Rabu, 27 November 2013

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Monohybrid Cross and Segregation Law

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Firstly, Mendel  studied  the inheritance  of seed shape. The hybridization involving  only one distinct trait  is called monohybrid cross. Parental  is symbolized  by P1, generation (first parental). The crossing  (hybridization) result between parental is called hybrid and symbolized by generation  (first filial). The seed of F1, is grown and becomes  F1, plant. The characteristics  of F1, generation can be looked at

THE OFFSPRING  OF F1, GENERATION



Moreover,  the F1, generation plants undergo self-pollination And produce  seeds of F2, generation. The ratio of F2, generation. (Second filial or the ratio of F2 or the crossing  result between  F1, generation)  is 3 : 1. The characteristics  of F, can be  looked at Table.

THE OFFSPRING  OF F2, GENERATION


When pure-breeding  tail stem plants crossed  with short  stem plant, all of the offspring  produced  are  tall stem plants. The  tall stem trait  that appears is considered as dominant, while short stem  trait  is recessive. This principal  is known as (dominancy  law.)

Dominant  trait is a trait that covers up another trait. Recessive  trait is a trait  that is suppressed by dominant  trait. The trait wiII appear  if there  is no dominant trait. Mendel  used capital  letter to symbolize dominant trait and  lowercase letter  to symbolize recessive  trait. The plants  in Mendel's experiment showed full dominancy because  therewas no  intermediate  trait appearing  in the offspring. It means,  there will be no rather  tall (intermediate  trait between  tall and short traits) in the offsprings.

Based  on  the experiment, Mendel  composed  some  hypotheses,  which are as follows.

  1. There are dominant-recessive  correlation  between  factors of trait determiner  (dominancy law).
  2. Heredity factors are segregated  during gamete  formation and each gamete  only carries  one copy of each  factor.
  3. Gamete conjugation occurs  randomly. 

Based on the above hypotheses, Mendel's  Law I, which is  Know as segregation  law, was discovered. Segregation  law

Monohybrid cross by observing

Monohybrid  cross and segregation

State that during  gamete  formation  there  occurs the process Of segregation  or segregation  of heredity  factors  randomly. Therefore, each gamete only carries one  factor  for each  trait.

Testcross

To test  the hypothesis on allele segregation (modern understanding) which  stated  that F1 was heterozygous,  Mendel crossed F, generation with homozygous  recessive  plant (short). Reason was that half  of  the offspring  (F2) was supposed to be Tall and the other half was short so that it would  create phenotype Ratio of 1 : 1. And actually  the test result supported Mendelian Segregation law. The cross  between  an unknown  organism with Recessive homozygous  organism  is known as testcross.


One exampte  of  testcross

In the early 1900, R.C. Punnet  introduced a method  to describe Generation probabilities  resulted  from genetic  hybridization.  In this case, Punned used a square diagram  (chess  board). The diagram  is later  known As Punnet square.


Punnet diagram, a method to describe genetic hybridization

Source: Biology. Concepts & Connections. 2006

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