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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