KEY- Chapter 15 Study Questions
1) What is “the chromosome theory of inheritance?”
Mendelian genes have specific loci on chromosomes, and it is the chromos that undergo segregation
and independent assortment
2) What are two advantages of using the fruit fly, Drosophila melanogaster, as an organism for genetics
studies?
-Single mating produces 100’s of offspring
- New generation every 2 weeks
- Only 4 pairs of chromos
3) Distinguish between wild type and mutant phenotypes. Give an example in fruit flies.
Wild Type = “normal” phenotype (red eyes)
Mutant Type = unusual or alternative phenotype (white eyes)
4) What is meant by “sex-linked genes?” Describe Morgan’s experiment which first proved that some
genes are X-linked.
-Genes located on sex chromos
-Self-crossed F1 and got F2 3:1 (red:white) ratio, but white eyes only found in males
5) What are linked genes? Give an example of two traits in fruit flies that are linked to one another.
-Genes located on the same chromo are linked (passed along as a unit)
Ex. Body color and wing size
6) What is meant by recombination? What is the physical basis of recombination between unlinked
genes? What is the physical basis (cause) of recombination between linked genes?
-Production of offspring w/new combos of traits inherited from parents
-Unlinked genes are physically located on different chromos and assort independently
-Recomb. Between linked-genes occurs due to crossing over
7) What is a “genetic map?” How did Sturtevant devise his method for creating a genetic map?
-Genetic map = an ordered list of genetic loci along a chromo
- Sturtevant related recombination frequency from breeding experiments to distances between
genes on a chromo. (farther apart 2 genes are, the higher the probability of crossing over
between them, leading to a higher recombination freq.)
8) Distinguish between a “linkage map” and a “cytogenetic map.”
- Linkage map – based on recomb. freq. and portrays gene sequence order (not precise location)
-Cytological map – locate genes w/respect to chromosomal features (stained bands; tip;
centromere) uses landmarks to get close to actual location
9) Explain nondisjunction and the 2 times that it can occur.
-When homolog. pairs fail to separate or sister chromatids fail to separate (anaphase I or II)
10) What is the difference between autosomes and sex chromosomes?
Autosomes: Pairs 1-22; need 2 copies of recessive for male and female
Sex Chromos: Pair 23; males only need one copy of recessive
11) In humans, when do the anatomical signs of sex begin to emerge? Describe the role of the SRY gene in sex determination.
*When the embryo is about 2 months old
*in the absence of SRY, the gonads develop into ovaries (presence or absence is the trigger) 
regulates many other genes
12) Why do more males have sex-linked recessive disorders than females? Draw a Punnett square showing the cross between a colorblind male
and a female with normal vision, but whose father was colorblind. What are the genotypes and phenotypes of all potential offspring of this
couple?
*males only need 1 copy because they only have 1 “X” sex chromosome
*Genotypes: XCXc : XcXc : XCY : XcY
*Phenotypes: normal female : color blind female : normal male : colorblind male
13) What is a “Barr body?” How does X-inactivation lead to traits such as the calico coat pattern in cats?
*condensed, inactive “X” (sex) chromosome (most genes are not expressed); condenses during
embryonic development
How many Barr bodies are there in each female somatic cell?
1
In each male somatic cell?
If chromosome pair #23 was XXX, how many Barr bodies would this female have in each somatic cell?
zero
2
How does X-inactivation lead to traits such as the calico coat pattern in cats?
*which “X” condenses is random (approx. ½ may express one allele and the other ½ will express
the other allele
14) Complete the table below for genetic disorders that are inherited as a result of sex-linkage or errors in meiosis.
Disorder
Cause
Symptoms
Duchenne muscular
dystrophy
X-linked recessive
Hemophilia
X-linked recessive
Rarely live past early 20’s;
progressive weakening of the
muscles & loss of coordination
Bleed excessively when injured;
blood won’t clot
Down syndrome
Nondisjunction
(aneuploidy)
Klinefelter syndrome
(XXY)
Nondisjunction of sex
chromosomes
Characteristic facial features;
short stature; heart defects;
mental retardation; increased #
of infections
Male sex organs but testes are
abnormally small & sterile
Other info. (% of
population, cure, etc.)
1/3500 males born in US
Hebrews (ancient) 
female family history, sons
not circumcised; common in
royals of Europe
1/700 in US
Extra #21 chromosome
1/2000; normal
intelligence; some female
characteristics
Turner syndrome
(XO)
Nondisjunction of sex
chromosomes
Female; some female sex organs
don’t develop; sterile; short
cri du chat
Specific deletion in
chromosome #5
Chronic myelogenous
leukemia (CML)
Chromosome
translocation of # 22
and #9
Genomic imprinting
Mentally retarded; unusual facial
features; cry sounds like a
mewing cat
Increased # of white blood cells
Mitochondrial
myopathy
1/5000; only known viable
monosomy in humans; most
have normal intelligence
Die in infancy or early
childhood
Mentally retarded
1/1500 males; 1/2500
females; more commonly
inherited from mom; tip of
“X” hangs by a thread;
maternal imprinting of the
abnormal allele causes
syndrome
15) Define the following terms describing errors and exceptions in chromosomal inheritance:
nondisjunction: members of a pair of homologous chromosomes do not move apart properly during Meiosis I
(Anaphase I) or sister chromatids fail to separate during Meiosis II (Anaphase II)
polyploidy: when an organism has more than 2 complete chromosome sets
translocation: a deleted chromosome fragment joins a nonhomologous chromosome
aneuploidy: abnormal # of chromosomes in offspring
deletion: chromosomal fragment lacking a centromere is lost during cell division
trisomic: when a chromosome is present in triplicate
duplication: a deleted fragment from 1 chromosome may join to the homologous chromosome
monosomic: organism is missing a chromosome of a homologous pair
inversion: if the deleted chromosome fragment rejoins to the original chromosome but in the reverse
orientation