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Genetics Notes
Aristotle: Greek philosopher – lived 2300 years ago
*believed traits were carried in the blood as pangenes and
transported to the reproductive organs
-expressions such as blood relative and bloodline come
from this idea
*proven wrong when the discovery was made that traits for
inheritance were found in each cell on chromosomes and
had nothing to do with the blood
Gregor Mendel – Austrian Monk
*worked on the scientific study of heredity in the late 1800’s
*unique background in math and physics
*applied mathematical background (probability) to
reproduction of pea plants
*studied 7 traits for many generations
*used probability to predict outcome of a cross
*hypothesized that each trait is controlled by something
called a “factor” – we now call these factors genes
*first to use dominant and recessive
Mendel’s Laws
1. The Law of Segregation: a pair of factors (alleles) is separated
during the formation of gametes (meiosis)
2. The Law of Dominance: One allele shows up in the phenotype
while the other is hidden
3. The Law of Independent Assortment: Factors (traits) are
distributed to gametes independently (they don’t travel
together)
P1 Generation: parental generation
F1 Generation: first filial generation – offspring from P1 generation
F2 Generation: second filial generation – offspring from F1 generation
Applied Genetics
Biotechnology is the application of biological science to solve practical
problems
*people have applied genetics to everyday use longer than they
have actually understood them
-the natives used controlled breeding to develop corn and
potatoes from wild plants
- the domestication of the dog from wild wolves
Selective Breeding: the breeding of animals and plants to produce a
desired trait by choosing which animals/plants will be bred to
which animals/plants
ex.
Sheep for wool
Cattle for beef
Inbreeding: the crossing of organisms with similar genotypes
*when you cross two closely related animals, after several
generations you will probably have homozygous alleles for the
most desired traits
*you can also have homozygous alleles for undesired traits
Inbreeding can cause problems
-may carry a recessive allele that produces harmful
traits when homozygous
ex. Deafness in dalmations
-offspring may be weak or unhealthy
ex. Royalty married royalty to keep bloodline
pure and passed on recessive trait for
hemophilia
Outbreeding: crossing of distantly related organisms
*may come from different species or more often from different
breeds within the same species
-in some cases, the cross may result in larger, healthier
offspring
ex. Mule
*when two species are crossed, offspring is almost
always sterile because chromosomes don’t match
*outbreeding is done in order to cross the best qualities of each
species
-used to improve cattle
*Brahman cattle: resistant to heat and insects
*Hereford cattle: high quality beef
Polyploidy: cells have more than two sets of chromosomes
*rare – almost always lethal in animal cells
*desired trait in plant cells
-produce double flowers and larger fruit
-can be the source of a new species of plant
-polyploidy is induced by applying a chemical called
colchicine – prevents the cell from dividing
after the chromosomes have doubled
Genetic Engineering: Modification of genes using biotechnology
*combining DNA from genes of different organisms
-called recombinant DNA
ex. Genetic engineers have transferred specific
human genes into bacteria enabling them to
produce proteins
*applications of genetic engineering
-medicine
-insulin, antiviral drugs (interferon), human growth
hormone, vaccines
-gene therapy for hereditary disease
-crops, food
Ethical Issues
Human Genetics
Difficult to study patterns of inheritance
-humans have few offspring
-too many years between generations
-most of what we know about human inheritance comes from
studying families and inheritance patterns in populations
Pedigree: chart that shows how a trait and the alleles that control
that trait are inherited in a family
Karyotype: photograph that shows an individuals chromosomes
-used to determine chromosomal abnormalities
*too few or too many chromosomes
*abnormal chromosome size or shape
-chromosomes are arranged by size from biggest (#1) to
smallest (#23)
Chromosomal Disorders
*mutations can occur spontaneously – in most cases they are not
harmful and do not show up in the phenotype
-in severe cases, the fetus does not survive
-in a few cases, the mutation is able to sustain and the
child is born mentally or physically challenged
Non-disjunction: the failure of one or more chromosome pairs
to separate during meiosis
-this can result in monosomy – 1 chromosome per pair
instead of 2 or trisomy – 3 chromosomes instead of 2
Examples:
Down Syndrome: Trisomy at site #21
-mildly to severely developmentally disabled
-almond eyes and enlarged tongue
-short, stocky bodies
-muscle development and coordination poor
-prone to heart defects
Klinefelter Syndrome: Trisomy at site #23
-caused by an extra X chromosome in male XXY
-sterile
-causes feminine characteristics
-breast development, feminine body shape
-in females – XXX usually sterile and
developmentally disabled
Turner Syndrome: Monosomy at site #23
-female X instead of XX
-normal childhood – normal intelligence
-don’t go through puberty – do not reach normal
height and fail to develop sexually
-sterile
Sometimes a person is born with a normal number of chromosomes but
the chromosome structure is abnormal.
*Inversion
*Missing end
*Mutation
Cri-du-chat syndrome: disorder caused by a defective chromosome
-large deletion from chromosome #5
-cri-du-chat means cry of the cat
-babies have a cry that sounds like a cat’s meow
-extremely small head
-seriously developmentally disabled
-seldom survive more than 1-2 years
Most genetic disorders in humans are caused by single allele mutations
Recessive Genetic Disorders
*all alleles are passed on even if not expressed – these alleles
remain in the gene pool
*recessive disorders are not as common as you must have two
recessive alleles to exhibit the disorder
Tay-Sachs Disease: fatal genetic disorder that results
in the inability to break down lipids causing
an accumulation of the substance in the brain
*as lipids build up in the brain, the
individual suffers blindness and seizures
*usually causes death within the first
months of life
*most often found in Ashkenazi Jews in
in central Europe
-incidence is 100 times greater than
among non-Jews
Cystic Fibrosis: disorder that results in excessive
secretion of thick mucus from the pancreas, lungs
and other organs
*leads to blockage of digestive tract,
congestion of lungs, pneumonia, and other
infections
*if untreated, individuals die at an early age
*current treatments lead to prolonged life
*usually found in Caucasians
Dominant Allele Disorders
Huntington’s Disease: fatal disorder that involves the
deterioration of the nervous system – particularly the brain
*individuals don’t develop symptoms until late 30’s or early
40’s
*by this time, most people have passed it on to their children
Achondroplasia: Dwarfism
*never reach more than 4 ft. 4 inches
*arms and legs are disproportionately short
Other characteristics caused by dominant allele:
*polydactyly (extra fingers and toes)
*dimples, freckles, widows peaks, ability to roll tongue
**A dominant allele disorder is not always more common than a
recessive allele disorder
X-Linked Traits (sex linked)
1. Colorblindness – can’t distinguish between certain colors
2. Hemophilia – occurs almost exclusively in males
-blood can’t clot
3. Duchenne Muscular Dystrophy – progressively weakens
and destroys muscle tissue
Co-Dominant Allele Disorders
Sickle Cell Anemia: abnormal form of hemoglobin – the protein
in red blood cells that carries oxygen
*sickle cell is co-dominant to normal allele
*individuals affected by this disease contain a
mixture of normal hemoglobin and sickle cell
hemoglobin
*sickle cell attack begins when oxygen supply
becomes low – sickle cells start to stick together
and become tangled masses
*clogs capillaries and stop blood flow
*extremely painful and can cause death
*primarily found in Africans and African-Americans
*often found in areas where malaria is common –
individuals with sickle cell have an increased
resistance to malaria