15 Jun Cattle coat color is determined by codominant alleles R and r. RR results in a reddish cow
Question
1. Cattle coat color is determined by codominant alleles R and r. RR results in a reddish cow, Rr
in a roan coloured cow, and rr in a white coloured cow. A population of 4000 cattle consisted of
2560 reds, 1280 roans and 160 white cattle. Calculate the gene frequency of the ‘R’ gene and the
‘r’ gene. If you fill in the Table below you’ll have your answer:
Phenotype
Number of cattle
160
Total
r
0
r
1280
White
R
2560
Roan
Gene Frequency
R
Red
Number of genes
0
4000
0
0
Hence the overall gene frequency (f(p)) of the red gene =
Hence the overall gene frequency (f(q)) of the white gene =
You can also solve this question using the Hardy Weinberg formula
2. PKU (phenylketonuria) occurs in about 1 out of 12,000 Caucasians. It is rarer in other ethnic
groups. Calculate the frequency of the recessive disease causing gene. Calculate the frequency
of the normal gene. Complete the following table:
Human Phenotypes
Number in a population of 12000
Normal
Human
Genotypes
Number in a population of 12000
15624
Normal
Carrier
Showing PKU
1
Showing
PKU
1
Hint: Here you must start the answer by calculating the frequency of the homozygous
recessive gene q and you know that homozygous individuals occur in the population once
every 12,000 individuals. Hence q2 = 1 in 15625.
3. 10% of human males in a sampled population were found to be red/green colorblind. This is a
recessive sex-linked condition. What proportion of human females are expected to be colorblind
in this population. Hint: You might find it easier to answer this question if you referred to the table
below, where Xb is the recessive X-linked colour-blind allele and XB is the dominant normal gene.
Sex
Category
Homozygous
Heterozygous
Homozygous
Dominant
Recessive
Genotype
XB XB
XB Xb
Xb Xb
Phenotype
FEMALES
Dominant
normal
normal
colour-blind
XB
XB and Xb
Xb
Genotype
XB Y
DOES
Xb Y
Phenotype
normal
NOT
colour-blind
Gamete Types
MALES
(Percentage)
(Then you can fill
this in)
Gamete Types
XB and Y
(First you must fill
this in)
EXIST
Xb and Y
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