytoplasmic inheritance -
Cytoplasmic inheritance is due to gene located in the cytoplasm. Generally gene present in the nucleus controls the character development but in case of cytoplasmic inheritance genes of cytoplasm also contributed in the character development. It was not discovered at the time of mendel.
Linkage -
Tendency of two or more than two genes to stay together during inheritance is known as linkage. it do not shows independent assortment or segregation. Which affects the dihybrid ration of 9:3:3:1. more in linkage and crossing over topic.
Gene interaction -
When expression of one gene depends on the the presence or absence of another gene is called as gene action.
When one gene affects the expression of another gene for same character is known as epistasis.
Duplicate gene action (15:1) -
Same phenotype produces by two dominant gene whether it remain together or alone with recessive allele of other gene.
Different phenotype produces when both gene remain in homozygous recessive state.
Awn character of rice is governed by two dominant genes (A and B). Awn produces when any two of them are present. Awn not produces only in such conditions where both are in homozygous recessive condition.
Complementary gene action (9:7) -
Phenotype produces when dominant alleles of both genes are present.
If any one of two or both gene present in honozygous recessive forms contrasting character will be form.
Any one of two gene unable to express himself when it in alone but dominant allele of two gene complement to each other to express.
Flower colour in sweet pea, when both genes present in dominant form (A and B) produces purple colour but when these two genes are separate (AAbb or aaBB) or recessive (aabb) they produces white colour.
Supplementary gene action (9:3:4) -
Concerned phenotype - When one dominant gene present.
Modified phenotype - When first genes recessive is in recessive state and the another in dominant state.
Prevents phenotype - Recessive prevents phenotype production that is why remain white colour.
In maize, purple colour produce when both gene remain together in dominant (A and B) condition.
Red colour in presence of B gene while white when both gene are in homozygous recessive (aabb) condition or “b”.
Inhibitory gene action (13:3) -
Concerned phenotype - dominant gene “A”
No colour production - Recessive of “A” that is “a”
No colour - another gene “B”
Prevent colour production - By “B” of “A” when both are in dominant condition.
UPSC Agriculture Optional Previous Year Questions Enlist Mendel’s different laws. Describe in detail about law of segregation with suitable examples. 2-18 Elaborate Mendel's law of 'independent assortment' by giving suitable example.10 2016 Laws of heredity 2015 Describe the laws of heredity and their significance in plant breeding (20). 2006 What were the reasons for Mendel's success in his genetic experiments? Explain how Mendelian inheritance differs from cytoplasmic inheritance. 2000 What are Mendel's laws of inheritance? Explain briefly the principles of dominance. 1998 Write Mendel in principles in your own words. 1997 White fruit colour in summer squash is dependent on a dominant allele (W), and coloured fruit on the recessive allele (w). In the presence of ww and a dominant gene (G), the colour is yellow, but when G is absent (i.e.. Gg), the colour is green.Give the F phenotyes and proportions that are expected from crossing a white- fruited (WWGG) with greenfruited (wwgg) plant. 1996 In certain breeding stocks of maize, assume that gene A for increased vigour is closely linked with a recessive gene w, which makes the plant weak and that gene B for another quality also results in increased vigour but is closely linked with a recessive gene 1 for low viability. 1994 I. Which combination would be most efficient for production of vigour? II. What kind of mating would produce the desired combination? III. What would be the likelihood of obtaining a pure breeding strain with superior vigour? |
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