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Ch 14
Mendelian Genetics
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Pre-Mendel belief in blending, child is a mix of parents problem = traits skipping generations Mendel – monk, mid 1800’s, bred pea plants
Terms Character = detectable, inherited feature, ex. color Trait = variant of an inheritable character, ex. green or
red color True-Breeding = always produce plants with same
traits as parents, self fertilization Cross-Breeding = cross parents with different traits to
create hybrids
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Generations are named
P = parental F1= results of PxP F2= results of F1 x F1
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Mendel’s experiment Mendel looked at 7 characteristics, each had 1
alternate form
hypothesis – if a cross of purple & white gives all purple, then a cross between F1’s would produce purple again
experiment – let F1’s self pollinate results – 3:1 ratio of purple to white flowers,
hypothesis wrong Mendel crossed true-bred peas, and never saw blending
conclusion – inheritable factor of white must be masked: purple is dominant, white is recessive
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So… there are alternate forms of the same gene = alleles,
p265
we inherit one allele from each parent if alleles are different, one is dominant (noted by
capital letter), one is recessive (lowercase letter) alleles segregate during meiosis
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More Terms homozygous – 2 identical alleles for a trait, ex. DD, dd
heterozygous – 2 different alleles for a trait, carrier, ex. Dd phenotype – organism’s expressed traits, ex. color, height genotype – organism’s genetic makeup, letters, ex. PP, Pp
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Testcross – a cross between a recessive and an unknown tells if it is homo or
heterozygous
monohybrid cross – dealing with 1 trait
dihybrid cross – 2 traits Trihybrid – 3 traits, ouch
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Law of Segregation = allele pairs separate randomly during meiosis, p. 266
There are 2 alleles for flower color, if 1 purple and 1 white: there is a 50% chance of getting either allele
Punnett square predict the
results
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Law of independent assortment p.268-269
when dealing with 2 or more traits, each allele of the different genes segregates independently of each other
If cross 2 dihybrid heterozygotes, get 9:3:3:1 ratio
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Probability
= mathematical chance of an event happening Rule of multiplication- probability of 2 events
occurring at the same time = product of their individual probabilities
Ex. 2 coins both coming up heads = ½ x ½ = ¼ Ex. DdRr x DdRr ? probability of getting
DDRR chance of DD = ¼, chance of RR = ¼ so ¼ x ¼ =
1/16
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Rule of addition – harder to define, p.270, probability that any one of two or more mutually exclusive events will occur is calculated by adding the individual probabilities Ex. cross of 2 heterozygotes, ? chance of result being hetero? Chance of recessive egg + dominant sperm = ½ x ½ = ¼ Chance of dominant egg + recessive sperm = ½ x ½ = ¼ chance of hetero child is ¼ + ¼ = ½
Use → trihybrid AaBbCc x AaBbCc ? chance of AabbCC
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Extensions:
found that Mendel’s laws were not perfect, in fact, he was lucky that he choose peas which have simple inheritance (except pod shape)
Incomplete dominance = 1 allele is not completely dominant over the other thus, there is a 3rd phenotype, intermediate, ex.Carnations/snapdragonsp. 271
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Codominance
= both alleles are expressed Level of expression varies at different levels Tay-sachs
- molecular level – looks codominant - biochemical level – looks like incomplete→ an
intermediate level of lipid-metabolizing activity - organismal level – hetero’s =symptom free, homo rec.
have
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Multiple Alleles = genes that have more than
2 alleles Ex. blood groups A, B, AB, O
(surface carbohydrates) blood type is the antigen present
on the RBC, p. 273
also contains Rh factor, + or -mendelian
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Pleiotropy = a single gene has multiple affects
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Epistasis = one gene effects the expression of another gene, Ex. pigments in mice
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Polygenic inheritance = many genes affect the same trait Ex. skin color, very dark to very light, p. 274
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Environment plays an important part in gene expression, how much is not exactly known, nature vs. nurture argument
Norm of Reaction = The phenotypic range for a genotype, p.275
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Humans Pedigree – family tree that shows inheritance over many
generations, shows patterns = male, O = female, ●= diseased, ○= non-diseased
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- usually caused by a defective protein - heterozygotes are carriers Cystic Fibrosis – most common, membrane protein
that controls Cl traffic, causes increase mucus in lungs
Tay-Sachs – higher in “Jews”, can’t break down a type of lipid
Sickle cell – substitution in one hemoglobin, causes RBC to sickle and clog, carriers are immune to malaria, p. 278
Consanguinity – mating with relatives
Recessive human disorders
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– rarer than recessive, not masked Achondroplasia – type of dwarfism Huntington's – late acting degeneration of
nervous system, chromosme #4
Multifactoral disorder- many different factors effect, ex. Heart disease, diabetes, cancer
Dominant inherited disorders
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Genetic testing and counseling 1) carrier recognition - help make decisions about
children
2) fetal tests amniocentesis – take amniotic fluid from around fetus, do
karyotype chorionic villus sampling (CVS) – take villi, do karyoptype, fast,
earlier, more risk, p. 280 ultrasound – imagery using sound waves, look for physical
problems fetoscopy – fiber optics
3) Newborn screening – ex. PKU
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Ch 15
Chromosomes and Inheritance
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Chromosome theory of inheritance: genes are located on
chromosomes, they segregate and independently assort
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T.H.Morgan
rediscovered Mendel’s work 1900’s, specific genes on specific chromosomes?
work on fruit fly, why? fast repro., easy to handle, 4 pairs of chromosomes (1 pair
are sex chromosomes) gene symbol is based on the mutant or recessive
ex. curly is recessive = Cy, if normal then Cy+ wild type is the type seen in nature = +
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Experiment- p 289 white eyed male (♂)→
crossed with a red eyed female (♀)→ in F2 only males had white eyes ?→ eye color and sex are linked
Linked = when genes are on the same chromosome, so they are inherited together (? crossing over), no independent assortment
Sex linked = located on a sex chromosome, p. 290, ex. Hemophilia
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Recombination = offspring with different combinations of traits than the
parents, caused by crossing over or mutations Parental types – same phenotype as a parent Recombinants – differ from parents, *p. 293-294
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Sturtevant made chromosome maps
find relative distance between farthest genes, find distance of an end and a middle, fill in other genes
double crossovers can occur too, throw # off a little 50% frequency of recombinants = 2 genes on different c’somes use recombination frequency to determine distance of genes 1% = 1 map unit = 1 cm (centimorgan), ex
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Heterogametic- produces 2 kinds of gametes Homogametic- produces only 1 kind of gamete Humans - ♂ is XY, ♀ is XX, other animals differ few genes on the Y, thus most sex-linked diseases are
seen in males b/c on the X (not masked), females often carriers, p. 290
X-inactivation = females inactivate one of their X’s, why? inactive X becomes a Barr body = Lyon hypothesis, p.291
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Nondisjunction –division error, chromosomes don’t separate, mitotic and meiotic, p. 297
Aneuploidy = having an abnormal # of chromosomes Trisomy – 3 copies of 1 chromosome Monosomy – 1 copy of the chromosome
Polyploidy = more than normal chromosome set Triploidy – 3 chromosome sets (3N)
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Mutations Deletion – chromosome loses a piece, p. 298 Duplication – double of gene Inversion – chromosome is in reverse Translocation – gene moves to another chromosome →caused by UV light, chemicals or random →effects can be silent, lethal or in between
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Down Syndrome – trisomy 21, female age makes more frequent?
Klinefelters – XXY, XXXY male, sterile, some female features
XYY – male, usually normal, XXX- female, usually normal Turner syndrome – X, female, sterile, few sexual features Some effects of chromosomal abnormalities depend on
what parent inherited by (genomic imprinting, p.300) - prader–willi – deletion of part of #15 from dad, retardation - angelman – deletion of same part of # 15 from mom, motor
issues