law of segregation
Mendel’s first law, stating that the two alleles in a pair segregate (separate from each other) into different gametes during gamete formation.
law of independent assortment
Mendel’s second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.
Tay-Sachs disease description
Brain cells of a child can’t metabolize certain lipids b/c a crucial enzyme doesn’t work properly; as these lipids accumulate in brain cells, the child suffers seizures, blindness, and degeneration of motor and mental performance and dies within a few years.
pleiotropy
The ability of a single gene to have multiple phenotypic effects. In humans, pleiotropic alleles are responsible for the multiple symptoms associated w/ certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.
Converse of polygenic inheritance.
epistasis
the phenomenon where the effect of one gene (locus) is dependent on the presence of one or more ‘modifier genes’
polygenic inheritance
Quantitative variation usually indicates polygenic inheritance, an additive effect of 2 or more genes on a single phenotypic character – the converse of pleiotropy.
norm of reaction
The range of phenotypes produced by a single genotype, due to environmental influences. Broadest for polygenic characters.
multifactorial
Referring to a phenotypic character that is influenced by multiple genes and environmental factors.
cystic fibrosis
- Normal allele codes for a membrane protein that functions in the transport of chloride ions btwn certain cells and the extracellular fluid.
- Homozygous recessive: these chloride transport channels are defective or absent in the plasma membrane.
- Result: abnormally high concentration of extracellular chloride, which causes the mucus that coats certain cells to become thicker and stickier than normal. This mucus builds up in the lungs, pancreas, digestive tract, and other organs, leading to pleiotropic effects – poor intestinal nutrient absorption, chronic bronchitis, recurrent bacterial infections.
Tay-Sachs disease at each level
- At the organismal level: recessive
- Biochemical level: incomplete dominance – the activity level of the lipid-metabolizing enzyme in heterozygotes is intermediate
- Molecular level: codominant – heterozygous individulas produce equal numbers of normal and dysfunctional enzyme molecules
What explains the prevalence of the sickle-cell allele?
The malaria parasite spends part of its life cycle in RBCs, and the presence of even heterozygous amounts of sickle-cell hemoglobin results in lower parasite densities and hence reduced malaria symptoms.
sickle-cell disease description
- Caused by the substitution of a single amino acid in the hemoglobin protein of RBCs. When the oxygen content of an affected individual’s blood is low, the sickle-cell hemoglobin molecules aggregate into long rods that deform the RBCs into a sickle shape.
- Sickled cells may clump and clog small blood vessels, leading to physical weakness, pain, organ damage, paralysis.
- Can be treated w/ regular blood transfusions and drugs.
achondroplasia
Form of dwarfism caused by a dominant allele.
Huntington’s disease
A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; noticeable at about 35 to 45 years old; usually fatal 10 to 20 years after the onset of symptoms.
PKU
phenylketonuria
- Children w/ this disease can’t properly metabolize the amino acid phenylalanine
- Phenylalanine and its by-product, phenylpyruvate, can accumulate to toxic levels in the blood, causing mental retardation
- Treatment: low-phenylalanine diet
ultrasound
Reflected sound waves are used to produce an image of the fetus by a simple noninvasive procedure.
fetoscopy
A needle-thin tube containing a viewing scope and fiber optics (to transmit light) is inserted into the uterus.
amniocentesis
- Needle inserted into uterus; 10 mL of amniotic fluid, the liquid that bathes the fetus, is extracted. Some genetic disorders can be detected from the presence of certain molecules in the amniotic fluid itself.
- Tests for other disorders are performed on the DNA of cells cultured in the lab (for several weeks), descendants of fetal cells sloughed off into the amniotic fluid.
- Karyotype produced
chorionic villus sampling (CVS)
- Narrow tube inserted through the cervix into the uterus and suctions out a tiny sample of chorionic villus tissue from the placenta.
- These cells are proliferating rapidly enough to allow karyotyping to be carried out immediately.
- This rapid analysis represents an advantage of amniocentesis, in which the cells must be cultured for several weeks before karyotyping.
