Download Chapter 14: Human Inheritance

Section 14-2:
Human Genetic Disorders
From Molecule to Phenotype
 There is a direct link between genotype and
phenotype
 Example:
 People of African or European descent are more likely to
have wet earwax (dominant) while those of Asian or
Native American ancestry have the dry form (recessive)
 The difference is in ONE BASE in the gene for a
membrane-transport protein - a change from a G to an A
 Changes in a gene’s DNA sequence can change
proteins by altering their amino acid sequences, which
may directly affect one’s phenotype
Disorders Caused by Individual
Genes
 Sickle Cell Disease
 Caused by defective allele for beta-globin, part of
hemoglobin
 Makes hemoglobin less soluble, molecules stick
together when blood’s oxygen level decreases
 Molecules clump into long fibers, forcing cells into a
sickle shape that are more rigid than normal RBCs, and
they
 Tend to get stuck in the capillaries
 Can result in damage to cells, tissues, organs
Disorders Caused by Individual
Genes
 Cystic fibrosis (CF)
 Most common among people of European ancestry
 Most cases result from the deletion of just 3 bases in the
gene for a protein called cystic fibrosis transmembrane
conductance regulator (CFTR)
 Amino acid phenylalanine is missing from the protein
 CFTR normally allows chloride ions (Cl−) to pass across
cell membranes - the removal of phenylalanine causes
the protein to misfold, and it is destroyed
Disorders Caused by Individual
Genes
 CF con’t
 Tissues malfunction because cell membranes can’t
transport Cl Children with CF have serious digestive problems and
produce thick, heavy mucus that clogs their lungs and
breathing passageways
 People with one normal copy of the CF allele are
unaffected by CF, because they can produce enough
CFTR to allow their cells to work properly
 Two copies of the defective allele are needed to produce
the disorder – it is recessive
Disorders Caused by Individual
Genes
 Huntington’s disease
 Caused by a dominant allele for a protein found in brain cells
 Allele contains a long string of bases in which the codon CAG
(glutamine) repeats over and over again – more than 40X
 Reason why is unknown
 Symptoms of Huntington’s disease do not appear until middle
age - mental deterioration and uncontrollable movements
 The greater the number of codon repeats, the earlier the
disease appears, and the more severe are its symptoms
Genetic Advantages
 Why are alleles for sickle cell disease or CF still around
if they can be fatal for those who carry them?
 Individuals with one sickle cell allele are resistant to
malaria
 Individuals with one CF allele have a harder time
developing typhoid fever
Chromosomal Disorders
 Usually caused by
nondisjunction
 Creates gametes with an
abnormal number of
chromosomes
Chromosomal Disorders
 If 2 copies of an autosome do not separate, a person
can be born with 3 copies of that chromosome –
known as trisomy
 The most common form of trisomy is Down
syndrome (trisomy 21)
 Mild to severe mental retardation and a high
frequency of certain birth defects
Chromosomal Disorders
 Nondisjunction of X chromosomes can lead to
Turner’s syndrome, where a woman only inherits one X
chromosome
 Called a monosomy
 Sex organs do not develop properly at puberty, sterile,
small/short stature, high voice
 Karyotype: 45, XO
Chromosomal Disorders
 Nondisjunction can also cause a male to inherit an
extra X chromosome
 Disorder called Klinefelter’s syndrome
 Also sterile
 Karyotype: 47, XXY