Cattle coat color is determined by codominant alleles R and r. RR results
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
Number of genes
Gene Frequency
R
r
R
r
Red
2560
0
0
Roan
1280
White
160
0
0
Total
4000
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
Human Genotypes
Number in a population of 12000
Normal
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, whereXb is the
recessive X-linked colour-blind allele and XB is the
dominant normal gene.
Sex
Category
Homozygous
Dominant
Heterozygous
Dominant
Homozygous
Recessive
FEMALES
Genotype
XB XB
XB Xb
Xb Xb
Phenotype
normal
normal
colour-blind
Gamete
Types
XB
XB and Xb
Xb
MALES
Genotype
XB Y
DOES
Xb Y
Phenotype
(Percentage)
normal
(Then you can fill this in)
NOT
colour-blind
(First you must fill this in)
Gamete
Types
XB and Y
EXIST
Xb and Y