1. Chromosomal
Inheritance

2. Chromosomal Theory
of Inheritance
• Chromosomes contain the units of heredity
(genes)
• Pair chromosomes segregate during meiosis,
each sex cell has half of the number of
chromosomes found in a somatic cell. (Mendel’s
law of segregation)
• Chromosomes assort independently during
meiosis (Mendel’s law of independent
assortment)
• Each chromosome contain many different genes

3. Sex Chromosomes
• Sex chromosomes (X and Y) vs. autosomes
(chromosomes 1-22), Sex cells and somatic cells.
• Homogametic sex -- that sex containing two like sex
chromosomes. In most animals species these are
females (XX). Each egg only contain one X
chromosome.
• Heterogametic sex --- that sex containing two
different sex chromosomes . In most animal species
these are XY males. Each sperm will contain either
an X or Y. Therefore the father determines whether
the offspring is a boy or a girl (50/50 chance)

4. Sex Linkage
• XA = Locus on X chromosome
• XX females
– XA XA, XaXa - homozygotes
– XA Xa – heterozygote (carrier)
• XY male
– XA Y, XaY
– no carriers in males, therefore they are more
susceptible to x-linked traits.

5. Red/white eye color in
Drosophila
• In females:
– XR XR , XR Xr = red-eye female
– Xr X r = white-eyed females
• In males:
– XR Y = red-eye male
– Xr Y = white-eyed male

6. Examples of Sex
Linked Traits
• Hemophilia - Recessive
• Red-Green Color Blindness - Recessive
• Muscular Dystrophy - Recessive
• Fragile X syndrome - Dominant

7. Nondisjunction
• abnormal number of autosomal
chromosomes when chromosomes fail
to separate during replication.
• 2n – 1 = monosomic
• 2n + 1 = trisomic

8. Nondisjunction

9. Nondisjunction -
Examples
• Down's -- trisomy 21 mean
life expectancy 17 years.
Short in stature, round face
and mental retardation
• Patau's -- trisomy 13 mean
life expectancy 130 days
• Edward's --- trisomy 18
mean life expectancy a few
weeks

10. Sexual Determination
- Males
Single Y = male, so XXY, XYY,
XXXY all male
• Klinefelter Syndrome – XXY or
XXXY. Male due to Y
chromosome, Testes and
prostrate underdeveloped, some
breast formation, no pubic or
facial hair, subnormal intelligence.
• Jacob’s Syndrome – XYY. Males
are usually taller than average,
and tend to have speech and
reading problems

11. Sexual Determination
- Females
• Turner’s Syndrome – X0. Female with
bull neck, short stature, nonfunctional
ovaries, no puberty
• Metafemale – 3 or more X
chromosomes. No apparent physical
abnormality except menstrual
irregularities.

12. Chromosomal
Mutation
• Permanent change in chromosome
structure.
• Caused by exposure to radiation,
organic chemicals, viruses, replication
mistakes.
• Only mutations in sex cells are passed
onto the next generation.

13. Structural Changes
in Chromosomes
• Inversion – occurs when a chromosome
segment turns around 180 degrees.
• Translocation – is movement of
chromosomal segments to another non-
homologous chromosome
• Deletion – occurs when a portion of the
chromosome breaks off.
• Duplication – when a portion of a
chromosome repeats itself.

14. Deletions and
Duplications

15. Inversions and
Translocations

16. Linkage
• When genes are on the same chromosome, they are
called linked. They can show departures from
independent assortment
• If genes on the same chromosome are sufficiently far
apart, they can segregate independently through
crossing over.

17. Gene Mapping
• By studying cross-over (recombination)
frequencies of linked genes, a
chromosomal map can be constructed
– Distant genes are more likely to be
separated by crossing-over than genes
that are closer together.
– Each 1% of recombination frequency is
equivalent to 1 map unit

18. Crossing Over
Produces
Recombinations

19. Constructing Gene
Maps
• Crossing over frequencies can be used
to construct gene maps. For example,
– Crossing over frequency of genes A and B
is 3%, genes B and C is 9% and genes A
and C is 12%.
3 mu 9 mu
A B C

20. Human Genome
Project
• Map of the all of the genes on the
human chromosomes.

21. Pedigree Analysis

22. Modes of Inheritance
• Autosomal dominant allele
[e.g., Huntington's Disease,
brown eyes]
– A phenotype associated with an
autosomal dominant allele will,
ideally, be present in every
individual carrying that allele. It
will be present in close to 50% of
the individuals.
– Affected children usually have
affected parents
– Two affected parents can
produce an unaffected child
– Both males and females are
affected equally.

23. Modes of Inheritance
• Autosomal recessive alleles [silent
carriers]
– albinism, cystic fibrosis, certain types of
hemophilia, Tay-Sachs disease, PKU,
blue eyes.
– A pedigree following a trait associated
with an autosomal recessive allele is
often marked by a skipping of
generations. That is, children may
express a trait which their parents do not.
– In such a situation, both parents are
heterozygote, also known as silent
carriers.
– Close relatives who reproduce are more
likely to have affected children.
– Both males and females will be affected
with equal frequency
– A low number of individuals normal
affected

24. Modes of Inheritance
• Sex-linked dominant alleles
[sex linkage]
– A sex linked dominant allele has a
variation on the pattern displayed by
autosomal dominant alleles. That is:
• one-half of the offspring of an afflicted
heterozygote female will be similarly
afflicted (gender independent).
• only the female progeny of males will be
afflicted (because the male donates an
X chromosome to his female progeny).
– As with any sex-linked allele, males
can pass the allele only on to their
daughters, not their sons.

25. Mode of Inheritance
• Sex-linked recessive alleles
– red-green color blindness,
certain types of hemophilia.
– More males affected than
females
– An affected son can have
parents who have the normal
phenotype
– For a female to have the
characteristic, her father must
also have it and the mother must
be a carrier.
– If a woman has the characteristic
all her sons will have it
– The characteristic often skips a
generation from grandfather to
grandson