I'm reading through some science that could come up in my GRE equivalency.

I've never been much of a biologist, more of a chemist/geologist....though it does interest me massively!

I'm just seeing this question in a textbook....

Q) Assume eye colour in humans is controlled by a pair of alleles of a gene where the allele giving brown eyes is dominant to the allele giving blue eyes. Both parents of a blue eyed man, John, were brown eyed. He married a brown eyed woman, Sara, whose father had brown eyes and mother blue eyes. Sara had a blue eyed sister. John and Sara had a brown eyed child.

Fill in the boxes and circles on the family tree below to show the genotype of each individual.

I just wanted to ask how exactly we mark out genotype in science? Do we use D's and r's?


  • 1
    $\begingroup$ Conventionally, a pair of contrasting alleles are shown with the first letter of the dominant trait as capital for dominant, and the first letter of the first letter of the dominant trait in lower case for recessive. For this case, it'll be B for brown and b for blue. Someone homozygous for blue would be bb, someone homozygous for brown would be BB and someone heterozygous (brown) would be Bb. $\endgroup$ May 19, 2016 at 12:47
  • 1
    $\begingroup$ @MalharKhushu -- You should put that in the answer box. :) It's correct AFAIK -- Capital letters for Dominant, lower-case for Recessive, and works fine as long as it's within a Mendelian framework. $\endgroup$
    – MCM
    May 19, 2016 at 14:02
  • $\begingroup$ Out of curiosity, what is a GRE equivalency? $\endgroup$
    – Remi.b
    May 19, 2016 at 15:57
  • $\begingroup$ Thanks, @MCM, but that would require citations, and so on, and for such a short answer, I don't really want to hunt out articles or studies $\endgroup$ May 20, 2016 at 9:51
  • $\begingroup$ How can I distinguish between who is homozygous and who is hetrozygous? $\endgroup$ May 20, 2016 at 22:09

1 Answer 1


[H]ow exactly we mark out genotype in science? Do we use D's and r's?

General rule

The general rule is to indicate the dominant allele with a upper-case letter and the recessive allele with a lower-case letter.

The most commonly used letter is the first letter of the alphabet. The two possible alleles are a and A and the four possible genotypes here are therefore aa, aA, Aa and AA. Only aa has blue eyes in your example. Note that it is very common to merge together Aa and aA (that is to not consider whether the A allele was inherited from the father or from the mother) and just call it Aa.

More than 2 alleles OR no clear dominance relationship

It is quite common to use A1 and A2 as well but when doing so, we don't implicitly indicate a relationship of dominance. This notation also has the advantage to be able to deal with cases where there are more than two alleles segregating in the population (A1,A2,A3). Note that in reality perfect dominance is quite rare.

More than 1 locus

When dealing with more than one locus, we generally use A/a for the first locus, B/b for the second locus, C/c for the third etc... However, in general when dealing with several loci, authors don't have identical loci ad therefore they directly name them and use appropriate letters. For example, in models of recombination under control of a modifier locus, the modifier locus can often take the values M and m, while a random linked locus under purifying selection would take the letters A and a.

  • $\begingroup$ Thank you, this is so incredibly helpful!! So brown eyes are dominant over blue eyes $\endgroup$ May 20, 2016 at 22:12
  • $\begingroup$ Just to make sure you got it: I am not stating that brown eye allele is dominant over blue eye allele (actually genetics of eye color is a little more complicated). I only described the standard notation used to designate different alleles. That was your question right? $\endgroup$
    – Remi.b
    May 20, 2016 at 22:17
  • $\begingroup$ Thanks for this, sorry silly comment by me. Am I right in suggesting that - John's parents are both Bb - John is bb. - Sara is Bb - Sara's mum is bb - Her sister is Bb - Her dad, is harder to figure out for.....is he BB or Bb and why? - Finally, the brown eyed child is Bb. $\endgroup$ May 20, 2016 at 22:20
  • $\begingroup$ I'm assuming that the father is Bb, because amongst his children there is one dominant allele and three recessive $\endgroup$ May 20, 2016 at 22:24
  • $\begingroup$ It looks like there were an implicit question in your post about the genotypes of the different individuals. I thought the post was about notation. Next time make sure to make your question clear and to limit your posts to only one question. If it was a homework question you should have used the homework tag too. $\endgroup$
    – Remi.b
    May 20, 2016 at 23:13

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