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Saturday, 10 March 2012

Monohybrid Inheritance

Mendel's monohybrid experiments-named after Gregor Mendel, a 19th century Austrian monk who first explained how heredity might work. He carried out breeding experiments on garden peas (Pisum sativum). 

For his experiments, Mendel selected several varieties of garden pea plants with one pair of contrasting characteristics or traits such as: 
  • plants that were either very tall or very short (dwarf)
  • plants that had either red or white flowers
  • plants that produced either yellow or green seeds; and
  • plants the produced either round or wrinkled seeds
Inheritance involving only one pair of contrasting traits is called monohybrid inheritance.
(Remember: alleles, which always occur in pairs, control the inheritance of various characteristics. Genes are always at the same position (locus) on homologous chromosomes.)

Mendel also used pure-bred varieties of pea plans. Pure-bred plants are plants which, when self-fertilised, produce offspring (progeny) that resemble their parent. For example, when tall plants self-fertilise, they produce tall offspring. 

Mendel's monohybrid experiments:
  1. In one experiment, Mendel crossed or cross-pollinated tall pea plants with pollen from dwarf plants and vice versa.
  2. Mendel planted the seeds from the cross and observed the characteristics of the resulting hybrids, which were tall. (TT x tt, results would be Tt, so offspring are tall) A hybrid is the offspring from 2 different varieties or species. Mendel called these hybrids the First Filial Generation or F
     generation. 
  3. He allowed the F
    1
      hybrids to self-fertilise and produce new seeds. These seeds gave rise to the F2 (second filial) generation. In the F2 generation, a ratio of 3 tall plants to one dwarf plant was observed. (3:1 ratio we talked about in class is hence derived)
Notes: 
In all his experiments, Mendel observed that one trait always appeared in the F
1
 hybrids (e.g. tall plants), Mendel called this trait dominant. The other trait (dwarf) seemed to disappear or 'recede'. Mendel called this trait recessive. The recessive trait reappeared in about one quarter of the total number of Foffspring. (3:1 ratio, the 1 out of four is a homozygous recessive.) 





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