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Ch. 15- The Chromosomal Basis of Inheritance- Guided Notes
What you must know:
How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to
Mendel’s laws of inheritance.
The unique pattern of inheritance in sex-linked genes.
How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can
cause genetic disorders.
How genetic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian
inheritance.
1. Chromosomal Theory of Inheritance
 Genes have specific locations (loci) on
chromosomes
 Chromosomes segregate and assort
independently
o Law of segregation- The two
alleles for each gene separate
during gamete formation
o Law of Independent Assortmentalleles of genes on
nonhomologous chromosomes
assort independently during
gamete formation
2. Thomas Hunt Morgan
 Drosophila melanogaster – fruit fly
o Fast breeding, 4 prs.
chromosomes (XX/XY)
 Sex-linked gene: located on X or Y chromosome
o Red-eyes = wild-type; white-eyes = mutant
 Specific gene carried on specific chromosome
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In one experiment, Morgan mated male flies with white eyes
(mutant) with female flies with red eyes (wild type)
-The F1 generation all had red eyes
-The F2 generation showed the 3:1 red: white eye ratio, but only
males had white eyes
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Are these results what you expect?
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Morgan determined that the white-eyed
mutant allele must be located on the X
chromosome
3.
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Sex Linked Gene
Sex-linked gene on X or Y
Females (XX), male (XY)
Eggs = X, sperm = X or Y
Fathers pass X-linked genes to
daughters, but not sons
Only mothers can pass X-linked to their
sons
X-linked recessive disorders are much
more common in males than in females
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For a recessive X-linked trait to be expressed
o A female needs two copies of the allele (homozygous)
o A male needs only one copy of the allele (hemizygous)
 Females can be affected or carrier
4. Transmission of Sex-Linked Recessive Traits
5. Sex Linked Disorders
a. Colorblindness
b. Duchenne muscular dystrophy
c. Hemophilia
6. X-Linked Genes Warm-Up Questions
 A white-eyed female fruit-fly is mated with a red-eyed male. What genotypes and phenotypes do you
predict for the offspring?
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Neither Tim nor Rhoda has Duchenne muscular dystrophy (X-linked recessive disorder), but their firstborn son
has it. What is the probability their 2nd child will have it?
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Colorblindness is a sex-linked recessive trait. A colorblind male and a female with normal vision have a son
who is colorblind. What are the parents’ genotypes?
7. Linked Genes
 If results do not follow Mendel’s Law of Independent Assortment, then the genes are probably linked
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Linked
genes: located on
same chromosome
and tend to be
inherited together
during cell division
8. Genetic Recombination: production
of offspring with new combo of
genes from parents
(when does this occur during
meiosis?)
 If offspring look like parents 
parental types
 If different from parents 
recombinants
9.
Crossing over: explains why some linked
genes get separated during meiosis the further
apart 2 genes on same chromosome, the higher
the probability of crossing over and the higher
the recombination frequency
10. Calculating recombination frequency
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Linkage Map: genetic map that is based on % of cross-over events
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Map unit: distance
between genes
o
1 map unit = 1%
recombination frequency
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Express relative distances
and order along chromosome
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50% recombination = far
apart on same chromosome or on
2 different chromosomes
11. Linked Genes Warm Up Questions
 How are linkage maps constructed? (See. Fig. 15.11 in Campbell 9th ed.)
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Determine the sequence of genes along a chromosome based on the following recombination frequencies: AB, 8 map units; A-C, 19 map units; A-D, 20 map units; B-C, 11 map units; B-D, 28 map units.
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What does a frequency of recombination of 50% indicate?
12. The Chromosomal Basis of Sex
 In humans and other mammals,
there are two varieties of sex
chromosomes: a larger X
chromosome and a smaller Y
chromosome
 Only the ends of the Y chromosome
have regions that are homologous
with corresponding regions of the X
chromosome
 The SRY gene on the Y chromosome
codes for a protein that directs the
development of male anatomical
features
13. Human development
 Y chromosome required for development of
testes
 Embryo gonads indifferent at 2 months
 SRY gene: sex-determining region of Y
 Codes for protein that regulates other genes
14. Sex determination varies between animals
15. X-Inactivation
 Barr body = inactive X chromosome;
regulate gene dosage in females during
embryonic development
 Cats: allele for fur color is on X
 Only female cats can be tortoiseshell or
calico.
16. Nondisjunction: chromosomes fail to separate
properly in Meiosis I or Meiosis II
 A monosomic zygote has only one copy of a
particular chromosome
 A trisomic zygote has three copies of a
particular chromosome
 Aneuploidy: incorrect # chromosomes
 Monosomy (1 copy) or Trisomy (3 copies)
 Polyploidy: 2+ complete sets of chromosomes;
3n or 4n
o Rare in animals, frequent in plants
 Karyotyping can detect nondisjunctions.
17. Aneuploidy of Sex Chromosomes
 Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
o Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals
o Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known
viable monosomy in humans
18. Alterations of Chromosome Structure
 Breakage of a chromosome can lead to four types of changes in chromosome structure
o Deletion removes a chromosomal segment
o Duplication repeats a segment
o Inversion reverses orientation of a segment within a chromosome
o Translocation moves a segment from one chromosome to another
 Chromosomal Mutations
19. Exceptions to Mendelian Inheritance
 Genomic Imprinting
o Genomic imprinting: phenotypic effect
of gene depends on whether from M or F
parent
o Methylation: silence genes by adding
methyl groups to DNA
20. Inheritance of Organelle Genes
 Extranuclear genes (or cytoplasmic genes)
are found in organelles in the cytoplasm
o Mitochondria, chloroplasts, and other
plant plastids carry small circular DNA
molecules
 Extranuclear genes are inherited maternally
because the zygote’s cytoplasm comes from
the egg
 Variegated (striped or spotted) leaves result
from mutations in pigment genes in
plastids, which generally are
inherited from the maternal parent.
21. Genetic Testing
 Reasons for Genetic Tests:
 Diagnostic testing (genetic disorders)
 Presymptomatic & predictive testing
 Carrier testing (before having
children)
 Pharmacogenetics (medication &
dosage)
 Prenatal testing
 Newborn screening
 Preimplantation testing (embryos)
22. Prenatal Testing
 May be used on a fetus to detect
genetic disorders
o Amniocentesis: remove
amniotic fluid around fetus to
culture for karyotype
o Chorionic villus sampling:
insert narrow tube in cervix to
extract sample of placenta
with fetal cells for karyotype
23. Review Questions
 How many chromosomes are in a human cell that is:
a) Diploid?
b) Triploid?
c) Monosomic?
d) Trisomic?
 What is a Barr body?
Chi-Square Analysis
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Used to determine if there is a significant difference between the expected and observed data
Null hypothesis: There is NO statistically significant difference between expected & observed data
o Any differences are due to CHANCE alone
Chi-Square (χ2) Formula
 How to use the Chi-Square Test
1. Determine null hypothesis
 All frequencies are equal –OR– Specific frequencies given already
2. Use formula to calculate χ2 value:
 n = # of categories, e = expected, o = observed
3. Find critical value using table (Use p=0.05).
 degrees of freedom (df) = n – 1
2
4. If χ < Critical Value, then ACCEPT null hypothesis. Differences in data are due to chance alone.
If χ2 > Critical Value, REJECT the null hypothesis: Differences in data are NOT due to chance alone!
 Q1: Chi Square
A hetero red eyed female was crossed with a red eyed male. The results
are shown below. Red eyes are sex-linked dominant to white, determine
the chi square value. Round to the nearest hundredth.
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Phenotype
# flies observed
Red Eyes
134
Chi Square Strategy
White Eyes
66
o Given—observed
o You have to figure out expected. Usually to do a Punnett square to figure this out
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Plug in
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Chi squared Practice Question
 Two true-breeding Drosophila are crossed: a normal-winged, red-eyed female and a miniature-winged,
vermillion-eyed male. The F1 offspring all have normal wings and red eyes. When the F1 offspring are
crossed with miniature-winged, vermillion-eyed flies, the following offspring resulted:
233 normal wing, red eye
247 miniature wing, vermillion eye
7 normal wing, vermillion eye
13 miniature wing, red eye
What type of conclusions can you draw from this experiment? Explain your answer.