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Just a basic question on alleles, I am just trying to learn the basic concepts.

So if we are to screen two parents, both with one dominant and one recessive allele for a given characteristic e.g. eye colour, does this mean that we can say for certain (given a correct screen) that the child has a 3/4 chance of having the dominant characteristic (expressed) and a 1/4 chance of having the recess characteristic?

Similarly, if both parents had 2 dominant alleles, does this mean it is certain for the child to exhibit said quality given that the screen was effective?

Also, are there only these two types of alleles? I heard something about gene driving creating 'super dominant' alleles and would appreciate if someone could explain this to me.

Finally, is it the case that all alleles have a 50% chance of being passed on?

(assume in this question there aren't any relevant mutations going on)

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It can be a little confusing. A genetic allele is just a portion of the genome responsible for an observable trait or phenotype. Examples of phenotypes consist of your example (eye color) and millions of others. As @swbarnes alluded to, most phenotypes are complex phenotypes, meaning they are not described by a single genetic locus. Height, weight, hair color, pulmonary function, and eye color are all examples of complex phenotypes (or traits); meaning that more than 1 genetic locus controls the observed phenotype. Other traits like PTC tasting bitter are determined by a single genetic locus; these are referred to as simple traits.

When it comes to dominant verses recessive, this classification is usually applied to simple traits, and refers to whether it requires two copies or one copy of a causal genetic variant to observe the phenotype. For example, a single copy of the genetic variant responsible for sickle cell disease does not cause the phenotype of sickle cell anemia. It takes two copies to cause it. Thus we label this genetic variant as a recessive allele. If a single copy of the genetic variant were sufficient to cause the observed phenotype, then we would label that variant a dominant allele.

Some traits are intermediate, in that a single copy of the allele causes an intermediate phenotype, that is more exaggerated if you inherit two copies. The PTC tasting bitter above is such a trait. If you have a single variant allele, it will taste bitter, if you have two copies of the variant allele, it will taste very bitter. If you have zero copies, you will wonder why your friend with two copies just made a horrible face.

IF you are referring to a simple trait (controlled by only one genetic region) and the allele controlling the observed trait is a dominant allele, this would mean a single copy of the variant allele is sufficient to see the trait. However, this also means that both parents could be heterozygous for the allele ( each possessing 1 copy of the variant and one copy of the normal). In this case, there is a 25% chance their offspring gets both normal alleles. So your supposition was correct if the trait is simple and dominant.

With regards to 50% chance of passing any given allele, yes. However, in complex traits, the math gets complex quick with regards to odds of any given phenotype.

Lastly, some traits are influenced by both a genetic determinant and an epigenetic determinant that can be environmentally driven which provides infinite more complexity. I'll let look you look that up or ask another question :) The same with "gene drives".

A neat listing of simple traits in humans and whether they are dominant or recessive comes from here:

Simple Traits in Humans

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does this mean that we can say for certain (given a correct screen) that the child has a 3/4 chance of having the dominant characteristic (expressed) and a 1/4 chance of having the recess characteristic?

If you are sure that the phenotype is 100% determined by that allele only, then yes, barring things like new mutations. Biology always has some exceptions.

Also, are there only these two types of alleles?

In real life, most characteristics are not well described with simple Mendelian genetics. Most traits are controlled by many genes, and the different alleles can interact in many potential ways.

Finally, is it the case that all alleles have a 50% chance of being passed on?

In general, yes. Meiosis puts half of your alleles into half your gametes, and half of your alleles in the other half.

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  • $\begingroup$ 1) Thanks 2) I understand that most characteristics (phenotypes?) will rely on a number of different types of genes and the exact types may not be understood as of yet. However for characteristics which are only affected by a single gene i.e. eye colour (?) does this mean that such alleles can only be recessive or dominant? Further, am I correct in saying that there are more than two possible types of each gene and that as a result of mutations there could be very large number of alleles? For each such allele, if it is say dominant or recessive, are all dominant genes 'equally dominant'? $\endgroup$
    – Kevin
    Jul 8, 2016 at 15:04
  • $\begingroup$ Eye color is affected by multiple genes, though if a particular family has only brown and blue eyes, that variation is likely caused by variation in a single allele, and should mostly behave in a Mendelian manner. $\endgroup$
    – swbarnes2
    Jul 12, 2016 at 21:06
  • $\begingroup$ We label one allele dominant to another if the heterozygous condition looks the same as the homozygous condition for the "dominant" allele. Like blood type; the AO phenotype is identical to the AA phenotype. A is classically dominant to O. But if a snapdragon with red flowers is bred with one with white flowers, the result is pink flowers. Neither allele is classically dominant to the other. People with AB blood type have a different phenotype than people with A or B blood. $\endgroup$
    – swbarnes2
    Jul 12, 2016 at 21:12
  • $\begingroup$ Just because 95% of your genetics homework involves traits that fit the Mendelian paradigm doesn't mean that anywhere near that much real biology does. Most alleles and traits simply do not fit the classical Mendelian paradigm. The results are more complicated to predict than "This allele always masks that allele" $\endgroup$
    – swbarnes2
    Jul 12, 2016 at 21:14
  • $\begingroup$ I understand that the vast majority of traits can not be characterised under one gene. Is there a more complex mechanic behind it, or would it say require information from n different genes (which would mean we could theoretically calculate it)? $\endgroup$
    – Kevin
    Jul 17, 2016 at 19:39
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I think the "gene drive" that you are talking about involves the CRISPR system, and it is an engineered run of code that deliberately makes that allele 'super dominant' (by making more CRISPR components that are loaded with the particular gene replacement instructions), like in a scenario where they are trying to replace the faulty gene for Huntington's disease or ALS in an adult.

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