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Genetics
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Mendel’s work
• Bred pea plants
• Cross-pollinated true breeding parents (P) then raised the seed & observed traits (F1)
• Allowed offspring to cross-pollinate & observed next generation (F2)
• In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties.
• The true-breeding parents are the P generation and their hybrid offspring are the F1 generation.
• Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.
What did Mendel’s findings mean?
• Traits come in alternative versions
o Purple vs. white flower color (Alleles)
� Different alleles vary in the sequence of
nucleotides at the specific locus of a gene
Traits are inherited as discrete units
• For each character, an organism inherits 2 alleles, 1
from each parent
• Diploid organism inherits 1 set of chromosomes from
each parent
o Homologous chromosomes
o Diploid = 2 sets of chromosomes
o Like having 2 different editions of an encyclopedia
What did Mendel’s findings mean?
• Some traits mask others
o Purple & white-flower colors are separate traits that do not blend
� Purple x white ≠ light purple
� Purple masked white
o Dominant allele
� Fully expressed
o Recessive allele
� No noticeable effect
� Non-functional protein
P = parents
F = filial
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Genotype vs. phenotype
• Difference between how an organism “looks” & its genetics
o Description of an organism’s trait = phenotype
o Description of an organism’s genetic makeup = genotype
Explain Mendel’s results using: Dominant & Recessive and Phenotype & Genotype.
Phenotype vs. Genotype
• 2 organisms can have the same phenotype
but have different genotypes.
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Mendel chose peas wisely
• Pea plants are good for genetic research
o Available in many varieties with distinct heritable features with different variations
� Flower color, seed color, seed shape, etc.
o Mendel had strict control over which plants mated with which
� Each pea plant has male & female structures
� Pea plants can self-fertilize
� Mendel could also cross-pollinate plants: moving pollen from one plant to another
Mendel chose peas luckily
• Pea plants are good for genetic research
o Relatively simple genetically
� Most characters are controlled by a single gene
� Each gene has only 2 alleles, one of which is completely dominant to the other
Mendel’s Law of Heredity (#1)
• Law of Segregation
o When gametes (eggs & sperm) are produced during meiosis, homologous chromosomes
separate from each other
o Each allele for a trait segregates (is packaged) into a separate gamete
o What meiotic event creates the law of segregation? _________________________
o And Mendel didn’t even know DNA or genes existed!
Test Cross
• It is possible to predict the genotype of an organism with a
dominant phenotype?
• Cross-breed the dominant phenotype — unknown genotype — with
a homozygous recessive (pp) to determine the identity of the
unknown allele.
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Monohybrid cross
• Some of Mendel’s experiments
followed the inheritance of single
characters
o Flower color
o Seed color
Dihybrid cross
• Some of Mendel’s experiments
followed the inheritance of 2
different characters
o Seed color & seed shape
• Wrinkled seeds in pea plants
with two copies of the recessive
allele are due to the
accumulation of
monosaccharides and excess
water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry.
• Both homozygous dominants and heterozygotes produce enough enzymes to convert all the
monosaccharides into starch and form smooth seeds when they dry.
Mendel’s Law of Heredity (#2)
• Law of Independent Assortment
o Each pair of alleles — for each trait — segregates into
gametes independently = independent assortment.
o 4 classes of gametes — YR, Yr, yR, yr — are
produced in equal amounts.
Review: Mendel’s Laws of Heredity
• Law of Segregation
o Monohybrid cross = single trait
o Each allele for a trait segregates (is packaged) into separate gametes
• Established by Meiosis 1
• Law of Independent Assortment
o Dihybrid (or more) cross
• 2 or more traits
o Each pair of alleles for each trait segregates into gametes independently
• Established by Meiosis 1
Some interesting historical facts
• While Mendel was acknowledged by his contemporaries as an outstanding plant breeder, his
revolutionary genetics work was overlooked for 34 years.
• Mendel published “Experiments on Plant Hybrids” in 1865. In 1900, 16 years after Mendel’s
death, a number of scientists independently rediscovered his work.
• Charles Darwin, a contemporary of Mendel, proposed that evolution by natural selection was
dependent on variation in the population, but Darwin was unable to propose a mechanism for how
this variation was transmitted.
• The key was Mendel’s work and nearly a century after Mendel published his findings historians
found a copy of Mendel’s paper in Darwin’s study. He presumably never read it.
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Chapter 14: Probability & Genetics
Mendel’s laws: Reflect same laws of probability that apply to tossing coins or rolling dice.
• Segregation
• Independent assortment
Probability & Genetics
• Calculating probability of making a specific gamete is just like calculating the
probability in flipping a coin
o Probability of tossing heads? 50%
o Probability making a P gamete…
• Outcome of 1 toss has no impact on the outcome of the next toss
o Probability of tossing heads each time? 50%
o Probability making a P gamete each time? 50%
Calculating Probability
Rule of multiplication
• Chance that 2 or more independent events will occur together
o probability that 2 coins tossed at the same time will land heads up
½ x ½ = ¼
o probability of Pp x Pp → pp
½ x ½ = ¼
Calculating dihybrid probability
• Rule of multiplication also applies to dihybrid crosses
o Heterozygous parent — YyRr
o Probability of producing yyrr?
� probability of producing y gamete = 1/2
� probability of producing r gamete = 1/2
� probability of producing yr gamete =
½ x ½ = ¼
� probability of producing a yyrr offspring =¼ x ¼ = 1/16
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Rule of Addition
• Chance that an event can occur 2 or more
different ways
o Sum of the separate probabilities
o Probability of Pp x Pp → Pp
Extending Mendelian genetics
• Mendel worked with a simple system.
o Peas are genetically simple.
o Most traits are controlled by a single gene.
o Each gene has only 2 alleles, 1 of which is completely dominant to the other.
• The relationship between genotype & phenotype is rarely simple.
Incomplete dominance
• Heterozygotes show an
intermediate phenotype
o RR = red flowers
o rr = white flowers
o Rr = pink flowers
� make 50% less
color
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Co-dominance
• 2 alleles affect the phenotype in separate, distinguishable ways.
o ABO blood groups
o 3 alleles = IA, I
B, and i.
� Both the IA & I
B alleles are dominant to the i allele
� IA & I
B alleles are codominant to each other.
o Determines presences of oligosaccharides on the surface of red blood cells.
Blood compatibility
• Matching compatible blood groups is critical for blood transfusions.
• A person produces antibodies against foreign blood factors = oligosaccharides.
o If donor’s blood has an A or B oligosaccharide that is foreign to the recipient, antibodies in
the recipient’s blood will bind to the foreign molecules.
o Cause the donated blood cells to clump together & can kill the recipient.
Pleiotropy
• Most genes are pleiotropic
o One gene affects more than one
phenotypic character
� Wide-ranging effects due to a
single gene:
� Dwarfism (achondroplasia)
� Gigantism (acromegaly)
Epistasis
• One gene masks another
o Coat color in mice = 2 genes
� Pigment (C) or no pigment
(c)
� More pigment (black=B) or
less (brown=b)
� cc = albino, no matter B
allele
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Polygenic inheritance
• Some phenotypes determined by additive
effects of 2 or more genes on a single
character
o Phenotypes on a continuum
o Human traits
� skin color
� height
� weight
� eye color
� intelligence
� behaviors
Nature vs. Nurture
• Phenotype is controlled by both
environment & genes
• A single tree has leaves that vary in size,
shape & color, depending on exposure to
wind & sun
• For humans, nutrition influences height,
exercise alters build, suntanning darkens
the skin, and experience improves
performance on intelligence tests
• Even identical twins — genetic equals —
accumulate phenotypic differences as a
result of their unique experiences
Thomas Hunt Morgan
• Morgan was an embryologist at Columbia
University
� 1st to associate a specific gene with a
specific chromosome
� Drosophila breeding
• prolific
• 2 week generations
• 4 pairs of chromosomes
• XX=female, XY=male
Morgan’s first mutant…
• Wild type fly = red eyes
• Morgan discovered a mutant white-eyed male
o Trace a gene for eye color to a specific
chromosome
• Discovery of Sex
Linkage
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Genes on sex chromosomes
• Y chromosome
o SRY: sex-determining region
� Master regulator for maleness
� Turns on genes for production of male hormones
• Pleiotropy!
• X chromosome
o Other traits beyond sex determination
� Hemophilia
� Duchenne muscular dystrophy
� Color-blind
Sex-linked traits summary
• X-linked
o Follow the X chromosomes
o Males get their X from their
mother
o Trait is never passed from father
to son
• Y-linked
o Very few traits
o Only 26 genes
o Trait is only passed from father
to son
o Females cannot inherit trait
X-inactivation
• Female mammals inherit two X
chromosomes
o One X becomes inactivated during embryonic development
• Condenses into compact object = Barr body
• X-inactivation & Tortoise shell cat
o 2 different cell lines in cat
Male pattern baldness
• Sex influenced trait
o Autosomal trait influenced by sex hormones
� Age effect as well: onset after 30 years old
o Dominant in males & recessive in females
� B– = bald in males; bb = bald in females
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Chapter 14: Studying Inheritance in Humans
Pedigree Analysis
• Pedigree analysis reveals Mendelian patterns in human inheritance.
• Data mapped on a family tree.
Recessive diseases
• The diseases are recessive because the allele codes for either a malfunctioning protein or no
protein at all.
o Heterozygotes (Aa)
� Carriers
� Have a normal phenotype because one “normal” allele produces enough of the
required protein.
Heterozygote Crosses
• Heterozygotes as carriers of recessive alleles.
Cystic Fibrosis
• Primarily whites of European descent
• Strikes 1 in 2500 births
o 1 in 25 whites is a carrier (Aa)
• Normal allele codes for a membrane protein that transports Cl- across cell membrane
o Defective or absent channels cause high extracellular levels of Cl-
o Thicker & stickier mucus coats around cells
o Mucus build-up in the pancreas, lungs, digestive tract & causes bacterial infections
• Without treatment children die before 5;
• With treatment can live past their late 20s
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Tay-Sachs
• Primarily Jews of eastern European (Ashkenazi) descent & Cajuns
o Strikes 1 in 3600 births
� 100 times greater than incidence among non-Jews or Mediterranean (Sephardic)
Jews
o Non-functional enzyme fails to breakdown lipids in brain cells.
� Symptoms begin few months after birth.
� Seizures, blindness & degeneration of motor & mental performance.
� Child dies before 5yo.
Sickle cell anemia
• Primarily Africans
o Strikes 1 out of 400 African Americans.
o Caused by substitution of a single amino acid in hemoglobin.
o When oxygen levels are low, sickle-cell hemoglobin crystallizes into long rods.
� Deforms red blood cells into sickle shape.
� Sickling creates pleiotropic effects = cascade of other symptoms.
o Substitution of one amino acid in polypeptide chain.
Sickle Cell Phenotype
• 2 alleles are codominant
o Both normal & abnormal hemoglobins are synthesized in heterozygote (Aa)
o Carriers usually healthy, although some suffer some symptoms of sickle-cell disease under
blood oxygen stress
• Heterozygote advantage
o Sickle cell frequency
� High frequency of heterozygotes is unusual for allele with severe detrimental effects
in homozygotes.
• 1 out of 400 African Americans
� Suggests some selective advantage of being heterozygous.
o Malaria
� Single-celled eukaryote parasite spends part of its life cycle in red blood cells.
o In tropical Africa, where malaria is common:
� Homozygous normal individuals die of malaria.
� Homozygous recessive individuals die of sickle cell anemia.
� Heterozygote carriers are relatively free of both.
o High frequency of sickle cell allele in African Americans is vestige of African roots.
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Genetics & culture
• Why do cultures have a taboo against incest?
o Laws or taboos forbidding marriages between close relatives are fairly universal.
• Fairly unlikely that 2 carriers of same rare harmful recessive allele will meet & mate.
o But mating between close relatives increase risk.
� Consanguineous
o Individuals who share a recent common ancestor are more likely to carry same recessive
alleles.
Chapter 15: Chromosomal Abnormalities
Chromosomal Abnormalities
• Incorrect number of chromosomes
o Nondisjunction
� Chromosomes don’t separate properly during meiosis.
o Breakage of chromosomes
� Deletion
� Duplication
� Inversion
� Translocation
Nondisjunction
• Problems with the meiotic spindle cause errors in daughter cells.
o Tetrad chromosomes do not separate properly during Meiosis 1.
o Sister chromatids fail to separate during Meiosis 2.
• Baby will have wrong chromosome number
o Trisomy
� Cells have 3 copies of a chromosome.
o Monosomy
� Cells have only 1 copy of a chromosome.
• Alteration of Chromosome Number
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Human Chromosome Disorders
• High frequency in humans
o Most embryos are spontaneously aborted
o Alterations are too disastrous
o Developmental problems result from imbalance
• Certain conditions are tolerated
o Upset the balance less = survive
o Characteristic set of symptoms = syndrome
• Down Syndrome
o Trisomy 21
� 3 copies of chromosome 21
� 1 in 700 children born in U.S.
o Chromosome 21 is the smallest human chromosome.
� But still severe effects.
o Frequency of Down syndrome correlates with the age
of the mother.
Sex chromosomes
• Human development more tolerant of wrong numbers in sex chromosome
• But produces a variety of distinct conditions in humans
o XXY = Klinefelter’s syndrome male
o XXX = Trisomy X female
o XYY = Jacob’s syndrome male
o XO = Turner syndrome female
• Klinefelter’s syndrome
o XXY male
o 1 in every 2000 live births
o Have male sex organs, but are sterile.
o Feminine characteristics, tall, normal intelligence.
• Jacob’s syndrome male
o XYY Males
o 1 in 1000 live male births
o Extra Y chromosome.
o Somewhat taller than average, more active.
o Slight learning disabilities, delayed emotional immaturity.
o Normal intelligence, normal sexual development.
• Trisomy X
o XXX
o 1 in every 2000 live births.
o Produces healthy females. Why? __________________
• Turner syndrome
o Monosomy X or X0
o 1 in every 5000 births
o Varied degree of effects
� Webbed neck
� Short stature, immature sterile females.
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Changes in Chromosome Structure
