Human Genetics
Chapter 12 continues
Pedigree Analysis
A flowchart showing who has mated with whom and which
children they had together
Tracks alleles within a family group
Interpretation of Pedigree
= unknown sex
Inheritance Patterns
Most human genetic disorders are caused by recessive alleles
Sickle-cell anemia (description to follow)
Albinism – lack of the pigment melanin
In these conditions, neither the homozygous dominant (AA) nor
the heterozygote (Aa) display the trait.
Only the homozygous recessive (aa) has the trait
Sickle Cell Anemia
SS = Normal blood cells
Dies from malaria
ss = Sickle cell
Dies from sickle cell
Ss = a few sickled cells, no major problem
Lives to reproduce!  Trait maintained in population
Dominant Inherited Genetic Defects
A few human genetic disorders are caused by dominant alleles
Normally die before reproducing - usually no phenotypic carriers
Sometimes die before birth (miscarriage)
Exception is Huntington's disease – Symptoms don’t appear until
later in life (time to reproduce and pass on the allele)
Deteriorative disease of the brain
Progressively less coordinated, awkward movements,
altered personality, eventual death
Sex-linked human disorders
Genes appearing on the sex chromosome (usually X)
Tend to skip a generation
Usually pass from mother to son
Females CAN inherit but must get two defective alleles to do so
e.g. color blind dad mates with carrier mom
Sex Linked …examples
Male Pattern Baldness – check your maternal grandfather!
Hemophilia – “bleeder’s disease” – a ‘royal’ problem
Red-green color blindness – are you? We’ll find out!
Even though these are generally recessive, males only have one
copy (just one X chromosome) so there is no chance of a dominant
allele to mask its effect
Females must inherit two defective alleles – rare occurrence
Inheritance of a sex-linked allele
Notice: affected individuals
tend to be male can occur
in female but far less often
Hemophilia traced through
a family
inherited from
Mom
*
*
* *
Color Blindness
is a sex linked
trait
The gene is carried
on the X chromosome
Ishihara Color-Blindness Test
Abnormal # Chromosomes
Disorders resulting from nondisjunction during meiosis
Abnormal number of sex chromosomes
Turner's syndrome (XO): sterile, short female; lacks Barr bodies
Trisomy X (XXX): fertile female; no detectable defects;
decreased intelligence
Klinefelter's syndrome (XXY): mixed secondary sex characteristics;
sterile male
XYY males: IGNORE BOOK’S STATEMENTS REGARDING
decreased intelligence; increased height, increased predisposition
for violence – OLD INCORRECT STUDY
Down’s Syndrome
Trisomy –21 caused by a non-disjunction
Gamete (egg usually) formed with an extra chromosome 21
resulting zygote has 3 rather than 2 copies of 21
Figure 12-20 p 222
That’s it!
Time for some experimentation
on each other!