Molecular Genetics

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Genetic characters are discrete entities whose phenotypes may be masked (recessive), but are revealed to segregate in subsequent generations. Segregation ratios are consistent with diploidy.

Facts

• Gregor Mendel observed (left) that some tall pea plants when self-fertilized produce seeds that all grow into tall plants. The seeds of others give a mixture of tall and short plants. (Interpretation)
• When Mendel crossed homozygous tall plants with homozygous short plants (right, image from activity.ntsec.gov.tw), the seeds produced all tall plants (ref). These plants when selfed gave seeds that produced both tall and short plants in the ratio of 3:1 (787 tall and 277 short). Two-thirds of the tall plants continued to segregate short plants on further selfing and analysis of the progeny. (Interpretation)

Interpretations-Observation 1

• Plants that do not give mixtures when selfed are said to be homozygous for the tall trait.
• Those that produce mixtures are said to be heterozygous.
• Heterozygous individuals produce segregating populations.

Interpretations-Observation 2

• The short character is said to be recessive, while the tall character is dominant.
• Recessive genetic characters are not modified when in the presence of the dominant character, but can be recovered on further crossing.
• The segregation of differentiating traits is Mendel's First Law.
• The 3:1 ratio is consistent with each plant having two copies of the "gene" and randomly passing one copy through the gametes (pollen or egg). Union of the haploid gametes results again in a diploid organism with two copies.

Further information

• Mendel did similar experiments with six other characters, including seed shape (round or wrinkled), with essentially identical results
• Many fundamental genetic concepts stem from G. Mendel's work on pea genetics. Mendel's first law gave rise to the concept of recessive and dominant markers.
• Mendel's stem length gene has been shown to code for an enzyme involved in plant growth hormone synthesis.
• One important exception to the rules of segregation has been documented in the progeny (F2) of Arabidopsis thaliana plants homozygous for recessive alleles of the HTH gene (ref). In an appreciable portion of these progeny, reversion to the alleles of the homozygous wild-type parent used in the original cross (F0) occurs. This reversion occurs at multiple loci. One interpretation is that the HTH gene functions to suppress an as yet undescribed non-Mendelian mechanism of inheritance.