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SOLUTION MANUAL
FOR GENETICS
FROM GENES TO
GENOMES, 7TH
EDITION, MICHAEL
GOLDBERG, JANICE
FISCHER, LEROY
HOOD, LELAND
HARTWELL
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chapter
1
Mendel’s Principles of Heredity
Synopsis
Chapter 1 covers the basic principles of inheritance that can be summarized as Mendel’s Laws of
Segregation (for one gene) and Independent Assortment (for more than one gene).
Key terms
genes and alleles of genes – A gene determines a trait, and different alleles or forms of a
gene exist. The color gene in peas has two alleles: yellow and green.
genotype and phenotype – Genotype is the genetic makeup of an organism (written as
alleles of specific genes), while phenotype is how the organism looks.
homozygous and heterozygous – When both alleles of a gene are the same, the
individual is homozygous for that gene (or pure breeding). If the two alleles are
different, the organism is heterozygous (also called a hybrid).
dominant and recessive – The dominant allele is the one that controls phenotype in the
heterozygous genotype; the recessive allele controls phenotype only in a homozygote.
monohybrid or dihybrid cross – a cross between individuals who are both heterozygotes
for one gene (monohybrid) or for two genes (dihybrid).
testcross – performed to determine if an individual with the dominant characteristic is
homozygous or heterozygous: An individual with the dominant phenotype but
unknown genotype is crossed with an individual with the recessive phenotype.
Key ratios
3:1 – Ratio of progeny phenotypes in a cross between monohybrids
[Aa × Aa → 3 A – (dominant phenotype) : 1 aa (recessive phenotype)]
1:2:1 – Ratio of progeny genotypes in a cross between monohybrids
(Aa × Aa → 1 AA : 2 Aa : 1aa )
1:1 – Ratio of progeny genotypes in a cross between a heterozygote and a recessive homozygote
(Aa × aa → 1 Aa : 1aa )
1:0 – All progeny are the same phenotype. Can result from either of two cases:
[AA × – – → A – (all dominant phenotype)]
[aa × aa → aa (all recessive phenotype)]
9:3:3:1 – Ratio of progeny phenotypes in a dihybrid cross
(Aa Bb × Aa Bb → 9 A – B – : 3 A – bb : 3 aa B – : 1 aa bb )
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chapter 1
Problem Solving
The essential component of solving most genetics problems is to DIAGRAM THE CROSS in a
consistent manner. Usually you will be given information about phenotypes, so the diagram would
be:
Phenotype of one parent × phenotype of the other parent → phenotype(s) of progeny
The goal is to assign genotypes to the parents and then use these predicted genotypes to generate
the genotypes, phenotypes, and ratios of progeny. If the predicted progeny match the observed
data you were provided, then your genetic explanation is plausible.
The points listed below will be particularly helpful in guiding your problem solving:
• Remember that two alleles of each gene exist when describing the genotypes of
individuals. But if you are describing gametes, remember that only one allele of each
gene is in a gamete.
• You will need to determine whether a character is dominant or recessive. Two main
clues will help you answer this question.
o First, if the parents of a cross are true breeding for the alternative characters of the
trait, look at the phenotype of the F1 progeny. Their genotype must be
heterozygous, and their phenotype is thus determined by the dominant allele of
the gene.
o Second, look at the F2 progeny (that is, the progeny of the F1 hybrids). The 3/4
portion of the 3:1 phenotypic ratio indicates the dominant character.
• You should recognize the need to set up a testcross (to establish the genotype of an
individual showing the dominant character by crossing this individual to a homozygote
for the recessive allele).
• You must keep in mind the basic rules of probability:
o Product rule: If two outcomes must occur together as the result of independent
events, the probability of one outcome AND the other outcome is the product of
the two individual probabilities.
o Sum rule: If there is more than one way in which an outcome can be produced,
the probability of one OR the other occurring is the sum of the two mutually
exclusive individual probabilities.
• Be aware that sometimes you need to use conditional probability, meaning that an
event’s probability is influenced by its relationship to another event that has already
occurred. You were introduced to conditional probability in Solved Problem III in this
chapter, and several of the problems in Section 1.3 require this kind of thinking. For
example, suppose you are given a pedigree diagram for a disease caused by a recessive
allele. You are asked to determine the chance that an unaffected individual is a carrier
(Dd ), when both parents are carriers. As the cross that produced the unaffected
individual is Dd × Dd, you would expect the chance of a Dd child to be 1/2. This is true,
but it was not the question you were asked! You know something about the individual in
question—which is that they are unaffected—they cannot be dd. This means that in this
case, the 1 DD : 2 Dd : 1 dd ratio changes to 1 DD : 2 Dd , and the chance is 2/3 that the
unaffected individual is a carrier. When solving probability problems in pedigrees,
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