Three-point cross
Encyclopedia
In genetics
, a three-point cross is used to determine the loci
of three genes
in an organism's genome
.
An individual heterozygous for three mutations is crossed with a homozygous recessive
individual, and the phenotypes of the progeny
are scored. The two most common phenotypes that result are the parental gametes; the two least common phenotypes that result come from a double crossover
in gamete formation
. By comparing the parental and double-crossover phenotypes, the geneticist can determine which gene is located between the others on the chromosome.
The recombinant frequency is the ratio of non-parental phenotypes to total individuals. It is expressed as a percentage
, which is equivalent to the number of map units (or centiMorgans
) between two genes. For example, if 100 out of 1000 individuals display the phenotype resulting from a crossover between genes a and b, then the recombination frequency is 10 percent and genes a and b are 10 map-units apart on the chromosome.
If the recombination frequency is greater than 50 percent, it means that the genes are unlinked - they are either located on different chromosomes or are sufficiently distant from each other on the same chromosome. Any recombination frequency greater than 50 percent is expressed as exactly 50 percent because, being unlinked, they are equally as likely as not to be separated during gamete formation.
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, a three-point cross is used to determine the loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
of three genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
in an organism's genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
.
An individual heterozygous for three mutations is crossed with a homozygous recessive
Recessive
In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...
individual, and the phenotypes of the progeny
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
are scored. The two most common phenotypes that result are the parental gametes; the two least common phenotypes that result come from a double crossover
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
in gamete formation
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. By comparing the parental and double-crossover phenotypes, the geneticist can determine which gene is located between the others on the chromosome.
The recombinant frequency is the ratio of non-parental phenotypes to total individuals. It is expressed as a percentage
Percentage
In mathematics, a percentage is a way of expressing a number as a fraction of 100 . It is often denoted using the percent sign, “%”, or the abbreviation “pct”. For example, 45% is equal to 45/100, or 0.45.Percentages are used to express how large/small one quantity is, relative to another quantity...
, which is equivalent to the number of map units (or centiMorgans
Centimorgan
In genetics, a centimorgan or map unit is a unit of recombinant frequency for measuring genetic linkage, defined as that distance between chromosome positions for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer...
) between two genes. For example, if 100 out of 1000 individuals display the phenotype resulting from a crossover between genes a and b, then the recombination frequency is 10 percent and genes a and b are 10 map-units apart on the chromosome.
If the recombination frequency is greater than 50 percent, it means that the genes are unlinked - they are either located on different chromosomes or are sufficiently distant from each other on the same chromosome. Any recombination frequency greater than 50 percent is expressed as exactly 50 percent because, being unlinked, they are equally as likely as not to be separated during gamete formation.