Introduction to genetics: Difference between revisions

Revision as of 09:32, 16 July 2023

Revision as of 07:48, 13 October 2023

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[[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 whatiswhat 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/>