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MCDB 1041 Class 13
Complex patterns of
inheritance, cont.
Problems in meiosis
Learning Goals:
•  Compare and be able to identify the different examples of inheritance
patterns.
•  Draw the ways in which non-disjunction can impact the chromosomal
content of gametes
•  Compare the results of non-disjunction in meiosis I with non-disjunction in
meiosis II
•  Determine from the genotype of a set of gametes whether nondisjunction happened in meiosis I or meiosis II
Complex pa*erns con/nued Another example of different allelic relationships:
One breed of dog, two very different phenotypes
“Powder Puffs”
Genotype is hh
“Chinese Crested”
Genotype is Hh
People love the hairless Chinese Crested dogs. However,
breeders have seen that in litters of pups from two
Chinese Crested dogs, there are frequently dead
puppies.
This can happen when a particular combination of alleles is
lethal. Which allele causes lethality when homozygous?
How would dog breeders get the most Chinese crested
hairless pups per mating, with the fewest dead
puppies?
a. 
b. 
c. 
d. 
e. 
mate two hairless dogs
mate two powderpuff dogs
mate a hairless dog with a powderpuff dog
a or c
b or c
Final example of complex inheritance patterns:
Mitochondrial Inheritance
The Mitochondria Mitochondria are organelles in the cytoplasm They have their own DNA (they were likely once independent organisms!) •  small circle of DNA with 37 genes. –  These genes func/on in energy produc/on and protein assembly •  Mitochondria are only passed along from the mother, because they are present in the egg at fer/liza/on, while the sperm contributes only its nucleus to the egg (no cytoplasm, and thus no organelles). Mitochondral disorders
Typically cause fatigue
Mitochondrial myopathies: muscular weaknesses (defective
mitochondria can t produce energy)
Can also cause degeneration of muscles or nerves
Example: In genera0on 3, which individuals have the chance of passing on the mitochondrial disorder to their offspring? a.  Anyone with an affected parent b.  Anyone who is affected (Individuals 4,5,6,10,11,12,13) c.  Individuals 4,10,13 d.  Individuals 5,6,11,12 Chromosomal abnormalities
Chromosomes contain many genes, each with a
unique function
The number of copies of each gene is important: in
general, we need two functional copies of every
gene to have a normal phenotype.
When there are too few or too many copies of a
chromosome, problems ensue.
Examples:
Phenotypes caused by abnormal chromosomal number
Turner
Syndrome : Only one X
Chromosome
Female
Short and have a low hairline
Broad chest and folds of skin
on neck
Normal intelligence
Sterility
Trisomy 21 (Down’s Syndrome): 3 copies of ch. 21
Developmental disabilities
Characteristic facial features
Decreased longevity
Heart defects
Chromosomal number abnormalities result from
mistakes in meiosis
•  EITHER a homologous
pair of chromosomes
or sister chromatids
can fail to be pulled
apart in meiosis I or
meiosis II
•  When this happens,
the gamete ends up
with too few or too
many of a particular
chromosome
This is called “non-disjunction” , which
means “failure to separate”
This is a relatively rare event, not
something that happens in the production of
all gametes
#1
A
#1
a
DNA replication
A
F
#2
#2
f
#1
#1
F
Spermatogonium
Metaphase of meiosis I
A
A
a
a
F
F
f
f
A
#1
#1
a
a
#2
#2
F
#2
#2
f
f
Errors during meiosis I a
a
A
A
F
F
A
A
a
a
F
F
f
f
f
f
a
a
A
F
A
f
f
F
Too many chromosomes!
Too few chromosomes!
Errors during meiosis II A
A
A
F
F
A
A
a
a
F
F
f
f
a
f
a
f
a
a
A
f
F
Too many
chromosomes!
F
Too few
chromosomes!
f
Normal
Work on handout Differences between meiosis I and II errors Errors in meiosis I
a
a
A
A
f
f
F
F
Too many chromosomes!
Too few chromosomes!
Errors in meiosis II
A
a
a
A
f
F
Too many
chromosomes!
F
Too few
chromosomes!
f
Normal
When nondisjunc0on happens in meiosis II rather than in meiosis I, the outcome is: a.  The same as when it happens in meiosis I b.  In meiosis II, all gametes are abnormal, while in meiosis I only ½ the gametes are abnormal c.  In meiosis II, only ½ the gametes are abnormal, while in meiosis I, all gametes are abnormal Prac0ce. You are presented with a pa0ent who has Down syndrome (3 copies of chromosome 21). The mother was Bb for a gene of interest on chromosome 21 The father was BB. You know the nondisjunc0on event occurred in the mother. If the resul0ng genotype of the pa0ent is BBb, when during meiosis did the nondisjunc0on event occur (assuming no recombina0on)? #21
B
a. During meiosis I b. During meiosis II #21
#21
c. It could have happened during meiosis I or II b
B