Transcript Inheritance Patterns - Santa Susana High School

Inheritance Patterns
• Things that cause alterations in chromosome
number or pattern lead to genetic disorders.
– Alterations in chromosome number - aneuploidy
• caused by nondisjunction
– failure of a chromosome to separate during meiosis
» if nondisjunction is of a single chromosome the zygote will
either be monosomic or trisomic
» if nondisjunction is of a entire genome the zygote will exhibit
popolyploidy
• Triploidy, tetraploidy, …
• Very common in plants but not viable in animals
monosomic
trisomic
Examples of Syndromes Associated
with Non-disjunction
– monosomic - Turner Syndrome (XO)
• only viable monosomic disorder in humans
• 1/500 births
• sterile, less developed secondary sex characteristics (better with estrogen therapy)
– trisomic
• Down Syndrome (trisomy 21)
–
–
–
–
1/700 births
part of the pre-birth screening panel
short stature, heart defects, prone to respiratory infections, mental retardation
risk increases dramatically for childbearing women over the age of 30
• XXX
– Normal outwardly
– 1/1000 births
• XXY
–
–
–
–
–
Klinefelter syndrome
under developed genitalia
sterile
some female sex characteristics - fleshy breasts
1/2000 live births
• XYY
• may be taller but otherwise indistinguishable
• homologous alterations often lethal
– deletion
• can cause a frame-shift
affecting all genes down
stream
• cri du chat syndrome
– deletion in chromosome 5
– cry sounds like a mewing
cat, microcephaly,
retardation, death in early
childhood
– duplication
• Caused by the attachment
of a deleted region during
meiosis
• Causes gene dosing errors
– inversion
• Reverse attachment of a
deleted region during
meiosis
• Causes reading frame
errors in genes
Alteration in
Chromosome
Structure
– non-homologous alterations
• translocation
– attachment of a deleted region from a non-homologous
chromosome
– reciprocal translocations occur when neighboring
chromosomes cross over
» gene number is conserved although gene dosing
secondary to gene position may be affected
– implicated in many cancers and leukemia
Exceptions to Mendelian genetics
• Genomic imprinting
– effect of the depends on the sex of the person inherited from
– occurs during the formation of gametes
– certain genes are turned off in the sperm & the ova
• insulin-like growth factor (used in prenatal growth)
– only the paternal version is expressed
• Inherited organelle genes (extranuclear genes)
– circular DNA found in the mitochondria and chloroplasts
– inherited in the cytoplasm of the ova (maternal)
– mitochondrial myopathy
• weakness, intolerance of activity, muscle deterioration