Have you ever wondered how you came to be? No I don’t mean by the stork or the birds and the bees. I mean how you ended up looking the way you do, especially how surprising it is that you don’t look the exact same as your siblings or your parents? Well it’s all down to a tight little unit called Deoxyribonucleic Acid, or like the foreign teacher’s name you can’t pronounce you can make it easy and call it DNA.
DNA Helix with nucleotide base pairs Source [astrochem.org]
As pictured above it twists and twirls around with little Lego brick like connections between, but however cool you thought the LEGO house you built when you were a kid was, DNA is much more significant. There are lots of these little helixes within us, where the A, T, C or G (Different Nucleotide Base pairs) connect up define everything about you. Whether you have blue eyes from your Mother or stop producing hair from your head an an early age like your Dad.
So what if we wanted to figure out which traits we would pass onto our own children, can it be done without rigorous testing? Yes, I tell you it can, all thanks to a man called Reginald Punnett. He came up with the idea of comparing traits from one parent against another, and once we know whether a trait is dominant or recessive we can see the probability of each outcome.
Punnet Square comparison of parental genes for outcomes of offspring Source[Fiesty]
-Dominant features require a capitol letter denotation E.g If dominant for Brown hair B
-Recessive features require a lower case letter denotation E.g If recessive for Red hair r
-Being a carrier states that you show one gene (the dominant), but hold the possibility to pass on another as well (recessive) E.g Has brown hair, but carries gene for red denoted by Br.
-Expression of a gene such as having brown hair is a Phenotype, Henderson’s Dictionary of Biology stating it is “The visible and biochemical characteristics of an organism resulting from the genotype” (Lawrence, 2011)
-The denotations of B or r or Br are a Genotype, the gene displayed for the trait.
Question – If a mother with brown hair which is a carrier for red hair, has a child with a man with the same traits what will the outcome be?
Table 1. Punnet Square of Parent 1 x Parent 2
From this we can see that for each child the couple have there is
-1/4 Chance of the baby having Brown Hair and not being a carrier for red
-1/4 Chance of the baby have Red hair and not being a carrier for brown
-1/2 Chance of the baby being the same as the parents with brown hair and the carrier gene.
Now let’s get you to try some traits yourself, DNA is a protein building molecule which humans rely on for everything and changes from person to person. Have a quick check of these features to see if they apply to you.
Are your earlobes connected or separate to the side of your head?
Earlobe possibilities in genetics Source [Nakita Mcneil]
Earlobes which are a detached are the dominant trait.
Are you able to roll your tongue?
Rolling of the tongue Source [Nakita Mcneil]
Tongue rolling is a dominant trait
Finally does your hairline display a widow’s peak (bottom of image), or a continuous line of hair(top of image)?
Widows peak as compared to a normal hairline Source [Nakita Mcneil]
Widows peak is the dominant trait.
However, not all traits are as simple as those shown in the Punnett square. They are usually multifactorial therefore require the influence of more than just one gene to occur. Reece, Meyers et al state that Inheritance Patterns are often more complex than predicted by simple genetics (Reece, Meyers et al 2015). Thus hereditary characteristics aren’t just down to one parent having a gene for blue eyes, it can be a combination of parents and grandparents, etc.
These differences occur in vitro, where DNA codes for certain proteins making either trait occur. In the case of earlobe formation or lack thereof it isn’t detrimental, however in the case of finger digit separation it can cause marked differences. Human hands in the womb have a webbing between fingers, as the fetus grows the cells in the webbing get DNA signals to die off. This causes the webbing to disappear, however if the DNA is faulty and the signal doesn’t get through a child may be born with Syndactyl hands where they still have some webbing.
Polydactyly examples Source [Darryl Leja, 2010]
Syndactyly examples Source [Boston Children’s hospital, 2016]
Alternatively DNA can overexcite the process of separation and cause extra segments to form. “Polydactyly is one of the most common hereditary limb malformations, featuring additional digits in hands and/or feet” (Malik, Clinical Genetics, 2014). It’s all down to DNA to make many simple and complex processes occur which can end up in your individual characteristics.
Keep posted for my next blog on the Genetics of Twins and how the many variations of twins occur
Boston Children’s Hospital, 2005, ‘Syndactyly’, The classifications of syndactyly correspond to the condition’s different types and degrees of complexity, http://www.childrenshospital.org/conditions-and-treatments/conditions/syndactyly/symptoms-and-causes retrieved April 10th 2016.
Birdbrain75, 1998-2000, ‘Punnet Square’, What if both parents are heterozygous for brown eyes? You’d get the following, http://feistyhome.phpwebhosting.com/genes.htm Retrieved April 10th 2016.
Campbell Biology 10th Edition, ISBN:9780321775658 by Reece, Jane, B,; Urry, Lisa A.; Cain, Michael L.; Waaserman Steven A.; Minorsky, Peter V.; Jackson, Robert N., published by Pearson Education, Inc, publishing as Benjamin Cummings, copyright © 2014, Essex UK.
Henderson’s Dictionary of Biology 15th Edition, ISBN 9781408234303 by Lawrence, E. Published by published by Pearson Education, Inc, publishing as Benjamin Cummings, copyright © 2014, Essex, UK.
Leija, D 2010, ‘Polydactyly’, Polydactyly is an abnormality characterized by extra fingers or toes, National Human Genome Research Insitute, Retrieved 10th April 2016.
Malik, S 2014, ‘Polydactyly: phenotypes, genetics and classification’, Clinical Genetics, vol. 85, no. 3, pp. 203-212
Mcneil, N 2008, How many of you (students) can role your tongue?: How many students have connected earlobes?: How many students have a widow’s peak?, http://www.scienceteacherprogram.org/biology/McNeil08.html Retrieved April 10th 2016