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I Errors and Exceptions in Chromosomal Inheritance
A. Possibilities
1. Alterations of chromosome number
a. Nondisjunction—means “not coming apart”; most common when homologous
chromosomes fail to separate in meiosis
b. Consequence of nondisjunction: one gamete gets 2 of same type of chromosome
& another gets no copy
c. Offspring from fertilization of normal gamete with one produced by
nondisjunction will have an abnormal chromosome number (called aneuploidy)
d. Terms: trisomic cell: has 3 copies of particular chromosome (2n + 1);
Monosomic cell: has only one copy of a particular chromosome (2n – 1)
e. Polyploid: organism having more than two complete sets of chromosomes
f. Polyploidy common in plants (important for evolution). Can be 3N or 4N; often
larger and stronger the 2N (diploid) plants. Examples: bananas & citrus fruits.
Occurs in some fish & amphibians too.
2. Alterations of chromosome structure
a. breakage of a chromosome can lead to 4 types of changes in chromosome
structure
(1) Deletion: when chromosome fragment lacking a centromere is lost
during cell division (will be missing certain genes—often fatal)
(2) Duplication: portion is repeated
(3) Inversion: fragment reattaches to original chromosome, but in reverse
order
(4) Translocation: fragment attaches to another (nonhomologous)
chromosome
b. Deletions and duplications are likely to occur during meiosis (especially during
crossing over)
3. All these can be increased by X-rays & other chemicals
B. Syndromes resulting from chromosome changes
1. Down Syndrome
a. aneuploid condition; most often from nondisjunction during game production in
one parent
b. due to 3 copies of chromosome 21 (trisomy 21)
c. affects 1 in 700 children in US
d. set of characteristic symptoms: characteristic facial features, short stature, heart
defects, susceptibility to respiratory infection, mental retardation, increased risk
of developing leukemia & Alzheimer’s. Also, most individuals are sexually
underdeveloped & sterile
e. frequency of Down syndrome increases with age of the mother
2. Klinefelter’s syndrome
a. XXY male; extra X is inactivated, but some female characteristics evident
b. Occurs once in every 2000 live births
c. Affected individuals have male sex organs, but are sterile
d. Affected individuals have normal intelligence
3. XYY
a. male
b. tend to be taller than average, but no defined “syndrome”
4. Trisomy X
a. XXX
b. Occurs once in every 2,000 live births
c. Produces healthy females
5. Monosomy X or Turner syndrome
a. XO
b. Occurs once in every 5,000 births
c. Female phenotype, but sterile because sex organs do not mature
d. Estrogen replacement therapy can be used to develop secondary sex
characteristics
e. Normal intelligence
6. Cri du Chat
a. deletion in chromosome 5
b. mentally retarded, small heads, unusual facial features, “cat cry”
c. fatal in infancy or early childhood
7. CML (chronic myelogenous leukemia)
a. large fragment of chromosome 22 switches places with small fragment from tip
of chromosome 9
b. resulting short chromosome 22 is called the Philadelphia chromosome
c. example of translocation implicated in a cancer
C. Genomic Imprinting
1. For a few dozen mammalian traits, phenotype varies depending on which parent passed
along the alleles for those traits
2. this variation in phenotype depending on whether an allele is inherited from the male or
female parent is called genomic imprinting
3. imprinted genes are NOT expressed ex. For maternally imprinted gene, only the
paternal allele is expressed. Example: insulin like growth factor
D. Extranuclear genes
1. found in small circles of DNA in mitochondria & chloroplasts/plastids
2. reproduce themselves & transmit their genes to daughter organelles
3. break Mendel’s “rules”
4. because all mitochondria is from the ovum, all mitochondrial genes in mammals come
from the mother—defects can decrease amount of ATP cell can make; nerves &
muscles are especially susceptable. Example: rare diseases like mitochondrial
myopathy