Mendelian Experiment

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Overview

This note explains how genetic crosses are done and what are their results. when a cross is made in between two pure contrasting characters in F1 generation one character is expressed while another one is hidden.

Mendelian Experiment
  1. Monohybrid cross:

    Let the genotypes for tall be TT and for dwarf tt.

    First, the parent plants are cross pollinated to provide first filial generation (F1) and then F1 plants self-pollinated to give second filial generation (F2)

    In first filial generation all the plants are tall (i.e. only dominant character is seen, but all plants are hybrid) then in second filial generation there are 3 tall and one short plant (Only phenotypically but genotypically the plant is one tall two-hybrid and one dwarf).

    Genotypic ratio 1:2:1 (Dominant: Hybrid: Recessive) Phenotypic ratio 3:1 (Dominant: Recessive). Same goes for any other characters.

    https://lh4.googleusercontent.com/sZd4XfukjT6-a00-CI3k1beIqr781nXIYJUWAqjXMAkYLo8icwdVKNarjj1hMcIoiCzMwRuEdaZ4zsd_w1FbDtIT-zVZrHcwiKAESImhILZGb5AvpdRTGTrAewniNXclx_gy9yvx064

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    https://lh4.googleusercontent.com/_KEjdovQiJpKJ0Y7m4FM2m9DzgWGm_rTdBYRLrcytwgu1s7jPt48deBnTvfa3Bk0Fa_tkIrvk1Jzw16n3neW5IgqllcHKImJOiCb26xAcbsMD7vDifJna01imzF8bjo9FdaU4IR_7hw

  2. Dihybrid Cross:

    Let the two characters be shape of seed and color of seed and genotype be RRYY for pure round and yellow and rryy for pure wrinkled and green. Then these parent plants were first cross-pollinated to give F1 and then F1 were self-pollinated to give F2 generations. Ratio for the cross for second filial generation will be:

    Round and yellow = 9 Round and green = 3 Wrinkled and yellow =3 Wrinkled and green = 1

    https://lh5.googleusercontent.com/WUDAVZXZZbP6hB5XsOTlfdSoxY9t9nq-ZlEkdf9UWCBsaXG87vXrSJ182pCeTHrBi2zxVu5td89iJ2m9sLrL1CPVFXI5mONYk-iJ1q5ZbrMLJ4HPDPU8rM-EK1F5Pz2EW4ZH1VKNzMg

    https://lh3.googleusercontent.com/Pf0oPl2wdNSet-HpV3I5QZJ6C5Elv02dqh6TmmM5NL1FEvmn7JJ13i9KL-Y5IWZP5VzGWW1UHhjKauegKM1_ozKd0L9EVbNsOFR7J2WDjjpfZ0wv3gjKtP_34KL6UcZBOwCs2_dqqeM

Mendelian Law:

On the basis of his experiments, Mendel came up with three conclusions on how characters behave when transmitted from generation to generation.

  1. Law of Dominance: when a cross is made in between two pure contrasting characters in F1 generation one character is expressed while another one is hidden. Such expressed character is called dominant character and the masked character is called recessive character.

  2. Law of purity of gametes: Though the genes, which control recessive character, are hidden in the hybrid organism it is present in the separate location of the chromosome, which later reappear while gametes are formed. This is also called the law of segregation as the heterozygous pair will separate during gamete formation.

  3. Law of independent assortment: When two pairs of independent unit characters are brought together in the hybrid plant but at the time of gamete formation they assort or segregate randomly.
Things to remember
  • Law of Dominance: When a cross is made in between two pure contrasting characters in F1 generation one character is expressed while another one is hidden. Such expressed character is called dominant character and the masked character is called recessive character.
  • Law of purity of gametes: Though the genes, which control recessive character, are hidden in the hybrid organism it is present in the separate location of the chromosome, which later reappear while gametes are formed. This is also called the law of segregation as the heterozygous pair will separate during gamete formation.
  • Law of independent assortment: When two pairs of independent unit characters are brought together in the hybrid plant but at the time of gamete formation they assort or segregate randomly.
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Questions and Answers

According to this law, when two plants one tall and another dwarf are cross-pollinated, only tall plants appears in first generation which is dominant:

In the living organisms the characters are transmitted from parents to their offspring as a gene, which occur in pair. Out of such two characters, only one becomes visible in first generation, though the next resides within the organism. Such a character that expresses itself is called dominant. On the other hand, the one which remains unexpressed in first generation but showed up after the hybrid is self-pollinated in the second generation is called recessive.

This law states that when two pairs of independent allele (different characters) are brought together in the hybrid(F1), they, at the same time of gamete formation, assort independently and freely. This law is illustrated from dihybrid cross.

Law of segregation is directly related to the result to Mendel's mono hybrid experiment. This law states that the two contrasting genes or characters originally found in parents (male of female) are transmitted to the offspring but remains unexpressed until they come in contact with others.

Two application of Mendel's law of inheritance are as follows:

  1. It gives a new idea about the new combination of hybrids.
  2. New types of plants with new combination can be formed.

Mendel's laws are as follows:

  1. Law of dominance
  2. Law of segregation or law of purity gametes
  3. Law of independent assortment.
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