the biology

genes genetices

Gene

A section of DNA that contains the instructions for a particular characteristic or trait.

Allele

A different version of a gene, such as an allele for brown eyes or blue eyes.

Autosomes

Chromosomes that are not sex chromosomes. Humans have 22 pairs of autosomes.

Sex chromosomes

The chromosomes that determine biological sex. Humans have one pair:

Chromosome

A long, tightly coiled structure made of DNA and proteins that carries genetic information.

Chromatid

One of the two identical copies of a chromosome after DNA has been replicated. The two chromatids are joined at the centromere.

Homologous chromosomes

A pair of chromosomes that contain the same genes in the same locations, with one inherited from each parent.

Dominant allele

An allele that is expressed in the phenotype when at least one copy is present. Represented by a capital letter, such as A.

Recessive allele

An allele that is only expressed when two copies are present. Represented by a lowercase letter, such as a.

Homozygous

Having two identical alleles for a gene.

Homozygous dominant

Having two dominant alleles, such as AA.

Homozygous recessive

Having two recessive alleles, such as aa.

Heterozygous

Having two different alleles for a gene, such as Aa.

Gamete

A reproductive sex cell, such as a sperm or egg cell. Gametes contain one set of chromosomes and are haploid.

Genotype

The combination of alleles an organism has for a particular gene, such as AA, Aa or aa.

Phenotype

The observable characteristic or trait of an organism, such as brown eyes.

Meiosis

A type of cell division that produces four genetically different haploid gametes. It reduces the chromosome number by half.

Mitosis

A type of cell division that produces two genetically identical diploid daughter cells. It is used for growth, repair and replacing cells.