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Transcript of Genetics
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M S . N A P O L I T A N O & M R S . H A A SC P B I O L O G Y
GENETICS
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INTRODUCTION TO GENETICS
• Every living thing inherits characteristics from its parent(s)
• Genetics – the scientific study of heredity
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GREGOR MENDEL & HIS PEA PLANTS
• Studied genetics of garden peas
• Started with true-breeding plants• If allowed to self-pollinate, would create
identical offspring• One only tall, one only short, one green, one
yellow, etc. (called traits)
• Self-pollinated the plants by hand• Controlled breeding• Called “cross-pollination”
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GENES & DOMINANCE
• Traits – specific characteristics that vary from one individual to another• Freckles, eye color, right vs. left handed, etc.
• Original cross – P generation• Mendel = true breeding plants
• Offspring = F1 generation
• Offspring of F1 = F2, etc.
• Hybrids – parents have different traits
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MENDEL’S PEA PLANTS
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MENDEL’S CONCLUSIONS
1. Inheritance determined by factors that are passed down from one generation to the next• Genes – chemical factors that determine traits• Alleles – different forms of a gene• Ex: brown vs. blue eyes, curly vs. straight hair, etc.
2. Principle of dominance – some alleles are dominant and others are recessive • Dominant (T) will always be expressed • Recessive (t) only expressed if dominant allele is absent
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MENDEL’S SECOND CROSS
• Mendel not satisfied – what happened to recessive alleles?
• Crossed F1 generations with themselves• Made F2 generations
• Results – traits from recessive alleles came back in ~¼ of plants
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SEGREGATION
• Segregation – separation of alleles• Happens during meiosis during formation of
gametes
Mendel’s conclusions:• When F1 plants produce gametes, alleles separate
so that each gamete carries only a single copy of the gene• F1 produces two types of gametes – one tall allele
& one short allele
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EXAMPLE OF SEGREGATION
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GENETICS & PROBABILITY
• Probability – the likelihood that a particular event will occur
• Probability is used to predict the outcomes of genetic crosses
• Punnett square – used to predict genetic variations• Homozygous – 2 identical alleles for a particular trait • TT or tt• Aka “true breeding”
• Heterozygous – 2 different alleles for a particular trait • Tt• Aka “hybrid”
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GENOTYPE & PHENOTYPE
• Phenotype – physical characteristics• Ex: Plant phenotype is tall or short
• Genotype – genetic makeup• Ex: Plant genotype is TT, Tt, or tt
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PUNNETT SQUARE – PEA SEED COLOR
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PUNNETT SQUARE – PEA SEED COLOR
1. What is the genotype of both parents?2. What is the phenotype of both parents?3. Are the parents homozygous or heterozygous?
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PROBABILITY & SEGREGATION
• In previous Punnett square, ¼ seeds are green and ¾ seeds are yellow • 3 dominant, 1 recessive• Ratio = 3:1
• In previous Punnett square, ¼ seeds are YY, ¼ seeds are yy, and 2/4 (1/2) seeds are Yy• Ratio = 1:2:1
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PROBABILITIES & AVERAGES
• Need a large sample size!• Probability cannot predict outcome of a single
event• Ex: In theory, if you flipped a coin twice, you
would get 1 heads and 1 tails – but won’t always happen• The larger the number of offspring, the closer the
resulting numbers will get to expected values
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INDEPENDENT ASSORTMENT
• Does segregation of one pair of alleles affect the segregation of another pair of alleles?
• Mendel – crossed true-breeding, round, yellow peas (RRYY) with true-breeding, wrinkled, green peas (rryy)
• Offspring: all round, yellow peas
• Therefore: Yellow & round dominant to wrinkled & green
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INDEPENDENT ASSORTMENT
• Mendel – now crossed new plants (RRYY x rryy)• 556 total seeds produced• 315 seeds round, yellow• 32 seeds wrinkled, green• 209 seeds had combinations not found in either parent
• Independent assortment – alleles segregate independently• In this example, color segregates independent of shape• In other words – the inheritance of one gene does not
affect the inheritance of the other
• Mendels results close to 9:3:3:1 ratio
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MENDEL’S SECOND CROSS
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GENETIC DISEASES
• Tay Sachs• Recessive • Progressively destroys nerve cells
• Sickle Cell Anemia• Recessive• Affects hemoglobin shape in red blood cells• Cannot get enough oxygen to cells
• Huntington’s Disease• Dominant (ah!)• Neurodegerative• Cognitive decline• Symptoms do not arise until ~35-44 years old
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EXCEPTIONS
• Genetics is very complicated in many organisms! • Many genes have more than 2 alleles
• Some alleles are neither dominant nor recessive
• Examples: • Incomplete dominance• Codominance
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INCOMPLETE DOMINANCE
• Example: A white & red flower cross, making pink• One allele is not completely dominant over
another• Traits “blend”
Parents:• 1 WW (white)• 1 RR (red)
Offspring• 4 RW (pink)
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CODOMINANCE
• Both alleles contribute to phenotype• Both traits fully expressed• Ex: In chickens, black feathers is codominant with
white feathers
Parents• 1 CBCB
• 1 CWCW
Offspring• 4 CBCW
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MULTIPLE ALLELES
• Genes with more than 2 alleles• More than 2 alleles possible in a population (Not
per organism! Remember – 1 from mom, 1 from dad)• Ex: Rabbit coat color
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SEX-LINKED GENES
• Sex-linked genes - genes located on sex chromosomes
• Mostly found on the X chromosome• Much bigger in size• Over 100 sex-linked diseases on X chromosome
• Males will express X-linked alleles because they have only 1 X chromosome!• Doesn’t matter if dominant or recessive
• In Punnett square, only the X chromosome gets the gene!• Ex: Parents will be XBXb and XbY
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SEX-LINKED PUNNETT SQUARE
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SEX-LINKED DISEASES
• Colorblindness• Inability to distinguish certain colors• Red-green colorblindness = 1 in 10 males in US• Only 1 in 100 females are colorblind (why?)
• Hemophilia• Blood-clotting protein missing, can bleed to death easily• 1 in 10,000 males have hemophilia
• Duchenne Muscular Dystrophy• Progressive weakening & loss of skeletal muscle• 1 in 3,000 males
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BLOOD TYPING
• Use ABO and Rh blood groups• Rh = either + or –, + is dominant
• 3 alleles: IA, IB, & i• IA and IB are codominant, and both dominant to
recessive i• Possible blood types:• IA IA – Type A• IA i – Type A• IB IB – Type B• IB i – Type B• IA IB – Type AB• i i – Type O
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BLOOD TYPE PUNNETT SQUARES
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POLYGENIC TRAITS
• Polygenic traits - traits controlled by 2+ genes• Ex: Human skin color (4 genes)
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HUMAN CHROMOSOMES
• Recall: Humans have 46 chromosomes
• Karyotype – picture of human chromosomes lined up in order
• 2 chromosomes = sex chromosomes• Determine gender• Females = XX• Males = XY
• Other 44 chromosomes = autosomes
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HUMAN KARYOTYPE
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THE “MAGIC” MOMENT
• Males & females born in ~50:50 ratio• All human egg cells carry a single X chromosome,
but only half of all human sperm cells carry a single X chromosome. The others carry a single Y chromosome.• Females = (23,X)• Males = (23,Y)
X
X Y
X+
+
=
=
Female
Male
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PEDIGREE CHARTS
• Show relationships within a family
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PEDIGREE EXAMPLE