Chi-Square Test for Goodness of Fit in a Plant Breeding Example Glossary

allele

One of the different forms of a gene (or marker) that can exist at a single locus. A single allele for each locus is inherited separately from each parent.

alleles

One of the different forms of a gene that can exist at a single locus. A single allele for each locus is inherited separately from each parent.

backcrossing

A breeding method used to move one or only a few desirable genes from an agronomically poor crop line to an elite line. This is done by crossing a donor parent to an elite line, and crossing offspring with the 'desired gene(s)' back to the elite parent.

chi-square

A statistical test used to measure if experimental data supports a particular hypothesis.

co-dominant

When both alleles present in a living thing are expressed.

cross

The deliberate mating of two parental types of organisms in genetic analysis.

crossing

The deliberate mating of two parental types of organisms in genetic analysis.

degrees of freedom

In a chi-square analysis this is the number of classes in the data set minus one.

DNA

(deoxyribonucleic acid) The molecule that encodes genetic information. DNA is a double-stranded molecule held together by weak bonds between base pairs of nucleotides. It is the fundamental substance of which genes are composed.

dominant

The allele that is expressed in a heterozygous organism.

expected

When conducting a chi-square test, this is the number of individuals anticipated for a particular phenotypic class based upon ratios from a hypothesis.  An example would be an F2 3:1 ratio of yellow to green plants.   Expected numbers from 100 F2 plants would then be 75 yellow plants and 25 green plants.

frequency

A measure of the number of times per second (units of sec-1) that the electric field in a photon vibrates.

gene

The fundamental unit of heredity that carries genetic information from one generation to the next. A gene is an ordered sequence of nucleotides located on a particular position on a particular chromosome that encodes a specific functional protein.

genome

All the genetic material in the haploid set of chromosomes for a particular organism.

goodness of fit

A statistical term which refers to how well numbers of what was expected and numbers of what is observed are the same.  How well does your experimental data fit or support your hypothesis?

heterozygous

Having unlike alleles at one or more corresponding loci (opposite of homozygous).

hybrids

Individuals produced by crossing two parents of different genotypes.

lines

Plants within a species that have the same genetic composition and are genetically pure, (i.e., inbred line). Lines are typically not agronomically competitive and are used only in plant breeding.

marker

An identifiable DNA sequence on a chromosome. A marker can be a gene, part of a gene, or a sequence in a non-gene region. There are many types of markers used in plant breeding programs. The terms, "molecular marker" and "DNA marker" refer to the same concept.

observed

When conducting a chi-square test, this is the number of individuals seen for a particular phenotypic class, ie. 78 yellow plants, 22 green plants.

P value

In a chi-square analysis, the p-value is the probability of obtaining a chi-square as large or larger than that in the current experiment and yet the data will still support the hypothesis.  It is the probability of deviations from what was expected being due to mere chance.  In general a p value of 0.05 or greater is considered critical, anything less means the deviations are significant and the hypothesis being tested must be rejected.

p-value

In a chi-square analysis, the p-value is the probability of obtaining a chi-square as large or larger than that in the current experiment and yet the data will still support the hypothesis.  It is the probability of deviations from what was expected being due to mere chance.  In general a p value of 0.05 or greater is considered critical, anything less means the deviations are significant and the hypothesis being tested must be rejected.

phenotype

The observable physical characteristics of an organism that are determined by a combination of the genetic composition (genotype) and the environment of the individual.

qualitative trait

A trait whose inheritance is due to one or a few genes working together.  Phenotypic classes are not continuous so classification into discrete categories is possible.  An example of this would be herbicide resistance in crop species

quantitative trait

A trait whose inheritance is due to several genes. Phenotypic classes are continuous so classification into discrete categories is not possible. An example of this would be plant height.

resistance

The ability of an organism to survive and thrive in the presence of something that would normally cause damage or death, i.e., herbicide-resistant corn, Roundup-ready corn.

resistant

The inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. In a plant, resistance may be naturally occurring or induced by such techniques as genetic engineering or selection of variants produced by tissue culture or mutagenesis.

selection

A natural or artificial process that favors or induces survival and perpetuation of one kind of organism over others that die or fail to produce offspring.

significant deviations

refers to when a value is outside the typical variation for a set of values

susceptible

Sensitive to a stimulus.