Definitions for terms in genetics problems


All the different forms of the same gene.

autosomal genes

All genes on chromosomes other than the sex chromosomes (X and Y).


One strand of a replicated chromosome as illustrated in the image of a chromosome at the right. A single strand by itself with it's own centromere is a chromosome and not a chromatid.


A single (before replication) or double (after replication) strand of DNA with only a single centromere. Chromosomes contain the loci for alleles of different genes. The illustration below shows a chromosome with the parts labeled before (on the left) and after (on the right) replication.

crossing over

A process that occurs during prophase I of meiosis in which genetic material from the chromatid of one chromosome exchanges places with the material from the same area of a chromatid on it's homolog. This process increases the variation in gametes produced by an individual. The images below illustrate a homologous pair of chromosomes before (on the left) and after (on the right) crossing over has occurred.

diploid (2n)

Cells which have two copies of a gene, on a pair of homologous chromosomes.


An allele which, if present, masks the effect of any recessive allele paired with it. Indicated by a capital letter.


first-generation offspring (children).


second-generation offspring (grand children).


The haploid cells produced by meiosis which later fuse to form the diploid zygote. In humans, these are the eggs and sperm.


Units of information about specific traits, passed from parents to offspring. Each gene has a specific location (locus) on a chromosome and may come in several forms (alleles).


The actual genes for a trait present in an individual.

genotypic ratio

The expected numbers of different genotypes produced by a particular cross. Example: 1 RR, 2 Rr, and 1 rr individuals could result from a cross of two Rr individuals. The genotypic ration is 1:2:1.

haploid (1n)

Cells which have only one allele from the originally homologous pair. In humans, gametes are the only haploid cells.


The two alleles of a pair are not identical (for example: one dominant and one recessive allele for the color trait in roses).

homologous chromosomes

A pair of chromosomes in the same individual that carry the same type of information (eye color) but not necessarily the same alleles (blue or brown). One of these "homologs" comes from the individual's mother and one from the father.


Both alleles of a gene in a homologous pair are identical.

linked genes

Genes that appear on the same chromosome and that do not sort independently during meiosis.


The physical location of the alleles of a gene on it's chromosome (See the definition for chromosome for an image).


A type of cell division that produces haploid gametes. The image below shows the very basic steps of meiosis and it's products.


A change in a gene's molecular structure and thus it's information about a trait.


parental generation


An individual's observable traits (how the organism looks, behaves, etc.).

phenotypic ratio

The expected numbers of different phenotypes produced by a particular cross. Example: 3 red flowered plants and 1 white flowered plant result from a cross of two red flowered plants. The phenotypic ratio is 3:1.

punnett square

A graphical representation of a cross between two individuals and the possible genotypes of the offspring produced. The gametes of one individual are placed across the top of the square and the gametes of the other individual are placed down the left side. The gametes are then combined in the cells of the square. Below is an example of a dihybrid cross between two individuals worked in the punnett square.

At At at at
At AAtt AAtt Aatt Aatt
At AAtt AAtt Aatt Aatt


An allele which must be homozygous for it's effect to be observed. Indicated by a lowercase letter.


sex-linked genes are those that are carried on the X chromosome. In humans, females carry 2 X chromosomes while males carry only one.