Chapter 15 Study Guide
Vocabulary:
Word Roots:
aneu- = without
cyto- = cell
hemo- = blood
mono- = one
non- = not
dis- = separate
poly- = many
re- = again
-com- = together
bin- = two at a time
trans- = across
tri- = three
soma- = body
Key Terms:
Aneuploidy: A chromosomal aberration in which one or more chromosomes are present in extra
copies or are deficient in number.
Barr body: A dense object lying along the inside of the nuclear envelope in female mammalian
somatic cells, representing an inactivated X chromosome.
Crossing over: The reciprocal exchange of genetic material between nonsister chromatids during
prophase I of meiosis.
Deletion: A deficiency in a chromosome resulting from the loss of a fragment through breakage.
Duplication: An aberration in chromosome structure due to fusion with a fragment from a
homologous chromosome, such that a portion of a chromosome is duplicated.
Genetic recombination: General term for the production of offspring that combine traits of the two
parents.
Hemizygous: Having only a single copy of a gene instead of the customary two copies.
Inversion: An aberration in chromosome structure resulting from reattachment in a reverse
orientation of a chromosomal fragment to the chromosome from which the fragment originated
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Linked genes: Genes located close enough together on a chromosome to be usually inherited
together
Monosomic: Referring to a cell that has only one copy of a particular chromosome, instead of the
normal two.
Mosaic: A person or a tissue that contains two or more types of genetically different cells.
Nondisjunction: An error in meiosis or mitosis, in which both members of a pair of homologous
chromosomes or both sister chromatids fail to move apart properly.
Parental type: An offspring with a phenotype that matches one of the parental phenotypes.
Polyploidy: A chromosomal alteration in which the organism possesses more than two complete
chromosome sets.
Recombinant: An offspring whose phenotype differs from that of the parents; also called
recombinant type.
Sex-linked gene: A gene located on a sex chromosome.
Translocation: An aberration in chromosome structure resulting from attachment of a chromosomal
fragment to a nonhomologous chromosome.
Trisomic: Referring to a cell that has three copies of a particular chromosome, instead of the normal
two.
Wild type: An individual with the normal (most common) phenotype.
X-linked: A gene located on the X chromosome.
Chapter 15 Important Points:
 The Chromosomal Theory of Inheritance is important because it addresses the fact that
genes have specific, predicted locations (loci) on chromosomes and that it is the whole
chromosome that is segregated during meiosis, not individual genes
o This allows for some genes to be linked together in their inheritance patterns
 Wild type refers to the phenotype typically seen by most individuals for a particular
character
Sex Chromosomes:
 Sex in mammals in determined by which combination of sex chromosomes an individual
possesses
o Human females have 2 X chromosomes (and therefore have 23 pairs of homologous
chromosomes)
o Human males have 1 X and 1 Y chromosome and therefore have 22 pairs of
autosomal homologous chromosomes but their sex chromosomes are not
homologous (meaning the X and Y gene do not carry the same gene loci)
 The SRY gene on the Y chromosome (which contains far fewer genes overall
compared to the X) codes for proteins that determine the male
characteristics
 Any individual who has a Y chromosome is therefore
chromosomally “male”
 X linked genes are only found on the X chromosome and females will have 2
copies
 Females can be homozygous or heterozygous for X linked genes
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 Females give their X linked genes to both daughters and sons
Males will have 1 copy of X-linked genes
 Males can not be homozygous or heterozygous (therefore can not
be carriers) for X linked genes but can be hemizygous
 Males only give their X linked genes to their daughters
 X-linked recessive diseases are more common in males than females
During development in female mammalian somatic cells, one of the 2 X
chromosomes becomes a Barr body and is deactivated
 Genes on the Barr body are not expressed
 Deactivation is random for each cell so adult somatic cells could be a mosaic
if a female was a heterozygote for an X-linked gene
 One functional X chromosome is required so normal human males will not
have any Barr bodies

o
Linked Genes:
 When genes are not linked (therefore on different pairs of chromosomes), they undergo
independent assortment and we expect equal probabilities of gametes that will create
parental and recombinant phenotypes
 If genes are linked, we would expect 100% parental phenotypes but that is not always the
case
o Crossing over between non-sister chromatids during Prophase I of Meiosis I can
create recombinant chromosomes
 A recombination frequency is the percentage of recombinant offspring out of all offspring
o Recombination frequency of 50% tells us 2 genes are either on completely different
chromosomes (and thus independent assortment applies) OR so far apart on the
same chromosome so that crossing over occurs 100% of the time between the 2
genes
o Any recombination frequency less than 50% implies 2 genes are linked
Chromosomal Errors:
 Non-disjunction occurs when homologous chromosomes in anaphase I meiosis I or sister
chromatids in mitosis or anaphase II of meiosis II fail to properly separate
o This causes incorrect chromosome numbers in the resulting daughter cells
(aneuploidy)
o For meiosis, if aneuploid cells are used for fertilization, it will create zygotes that are
potentially monsomic, 2n-1, (have one fewer chromosome than normal with one
pair of homologous chromosomes lacking a partner) or trisomic, 2n+1, (having 3
chromosomes for a given pair)
o Polyploidy refers to having extra entire sets of chromosomes
o Aneuploidy in humans often results in miscarriage or severe defects often causing
early death
o Monosomy in humans is lethal except for the X chromosome (see below)
 A common human condition caused by non-disjunction is Down Syndrome in which an
individual has 3 copies (trisomy, 2n+1) for chromosome #21
o Most frequently caused by older maternal age but all ages of mothers can have a
Down baby
 Non-disjunction of the sex chromosomes results in a viable offspring but with various
phenotypic effects (except for males missing the X—OY which is nonviable)
o Monosomy for the X chromosome in human females causes Turner’s Syndrome (XO)
which causes sterility and other issues
o An extra X in human males is XXY and causes Kleinfelter Syndrome; individuals are
male but sterile and have feminized characteristics
 Chromosome structure can also be altered by mistakes during crossing over
o Many genes are typically involved in these types of errors often leading to
significant phenotypic effects
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Deletion: Missing chromosomal segments
Duplication: Segments of chromosomes are improperly duplicated
Inversion: Segments are present but in reverse order
Translocation: Segments of chromosomes are not found on the correct
chromosome but attached to other pairs of homologous chromosomes
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