1. The Chromosomal basis of Inheritance
It’s where the genes are….

2. The Chromosomal Theory of Inheritance
Genes have specific loci on chromosomes
Chromosomes undergo segregation and independent assortment

3. Chromosomal Linkage Thomas Morgan XX (female) vs. XY (male)
Worked with Drosophilia melanogaster
XX (female) vs. XY (male)
Some other organisms have a different system (link)
Sex-linkage: genes located on a sex chromosome Animate!
Linked genes: genes located on the same chromosome that tend to be inherited together

4. Genetic recombination
Crossing over of linked genes- Now, if linked genes get separated during crossover, they DO assort independently of each other!
Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency. (Think of the likelihood of being on the same ride the further apart you are in line….)
Linkage maps Genetic map based on recombination frequencies
Animate!

5. Human sex-linkage
SRY gene: gene on Y chromosome that triggers the development of testes
Fathers= pass X-linked alleles to all daughters only (but not to sons)
Mothers= pass X-linked alleles to both sons & daughters
Sex-Linked Disorders: Color-blindness; Duchene muscular dystrophy (MD); hemophilia
X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)

6. Chromosomal Alterations
Non-disjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II.
Aneuploidy: abnormal number of a certain chromosome. Examples:
Monosomy X ~ Turner Syndrome
(only viable human monosomy)
Trisomy 21~ Down Syndrome
Trisomy 13 ~ Patau Syndrome
Trisomy 18 ~Edward’s Syndrome
Polyploidy: more than 2 complete sets of chromosomes.
Comes from non-disjunction of all chromosomes
Common in Plant Evolution.

7. Alterations in Chromosome Structure
Can result from non-reciprocal crossovers or weak area of chromosomes.
Deletion: removal of a chromosomal segment
Duplication: repeats a chromosomal segment (a fragment may join a homolog)
Inversion: segment reversal in a chromosome (reattaches in reverse)
Translocation: movement of a chromosomal segment to another

8. Various effects of Chromosomal Alterations:
-Homozygous deletions are usually lethal
-Duplications & translocations usually deleted
-Some reciprocal translocations can alter phenotype because of
“POSITION EFFECT”:
-The influence on a gene’s expression
because of its location among neighboring genes.

9. Genomic imprinting
Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
“Something” happens to the gene after they arrive to form the zygote
May involve degrees of methylization (CH3 groups)
Example:Fragile X syndrome: higher prevalence of disorder and in males

10. GENOMIC IMPRINTING Prader-Willi / Angelman Syndrome-
Another example of
GENOMIC IMPRINTING
Prader-Willi / Angelman Syndrome-
Both disorders are caused by a deletion on
Chromosome 15 but:
If the altered chromosome came from Dad:
Prader-Willi-
Mentally handicapped, short, obese
If the altered chromosome came from Mom:
Angelman’s Syndrome-
motor/mental symptoms, jerky, uncontrolled laughter

11. Most (but not all) Disorders are recessively Inherited.
Defective alleles code for either a malfunctioning protein….
or no protein at all.
Heterozygotes can be phenotypically normal if
one of the normal copies is all that is needed.
They are, of course, carriers.
Most people with recessive disorders are born to
“normal” parents. (carriers)
Probability is ¼ that the mating of 2 carriers will produce
a homozygous recessive.
…and 2/3 that (s)he will be a carrier.
Consanguinity: condition that results from shared ancestry
Results in more homozygous recessives. (Stearns County Syndrome)
REMEMBER!- not all disorders are recessive
some are “late-acting” & occur after sexual maturity.
Example- Huntington’s Disease

12. Some other Genetic disorders:
Cri-du-Chat: A specific deletion in Chromosome 5
mentally handicapped, distorted face, cry that sounds like a cat
Chronic Myologenous Leukemia-
Portion of Chromosome 22 translocates with portion of chromosome 15
Patau Syndrome: Trisomy 13
Severely mentally handicapped, cleft lip
Edward’s Syndrome: Trisomy 18
Low ears,deformed fingers, heart problems