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Chapter 12
Chromosomes & Human
Inheritance
OCC BIO-114
In this chapter…
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you will learn how biologists use their
knowledge of DNA & chromosome
behavior to study how traits are inherited &
expressed.
Sex-determining Chromosomes and
Linkage
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sex chromosomes
autosomes
Sex Chromosomes
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It’s all about the “X”
and the “Y”
Sex determination and Inheritance
of Sex-linked genes:
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Mammals/ many insects= male/female same number of
chromosomes
-one pair= sex chromosomes
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different appearance and genetic composition
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XX= female
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XY= male
Humans:
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Autosomes = # 1-22
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Sex chromosome = #23
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sex chromosome carried by sperm determines sex of offspring ( X or Y
sperm)
Sex-Linkage
In 1910, Thomas Hunt Morgan - Studied
inheritance patterns of fruit fly, Drosophila
melanogaster, discovered presence of a
white eye in certain individuals.
 Since this was a distinctive feature,
Morgan decided to study the inheritance
pattern for this recessive eye color.
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Sex-Linkage
Morgan made several crosses using a
white-eyed male, expecting the standard
Mendelian results. He did not get them.
 While the ratio of 3:1 was obtained, all of
the white-eyed second generation
offspring were male flies.
 All females had red eyes (and 25% of the
males also had red eyes).
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Sex-Linkage
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Morgan did a series of reciprocal crosses of white-eye
males with red-eye females and red-eye males with
white-eye females.
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He concluded that the gene for eye color was located on the X
chromosome.
Males passed the trait their daughters (on their solitary
X chromosome) & mothers passed the trait sons.
White eyed females could also pass the white eye allele
to their daughters, but if the father fly had red eyes, the
eye color of the daughters would be red, while the eye
color of the sons of white-eyed females would always be
white.
Sex-Linkage
Morgan concluded that eye color was
related to sex, & that the sex-determining
chromosomes also had genes that were
unrelated to gender determination.
 Prior to Morgan's discovery, no one knew
that genes unrelated to gender were also
located on these chromosomes.
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Sex-Linkage
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These other traits are said to be sex-linked
because they are inherited along with the sex of
the individual.
Because the X and Y chromosome are not
exactly matching, the X chromosome can have
genes that are not located on the Y
chromosome, and vice-versa.
Some of these genes are unrelated to the sexual
characteristics, but are inherited with the sexdetermination. This is referred to as sexlinkage.
sex-linked (X-linked)
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genes on one sex chromosome; not on other
Y= carries relatively few genes
X= many genes; some not specifically related to female traits
Ex: genes for:
color vision;
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blood clotting; structural proteins
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female XX= homozygous or hybrid (dominant/recessive)
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male XY= fully expresses all alleles on single X (whether dominant
or recessive)
Ex: color blindness; hemophilia; muscular dystrophy
Some human sex-linked traits are
Hemophilia (X)
 Hairy ear rims (Y)
 Red-green color blindness (X) (fig.12.9)
 Duchene muscular dystrophy (X)
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X-Linked
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Only found on X chromosome
Colorblindness – red/green, most common
in males (8%)
 Hemophilia – blood clotting, affects males
 Duchene Muscular Dystrophy – weakens &
destroys muscle tissue
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*Not all X-Linked traits are diseases = Only a
few of hundereds of genes on X chrom.
Others code for normal functioning proteins
The Effect of Recombination on Gene Linkage and
Inheritance
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Meiosis results in exchange of bits & pieces of
DNA between homologous pairs of
chromosomes at the chiasmata during
prophase I of meiosis.
This process of recombination results in
gametes (or meiotic products) that are not
identical; some of the linkage groups have been
changed by the crossing-over.
As a result of recombination, new allele
combinations are formed, and we have more
genetic variation.
crossing over
segments of homologous chromosomes
are exchanged w/ each other at site called
chiasmata during meiosis I
 forms new gene combinations on both
homologous chromosomes
 gene combinations for daughter cell
different from parent cell
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Crossing Over and Recombination
Cell Mutations
Germ-cell mutation – gametes, passed
on to offspring
 Somatic mutations – body cells, affect
organism. Not passed on to offspring. i.e.
leukemia
 Lethal mutations – cause death. Are they
beneficial???
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Chromosome mutations(12.5)
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Deletion – piece of chrom breaks off & is lost
(end); due to virus, radiation, chemicals, or envir
factors. Most are lethal
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Inversion – breaks off & reattaches in reverse
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Ex:ABCDEFG becomes ABCFG, Cri-du-chat
syndrome (p.192)
Ex: ABCDEFG becomes ABGFEDC
Translocation – breaks off & reattaches to
different (non-homologous chrom)
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Ex: ABCDEFG becomes ABCDLMNOP
Changes in Chrom #
Nondisjunction – chrom fail to separate
during Anaphase (gamete formation)
(p.192)
 Aneuploidy – 1 extra or less chrom
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Monosomic-1 less, Turner syndrome (XO)
 Trisomic-1 more
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Polyploidy – 3 or more of each type of
chrom; lethal for humans
Inheritance of Recessive Alleles
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Any alteration of a gene, called a mutation, has the
potential to inhibit the formation of a needed enzyme.
With diploid organisms, however, a mutation most likely
affects just one of the homologues, and the second can
still code for the appropriate enzyme with little or no
phenotypic effect on the individual.
Gene alterations that affect health are called genetic
disorders (Table 12.1, p.196).
Those that are just "abnormal" but do not affect
physiological health, are called genetic abnormalities
i.e. 6 toes
When the genetic alteration causes a host of symptoms,
it may be called a syndrome.
Disease –illness caused by infections, not by inheritance
Single Allele Traits
Controlled by a single allele of a gene.
 >200 human traits governed by single
dominant allele.
 Ex: Huntington’s Disease (HD) caused by
dominant allele located on an autosome =
autosomal dominant pattern of inheritance
 Read description of HD on p.196, Table
12.1
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HD continued
Geneticists discovered a genetic marker for
HD allele.
 Genetic marker is a short section of DNA
known to have a close association w/ a
particular gene located nearby. Easy to I.D.
the HD allele.
 If marker is present = 96% chance of dev HD
 Parents can be tested for marker before
conceiving a child.
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Other Single-Allele Traits
(Table 12.1)
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Homozygous recessive (must have 2
copies of recessive allele):
Cystic Fibrosis
 Sickle Cell Anemia
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*these are recessive alleles located on
autosomes = Autosomal Recessive
pattern of inhertiance
polygenic inheritance
 Trait
that is controlled by 2 or more
genes = many different variations
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Ex: Skin Color-influenced by additive effects of 3 to
6 genes. Each gene results in certain amount of
melanin (brownish-black pigment)
More melanin=darker
Ex: eye color. Light blue eyes=very little melanin
Human height – polygenic, but influenced by
environmental factors, such as disease & nutrition
Disorders due to Nondisjunction
Occurs during meiosis causes gametes to
lack a chromosome or have an extra one
(fig 12-10, p. 231)
 A zygote w/ 45 chrom. = monosomy
 47 chrom. = trisomy
 Often lethal
 Trisomy 21 = Down Syndrome – mild to
severe mental retardation…
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Nondisjunction cont.
Males w/ 1 extra X chrom. = Klinefelter’s
syndrome (XXY). Some feminine
characteristics, mentally retarded & infertile
 Turner’s Syndrome – Have a single x
chrom. (XO) = female appearance, but do
not mature sexually & remain infertile. What
happens if zygote only receives a Y chrom.?
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Detection (fig.12.21, 12.22)
Genetic Screening = Karyotype (picture of
person’s chrom.
 Amniocentesis – procedure removes
amniotic fluid from fetus & tested (14th-16th
week of pregnancy)
 Chorionic Villi Sampling – Removes fetal
cells from chorion fluid (between mothers
uterus & fetus)
 Ultrasound – Sound waves to observe
fetus
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Detection Cont.
In U.S. 1 out of 10,000 babies is afflicted
w/ phenylketonuria (PKU) – body cannot
metabolize amino acid phenylalanine =
brain damage.
 Genetic Counseling – medical guidance
for couples at risk
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12-10 Human Genetics
Humans have up to 20 times as many
genes as Drosophila, & our 23 pairs of
chromosomes & are made up of about
100,000 genes.
 Geneticists focus on disease-causing
genes b/c of concern for human pop.
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Pedigree Analysis
Pedigree – a family record that shows
how a trait is inherited over several
generations (fig 12-19).
 Certain phenotypes are usually repeated
in predictable patterns from 1 generation
to the next = patterns of inheritance.
 Carriers – individuals who have 1 copy of
a recessive allele, but can pass it along to
their offspring.
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How to read a Pedigree
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Human pedigrees
Before we consider human Mendelian
inheritance it is convenient to consider the
symbols used to draw pedigrees.
Generations are numberered from the top of the
pedigree in uppercase Roman numerals, I, II, III
etc. Individuals in each generation are
numbered from the left in arab numberals as
subscripts, III1 , III2, III3 etc.
Hemophilia pedigree of the
European Royal Families
Practice Problems
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Glencoe
References
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http://www.scidiv.bcc.ctc.edu/rkr/Biology10
1/lectures/pdfs/HumanInheritance101.pdf