Section 5.1 Questions
(Page 193)
1. The pea plant was an excellent choice as the subject for Mendel’s hereditary experiments because it
produces many offspring in a short amount of time, it does not need a lot of space to grow, it is easily
obtainable, and it has a number distinct traits for which only two phenotypes are expressed.
2. It was important that Mendel experimented with true-breeding variety plants because it ensured that the
traits of any offspring in his crosses would represent the hybrid condition. If the parents were not truebreeding, it would be unclear if the traits of the offspring were caused by the hybrid condition or by
variation in the genetic makeup of the parents.
3. For Mendel’s F1 crosses, the phenotypic ratio was 3:1 (3 plants expressing the dominant phenotype to 1
expressing the recessive phenotype) and the genotypic ratio was 1:2:1 (1 homozygous dominant to 2
heterozygous to 1 homozygous recessive).
4. Mendel concluded the following from his experiments:
x The inheritance of each trait is determined by discrete heredity units, which he called “factors.”
Although these factors might not be expressed in an individual, they can still be passed on.
x A factor may be dominant or recessive. A factor expressed in all F1 individuals is the “dominant
factor.” The factor that remained hidden but was expressed in the F2 generation is the “recessive
factor.”
x For each characteristic, an organisms carries two copies of each factor; one from each parent.
x Parent organisms donate only one copy of each factor to their offspring.
Today, Mendel’s “factors” are called genes, and each variation of a factor is called an allele. An allele is
dominant or recessive to other alleles. An organism carries two copies of each gene, which may be the
same or different alleles. One copy of a gene is inherited from the father and the other from the mother.
5. (a) Dominant or recessive may refer to alleles of a genotype or traits of a phenotype. A dominant allele is
always expressed and always gives rise to the dominant trait .A recessive allele is expressed only in the
absence of a dominant allele. There must be two copies of a recessive allele to give rise to the recessive
trait. If there is only one copy of the recessive allele, the recessive trait will not be expressed.
(b) A gene determines the expression of a characteristic, such as flower colour. An allele is a specific
copy of the gene and determines a trait, such as red. In Mendel’s pea plants, the alleles for height were tall
and dwarf.
(c) Homozygous means that the two copies of a gene are the same allele. Heterozygous means that the two
copies of a gene are different alleles.
6. The law of segregation states that: (1) organisms inherit two copies of genes, one from each parent, and (2)
organisms donate only one copy of each gene to their gametes because the genes separate during gamete
formation.
7. Sample size is important because it produces more accurate results, since the effects of random variation
are reduced. This helped Mendel to see the mathematical patterns in his data, since the observed numbers
would have approached theoretical numbers more closely.
8. (a) The predicted genotypic ratio of the offspring is 1:1 (Rr:rr). Students’ diagrams should be similar to
the following:
(b) The predicted phenotypic ratio of the offspring is 1:1 (1 smooth: 1 wrinkled).
(c) The predicted number of wrinkled pea seeds is 25.
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Chapter 5: Mendelian Genetics—Patterns of Inheritance 5.1-3
9. The probability that they will have children that suffer from this disorder is 50 % (1:1). Students’ diagrams
should be similar to the following:
10. (a) The bull is heterozygous (Rr).
(b) Since red and white spots are recessive, the genotype of the cow must be rr. The phenotypic ratio of
the offspring is 1:1 (1 black and white : 1 red and white). Students’ diagrams should be similar to the
following:
11. (a) Answers may vary with students’ research findings and development in this field. Sample answer:
Table: Advantages and Disadvantages of Artificial Insemination in Cattle Breeding
Advantages
Disadvantages
the best bulls can sire very large numbers
can be very costly, depending on the bull
offspring
semen used
increases the probability of getting desirable
traits of offspring also determined by cows,
traits in a herd
so may not get the traits that are desired
avoids sexually transmitted diseases in a herd if only a few prize bulls are used most often,
can reduce the genetic diversity of overall
cattle population
avoids the expense and effort of keeping own requires specialized training and hands-on
bulls for breeding, and can make breeding
management of animals and their
safer, since bulls may be aggressive
reproductive cycle
any two animals in the world can be bred
semen must be transported and stored under
without having to transport the animals
controlled conditions to remain viable
(b) Answers will vary throughout the life of the resource. In 2002, artificial insemination accounts for 10
to 20 % of all first inseminations of cows worldwide, and was increasing.
12. (a) Answers will vary. Sample answer: Giberellins (GAs) are a class of plant hormones that play a role
in plant height. In many plants, gibberellins promote stem elongation, which is growth of the length of the
stem but not the diameter. Many dwarf species produce lower levels of GAs. In fact, one of the seven pairs
of traits used by Mendel to work out the basic rules of inheritance, dwarf-tall, is regulated by a recessive
gene, called le, that is involved in GA production. In dwarf plants, le codes for a defective enzyme that
reduces the ability of the plant to synthesize GA. The dominant gene, Le, encodes a functional enzyme that
permits normal GA synthesis, giving the "tall" phenotype.
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Chapter 5: Mendelian Genetics—Patterns of Inheritance 5.1-4