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Mendelian Inheritance

Mendelian inheritance refers to the certain patterns that how characters (factors) are carried forward from parents to offspring. Gregor Mendel is the “Father of Modern Genetics.” He was an Austrian monk who studied heredity in pea plants. His work was published in 1865. He described “factors” that were passed between generations of plants.  We now know the factors are genes: chemical factors that determine characteristics. His experiments brought forth two generalizations which later were known as Mendel’s principles of heredity or Mendelian inheritance. These were described in his essay “Experiments on Plant Hybridization” that was read to the Natural History Society of Brunn on February 8 and March 8, 1865, and was published in 1866.

Table of Contents

Mendel’s Experiment

  • Mendel selected pea (Pisum sativum; Papilionaceae) as his experimental plant. This is might be due to the reasons.
    1. the plant was easy to cultivate and this plant completes its life cycle in short duration.
    2. the plant had discontinuous characteristic such as flower colour and pea texture.
    3. pollination can easily be controlled because of its anatomy and thus is flexible for both self or cross pollinations.
  • Most important feature of this plant is that it exhibits several pairs of contrasting characters for study.
  • Therefore, Mendel analyzed seven traits that are easily recognized and apparently only occur in one of two forms. These are:-
    1. Stem length (long or short);
    2. Flower position (axillary or terminal);
    3. Pod shape (inflated or constricted);
    4. Pod colour (yellow or green);
    5. Seed shape (round or wrinkled);
    6. Seed colour (yellow or green);
    7. Flower colour (purple and white).

Genetics Terminology

Before starting Mendel’s principles of heredity, it is crucial to understand some of the genetic terminologies. Mendel observed true-breeding pea plants produced genetically identical offspring. ex. Tall plants produced tall offspring, short produced short. True-breeding plants self-pollinate. (have both male and female parts).

  1. Traits
    • Traits are inherited characteristics that vary from individual to individual. Each trait each had two different forms or alleles. Pea plant height can be either tall (T) OR short (t). The T allele is the dominant allele and represents tall height  (TT or Tt). The t allele is the recessive allele. A plant with both recessive alleles (tt) gives dwarf plants.
  2. Homozygous
    • Homozygous means to have two identical alleles for a trait. Ex. TT or tt True-breeding pea plants are homozygous.
  3. Heterozygous
    • Heterozygous means to have two different alleles for a trait. Ex. Tt Hybrid plants are heterozygous.
  4. Genotype
    • Genotype is the genetic makeup of an organism. The genotype consists of the alleles that the organism inherits from its parents. For example, the pea plant height could be TT, Tt or tt.
  5. Phenotype
    • Phenotype is the physical manifestation or appearance of an organism as a result of its genotype. In the above example, the phenotype of a TT or Tt pea plant is ‘tall plants’. The phenotype of a tt pea plant is ‘dwarf plants’.
      • P = Parent generation = your parents (P1 & P2)
      • F1 = First generation offspring. = you
      • Produces hybrids = crosses between parents with different traits (Tall x short) (TT x tt) F2 = Second
      • generation offspring. = your kids Formed from hybrid x hybrid. (Tt x Tt) (F1 x F1)
  • Mendel crossed pea plants with pairs of differential or contrasting characteristics.
  • By crossing plants of parental generation (P1&P2), he observed the resulting hybrids in the first filial generation (F1).
  • Then he crossed the hybrids (F1s) among themselves and studied their progeny in the second filial generation (F2). So, let’s consider Mendel experiment one by one.

Monohybrid Cross

  • Mendel crossed pea plants having tall plants with those having dwarf plants in his first experiment.
  • In other words Mendel considered one pair of contrasting characters and studied their inheritance and variation pattern, if any. Such crosses are called monohybrid cross.
  • In the first filial generation Mendel found that all the hybrids had tall plants and, thus, the character of only one parent is inherited.
  • When these plants are selfed, in the next generation (F2) the characteristics of both parents reappeared in the proportion of 75% and 25% or in the ratio of 3:1.
  • Mendel postulated that the each character is controlled by a pair of factors which are transmitted through the gametes. For e.g. the plant height was controlled by a “factor” that was transmitted to the offspring by means of the gametes.
  • This hereditary factor (which is now called the gene) could be transmitted with other genes as shown in figure below
  • In the first generation seeds (F1) both TT and tt factors were present, but only T is revealed because it is dominant factor;
  • while factor t remained hidden (not expressed) and is called recessive factor.
  • Due to meiotic cell division during gamete formation, the two factors segregate leaving one in each gamete.
  • When these gametes fuse, the resultant zygote has a genotype of Tt, with one dominant (T) and one recessive (t) factor being contributed by each gamete.
  • When the F1 progeny are selfed, each individual produce two types of gametes in each sex, there are four possible combinations in (F2) viz TT (homozygous dominant; tall), tT (heterozygous dominant; tall), Tt(heterozygous dominant; tall) and tt (homozygous recessive; dwarf) in equal proportion of 25% each.
  • This gives us a result of the the phenotypic proportion of 3(tall):1(dwarf) and genotypic ratio of 1(TT) : 2 (Tt) : 1(tt). However when the 25% of plants in (F2) with green seeds were crossed among themselves, they always produced dwarf plants. Thus they are pure strain for this character (Figure below).

Monohydrid cross Result

  • Mendel’s results can now be explained in terms of the behavior of chromosomes and genes. The genes present in the chromosomes are found in pairs called alleles.
  • In each homologous chromosome the gene for each trait occurs at a particular point called a locus.
  • The individual having two similar alleles are called homozygous (i.e. TT or tt) and those with different allele are heterozygous (i.e. Tt).

Dihybrid Cross

  • In a related experiment Mendel crossed pea plants between plants having yellow and round cotyledons and plants having green and wrinkled cotyledons.
  • In other words Mendel considered two pairs of contrasting characters and studied their inheritance and variation pattern, if any. Such crosses are called dihybrid cross.
  • The F1 hybrids all had yellow and round seeds.
  • When these F1 plants were self fertilized they produced four types of plants in the following proportion:
    1. Yellow and round 9
    2. Yellow and wrinkled     3
    3. Green and round          3
    4. Green and wrinkled      1     
  • The above results indicate that yellow and green seeds appear in the ratio of 9 + 3 : 3 + 1 = 3 : 1.
  • Similarly, the round and wrinkled seeds appear in the ratio of 9 + 3 : 3 +1 = 12:4 or 3 :1.
  • This indicates that each of the two pairs of alternative characters viz. yellow-green cotyledon colour is inherited independent of the round-wrinkled character of the cotyledons.
  • It means at the time of gamete formation the factor for yellow colour enters the gametes independent of R or r, i.e, gene Y can be passed on to the gametes either with gene R or r.
  • In the above experiment yellow and round characters are dominant over green and wrinkled characters which can be represented as follows:
    1. Gene for yellow colour of cotyledons Y
    2. Gene for green colour of cotyledons y
    3. Gene for round character of cotyledons R
    4. Gene for wrinkled character of cotyledons r
  • Therefore, plants with yellow and round cotyledons will have their genotype YYRR and those with green and wrinkled cotyledons will have a genotype yyrr.
  • These plants will produce gametes with gene YR and yr respectively.
  • When these plants are cross pollinated, the union of these gametes will produce F1 hybrids with YyRr genes.
  • When these produce gametes all the four genes have full freedom to assort independently and, therefore, there are possibilities of four combinations in both male and female gametes.
    1. RY
    2. Ry
    3. rY
    4. ry

This shows an excellent example of independent assortment. These gametes can unite at random producing in all 16 different combinations of genes, but presenting four phenotypes in the ratio of 9: 3: 3: 1.


Mendel summarized his findings in two laws; the Law of Segregation and the Law of Independent Assortment.

Law of Segregation (The “First Law”)

  • The Law of Segregation states that when any individual produces gametes, the copies of a gene separate, so that each gamete receives only one copy.
  • A gamete will receive one allele or the other. The direct proof of this was later found when the process of meiosis came to be known.
  • In meiosis the paternal and maternal chromosomes get separated and the alleles with the characters are segregated into two different gametes.
  • This can be more clearly understood by keeping in mind the monohybrid cross.
  • It is obvious that though in F1 the dominant phenotype appears, the recessive phenotype is not lost but reappears in F2.
  • This suggested that there is no blending of Mendelian factors in F1, but that they stay together while one of them gets expressed.
  • At the time of formation of gametes, these two factors obviously separate or segregate, failing which recessive factors will not appear in F2.
  • Thus due to segregation of factors the gametes which formed are always pure for a particular character. Hence it is called law of purity of gametes or principle of segregation.

Law of Independent Assortment (The “Second Law”)

  • The F2 ratio of 9:3:3:1 in a dihybrid cross would be expected if the two pairs of characters are believed to behave independent of each other. In this case it can be seen that the two characters under consideration viz. (i) seed color and (ii) seed shape are assorting in an independent manner.
  • This phenomenon is known as Principle of Independent Assortment. Factors that lie in separate chromosomes are independently distributed during meiosis.
  • The resulting offspring is a hybrid at two loci.
  • The Law of Independent Assortment, also known as “Inheritance Law”, states that alleles of different genes assort independently of one another during gamete formation.
  • While Mendel’s experiments with mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios.
  • But the 9:3:3:1 table shows that each of the two genes are independently inherited with a 3:1 ratio.
  • Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cat’s color and tail length.
  • This is actually only true for genes that are not linked to each other.
  • For more clear understanding, the process of meiosis must be very clear.
    • Independent assortment occurs during meiosis I in eukaryotic organisms, specifically metaphase I of meiosis, to produce a gamete with a mixture of the organism’s maternal and paternal chromosomes.
    • Along with chromosomal crossover, this process aids in increasing genetic diversity by producing novel genetic combinations.
  • In independent assortment the chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes.
  • Because gametes end up with a random mix instead of a pre-defined “set” from either parent, gametes are therefore considered assorted independently.
  • As such, the gamete can end up with any combination of paternal or maternal chromosomes.
  • Any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency.
  • For human gametes, with 23 pairs of chromosomes, the number of possibilities is 2^23 or 8,388,608 possible combinations.
  • The gametes will normally end up with 23 chromosomes, but the origin of any particular one will be randomly selected from paternal or maternal chromosomes.
  • This contributes to the genetic variability of progeny.

Confirming the Principle of Segregation

  • A more common way to determine whether an individual with the dominant phenotype is homozygous or heterozygous is to perform a testcross.
  • By definition you must cross the individual with one that is homozygous recessive.
  • A test cross allows one to determine the alleles carried by the F1 parent.
  • Because the test cross parent can only contribute the recessive allele – is ALWAYS homozygous recessive.
  • Thus, utilizing test cross the phenotypes of the resulting progeny allow you to determine the genotype of the F1 parent.
Cross A
  • If an individual with the dominant phenotype is homozygous.
  • Then test cross of YY x yy will result in 100% of progeny with dominant phenotype
Cross B
  • If an individual with the dominant phenotype is heterozygous.
  • Then test cross of Yy x yy will result in 50% of progeny with recessive phenotype (yy) and  50% are Yy.
Cross C
  • If an individual with the recessive phenotype is homozygous.
  • Then test cross of yy x yy will result in 100% of progeny with recessive phenotype