Chromosomes contain genes and determine inheritance patterns. During meiosis, chromosome pairs segregate and assort independently. Sex chromosomes determine biological sex - females are XX and males are XY. X-linked traits affect males more due to hemizygosity. Nondisjunction can result in chromosomal abnormalities like Down syndrome. Structural changes like inversions, translocations, deletions and duplications also impact inheritance. Linkage maps gene position and crossover frequency. Different modes of inheritance exist like autosomal dominant, recessive, and X-linked traits.

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