The Biology Project: Vocabulary

Terms to know in Mendelian Genetics

The different forms of a gene. Y and y are different alleles of the gene that determines seed color. Alleles occupy the same locus, or position, on chromosomes.
A locus on any chromosome but a sex chromosome. Not sex-linked.
co-dominant alleles
Two different alleles at a locus are responsible for different phenotypes, and both alleles affect the phenotype of the heterozygote. For example, consider the situation where there are three alleles A,B, and O that determine human blood type. Three possible genotypes are AA, BB, OO that correspond to the phenotypes of blood type A, B, and O respectively; Two other genotypes are AO and BO that correspond to blood types A and B, respectively because the O allele is recessive, The remaining genotype is AB, corresponding to blood type AB. Both the A and B alleles contribute to the phenotype of the heterozygote. Thus the alleles A and B are said to be co-dominant.
complete linkage.
Complete linkage describes the inheritance patterns for 2 genes on the same chromosome when the observed frequency for crossover between the loci is zero.
Organisms produce only one type of gamete; i.e. humans
dominant trait.
A trait expressed preferentially over another trait.
Drosophila melanogaster
The fruit fly, a favorite organism for genetic analysis.
One gene masks the expression of a different gene for a different trait.
F1 generation
Offspring of a cross between true breeding plants, homozygous for the trait of interest
F2 generation
Offspring of a cross involving the F1 generation.
The genetic constitution of an organism with respect to a trait. For a single trait on an autosome, an individual can be homozygous for the dominant trait, heterozygous, or homozygous for the recessive trait. Yellow seeds are dominant, but yellow seeded plants could have a genotype of either YY or Yy.
If there is only one copy of a gene for a particular trait In a diploid organism, the organism is hemizygous for the trait, and will display a recessive phenotype. X-linked genes in fly or human males are hemizygous.
Differing alleles for a trait in an individual, such as Yy.
homologous chromosomes
The pair of chromosomes in a diploid individual that have the same overall genetic content. One member of each homologous pair of chromosomes in inherited from each parent.
Both alleles for a trait are the same in an individual. They can be homozygous dominant (YY), or homozygous recessive (yy).
heterozygous; usually referring to the offspring of two true-breeding (homozygous) individuals differing in the traits of interest.
incomplete dominance
Intermediate phenotype in F1, parental phenotypes reappear in F2. The flowers of the snapdragon plant can be red, pink, or white. Color is determined at a single locus. The genotype RR results in red flowers and rr results in white flowers. The heterozygote genotype of Rr results in pink flowers. When the heterozygote has a different, intermediate phenotype compared to the homozygous dominant or homozygous recessive individuals, this is said to be incomplete dominance.
lethal alleles.
Mutated genes that are capable of causing death.
genes that are inherited together on the same chromosome. Three inheritance patterns are possible: non-linkage, Partial linkage, and complete linkage.
mendel's law of independent assortment of alleles.
Alleles of different genes are assorted independently of one another during the formation of gametes.
mendel's law of segregation
Alleles segregate from one another during the formation of gametes.
Organisms produce both male and female gametes; i.e. garden pea.
monohybrid cross.
Cross involving parents differing in only one trait.
Change in the DNA sequence of a gene to some new, heritable form. Generally, but now always a recessive allele.
Non-linkage describes the inheritance patterns for 2 genes on the same chromosome, when the expected frequency for crossover between the loci is at least one. The observed inheritance patters for non-linked genes on the same chromosome is the same as for 2 genes on different chromosomes.
partial linkage.
Partial linkage describes one of the inheritance patterns for 2 genes on the same chromosome, when the expected frequency for crossover between the loci is greater than zero but less than one. From partial linkage analysis we can learn about the order and spacing of genes on the same chromosome.
The physical appearance of an organism with respect to a trait, i.e. yellow (Y) or green (y) seeds in garden peas. The dominant trait is normally represented with a capital letter, and the recessive trait with the same lower case letter.
A single gene determines more than one phenotype for an organism.
recessive trait.
The opposite of dominant. A trait that is preferentially masked.
reciprocal cross
Using male and female gametes for two different traits, alternating the source of gametes.
sex chromosomes
Sex determination is based on sex chromosomes
A gene coded on a sex chromosome, such as the X-chromosome linked genes of flies and man.
test cross
Generally a cross involving a homozygous recessive individual. When a single trait is being studies, a test cross is a cross between an individual with the dominant phenotype but of unknown genotype (homozygous or heterozygous) with a homozygous recessive individual. If the unknown is heterozygous, then approximately 50% of the offspring should display the recessive phenotype.
Homozygous for the true-breeding trait.
wild-type allele
The non-mutant form of a gene, encoding the normal genetic function. Generally, but not always a dominant allele.

The Biology Project
University of Arizona
Tuesday, August 13, 1996
All contents copyright © 1996. All rights reserved.