phenotype ratio

Question

Two dogs with black, short haired coats have been crossed several times and produced many litters of puppies. Altogether, they have produced puppies of the following phenotypes:
18 with black, short haired coats
5 with white, short haired coats
7 with black, long haired coats
2 with white, long haired coats
To achieve D2 do the following:
• Use genetic diagrams (Punnet square) to explain and calculate the expected ratios;
• Use chi-test to analyze whether the difference between the expected ratio and the observed ratio is significant of just due to chance;
• Explain why chi-test is the appropriate method to use in this case;

Answer 1

Hannah,

 

I shall give you some clues to answer this question yourself with Punnet square.

 

Two dogs with black coat color are producing both black and white puppies after mating. This fact indicates that black coat color (B) is a dominant trait and both dogs are heterozygous for this trait with genotype: B/b⁺ ;  B gives black coat color and b⁺/b⁺ gives white coat color. Please note that number of white coat color puppies is about half than black coat color puppies.

 

Both these dogs have short hair coat yet they produce puppies with  two kinds of coat hair: long hair coat and short hair coat. In total  there are 23 short hair coat  puppies and only 9 long hair puppies.  Short hair coat trait(SH) also appears as a dominant trait from this data and the mating pair appears heterozygous with genotype: SH/sh where sh/sh would give long hair coat.

 

Two coat color traits are segregating independently and that indicates that they are not linked.

 

Based on these  you can think of genotypes of  gametes from this parent pair and make a Punnet square. It would give you the expected ratio.

 

You make a table of expected ratio and another one of the ratio that is obtained in the question.

 

With these tables,  you can do the chi square test or use chi square calculator.

 

The ratio in the question is definitely skewed and therefore, chi square test is required to understand the significance of results.

 

Hope this helps.