PCB 3063: BASIC PRINCIPLES OF HEREDITY UPDATED
EXAM WITH SOLVED QUESTIONS GRADED A+ -
VERIFIED EXAM
Chapter Three: Basic Principles of Heredity
COMPREHENSION QUESTIONS
Section 3.1
1. Why was Mendel’s approach to the study of heredity so successful?
Solution:
Mendel was successful for several reasons. He chose to work with the plant Pisum
sativum, which was easy to cultivate, grew relatively rapidly, and produced many
offspring whose phenotype was easy to determine, which allowed Mendel to detect
mathematical ratios of progeny phenotypes. The seven characteristics he chose to
study were also important because they exhibited only a few distinct phenotypes and
did not show a range of variation. Finally, by looking at each trait separately and
counting the numbers of the different phenotypes, Mendel adopted a reductionist
experimental approach and applied the scientific method as well as mathematical
methods. From his observations, he proposed hypotheses that he was then able to test
empirically.
2. What is the difference between genotype and phenotype?
Solution:
Genotype refers to the genes or the set of alleles found within an individual.
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Phenotype refers to the manifestation of a particular character or trait.
Section 3.2
3. What is the principle of segregation? Why is it important?
Solution:
The principle of segregation, or Mendel’s first law, states that an organism possesses
two alleles for any one particular trait and that these alleles separate during the
formation of gametes. In other words, one allele goes into each gamete. The principle
of segregation is important because it explains how the genotypic ratios in the haploid
gametes are produced.
4. How are Mendel’s principles different from the concept of blending inheritance
discussed in Chapter 1?
Solution:
Mendel’s principles assert that the genetic factors or alleles are discrete units that
remain separate in an individual organism with a trait (or phenotype) encoded by the
dominant allele being the only one observed if two different alleles are present.
According to Mendel’s principles, if an individual contains two different alleles, then
the individual’s gametes could contain either of these two alleles (but not both).
Blending inheritance proposes that offspring are the result of blended genetic material
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36 Chapter Three: Basic Principles of Heredity
from the parent and the genetic factors are not discrete units. Once blended, the
combined genetic material could not be separated from each other in future
generations.
5. What is the concept of dominance?
Solution:
The concept of dominance states that when two different alleles are present in a
genotype, only the dominant allele is expressed in the phenotype.
6. What are the addition and multiplication rules of probability and when should they be
used?
Solution:
The addition and multiplication rules are two rules of probability used by geneticists
to predict the ratios of offspring in genetic crosses. The multiplication rule allows for
predicting the probability of two or more independent events occurring together.
According to the multiplication rule, the probability of two independent events
occurring together is the product of their probabilities of occurring independently.
The addition rule allows for predicting the likelihood of a single event that can
happen in two or more ways. It states that the probability of a single, mutually
exclusive event can be determined by adding the probabilities of the two or more
different ways in which this single event could take place. The multiplication rule
allows us to predict how alleles from each parent can combine to produce offspring,
while the addition rule is useful in predicting phenotypic ratios once the probability of
each type of progeny can be determined.
7. Give the genotypic ratios that may appear among the progeny of simple crosses and
the genotypes of the parents that may give rise to each ratio.
Solution:
Genotypic ratio Parental genotype
1:2:1 Aa × Aa
1:1 Aa × aa
AA × Aa
Uniform progeny AA × AA
aa × aa
AA × aa
8. What is the chromosome theory of heredity? Why was it important?
Solution:
Walter Sutton developed the chromosome theory of inheritance. The theory states that
genes are located on the chromosomes. The independent segregation of pairs of
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Chapter Three: Basic Principles of Heredity 37
homologous chromosomes in meiosis provides the biological basis for Mendel’s two
rules of heredity.
Section 3.3
9. What is the principle of independent assortment? How is it related to the principle of
segregation?
Solution:
According to the principle of independent assortment, genes for different
characteristics that are at different loci segregate independently of one another. The
principle of segregation indicates that the two alleles at a locus separate; the principle
of independent assortment indicates that the separation of alleles at one locus is
independent of the separation of alleles at other loci.
10. In which phases of mitosis and meiosis are the principles of segregation and
independent assortment at work?
Solution:
In anaphase I of meiosis, each pair of homologous chromosomes segregate
independently of all other pairs of homologous chromosomes. The assortment is
dependent on how the homologs line up during metaphase I. This assortment of
homologs explains how genes located on different pairs of chromosomes will
separate independently of one another during anaphase I. Anaphase II results in the
separation of sister chromatids and subsequent production of gametes carrying single
alleles for each gene locus as predicted by Mendel’s principle of segregation.
Mendel’s principles of independent assortment and segregation do not apply to
mitosis, which produces cells genetically identical to each other and to the parent cell.
Section 3.4
11. How is the chi-square goodness-of-fit test used to analyze genetic crosses? What does
the probability associated with a chi-square value indicate about the results of a
cross?
Solution:
The goodness-of-fit chi-square test is a statistical method used to evaluate the role of
chance in causing deviations between the observed and the expected numbers of
offspring produced in a genetic cross as predicted from the null hypothesis. The
probability value obtained from the chi-square table refers to the probability that
random chance produced the deviations of the observed numbers from the expected
numbers.
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