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BASIC GENETICS REVIEW
Genetics
 study of heredity (inheritance of traits).
Father of
 Austrian monk born in 1822.
genetics
 high school teacher and gardener.
Gregor Mendel
 studied inheritance in garden pea plants.
What did
Mendel
know?
What did
Mendel do?
 flowers have male and female parts.
 produce pollen and egg cells.
 fertilization: male and female sex cells join
to form new cell
 peas are self-pollinating.
 true-breeds: produce offspring like
themselves when s-p.
 crossed true-bred peas: fertilized one plant with
pollen from another.
 hybrids: offspring (F1) of crosses between
parents (P) with different traits.
 true-bred short male(tt) X true-bred tall mom(TT)
 hybrid offspring = Tt
Trait
 specific characteristic varies betw/ individuals.
Heritable trait  trait that can be passed on to offspring.
What did
Mendel
conclude?
Principle of
dominance
What did
Mendel do
next?
Law of
Segregation
 factors that control traits are passed from
generation to generation.
 genes: chemical factors that determine traits
 alleles: contrasting forms of a gene.
 e.g. gene for “tallness” or “shortness”
 some alleles are dominant; some are recessive.
 dominant allele: if present this trait is seen.
 recessive allele: seen if dominant allele is
not present.
 e.g. trait = plant height
tt = short plant
Tt or TT = tall plant
 crossed hybrid pea plants (Tt X Tt)
 recessive trait not seen in F1.
 recessive trait seen in F2!
 alleles separate when gametes (sex cells) are
formed (during meiosis).
 SO…..4 possible combinations
of gametes during fertilization
Genetics and Probability
Probability
 likelihood that a particular event will occur.
 each toss is separate: heads = ½ each toss.
Genetics?
 probability can be used to predict outcomes of
genetic crosses.
 predictions are averages not absolutes
 larger sample sizes closer to predictions.
Punnett Square  diagram used to record possible gene combinations
from genetic crosses.
Vocabulary
genotype: genetic makeup: write out alleles:“3Tt: 1tt”
phenotype: physical characteristics: “3 tall: 1 short”
homozygous: 2 identical alleles for a trait: TT or tt
heterozygous: 2 different alleles for a trait: Tt
Monohybrid
cross
example
 cross 2 heterozygous tall plants (Tt x Tt)
 Offspring genotypes:
 ¼ homozygous tall (TT);
 2/4 heterozygous tall (Tt);
 ¼ homozygous short (tt)
 offspring phenotypes: ¾ tall; ¼ short
Dihybrid cross  cross true-bred pea plants with 2 traits
Example
 dominant: round(R); yellow(Y)
 recessive: wrinkled (r); green (y)
Parents (P)
Filial (F1)
 RRYY (round/yellow) X rryy(wrinkled/green)
 offspring: RrYy (hybrid)
 cross 2 F1 plants RrYy X RrYy
 possible gametes (FOIL): RY, Ry, rY, ry
2nd generation
(F2)
Principle of
Independent
Assortment
Summary of
Mendel’s
Principles
 genes for different traits segregate (separate)
independently during formation of gametes.
 creates variation in offspring.
1.
Inheritance determined by genes passed from parents to
offspring.
2.
Alleles of genes can be are dominant or recessive.
3.
Somatic cells have 2 copies of each gene (1 from mom; 1
from dad).
4. Genes segregate independently of each other (RrYy) =
RY, Ry, rY, ry.
OTHER PATTERNS OF INHERITANCE
It’s not always so simple……
Incomplete
Dominance
Example
 no allele is completely dominant.
 heterozygote is blend of 2 homozygous phenotypes.
 red carnation X white carnation = pink carnation.
P genotype: ___X___
F1 genotype: ______ phenotype: ______
Codominance
Example
 both alleles dominant: both show up in phenotype of offspring.
 red cow X white cow = roan cow (red and white).
P genotype: ___X___
F1 genotype: ______ phenotype: ______
Multiple
Alleles
Example
Blood type
 more than 2 forms of a gene
 more than one allele is dominant.
 more than 2 phenotypes in population.
 alleles: dominant = A B; recessive = O
possible blood types
Example
P phenotype: ____X____
F1 genotype: ________
F1 phenotype: _______
genotype
AA
AO
BB
BO
AB
OO
phenotype
A
A
B
B
AB*
O*
Sex linked
genes
 Gene located on X or Y chromosome (more commonly X…the Y
chromosome is tiny)
Example
1. Colorblindness (recessive gene) located on X chromosome
 possible genotypes: XC Xc , XCXC, XcXc, XCY, XcY
 to be colorblind females need XcXc, males only need XcY
P genotype: ____X_____
F1 genotype: ____________ F1 phenotype: __________
2. Hemophilia carried on X chromosome
 possible genotypes:XH Xh, XHXH, XhXh, XHY, XhY
P genotype: ____X_____
F1 genotype: ____________ F1 phenotype: __________
Polygenic
Traits
Epigenetics
(epi = above)
 traits controlled by 2 or more genes.
 human skin color, eye color, height.
 AABBCCDD X aabbccdd…..many combinations.
 MANY intermediate phenotypes possible
 traits determined by interaction of genes and environment.
 Genes are the PLAN.
 ENVIRONMENT determines how plan unfolds.
 smoking, diet, stress, prenatal nutrition, trauma, etc. can
affect which genes are turned on or off.
 chemical “tags” on DNA or change in histones.
 may be inherited over multiple generations.
 can change over a lifetime.
MEIOSIS
Mitosis
Review
Meiosis
WHY?
 1 cell division results in 2 genetically identical daughter cells
 makes cells for growth, repair, asexual reproduction
 occurs in somatic (body) cells
 starts and results in diploid cells (2n)
 2 sets of homologous chromosomes
 1 set from mom; 1 set from dad
 2 cell divisions results in 4 daughter cells with ½ the number
of chromosomes
 makes gametes (sex cells)
 occurs in sex organs of animals, plants, fungi, etc.
 starts with diploid cell (2n)
 results in 4 haploid cells (n)
Chromosomes  different number for each species.




humans: 2N = 46; 1N = 23
dog: 2N = 78; 1N = 39
shrimp: 2N = 254; 1N = 127
pineapple: 2N = 50; 1N = 25
Crossing over  homologous chromosomes
exchange parts during Prophase I
of meiosis
 increases genetic variation
Gene Linkage
and Mapping  genes can be mapped to specific locations on chromosomes.
 figured out with fruit flies.
 linked traits are on the same chromosome.
 linked genes not always inherited together
 chromosomes cross-over during meiosis
 linkage maps show relative location of genes
 lower cross-over frequency = closer
 cross-over frequencies: A-B=8%
B-C=10%
C-A=2%