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Chromosomes and
Human Genetics
Chapter 12
Chromosome Theory of
Inheritance
• Genes have specific locations (loci) on
chromosomes.
• It is the chromosomes that separate and
assort independently during meiosis.
• This explains Mendel’s laws of
inheritance.
Karyotype
• A “picture” of chromosomes arrested in metaphase and sorted
by length, centromere location or other defining features
• Cultured cells are arrested at metaphase by adding colchicine
• This is when chromosomes are most condensed and easiest to
identify
• Used to help answer questions about an individual’s
chromosomes
– Lets us see sex chromosomes and look for abnormalities in
sex chromosomes or autosomes
1
2
3
4
13
14
15
16
5
17
6
7
8
9
18
19
20
21
10
22
11
12
XX (or XY)
• Linkage is the tendency of genes located on the same
chromosome to be transmitted together in inheritance.
• Linkage can be disrupted by crossing over—the
exchange of parts of homologous chromosomes.
• Certain alleles that are linked on the same
chromosome tend to remain together during meiosis
because they are positioned closer together on the
chromosome.
• This eventually led to the generalization that the
probability that a cross over will disrupt the linkage of
two genes is proportional to the distance that separates
them.
• The careful analysis of recombination patterns in
experimental crosses has resulted in linkage mapping
of gene locations.
Crossover Frequency
• Proportional to the distance that separates
genes
• The closer the genes are to each other, the
less likely crossing over will occur
A
B
C
D
• Crossing over will disrupt linkage between A
and B more often than C and D
In-text figure
Page 201
Sex Chromosomes
• Discovered in late 1800s
• Mammals, fruit flies
– XX is female, XY is male
• Butterflies, moths, some birds and some fish
– XX is male, XY female
• Human X and Y chromosomes function as
homologues during meiosis
– Differ physically, one is shorter (Y)
– Differ in the genes they carry
Sex
Determination
•Male gamete determine
sex of offspring in
mammals and fruit flies
Figure 12.5
Page 198
female
(XX)
male
(XY)
eggs
sperm
X
x
Y
X
x
X
X
X
X
XX
XX
Y
XY
XY
The Y Chromosome
• Fewer than two dozen genes identified
• One is the master gene for male sex determination
– SRY gene (sex-determining region of Y)
• SRY present, testes form
• SRY absent, ovaries form
The X Chromosome
• Carries more than 2,300 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed in both
males and females
SexLinkage
homozygous dominant
female
recessive male
x
Gametes:
X
X
X
Y
All F1
have red eyes
Gametes:
x
X
X
1/2
1/2
1/4
1/2
F2
generation:
X
1/2
1/4
1/4
1/4
Figure 12.7
Page 200
Y
X-Linked Recessive
Inheritance
• The characteristics of this condition are:
– The mutated gene occurs only on the X chromosome.
– Heterozygous females are phenotypically normal; males
are more often affected because the single recessive
allele (on the X chromosome) is not masked by a
dominant gene.
– A normal male mated with a female heterozygote have a
50 percent chance of producing carrier daughters and a
50 percent chance of producing affected sons. In the case
of a homozygous recessive female and a normal male, all
daughters will be carriers and all sons affected.
X-Linked Recessive
Inheritance
• Males show
disorder more
than females
• Son cannot inherit
disorder from his
father
Figure 12.12a
Page 205
Examples of X-Linked Traits
• Color blindness
– Inability to distinguish among some of all
colors
• Hemophilia
– Blood-clotting disorder
– 1/7,000 males has allele for hemophilia A
– Was common in European royal families
Fragile X Syndrome
• An X-linked recessive disorder
• Causes mental retardation
• Mutant allele for gene that specifies a
protein required for brain development
• Allele has repeated segments of DNA
Autosomal Recessive Inheritance
1. The characteristics of this condition are:
a. Either parent can carry the recessive allele
on an autosome.
b. Heterozygotes are symptom-free;
homozygotes are affected.
c. Two heterozygous parents have a 50
percent chance of producing heterozygous
children and a 25 percent chance of producing
a homozygous recessive child. When both
parents are homozygous, all children can be
affected.
2. Human examples: CF, PKU, Tay Sachs
Autosomal Recessive
Inheritance Patterns
• If parents are
both
heterozygous,
child will have a
25% chance of
being affected
Figure 12.10a
Page 204
Autosomal Dominant Inheritance
• The dominant allele is nearly always expressed and
if it reduces the chance of surviving or reproducing,
its frequency should decrease; mutations,
nonreproductive effects, and postreproductive onset
work against this hypothesis.
• If one parent is heterozygous and other
homozygous recessive, there is a 50 percent
chance that any child will be heterozygous.
• Human examples: Huntingddon’s Disease (nervous
system deterioration, death, Symptoms don’t usually
show up until person is past age 30, People often
pass allele on before they know they have it) and
Achondroplasia (a form of dwarfism)
Autosomal
Dominant Inheritance
Trait typically
appears in
every
generation
Figure 12.10b
Page 204
Pedigrees
• A chart of genetic connections among
individuals
• A graphic representation of a families traits
• Provides data on inheritance patterns
through several generations.
• Knowledge of probability and Mendelian
inheritance patterns is used in analysis of
pedigrees to yield clues to a trait's genetic
basis.
Pedigree
Symbols
male
female
marriage/mating
offspring in order of birth,
from left to right
Individual showing trait
being studied
sex not specified
I, II, III, IV...
Figure 12.9a
Page 202
generation
Pedigree for Polydactyly
female
I
male
II
5,5
6,6
*
III
IV
5,5
6,6
6
6,6
5,5
7
5,5
6,6
5,5
6,6
5,5
6,6
5,5
6,6
5,6
6,7
12
V
*Gene not expressed in this carrier.
Figure 12.9b
Page 202
6,6
5,5
6,6
6,6
Changes in Chromosome Structure
• Portions of chromosomes may be lost or
rearranged.
• Results in chromosomal mutations.
• 4 types:
– Duplications, deletions, inversions or
translocations
Duplication
Gene sequence that is repeated several to hundreds
of times
normal chromosome
one segment
repeated
three repeats
Inversion
• A linear stretch of DNA is reversed
within the chromosome
• alters the position and sequence of the genes
so that gene order is reversed.
segments
G, H, I
become
inverted
In-text figure
Page 206
Translocation
•A piece of one chromosome becomes attached to
another nonhomologous chromosome a part of one
chromosome is transferred to a nonhomologous
chromosome
•Most are reciprocal
one chromosome
a nonhomologous
chromosome
In-text figure
nonreciprocal translocation
Page 206
In-text
figure
Page 206
Deletion
• Loss of some segment of a chromosome
• Most are lethal or cause serious disorder
Changes in Chromosome Number
Nondisjunction
• Failure of chromosomes to separate properly
• Results in gametes or daughter cells with
abnormal number of chromosomes.
• If a gamete with an extra chromosome (n + 1)
joins a normal gamete at fertilization, the diploid
cell (zygote )will be 2n + 1; this condition is called
trisomy.
• If an abnormal gamete is missing a (n - 1) joins a
normal gamete at fertilization, the diploid cell
(zygote )will be 2n – 1; this condition is called
monosomy.
Nondisjunction
n+1
n+1
n-1
chromosome
alignments at
metaphase I
n-1
nondisjunction alignments at
at anaphase I metaphase II
anaphase II
Page 208
Aneuploidy
• Individuals have one extra or less chromosome
• (2n + 1 or 2n - 1)
• Major cause of human reproductive failure
• Most human miscarriages are aneuploids
Polyploidy
• Individuals have three or more of each type of
chromosome (3n, 4n)
• Common in flowering plants
• Lethal for humans
– 99% die before birth
– Newborns die soon after birth
Changes in the Number of Autosomes
• Down Syndrome - Trisomy of chromosome 21
Changes in the Number of Sex
Chromosomes
• Turner Syndrome - females whose cells have only one
X chromosome (designated XO). (monosomy)
• XXX Syndrome
• Klinefelter Syndrome - Nondisjunction results in an extra X
chromosome in the cells (XXY)
• XYY Condition -Extra Y chromosome in these males (does
not affect fertility)
Chi-Square statistical analysis
of genetics data
•X2 = ∑ (o-e)2
e
•o = observed numbers
•e = expected numbers
•Solve the equation
•Find the degrees of freedom (one less than the total
number of choices – types of offspring phenotypes)
•Look at the critical value table
• if your answer is less than .05 value, then your results
deviate from the expected only from chance
•If your answer is greater than the .05 value, then your results
deviated from the expected for some other reason and you
have to figure out why