3.E: Genetic Analysis of Single Genes (Exercises)

These are homework exercises to accompany Nickle and Barrette-Ng's "Online Open Genetics" TextMap. Genetics is the scientific study of heredity and the variation of inherited characteristics. It includes the study of genes, themselves, how they function, interact, and produce the visible and measurable characteristics we see in individuals and populations of species as they change from one generation to the next, over time, and in different environments.

Q3.1

hat is the maximum number of alleles for a given locus in a normal gamete of a diploid species?

Q3.2

Wirey hair (W) is dominant to smooth hair (w) in dogs.

1. If you cross a homozygous, wirey-haired dog with a smooth-haired dog, what will be the genotype and phenotype of the F1 generation?
2. If two dogs from the F1 generation mated, what would be the most likely ratio of hair phenotypes among their progeny?
3. When two wirey-haired Ww dogs actually mated, they had alitter of three puppies, which all had smooth hair. How do you explain this observation?
4. Someone left a wirey-haired dog on your doorstep. Without extracting DNA, what would be the easiest way to determine the genotype of this dog?
5. Based on the information provided in question 1, can you tell which, if either, of the alleles is wild-type?

Q3.3

An important part of Mendel’s experiments was the use of homozygous lines as parents for his crosses. How did he know they were homozygous, and why was the use of the lines important?

Q3.4

Does equal segregation of alleles into daughter cells happen during mitosis, meiosis, or both?

Q3.5

If your blood type is B, what are the possible genotypes of your parents at the locus that controls the ABO blood types?

Q3.6

In the table below, match the mouse hair color phenotypes with the term from the list that best explains the observed phenotype, given the genotypes shown. In this case, the allele symbols do not imply anything about the dominance relationships between the alleles. List of terms: haplosufficiency, haploinsufficiency, pleiotropy, incomplete dominance, co-dominance, incomplete penetrance, broad (variable) expressivity.

Table for Question 3.6

	A1A1	A1A2	A2A2
1	all hairs black	on the same individual: 50% of hairs are all black and 50% of hairs are all white	all hairs white
2	all hairs black	all hairs are the same shade of grey	all hairs white
3	all hairs black	all hairs black	50% of individuals have all white hairs and 50% of individuals have all black hairs
4	all hairs black	all hairs black	mice have no hair
5	all hairs black	all hairs white	all hairs white
6	all hairs black	all hairs black	all hairs white
7	all hairs black	all hairs black	hairs are a wide range of shades of grey

Q3.7

A rare dominant mutation causes a neurological disease that appears late in life in all people that carry the mutation. If a father has this disease, what is the probability that his daughter will also have the disease?

Q3.8

Make Punnett Squares to accompany the crosses shown in Figure 3.10.

Q3.9

Another cat hair colour gene is called White Spotting. This gene is autosomal. Cats that have the dominant S allele have white spots. What are the possible genotypes of cats that are:

1. entirely black
2. entirely orange
3. black and white
4. orange and white
5. orange and black (tortoiseshell)
6. orange, black, and white (calico)

Q3.10

Draw reciprocal crosses that would demonstrate that the turkey E-gene is on the Z chromosome.

Q3.11

Mendel’s First Law (as stated in class) does not apply to alleles of most genes located on sex chromosomes. Does the law apply to the chromosomes themselves?

Q3.12

What is the relationship between the O⁰ and O^B alleles of the Orange gene in cats?

Q3.13

Make a diagram similar to those in Figures 3.9, 3.11, and 3.13 that shows the relationship between genotype and phenotype for the F8 gene in humans.

Answers

3.1 There is a maximum of two alleles for a normal autosomal locus in a diploid species.

3.2 a) In the F₁ generation, the genotype of all individuals will be Ww and all of the dogs will have wirey hair.

b) In the F₂ generation, there would be an expected 3:1 ratio of wirey-haired to smooth-haired dogs.

c) Although it is expected that only one out of every four dogs in the F2 generation would have smooth hair, large deviations from this ratio are possible, especially with small sample sizes. These deviations are due to the random nature in which gametes combine to produce offspring. Another example of this would be the fairly common observation that in some human families, all of the offspring are either girls, or boys, even though the expected ratio of the sexes is essentially 1:1.

d) You could do a test cross, i.e. cross the wirey-haired dog to a homozygous recessive dog (ww). Based on the phenotypes among the offspring, you might be able to infer the genotype of the wirey-haired parent.

e) From the information provided, we cannot be certain which, if either, allele is wild-type. Generally, dominant alleles are wild-type, and abnormal or mutant alleles are recessive.

3.3 Even before the idea of a homozygous genotype had really been formulated, Mendel was still able to assume that he was working with parental lines that contained the genetic material for only one variant of a trait (e.g. EITHER green seeds of yellow seeds), because these lines were pure-breeding. Pure-breeding means that the phenotype doesn’t change over several generations of self-pollination. If the parental lines had not been pure-breeding, it would have been very hard to make certain key inferences, such as that the F₁ generation could contain the genetic information for two variants of a trait, although only one variant was expressed. This inference led eventually to Mendel’s First Law.

3.4 Equal segregation of alleles occurs only in meiosis. Although mitosis does produce daughter cells that are genetically equal, there is no segregation (i.e. separation) of alleles during mitosis; each daughter cell contains both of the alleles that were originally present.

3.5 If your blood type is B, then your genotype is either I^Bi or I^BI^B. If your genotype is I^Bi, then your parents could be any combination of genotypes, as long as one parent had at least one i allele, and the other parent had at least one I^B allele. If your genotype was I^B I^B, then both parents would have to have at least one I^B allele.

3.6 case 1 co-dominance

case 2 incomplete-dominance

case 3 incomplete penetrance

case 4 pleiotropy

case 5 haplosufficiency

case 6 haploinsufficiency

case 7 broad (variable) expressivity

3.7 If the gene is autosomal, the probability is 50%. If it is sex-linked, 100%. In both situations the probability would decrease if the penetrance was less than 100%.

3.8

3.9 Note that a semicolon is used to separate genes on different chromosomes.

	Phenotype	Genotype(s)
a	entirely black	O^B / O^B; s / s O^B / Y ; s / s
b	entirely orange	O^0 / O^0 ; s / s O^0/ Y ; s / s
c	black and white	O^B / O^B ; S / _ O^B / Y ; S / _
d	orange and white	O^0 / O^0 ; S / _ O^0/ Y ; S / _
e	orange and black (tortoiseshell)	O^0 / O^B ; s / s
f	orange, black, and white (calico)	O^0 / O^B ; S / _

3.10

3.11 Because each egg or sperm cell receives exactly one sex chromosome (even though this can be either an X or Y, in the case of sperm), it could be argued that the sex chromosomes themselves do obey the law of equal segregation, even though the alleles they carry may not always segregate equally. However, this answer depends on how broadly you are willing to stretch Mendel’s First Law.

3.12 Co-dominance

3.13 People with hemophilia A use injections of recombinant Factor VIII proteins on demand (to control bleeding) or regularily (to limit damage to joints).