| U |
| uracil or uridine. |
| underdominance |
| A condition in which the phenotypic expression
of the heterozygote is less than that of either homozygote. |
| unequal crossing over
|
| Non-reciprocal crossing over caused by mismatching
of homologous chromosomes. Usually occurs in regions of repetitive
DNA. |
| unequal crossover |
| A crossover between homologs that are not
perfectly aligned. |
| uninemic chromosome |
| A chromosome consisting of one double helix
of DNA. |
| uniparental inheritance
|
| The transmission of certain phenotypes from
one parental type to all the progeny; such inheritance is generally
produced by organelle (mitochondria, chloroplast) genes. |
| unique DNA |
| A length of DNA with no repetitive DNA sequences.
|
| unmixed codon family
|
| Group of four codons sharing their first
two bases and coding for the same amino acid. |
| unstable mutation |
| A mutation that has a high frequency of
reversion; a mutation caused by the insertion of a controlling element,
whose subsequent exit produces a reversion. |
| unusual bases |
| Other bases in addition to the normal adenine;
cytosine; guanine; and uracil. Found primarily in tRNAs and produced
by post transcription modification of one of the normal bases. |
| upstream |
| A convention on DNA related to the position
and direction of transcription by RNA polymerase (5'>3'). Downstream
(or 3' to) is in the direction of transcription whereas upstream
(5' to) is in the direction from which the polymerase has come.
|
| uracil |
| A pyrimidine base that appears in RNA in
place of thymine found in DNA. See pyrimidine. |
| URF |
| Unassigned reading frame. An open reading
frame (ORF) whose function has not yet been determined. |
| uridine |
| The nucleoside having uracil as its base.
|
