How genetic traits are inherited?

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My father has brown eyes and my mother green eyes, what eye color would their children have?

Well, my eyes are brown, my brother's are blue and my sister's are green!

To explain how this happens first we need to introduce some definitions:

ALLELE: For a gene in a specific position there could be more than one variant. These variants are called alleles. For example for the gene of the eye color, the version of the gene that encodes for brown eyes is one allele and the version that encodes for blue is another allele of this gene.

GENOTYPE / PHENOTYPE: Each one of us has two complete set of genes, one inherited from the mother and the other from the father. The specific alleles of your genes are the genotype. For example you have inherited the brown (B) allele from your father and the blue (b) allele from you mother, so your genotype is Bb. The phenotype is the expression in your body of your genotype combined with the environment. In the previous example if the genotype for eye color is Bb and you have the brown and the blue alleles the phenotype would be brown eyes because this allele is dominant. An example to illustrate the effect of the environment is height: you may have inherited from your parents a genotype that predisposes you to be taller than the average, but if during your childhood yo do not have access to enough food and nutrients (environment), the resulting phenotype is that you will not be as tall as you could have been just according to your genotype.

HOMOZYGOUS / HETEROZYGOUS: Homozygous for one gene is when you have both alleles (the one from the father and the one from the mother) identical. For example regarding to eye color being BB (two brown alleles) or bb (two blue alleles). Likewise, heterozygous is when you have two different alleles. For example Bb (one brown allele and one blue allele). The phenotype of an heterozygous depends on the dominance.

DOMINANT / RECESSIVE: A dominant allele masks the expression of a recessive allele. For example the phenotypic eye color of the heterozygous Bb is brown like the phenotype of the homozygous BB. In this case we can say that the brown allele B is dominant over the blue allele b (usually the dominant allele is typed in uppercase and the recessive allele in lowercase). The phenotype of the recessive allele will only be observed as a phenotype in the homozygous recessive bb.

AUTOSOMAL / X-LINKED: Humans have 23 pairs of chromosomes: 22 pairs of autosomes and a pair of sex chromosomes (women have XX and men XY). The two chromosomes of each pair of autosomes and each X in the XX pair are equivalent, however the pair XY in the men is not equivalent since the Y chromosome is significantly smaller that the X chromosome. As a consequence, some of the genes in the X chromosome of a man are in single copy as they are not present in the Y chromosome. This makes the inheritance partters of autosomal and x-linked genes different.

Coming back to the eye color inheritance, I have to say that eye color is a polygenic trait, so it is the interaction of several genes that originates the phenotypic eye color that we can see, but it can be roughly simplified to a gene with three alleles for the shake of making an easily understandable example to explain the concepts involved in genetic inheritance. We can assume that there are three different alleles: brown (B), green (g) and blue (b); being brown dominant over green and blue, green dominant over blue, and blue always recessive.

In the following figure you can see the phenotype and the genotype of the eye color in my family:

The phenotype is obvious, is the eye colour that you can see, but how is it possible to know the genotype?

Well, it is just logic deduction. For every person there are two alleles and we know at least one that it is the same as the phenotype. For example, since my father has brown eyes I know he has one brown allele B, but the second allele could be B, g or b since the 3 genotypes BB, Bg and Bb would express the same phenotype of brown eyes. Likewise, I know that my mother has one green allele g because her eyes are green, but the second allele could be g or b (it could not be B or she would have brown instead of green eyes). The key is my brother. He has blue eyes and b is recessive compared to brown and green so his genotype has to be bb. The only way he can be bb is that he has inherited one b allele from each parent, so now I know that my father is Bb and my mother gb. My sister is gb since she has inherited the green allele from my mother and the blue one from my father (it is the only combination compatible with her green eyes phenotype) and I am the only one that I can not tell the genotype for sure. I have a brown allele from my father for sure, since my eyes are brown, but I can not know if I have inherited the blue or the green allele from my mother. I will have to wait to have children to have more data. :)

After this introduction to how the genetic traits are inherited, in future posts we will see in detail the different genetic inheritance patterns:

• Autosomal dominant inheritance.
• Autosomal recessive inheritance.
• X-linked dominant inheritance.
• X-linked recessive inheritance.
• Mitochondrial inheritance.

If you want to read about genetic inheritance patterns in Spanish, you can follow the link patrones de herencia genetica and mutacion de novo.