Chromosome Mapping in Eukaryotes – Vocabulary Flashcards

These vocabulary flashcards review the essential terms and principles from Chapter 5, Chromosome Mapping in Eukaryotes, including linkage concepts, recombination, test crosses, mapping strategies, and key historical contributions.

Independent assortment

Random segregation of allele pairs for genes on different chromosomes (or very far apart), giving equal gamete types.

Genetic linkage

Tendency of genes located on the same chromosome to be inherited together because they do not assort independently.

Linkage group

Set of genes on one chromosome that usually travel together during meiosis; number equals the haploid chromosome count.

Complete linkage

Condition in which two linked genes are so close that virtually no crossovers occur, producing only parental gametes.

Linkage ratio

Characteristic F2 phenotypic ratio (1 : 2 : 1) obtained from a dihybrid cross with complete linkage, versus 9 : 3 : 3 : 1 for unlinked genes.

Crossing over

Exchange of DNA segments between non-sister chromatids of homologous chromosomes during prophase I; source of recombination.

Recombinant gamete

Gamete that carries a new combination of alleles created by crossing over between linked genes.

Noncrossover (parental) gamete

Gamete whose allele combination matches one of the original parental chromosomes; produced when no crossover occurs.

Recombination frequency

Percentage of recombinant offspring (recombinants ÷ total progeny × 100); used as a measure of genetic distance.

Map unit (centimorgan)

Unit of genetic distance equal to 1 % recombination frequency between two loci.

Chromosome map

Linear diagram showing the order and relative distances of genes on a chromosome, built from recombination data.

Two-point test cross

Cross between a double-heterozygote and a double-recessive tester used to measure recombination between two genes.

Three-point test cross

Cross between a triple-heterozygote and a triple-recessive tester to deduce gene order and distances among three linked loci.

Gene order

Linear arrangement of loci along a chromosome, inferred from crossover patterns in mapping experiments.

Single crossover (SCO)

One exchange event between homologous chromatids; produces 50 % recombinant gametes for the flanking genes involved.

Double crossover (DCO)

Two separate exchange events within the same chromatid pair; used in three-point mapping to locate the middle gene.

Chiasma

Visible physical site where crossing over occurs between homologous chromatids during meiosis.

Distance–recombination relationship

Principle that the farther apart two genes are, the more likely a crossover will occur between them.

Maximum recombination frequency

Upper limit of 50 %; observed for genes on different chromosomes or very far apart on the same chromosome.

Sturtevant

Student of T. H. Morgan who first constructed genetic linkage maps by interpreting additive recombination frequencies.

Additivity of recombination frequencies

Concept that genetic distances between distant loci can be estimated by summing the smaller intervals between them.

Test cross

Cross of an individual with unknown genotype to a homozygous recessive to reveal the gametes it produces.

Parental phenotype

Offspring phenotype identical to one of the P-generation combinations; appears most frequently when genes are linked.

Non-parental (recombinant) phenotype

Offspring phenotype with a new allele combination not seen in the parents; frequency reflects crossover events.

Chromosomal theory of inheritance

Idea that genes reside on chromosomes, which segregate and assort during meiosis, explaining Mendelian patterns.

Pseudo-autosomal region

Homologous segments on X and Y chromosomes where crossing over can occur during male meiosis.

Dihybrid cross

Cross between individuals heterozygous for two genes; segregation pattern depends on whether the genes are linked.

Independent assortment ratio

Classic 9 : 3 : 3 : 1 F2 phenotypic distribution obtained when two genes assort independently.

Heterozygous requirement for mapping

Parent in a mapping cross must be heterozygous at all loci so that crossover products can be detected.

Genetic distance formula

(SCO + 2 × DCO) ÷ total progeny × 100 = map units between two genes in three-point mapping.

Linkage without crossing over

Condition yielding only parental gametes and proving the concept of complete linkage.

Gene mapping

Process of determining the relative positions and distances of genes on chromosomes using recombination data.

Genes close together

Loci separated by only a few map units; show strong linkage and very low recombination frequencies.

Genes far apart

Loci wide apart on the same chromosome; multiple crossovers make them behave as if unlinked (≈50 % recombination).

Chromosomal basis of recombination

Physical exchange of DNA between homologs during meiosis explains recombinant progeny seen in genetic crosses.