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Genetics of Medicago sativa L. I. Inheritance of Dwarf Character, Dw₁¹

Cytological studies have indicated that alfalfa is a tetraploid. Recent reports have suggested that it behaves genetically and cytologically as an autotetraploid. The hypothesis, that meiosis in alfalfa is identical to that expected of an autotetraploid displaying random chromosome segregation, was tested by observing the segregation for dwarf plants in a source of plant material from Medicago sativa L.

Dwarf phenotype was found to be controlled by a single recessive gene, inherited tetrasomically. The symbols Dw_l and dw_l were proposed for dominant and recessive forms of the gene at this locus.

The conclusion was based upon a positive test for tetrasomic inheritance. Fifty-three normal plants, from the self progeny of a plant that segregated 3 normal:l dwarf upon selfing, were analyzed genetically by test crossing. Dwarf plants were homozygous recessive and were used as male testcross parents. Upon testcrossing, 52 of the 53 normal plants segregated with ratios that could likely occur only under tetrasomic inheritance.

There were significant deviations from the expected 5:1 ratio in testcross progenies of plants assumed to be duplex. Preferential pairing of chromosomes was considered the most likely cause of these deviations, but differential transmission of gametes could not be ruled out. If preferential pairing occurred, this would indicate that alfalfa deviates from true autotetraploid behavior in the direction of allotetraploid behavior.

² Graduate Assistant and Professor of Agronomy, Department of Agronomy, Iowa State University, Ames, Iowa.

Received for publication January 24, 1966.