Rules for Genetic Crosses: Trihybrid Cross

Three Mikan

So, you’ve read the article on dihybrid crosses, you’ve red the article on monohybrid crosses, and now you want to move on to polyhybrid crosses (but take it slow). Well, far be it from me to keep you from learning. Let’s see if you’ve got it…

In the hypothetical MPSN world, goldfish only have three traits: size, color, and teeth. The size gene comes in two flavors, big (B) and small (b). The color and teeth genes are similar, coming in gold(G) or black(g) and teeth(T) or no teeth(t), respectively. Let’s cross a homozygous dominant male with a homozygous recessive female. What are the phenotypes of the F1 offspring (the children)? And if we cross the F1 offspring with each other, what are the phenotypes and ratios of the F2 offspring (the grandchildren)?

Can you handle it? Now don’t scroll down until you think you have answered all the questions. Ready? GO!

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Step 1: Gather the Known Information

• Mom = (bbggtt)
• Dad = (BBGGTT)

Step 2: Find Unknown Information

The first thing we’ll need to do, now that we know our parental phenotypes is to figure out what types of gametes that they can produce. Since we know that mom ONLY HAS (b), (g), andt (t), then (bgt) is the only gamete she can produce. Likewise for dad: only his gametes will each be dominant (BGT).

If we cross these, we end up with 100% (BbGgTt) offspring in the F1 (children) generation. Thankfully since mom and dad only have dominant or recessive alleles to give, we can deduce that the offspring will each be heterozygous WITHOUT USING A PUNNETT SQUARE! Woot!

HOWEVER, to answer the second question, we must employ the help of a punnett square. Here goes:

So, what are the F1 gametes? Since they are all (BbGgTt), then they will all have the same set of gametes. (But those gametes can combine differently, so don’t assume that all the offspring of the F2 generation (grandchildren) will be the same like in the F1 generation.) Using what we know about permutations, we can calculate that there will be 8 different types of gametes. These F1 gametes are: (BGT), (BGt), (Bgt), (bgt), (bGt), (bgT), (BgT), (bGT). Now, to find out the phenotypes, we have to cross them. 123GO.

OY. That took me 20 minutes to type up… such a pain… But anyway, here’s what we found:

• Big, Gold, and with Teeth      = 27
• Big, Gold, and toothless        = 9
• Big, black, and with Teeth     = 9
• Big, black, and toothless       = 3
• small, Gold and with Teeth    = 9
• small, Gold, and toothless     = 3
• small, black, and with Teeth  = 3
• small, black, and toothless    = 1

As you can see, both for populations of Big F2 offspring (grandchildren) and small F2 offspring (grandchildren), we can see a difinite 9:3:3:1 ratio. [As you travel up the corporate ladder into bigger polyhybrid crosses, be on the look out for that 9:3:3:1 ratio]

Step 3: Check Your Work.

The original problem asked 2 questions: “What would the F1 phenotype be?” and “What would the F2 phenotypes and ratios be?” Have we answered both of those questions? Yes? Cool. Good work.

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Best of Luck,

Grey