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Although the red color allele is still there in this brown-haired girl, it doesn't show. This is a difference between what is seen on the surface (the traits of an organism, called its [[phenotype]]) and the genes within the organism (its [[genotype]]). In this example, the allele for brown can be called "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.
Now imagine that this woman grows up and has children with a brown-haired man who also has a Bb genotype. Her eggs will be a mixture of two types, one sort containing the B allele, and one sort the b allele. Similarly, her partner will produce a mix of two types of sperm containing one or the other of these two alleles. When the transmitted genes are joined up in their offspring, these children have a chance of getting either brown or red hair, since they could get a genotype of BB = brown hair, Bb = brown hair or bb = red hair. In this generation, there is, therefore, a chance of the recessive allele showing itself in the phenotype of the children—some of them may have red hair like their grandfather.<ref name=OMIM/> In the case of two parents with one recessive allele and one dominant allele, their offspring will have a 25% chance of inheriting BB, a 25% chance of inheriting bb, and a 50% chance of inheriting Bb.
Many traits are inherited in a more complicated way than the example above. This can happen when there are several genes involved, each contributing a small part to the result. Tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. However, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or red hair. This is because of the large number of genes involved; this makes the trait very variable and people are of many different heights.<ref>[http://www.childrensnyp.org/mschony/P02134.html Multifactorial Inheritance] Health Library, Morgan Stanley Children's Hospital, Accessed 20 May 2008</ref> Despite a common misconception, the green/blue eye traits are also inherited in this complex inheritance model.<ref name=Athro>[http://www.athro.com/evo/gen/inherit1.html#uncertainty Eye color is more complex than two genes], Athro Limited, Accessed 27 November 2010</ref> Inheritance can also be complicated when the trait depends on the interaction between genetics and environment. For example, malnutrition does not change traits like eye color, but can stunt growth.<ref>{{cite web |url=http://www.med.umich.edu/opm/newspage/2003/kidheight.htm |title=Low income kids' height doesn't measure up by age 1 |publisher=University of Michigan Health System |access-date=May 20, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080526034018/http://www.med.umich.edu/opm/newspage/2003/kidheight.htm |archive-date=26 May 2008 |df=dmy-all }}</ref>
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