Monohybrid Inheritance

19th century → Gregor Mendel → first explained how hereditary might work.

• carried out breeding experiments on garden pea plants.

first he experimented with plants with 1 pair of contrasting traits, such as:

• tallness and shortness of plant
• plants that either had red or white flowers
• plants which produced seeds that were either yellow or green, or round or wrinkled.

Mendel also used → pure bred varieties of pea plants.

• pure bred plants → are plants which when self pollinated → produce offspring (progeny) → that resemble their parent.
  • e.g. when pure bred tall plants self fertilise, they produce only tall offspring

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Mendel’s Monohybrid Experiments

Step	Image
1. In one experiment, Mendel crossed (cross pollinated) → tall pea plants with pollen from dwarf plants and vice versa
2. Mendel planted seeds from the cross and observed the traits of the resulting offspring → which were all tall. - Mendel called these offspring $F_1$ (first filial) generation. - $F_1$ generation only applies to the first generation offspring of a set of parents.
3. He allowed $F_1$ offspring → to self fertilise and produce seeds. - these seeds gave rise to the $F_2$ (second filial) generation. - in the $F_2$ generation → ratio of about 3 tall plants to 1 dwarf plant was observed.

In all his experiments → Mendel observed → that 1 trait always appeared in $F_1$ offspring (long stems). Mendel called this trait dominant. The other trait (short stems) → seemed to disappear or recede Mendel called this trait recessive Recessive trait reappeared in about 1/4 of the total number of $F_2$ offspring

Mendel also made crosses using 6 other pairs of contrasting traits in pea plants. results of these crosses:

Why does observed ratio often differ from expected ratio

• in above table → ratio in $F_2$ generation becomes closer to expected ratio of 3 : 1 → when larger number of plants are used in experiment.

• If smaller number of plants are used → observed ratios for the $F_2$ generation are not accurate.

• Statistically → ratios are often inaccurate when sample numbers are small.

• Observed ratio is also based on change and probabilities → due to the random nature of fertilisation.

• hence → actual number of offspring would be unlikely to match expected ratio precisely

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Mendel’s Model of Hereditary

• hereditary factors are responsible → for the transmission of characteristics. These hereditary factors are now known as genes

Definition

Gene is a sequence of DNA nucleotides that control the formation of a single polypeptide. It is also a unit of inheritance. #BiologyDefinitions

• each characteristic is controlled by a pair of factors in the cells of an organism.

  • e.g. height of a pea plant → is controlled by a pair of factors. if the 2 factors differ, only the dominant 1 will show its effect.
  • so if a pea plant contains 1 factor of tallness and one for dwarfness, only the tall factor will show its effect

• the 2 factors → in each pair segregate (separate) → during gamete formation.

  • each gamete will now contain only one factor. this is known as Mendel’s first law — Law of segregation

• fusion of gametes → at fertilisation → restores the diploid concentration in the zygote → that is, the zygote contains 2 factors (one factor from each parent) ⇒ for a particular characteristic.

• gametes unite at random → so that the ratio of characteristics among the offspring can be predicted.

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