MONOHYBRID CROSSES 

1. Long hair is dominant to short hair in guinea pigs.  Show the results of a cross between a homozygous short haired male and a pure long haired female.

L = long hair and dominant. l is recessive

ll x LL gametes: l and L

All offspring (F1’s) are Ll They are all long haired.

2. Self the offspring from #1 among themselves and show the expected results.

Ll x Ll (Each F1 produces L and l gametes) => LL: 2 Ll : ll 3 long haired ( LL and 2 Ll) and 1 short haired (ll)

3. The allele for purple flowers is dominant over the allele for white flowers.  What type of offspring would you expect if you crossed a pure line purple flower with one that is white?  Show both genotypic and phenotypic ratios.

Pure breeding purple x pure breeding whitePP x pp => Pp only (all purple because P is dominant) So there are no ratios here as both the genotypes and phenotypes are 100%)

4. Cross the offspring from the preceding problem.  Show genotypic and phenotypic ratios.

Pp x Pp => 1 PP: 2 Pp: 1 pp 1:2:1 genotypes 3 purple (PP and 2 Pp): 1 white (pp) ;3:1 purple to white

5. In Holstein cattle the spotting of the coat is due to a recessive allele while the solid colored coat is controlled by a dominant allele.  What types of offspring might be produced by a cross between two spotted animals?

Ss = spotted and S is dominant for solid coat

ss x ss => only ss offspring which are all spotted

6. The polled (hornless) condition in cattle is dominant over horned.  A cattleman in Texas has a range stocked with polled cattle only, but some horned cattle occasionally appear.  These are removed from the range before they are allowed to breed.  Assuming that this man has a good fence which can keep out stray bulls, how can this be explained?  How could the cattleman manage his herd in the most efficient way to eventually eliminate horned offspring? Explain how he could carry this out.

Some of the polled cattle are heterozygous (Pp). Occasionally a Pp x Pp mating occurs. There is a 25% chance that a pp (horned calf) is produced from this cross.

To eliminate the p gene he could:

a. Remove any horned offspring (sell them)b. Keep records of the mating and don’t let the parents of the horned cattle mate again (sell these parents)c. Don’t let any of the horned cattle’s siblings mate (sell them too) Barbecue?

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7. Suppose a red flower and a white flower was cross-pollinated.  The flowers from the resulting seeds were neither red nor white but pink.  Explain.

This is likely due to incomplete dominance. In some flowers, such as snapdragons, the heterozygous state is phenotypically an intermediate color.

8. Cross the offspring from the preceding problem.  Give the phenotypic and genotypic ratios.

Usually the phenotypic ratio is 1:2:1 for red: pink: white. Genotypically 1 RR: 2 Rr : 1rr where R = red and r = white

 9. An albino (lack of pigmentation) man marries a normally pigmented woman who had an albino

mother.  Show the types of children that this couple may have and the proportions of each.

aa = >albinism and AA or Aa => normal pigmentation

Albino father x carrier mother (inherited one a gene from her mother)

aa x Aa => 1aa: 1Aa in offspring. So the changes of having an albino child is 50% and normal pigmented child 50%

  

10.    In shorthorn cattle, coat color may be red, white, or roan.  Roan is an intermediate phenotype expressed as a mixture of red and white hairs.  The following data were obtained from various crosses:

                                                             i.      Red x White = All Roan                                                           ii.      White x White = All White                                                        iii.      White  x  Red = All Roan                                                          iv.      Roan x Roan = ¼ Red, ½ Roan, ¼ White

How is coat color inherited?  What are the genotypes of the parents and offspring for each cross?

The coat color is inherited as incomplete dominance. RR = red; Rr = roan and Rr = roan

i. RR x rr => offspring all Rrii. rr x rr => offspring all rriii. rr x RR => offspring al Rriv. Rr x Rr => offspring 1 RR 2Rr 1 rr

   

http://www.jcseagles.org/cmorris/Chapternotes/Genetics%20Problems.htm

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