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Take the following family tree, which is meant to show individuals exhibiting in their phenotype a particular genetic trait.

Family tree

  • White boxes (A, B, C, D, E, F, J, K) are unknown or undecided, and could be decided either way as necessary
  • Yellow boxes (G, N, S, T, V, W) are individuals that definitely do not exhibit the trait (let's call those individuals yellow)
  • Blue boxes (H, L, M, P, R) are individuals that definitely do exhibit the trait (let's call those individuals blue)

Based on this admittedly very limited sample, and assuming no other factors at play, what can be said about genetic inheritance of the trait in question?

A and B are parents of G. C and D are parents of H and L. E and F are parents of M. G and H are parents of N. J and K are parents of P. L and M are parents of R, S and T. N and P are parents of V and W.

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  • $\begingroup$ (Yes; graphically, it stinks. Live with it, or make a better one yourself. :)) $\endgroup$
    – user
    Mar 30, 2018 at 18:23
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    $\begingroup$ This trait is dominant? Looks to me like a question from a biology quiz... $\endgroup$
    – Alexander
    Mar 30, 2018 at 18:51
  • $\begingroup$ Really this belongs on Biology stack. But Bio stack can be such a bummer. $\endgroup$
    – Willk
    Mar 30, 2018 at 22:36
  • $\begingroup$ This question is the subject of a Worldbuilding Meta discussion at How is the “phenotype inheritance” question on-topic? $\endgroup$
    – user
    Apr 1, 2018 at 18:20
  • $\begingroup$ Isn't this a real-world-fact question, and therefore off-topic? $\endgroup$
    – JBH
    Apr 8, 2018 at 5:51

3 Answers 3

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It appears that Blue is the dominant trait.

Let "B" be the gene for Blue and "b" be the trait for yellow.

Since Blue appears in all generations and in combinations with two blue parents, Blue is dominant and yellow is recessive.

Traits are typically discussed in pairs of BB, Bb, or bb. A dominant trait will be present in either paring of BB or Bb. A recessive trait will only show in bb. Each child will have a 50% of being Bb, a 25% chance of BB, and a 25% of being bb. This means a child has a 75% chance of being blue and a 25% chance of being yellow.

As these are individual rolls and not statistical, it is okay to have a wider spread of bb Yellows than Blues so having 2 yellows and a blue is probable.

In order for each tree to be valid, Parents A and B must both be carriers (Bb at least) of "b" genes. BB will not produce that child. Parents that are both Bb/Bb, Bb/bb, or both bb are valid

In the case of C, D, E, F, J, and K at least one member in each paring must be a carrier of "b". As all their children are Bb in order to both be Blue and produce Yellow offspring. A combination of BB/Bb, Bb/Bb or BB/bb is possible, with the latter more likely to produce the genes present in their grandchildren. They cannot be BB/BB or bb/bb.

Please note that this cannot be reversed as while the lines increasingly favor Yellow, a Blue cannot both be recessive and produce a child of either color. A child of a bb/bb will always be yellow. For blue to be recessive, than all children of L and M will be blue. Also while this is basically how phenotypes work, sometime the coloration is controlled by two or more genes.

For example, human hair has two genes, one which controls Dark vs. Light and the other which controls Red vs. Light and depending on the parents can run a gamut of colors.

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  • $\begingroup$ See parents G and H, this indicates blue isn't dominant. It's a bit tricky following the diagram. $\endgroup$
    – a4android
    Mar 31, 2018 at 1:24
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    $\begingroup$ @a4android No, it still fits. H can be Bb, with N inheriting the b $\endgroup$
    – Andon
    Mar 31, 2018 at 6:34
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To make things simple :

As it seems highly probable that "blue" is dominant and "yellow" is recessive, the following rules should apply :

- If at least one parent is blue, children can be blue or yellow ;

As the blue allele is dominant, it will mask the yellow allele.

In this case, blue + yellow doesn't make green, it only makes blue.

That's why blue people can eventually carry a hidden yellow version of the gene and have yellow kids.

- If both parents are yellow, children can only be yellow.

As the yellow allele is recessive, yellow people can't carry a blue version of the gene (they would have the blue trait instead).

That's why they can't have blue kids.

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  • $\begingroup$ See parents G and H, this indicates blue isn't dominant. It's a bit tricky following the diagram. $\endgroup$
    – a4android
    Mar 31, 2018 at 1:24
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    $\begingroup$ @a4android, parent G (bb) + parent H (bB) can produce any of (bb) or (bB) genotypes in their offspring, which express as the Y or B phenotypes respectively. $\endgroup$
    – Mark
    Mar 31, 2018 at 4:56
  • $\begingroup$ @a4android Mark is right. In genetic, "dominant" doesn't mean that the trait will be transmitted automatically to the offspring. It only means that if you've got the 2 versions of the gene (blue and yellow), the blue wins over the yellow... and you'll be blue. But you can still transmit the yellow gene to your kids. If the "yellow" trait was a disease (genetic disorder), H would be an asymptomatic carrier. $\endgroup$
    – Ghajini
    Mar 31, 2018 at 12:18
  • $\begingroup$ @Ghajini I already knew that, because it's they taught us in Genetics. It's all about the alleles. $\endgroup$
    – a4android
    Apr 1, 2018 at 3:55
  • $\begingroup$ So, if we follow the basics (1 gene with 2 alleles) then G+H and their offspring indicate that blue IS dominant. $\endgroup$
    – Ghajini
    Apr 1, 2018 at 8:26
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The blue trait has a recessive gene. This is demonstrated by the fact that where both parents are blue non-trait (J & K) and trait (L & M), at least, one progeny has the trait. This is simple Mendelian genetics.

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    $\begingroup$ J & K are unkown, you can't tell anything from them. If the blue allele of the gene was recessive, R S & T should be blue. The fact that S and T are yellow (when both parents are blue) proves that the yellow allele is recessive. Both L & M have a yellow allele of the gene, and they transmitted it to their kids. But L & M also have a blue allele, and the blue masks the yellow. That is why blue is dominant and yellow is recessive. $\endgroup$
    – Ghajini
    Mar 31, 2018 at 12:42

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