## Natshane 3 years ago Is there a simple way to understand test cross of dihybrid plants? involving punnette square?

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1. yukitou

punnett square I always use this method for my genetic question.

2. blues

Yes - Punnett squares are very useful for this. A test cross is recessive for all traits of interest - so for a dihybrid cross that means, aabb where a is the first gene and b is the second.

3. Natshane

thank you! However my teacher loves to give us questions where she provide us with the ratio of offsprings and we have to find the parental genotype. How do I do this kind of questions??

4. jazrio

even my teacher @Natshane

5. Natshane

hehehehe....I'm asking YY to teach me after class tests...

6. yukitou

Use punnett square. Let say a couple has one AB type child and one O type child. You draw punnett square and filling in the child's genotype. The middle box is filled by AB and the bottom right is by O-type. Since the child receive each allele from each parent, you can 'trace back' the parental genotype. If the child is AB, one parent will have one A allele and the other will have B allele. If the child is O type, he receive each O allele from each parent. Now you can roughly figure the parent's genotype. |dw:1339531802278:dw| I hope this helps.

7. Natshane

Oh! This seems to be so much easier to understand! Thank you @yukitou! I'll try to do it this way for my other questions. But what if the question is given ratio 148 wrinkled, yellow, 24 wrinkled green that kind of question? isn't it harder to find using a punnett square?

8. yukitou

you mean dihybrid cross? The punnett square is still very useful. Let say parental genotypes are RR/yy and rr/YY where R is round, r is wrinkled, y is green and Y is yellow. When they cross, F1 genotype is Rr/Yy. |dw:1339599269501:dw| then, you self-cross this F1to get F2 genotype.|dw:1339599523841:dw| Well, you will get 4 different phenotype with 9 different genotype. The phenotype ratio is 9:3:3:1 where by 9 is R-/Y-, 3 is R-/yy, another 3 is for rr/Y- and the 1 is rr/yy. The dash means the allele can be either dominant or recessive because it still won't affect the effect of dominant allele. i hope this helps.

9. Natshane

Thank you! I'll try it on my exercise questions now.