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===Genes and inheritance===
[[File:ADN animation.gif|frame|150px|left|A section of [[DNA]]; the sequence of the plate-like units ([[nucleotide]]s) in the center carries information.]]
[[File:Redhead close up.jpg|thumb|180px|left|Red hair is a [[Dominance relationship#Recessive allele|recessive]] trait.]]▼
Genes are pieces of DNA that contain information for synthesis of ribonucleic acids (RNAs) or polypeptides. Genes are inherited as units, with two parents dividing out copies of their genes to their offspring. This process can be compared with mixing two hands of cards, shuffling them, and then dealing them out again. Humans have two copies of each of their genes, but each egg or sperm cell only gets ''one'' of those copies for each gene. An egg and sperm join to form a complete set of genes. The resulting offspring has the same number of genes as their parents, but for any gene one of their two copies comes from their father, and one from their mother.<ref name=Utah/>
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The effects of this mixing depend on the types (the [[allele]]s) of the gene. If the father has two copies of an allele for red hair, and the mother has two copies for brown hair, all their children get the two alleles that give different instructions, one for red hair and one for brown. The hair color of these children depends on how these alleles work together. If one allele [[dominance (genetics)|dominates]] the instructions from another, it is called the ''dominant'' allele, and the allele that is overridden is called the ''recessive'' allele. In the case of a daughter with alleles for both red and brown hair, brown is dominant and she ends up with brown hair.<ref name=OMIM>[https://www.ncbi.nlm.nih.gov/omim/155555 Melanocortin 1 Receptor], Accessed 27 November 2010</ref>
[[File:Hair colors punnett.png|thumb|right|A [[Punnett square]] showing how two brown haired parents can have red or brown haired children. 'B' is for brown and 'b' is for red.]]
▲[[File:Redhead close up.jpg|thumb|180px|left|Red hair is a [[Dominance relationship#Recessive allele|recessive]] trait.]]
Although the red color allele is still there in this brown-haired girl, it doesn't show. This is a difference between what you see on the surface (the traits of an organism, called its [[phenotype]]) and the genes within the organism (its [[genotype]]). In this example you can call the allele for brown "B" and the allele for red "b". (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown hair daughter has the "brown hair phenotype" but her genotype is Bb, with one copy of the B allele, and one of the b allele.
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