Question

Cat worksheet: use in combination with the cat photos provided in the module to practice your dihybrid cross skills

**refer to text sections 9.4 & 9.5 to help you during this exercise**

1. Look at the photographs of domestic cats found in this module and take a look at the information on Mendelian traits that control the appearance of cats’ coats found below:

1. Coat uniformity is controlled by genes at the P locus: genotype (P--) = white patches of fur & genotype (pp) = solid colored fur

2. Hair length is controlled by genes at the H locus: genotype (H--) = short hair & genotype (hh) = long hair

KEY: P-- means PP or Pp, and H-- means HH or Hh: you’ll need to choose one or the other

2. Use the information about these two genes and the pictures of cats to answer the questions that follow.

Questions:

1. What does the gene at the H locus control? Which allele is dominant and which is recessive?

Hair length, H is dominant while h is recessive.

2. What does the gene at the P locus control? Which allele is dominant and which is recessive?

Coat uniformity, P is dominant while p is recessive.

3. Look at the photos of cats and figure out all the possible phenotype combinations an individual cat could have for the two traits of hair length and coat uniformity. Use words, not letters to describe these phenotypes (i.e. solid colored long hair, solid colored short hair, etc..).

First cat: Solid colored long hair

Third cat: Solid colored short hair

Third cat: White patches short hair

Fourth cat: White patches long hair

4. For each of these genes, how many alleles does an individual cat have? In other words how many alleles that determine hair length and how many that determine coat uniformity?

Two

5. Choose four cat photos and fill in the blanks below for each. This helps you practice translating what you see (phenotype) into genotype.

1. Cat #1 phenotype: Solid colored long hair

genotype: pphh

2. Cat #2 phenotype: Solid colored short hair

genotype: ppHh

3. Cat#3 phenotype: White patches short hair

genotype: PpHh

4. Cat#4 phenotype: White patches long hair

genotype: Pphh

6. Now choose any two cats and designate one male and the other female. Both are heterozygous for hair length and coat uniformity.

1. What are the genotypes of each cat?

Male: PpHh

Female: PpHh

2. What are their phenotypes?

Male: White patches short hair

Female:White patches short hair

7. Write the genotypes of all the gametes that each of these two cats can produce. (NOTE: this can be tricky at first, if needed refer to your dihybrid cross Amoeba Sisters video in the PreLab to see an example of how it’s done).

1. Male’s gametes:

2. Female’s gametes

8. Now you’re ready to take the gametes you’ve created above and construct a dihybrid cross Punnett square to predict the phenotypic and genotypic outcomes of possible offspring (kittens!). Use the space below to construct your Punnett.

*Text section 9.5 & figure 9.5A are a good reference to use as you construct your Punnett. Doublecheck your gametes to be sure they each contain only ONE allele (alphabet letter) for each trait.

9. Using the possible phenotype combinations you listed in Question 3, write down the number of individuals in each category you expect to see based on your Punnett square. Can you express this as a ratio? (see fig 9.5A text)

10. Does a Punnett square show expectations for a whole population or just for one family? Explain your answer in relation to the cats in the photos and the Punnett square you just made.

Answer 1

In the given question, the genotype of Male: PpHh (White patches short hair) and Female: PpHh (White patches short hair). We know that the gametes separate during meosis via segregation and independent assortment. So the gametes will be :

1. Male’s gametes: PH, Ph, pH, ph

2. Female’s gametes: PH, Ph, pH, ph

Constructing a Punnett square we get,

	PH	Ph	pH	ph
PH	PPHH	PPHh	PpHH	PpHh
Ph	PPHh	PPhh	PpHh	Pphh
pH	PpHH	PpHh	ppHH	ppHh
ph	PpHh	Pphh	ppHh	pphh

The numer of individuals in each phenotypic category is : White patches short hair=9; White patches and long hair= 3; solid colored fur and short hair= 3; solid colored fur and long hair=1

Here we can see that the phenotypic ratio is : 9:3:3:1

And genotypic ratio is : 1:2:1:2:4:2:1:2:1

This ratio gives the expectations for a whole population and not just for one family. This is because the Punnett Square represents all the possible combinations of Mendellian inheritance which will be followed irrespective of the population size.

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