# Genetics Practice Problems and Answers | Chargaff rules and Probability

1. The ability to taste a chemical called PTC is inherited as an autosomal dominant allele. What is the probability that children descendant from parents both heterozygous for this trait can taste PTC
a) 0
b)1
c)3/4
d)1/2
Answer. If you let T represent allele for the ability to taste PTC, then the cross would be Tt xTt. The Punnet square that follows show that ¾ of the offspring have ability to taste PTC (1/4 TT +1/2 Tt)
 T t T TT Tt t Tt tt
Ans: 3/4

2. Which of the following is true of gametes produced by an individual with genotype Dd?
a)1/2D and 1/2D
b) 1/2D and 1/2d
c) 1/2Dd and 1/2dD
d)All Dd
Ans: d)All Dd

3. Three genes A,B,C  are located on a chromosome. The cross over percentage between A and B is 10% while that between A and C is 8% while that between B and C exhibits a cross over percentage of 18%. What will be the correct order of these genes in the chromosome.
a) XYZ
b)YXZ
c)YZX
d)none of these
Ans: b)YXZ

4. What is the cross over percentage between two genes which are 10 map units apart?
a) 5%
b) 10%
c) 20%
d) 100%
Ans: The actual distance between two genes is represented as map unit. One map unit is equivalent to the percentage (%) of crossing over between the two genes.
Ans: b) 10%

5. For the cross AABBCCDd X AAbbCcDd, what is the probability that an offspring will be?
a) 1/16 b) 1/8  c) ¼ d) 3/8
It is not practical to make Punnett square for genotypes involving more than two genes. In this problem, you asked about the frequency of one specific offspring, AABbCcDd. To solve this problem, look at each gene separately.
·         Looking at the first gene, the parents are AA X AA and all offspring will be AA (frequency of 1)
·         For the second gene, BB X bb, all offspring will be Bb (frequency of 1)
·         For the third gene, the parents are CC x Cc, which products ½ CC and ½ Cc.
·         For the fourth gene, Dd x Dd, produces ¼ DD, ½ Dd and ½ dd
To find the probability of AA is 1, of Bb is 1, of Cc is ½, and of Dd is ½.
Then find the product of these frequencies.
·         For AABbCcDd the product is 1 X 1 X ½ X ½ =1/4

6. If you roll a pair of dice, what is the probability that they will both turn up a three?
a)1/2
b)1/4
c)1/16
d)1/36
The chance that one dice will turn up a three is 1 in 6, or 1/6.
For both dice to turn up a three, the probability is determined by multiplying the probability of each event happening independently, or 1/6 x 1/6 =1/36

7. A mixture of DNA lysate has been analyzed for nucleotides. It has been found that the Percentage of Guanine in that mixture was 18%, what will be the percentage of Thymine in that mixture by considering Chargaff rules?

a) 18% b) 9% c) 32% d) 30%

Ans: c) 32%