When you clasp your hands together and interlock your fingers where do you place your thumb? Is the left over the right or the right over the left? Whichever position you feel most comfortable with is dictated by your genes.
First of all the basics. We have two different copies of each gene. We inherit one from our mum and one from our dad, and alternate versions of genes are known as alleles. An allele can be either dominant or recessive, and in a heterozygous genotype, it is the dominant gene that causes the phenotype (or trait/characteristic) to be seen. A heterozygous genotype has a dominant and a recessive allele. A homozygous genotype has either two dominant alleles or two recessive alleles. In genetics a dominant trait is represented by an upper case letter, and a recessive trait is expressed with a lower case letter.
People who place their left thumb over their right thumb when they interlock their fingers, possess either one or two dominant versions of the gene (F). Whilst people who place their right thumb over the left possess two recessive genes (f).
So if a heterozygous dominant parent has a child with a homozygous recessive parent then there is a 50% chance that their children will place the left thumb over the right.
If a homozygous dominant parent has a child with a homozygous recessive parent then all of the children will place the left thumb over the right.
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It's Not All Genetics
What we have demonstrated so far is a simple Mendelian dominant inheritance pattern. However, with genetics things are not always so simple and black and white. For example, if interlocking fingers was purely a genetic trait then you would expect to see identical twins sharing the same propensity for where they position their thumb when they clasp their hands together. However, this has not always been found to be the case, so there must be something else going on. And that could be where environmental factors come into play and interfere with the expression of genes. And those environmental factors are potentially numerous and can be cellular, sub-cellular (such as other genes) or factors that exist right out here in the macro environment.