debbie394
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This is the question:

In guinea pigs, the genes for coat texture and coat colour are found on separate chromosomes. The allele for rough coat is dominant to the allele for smooth coat. The allele for black coat is dominant to the allele for white coat.
A black guinea pig with a rough coat was crossed with a white guinea pig with a rough coat.
The cross was repeated on a number of occasions and the phenotypes of the offspring were as follows:
28 rough and black coats
31 rough and white coats
11 smooth and black coats
10 smooth and white coats

Complete the genetic diagram to explain this cross.
Use the following symbols to represent the alleles:
R = rough r = smooth
B = black b = white
Parental phenotypes: rough and black coat × rough and white coat
Parental genotypes: ...............RrBb.................. ...Rrbb.........................
Gametes: ................................. ..................................

Query:
why for the genotypes for the rough coats and the black coat is one recessive allele and one dominant allele is used?
why aren't there two dominant alleles?
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Amefish
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(Original post by esmeralda123)
Query:
why for the genotypes for the rough coats and the black coat is one recessive allele and one dominant allele is used?
why aren't there two dominant alleles?
Some of the offspring have soft coats. This could only have occurred with two recessive alleles. Think about if either parent had two dominant alleles for the rough coat (RR), then what would be the possible offspring genotypes & phenotypes?
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macpatgh-Sheldon
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Hi,

It is quite difficult to remember the various ratios of offspring with two variables (phenotype variants), and when one is dominant or otherwise.

The ratios of 1:3:3:1 are known to be the case where there are two dominant alleles for different phenotypes, and where one parent is heterozygous for both phenotype pairs, and the other parent is heterozygous for one phenotype pair and homozygous recessive for the other phenotype pair.

The only way to confirm this easily is with a quick punnet diagram (should not take too long by hand, but I have done here in Excel to illustate for you).

spotted grey = rough black
plain grey = smooth black
spotted yellow = rough white
plain yellow = smooth white

Name:  Punnet Sq double.jpg
Views: 424
Size:  133.8 KB

You will see that the ratios are 1:3:3:1


M (specialist biology tutor)
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