section 14-2: human genetic disorders. from molecule to phenotype there is a direct link between...
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Section 14-2:Human Genetic Disorders
From Molecule to PhenotypeThere is a direct link between genotype and
phenotypeExample:
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 GenesSickle 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 capillariesCan result in damage to cells, tissues, organs
Disorders Caused by Individual GenesCystic fibrosis (CF)
Most common among people of European ancestryMost 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 GenesCF 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 GenesHuntington’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 unknownSymptoms 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 AdvantagesWhy 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 DisordersUsually caused by
nondisjunctionCreates gametes
with an abnormal number of chromosomes
Chromosomal DisordersIf 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 DisordersNondisjunction of X chromosomes can lead to
Turner’s syndrome, where a woman only inherits one X chromosome
Called a monosomySex organs do not develop properly at
puberty, sterile, small/short stature, high voice
Karyotype: 45, XO
Chromosomal DisordersNondisjunction can also cause a male to
inherit an extra X chromosomeDisorder called Klinefelter’s syndromeAlso sterileKaryotype: 47, XXY
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