Glossary Plant Breeding

Law of Segregation. The first law of Mendel. It states that the two dissimilar alleles of a
gene pair separate from each other during gametogenesis and distributed equally and
randomly into gametes. The law is true in all those organisms which utilize meiosis at
least in some part of its life cycle. It does not hold true if there is non-disjunction
during meiosis.
LD 50 . The dose of a mutagen at which 50% of the mutagenized individuals die (before
reproduction). It is used as an important parameter for effectively inducing mutation.
It varies with genotypes, plant species, and the like.
Leader Sequence. The sequence at the 5 end of an mRNA that is not translated into
protein.
Leading Strand. The strand of the DNA double helix that is synthesized continuously in
the 5 to 3 direction during DNA replication.
Leaky Mutation. A mutation that still retains a low but detectable wild-type function
besides displaying mutant phenotype.
Lesion. A damaged area in a gene (a mutant site), a chromosome or a protein.
Lethal. Of a gene (genotype) which, when expressed, is fatal to its carrier.
Lethal Gene. A gene whose expression results in the death of the carrier individual.
Leucoplast. A plastid containing no visible pigments. It is found in embryonic, germ,
and fully differentiated cells. Leucoplasts are also found in meristematic cells and in
those regions of the plant not receiving light. Leucoplasts in certain differentiated
zones of the root produce starch granules, called amyloplasts. They do not have
thylakoids and ribosomes.
Level of Significance. Also called confidence limit; the frequency at which chance errors
in performance are allowed to occur beyond which any variation is supposed to be
real one and hence significant. In other words, the probability chosen for deciding a
deviation to be significant (or non-significant) is called level of significance.
Conventionally, 1% or 5% (or in some genetical experiment, even 50%) error is
allowed. Thus, 99% or 95% is the confidence limits, respectively. In other words, we
can assert with confidence that in 99% or 95% cases, variation observed among
treatments is real (genetic) and not due to chance error. In all biological experiments,
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