Answer:
Both parents have white fur. <em>The only</em> way that this can happen is if their genotypes are both <em>homozygous (bb)</em>. Therefore, both parents pass the (b) allele to every offspring. When you combine two of these alleles, the resulting genotype is <em>(bb);</em> therefore, the resulting offspring have white fur.
Explanation:
A homozygous trait can be described as a trait in which both the alleles for a gene are similar. A heterozygous trait can be described as a trait in which both the alleles of a gene are different. If an allele masks the effect of another allele, it is said to be dominant. the allele that gets suppressed is termed as recessive.
Hence, for a recessive trait to occur both the alleles of a gene should be homozygous recessive.
The punnet square for the above cross is shown below:
b b
b bb bb
b bb bb
The pair of alleles would be heterozygous
A karyotype involves the arrangement of a complete set of an individuals chromosomes (see attached image)
During interphase you cannot see the chromosome because they aren't condensed to the shape you can see through a microscope.
During prophase the Chromosomes become condensed and are able to be seen.
So, to answer your question, you cannot make a proper karyotype while cells are in interphase. Therefore, they would need to be in mitosis phase.
I hope this helps! I'm happy to answer any other questions you might have :)
[image sourced from: http://www.spectral-imaging.com/applications/cytogenetics/karyotyping]