1. Chromosomal Inheritance
Biology 30

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)
Chromosomes assort independently during meiosis (Mendel)
Each chromosome contain many different genes

3. Sex Chromosomes
Sex chromosomes (X and Y) vs. autosomes (chromosomes 1-22)
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 XY male
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. 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

7. 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.

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

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

10. Nondisjunction

11. 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

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.
Crossing over% = # of recombinations
total # of offspring

18. Crossing Over Produces Recombinations

19. Gene Mapping - Example Crossing over% = # of recombinations
total # of offspring
Crossing over % = = 53% = 53mu
381
Therefore genes p and l are 53 map units apart.

20. 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

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

22. Pedigree Analysis

23. 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.

24. 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

25. 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 is 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.

26. 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 heterozygotes, 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