In this article, we are going to study Mendelian Inheritance its laws, and applications.
Gregor Johann Mendel and Mendelian Inheritance
“Gregor Johann Mendel is called the father of genetics.”
Mendel was born on July 22, 1822, in Heinzendorf village in Austria (now in Czechoslovakia).
He was a monk in a monastery at Brunn, Austria. In 1840 he completed his graduation and became a priest in 1847.
He worked as a teacher in a preparatory school and he was very much interested in plant hybridization.
He performed his experiments on the sweet pea plants (Pisum sativum).
After seven years of experimental work in the monastery garden, he published his results in ‘Annual proceedings of Natural History Society’ Brunn (1866). Unfortunately, this work remained neglected until 1900.
In 1868 he became ‘Abbot’. He died in 1884.
In 1900 three botanists, De Vries of Holland, Carl Correns of Germany, and Tschermak of Austria independently discovered similar conclusions as those of Mendel.
De Vries republished Mendel’s research in 1901 and the value of his work was appreciated long after his death.
Why Sweet pea plant?
- Pea is an annual plant and easy to cultivate.
- It has a short life span. Thus many generations can be studied in a short time.
- It is a naturally self-pollinating plant hence pure lines of characters are maintained.
- It is easy to cross-pollinate and produce fertile offspring after cross-pollination.
- The flowers are large enough or easy to carry out the emasculation process.
- Many verities are available with easily distinguishable morphological characters.
Selection of characters
Mendel selected pea plants with seven contrasting characters. Let’s see these 7 contrasting characters…
| Sr. No. | Character | Dominant | Recessive |
|---|---|---|---|
| 1 2 3 4 5 6 7 | Stem length Flower colour Flower position Pod shape Pod colour Seed form Cotyledon colour | Tall (T) Red (R) Axial(A) Inflated(I) Green (G) Round (R) Yellow (Y) | Dwarf (t) White (r) Terminal (a) Constricted(i) Yellow (g) Wrinkled (r) Green (y) |
Reasons for Mendel’s success
- Careful and proper selection of pea plants with advantageous characters like self-pollinating but can be easily cross-pollinated, short life span, the large size of flowers, etc.
- He concentrated on only one contrasting character at a time.
- He performed a large number of crosses to avoid chance factors.
- He maintained a pedigree record of every generation.
- He analyzed the data statistically.
- One form of character was completely dominant over the other.
- Luckily seven characters selected by Mendel were present on seven different
chromosomes. (He was unknown about that fact).
Mendel’s Laws of Inheritance
Mendel established Three laws of inheritance from his experiments of hybridization. They are as follows-
- Law of dominance
- Law of segregation or purity of gametes
- Law of independent assortment
Law of Dominance
Statement – “Law of dominance states that in the crosses between organisms for contrasting characters, only one character of a pair appears in the (F1) first generation that character is called a dominant character and another character which is not expressed is called a recessive character.”
Example – In the crosses between Tall and Dwarf plants, all plants were tall in F1 generation but in F2 generation they were tall as well as a dwarf.
In the crosses between red and white flower plants, all the plants in the F1 generation were with red flowers and F2 generation showed both the red flower plants and the white flower plants.
In the cross between tall and dwarf plants, tallness appears in the F1 generation and suppresses the dwarfness hence tall character is dominant and the dwarf character is recessive.
Similarly, in the cross between the red flower plant and the white flower plant, the red color of the flowers is dominant as it appears in the F1 generation while the white color of the flowers is recessive as it is not expressed in the F1 generation.
The contrasting characters are determined by factors. “The term factor is coined by Mendel, now it is called a gene.“
The genes of contrasting characters are called alleles or allelomorphs. The gene or factor is represented by the appropriate alphabet.
For the dominant character, a capital letter is used while for the recessive character corresponding small letter is used. Thus tallness is represented by letter ‘T’ and dwarfness is represented by the letter ‘t’.
Each character is controlled by a pair of genes or factors. Hence genotype of a pure tall parent is ‘TT’ and of the pure dwarf parent is ‘tt’.
When both the factors in a parent plant are identical i.e. either dominant (TT) or recessive (tt) then it is called homozygous or true breeding or pure breeds. When a genotype contains one dominant and one recessive gene or factor i.e. ‘Tt’ then it is called hybrid or heterozygous.
Monohybrid cross and ratio –
Definition – “A cross between two parents considering only one contrasting character at a time is called monohybrid cross and the phenotypic ratio of F2 individuals obtained from this cross is called monohybrid ratio.”
Example – A cross between pure tall and pure dwarf plants.
Phenotype – Tall X Dwarf
Genotype – TT tt
Gametes – T t
F1- generation – Tt (All tall)
Selfing of F1 Tt X Tt
Gametes – T t T t
Punnet square –
Phenotypic ratio – 3:1
Genotypic ratio – 1:2:1
On selfing F1 plants, 3 Tall : 1 Dwarf ratio is obtained in the F2 generation. F1 plants produce two types of gametes i.e. T and t.
The results of the F2 generation are represented by using Punnet square. In the F2 generation, tall and dwarf plants appeared in the 3:1 ratio which is called a phenotypic ratio.
Among all tall plants in the F2 generation, 1/3 are homozygous and 2/3 are heterozygous (hybrid tall). Thus genotypic ratio can be represented as 1 homozygous Tall : 2 heterozygous Tall : 1 homozygous Dwarf.
Law of segregation or law of purity of gametes
Statement – “Law of segregation states that when hybrid forms gametes, the contrasting factors or alleles segregate and enter into different gametes. The gametes forms are pure and never hybrid. Hence it is also called the law of purity of gametes.’’
Example – In the cross between tall and dwarf plants, F1 generation plants are heterozygous i.e. hybrid containing both the factors for tallness and dwarfness. These factors remain unaffected by each other without mixing.
At the time of gamete formation, these factors segregate from each other, hence one gamete receives a factor for tallness and the other gamete receives a factor for dwarfness.
Thus gamete contains only one gene of a pair of an allele for any single character. Hence gametes are pure for any gene and never hybrid.
Selfing of F1 Tt X Tt
Gametes – T t T t
Punnet square: F2 – generation
table
A dwarf plant with ‘tt’ factors obtained in F2 generation must have received one
gamete with ‘t’ (dwarfness) from each of the hybrid parents. This is possible only when two
factors of a trait (Tt) in hybrid segregate or separate at gamete formation.
Law of independent assortment
Statement – “Law of independent assortment states that the genes for each character separate and enter the gametes independently of the genes of another character.”
Mendel studied the law of independent assortment in dihybrid cases.
Dihybrid Cross and Ratio
Definition – “A crossing between two parents considering two contrasting characters at a time is called dihybrid cross and the phenotypic ratio of F2 individuals obtained from this cross is called dihybrid ratio.”
Example – A cross of pea plants having yellow and round seeds with green and wrinkled seeds. This cross produces yellow and round seeds in the F1 generation indicating this character as a dominant character.
When F1 plants were self-fertilized, in F2 generation 4 kinds of plants were produced in the ratio of 9:3:3:1.
Phenotype – Yellow Round X Green Wrinkled
Genotype – YYRR yyrr
Gametes- YR yr
F1 – generation – YyRr (Yellow Round)
Selfing of F1 – YyRr X YyRr
Gametes – YR Yr yR yr YR Yr yR yr
Dihybrid ratio – 9 (Yellow round): 3 (Yellow wrinkled) : 3 (Green round) : 1 Green wrinkled. (i.e. 9 : 3 : 3 : 1)
Explanation: The genotype of a plant having yellow round seeds is YYRR and that of a plant having green wrinkled seeds is yyrr. Gametes produced by YYRR plant are with ‘YR’ alleles and gametes of yyrr plant are with ‘yr’ alleles.
After fertilization hybrid plant with ‘YyRr’ is produced in the F1 generation. This plant will produce yellow and round seeds as ‘Y’ and ‘R’ are dominant over ‘y’ and ‘r’.
On selfing of F1 hybrid plants, they can produce four possible gametes i.e. YR, Yr, yR, and yr. Thus factors of characters are assorted independently of the other pair of factors.
E.g. the gene ‘Y’ may combine with dominant ‘R’ or with recessive ‘r’ and enter a gamete. Similarly gene ‘y’ may combine with the dominant gene ‘R’ or with the recessive gene ‘r’ and enter the gamete.
Four types of gametes from each dihybrid plant, on fertilization, can produce sixteen plants in the F2 generation.
Out of 16 plants, 9 plants produce yellow round seeds, 3 plants produce yellow wrinkled seeds, 3 plants produce green round seeds and 1 plant produces green wrinkled seeds. Thus in F2 generation plants occur in the ratio 9:3:3:1.
Back cross
Definition: “A cross of an F1 hybrid with any one of its parents is called back cross.” It is of two types –
Dominant back cross
“A cross of an F1 hybrid with the dominant parent is called a dominant back cross.” All the resulting offspring in this cross will possess the dominant character; i.e. 100% of the dominant character.
E.g. cross between homozygous tall (TT) and F1 hybrid tall (Tt).
Parents – Tall X Tall (F1)
Genotype – TT Tt
Gametes – T T t
F1- generation – TT Tt
Recessive back cross or Test cross
“If the F1 hybrid is crossed with the recessive parent, the cross is called a recessive back cross.” In this cross two types of offspring are obtained in equal numbers; i.e. 50% dominant and 50% recessive (1:1 ratio).
“The recessive back cross helps to identify the heterozygosity of the hybrid. Hence this cross is also called test cross.” In the test cross, if all resulting offsprings are tall (100% tall), the test plant is homozygous tall (TT). If the resulting offsprings contain 50% tall and 50% dwarf i.e. 1:1 ratio, the test plant is heterozygous (Tt).
Parents – Dwarf X F1 Tall
Genotype – tt Tt
Gametes – t T t
F1- generation – Tt tt
Significance of Test cross
- Test cross helps to verify the law of inheritance.
- It is used to check unknown genotypes of the F1 hybrid.
- It is a quicker method of improving a variety of crop plants.
- It is an easier and rapid method to obtain a desirable trait in homozygous conditions.
- It is useful to breeders and geneticists in determining the genotype constitution of any organism.
Practical applications of Mendel’s laws
Mendel’s law of dominance is important for eliminating recessive genes in human beings by intercaste marriages. Such marriages reduce the chances of birth of children with hereditary – genetic defects caused due to homozygous recessive genes.
The same is also true in the case of plants.
Mendel’s law of independent assortment is helpful in finding new combinations that will appear in the progeny of hybrids and enable us to predict their frequency.
This plant breeder can produce new varieties of plants having new combinations of useful characters by hybridization. This is also important in animal breeding experiments.
Important Related Terms
Father of genetics: Gregor Johann Mendel.
Emasculation: Removal of stamens or anthers from a flower.
Dominant: Character that expressed in the F1 generation.
Recessive: Character that does not express in the F1 generation.
Alleles or Allelomorphs: The genes of contrasting characters.
Back Cross: A cross of an F1 hybrid with any one of its parents.
Test Cross: The recessive back cross helps to identify the heterozygosity of the hybrid.
References
- Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S. Chand and Company Ltd.
- Gardner, E. J., Simmons, M. J., & Snustad, D. P. (1991). Principles of genetics. New York: J. Wiley.
Sources
- <1% – http://kmbiology.weebly.com/mendel-and-genetics—notes.html
- <1% – http://knowgenetics.org/mendelian-genetics/
- <1% – https://en.wikipedia.org/wiki/Mendelian_inheritance
- <1% – https://www.genome.gov/genetics-glossary/Mendelian-Inheritance
- 2% – https://www.embibe.com/study/examples-on-back-cross-concept
- 9% – https://dfs-contest.com/mendel’s-3-laws-of-inheritance
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