genetics_ottolini_biology
TRANSCRIPT
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Genetics
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Mendel &
Simple Patterns of Inheritance
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Human Traits
• Your physical traits resemble those of your parents.
• Heredity = the passing of traits from parents to offspring
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Gregor Johann Mendel• Mendel was an Austrian
monk who conducted breeding experiments with garden peas.
• Developed rules which accurately predict patterns of heredity
• Genetics = the branch of biology that focuses on heredity
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Useful Features in Peas 1. Many traits that have two clearly different forms (no
intermediate forms) 2. Mating can be easily controlled
– Self-fertilization– Cross-fertilization
3. Small, grows easily, matures quickly & produces many offspring
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Self-Pollination
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Cross-Pollination Cross= mate or breed two
individuals
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Nature’s Pollinators
• Insects (bees)
• Vertebrates (birds & bats)
• Wind
• Water
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Mendel’s Observations: Traits are Expressed as Simple Ratios
• First experiments involved monohybrid crosses
• Monohybrid cross = a cross that involves one pair of contrasting traits
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Mendel’s Experiments: Step #1• Self-pollinate each pea
plant for several generations
• True-breeding= all offspring display only one form of a particular trait
• P generation = parental generation, the first two individuals crossed in a breeding experiment
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Mendel’s Experiment: Step #2• Cross-pollinate the P
generation plants with contrasting forms of a trait
• F1 generation = the first filial generation, the offspring of the P generation
• Characterize and count plants
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Mendel’s Experiment: Step #3• Allow the F1 generation
to self-pollinate
• F2 generation = the second filial generation, the offspring of the F1 generation plants
• Characterize and count plants
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Mendel’s Results• F1 generation showed only one form of the trait
(e.g. purple flowers) – The other form of the trait disappeared (e.g. white
flowers) – Reappeared in the F2 generation
• 3:1 ratio of the plants in the F2 generation
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Expressing Ratios: Mendel’s F2 Generation
705 purple-flowered plants; 224 white-flowered plants
• Reduce the ratio to its simplest form: 705/224 = 3.15 Purple 224/224= 1 White– Ratio can be written in a few different ways:
• 3:1• 3 to 1• 3/1
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Are offspring simply a blend of their parents’ characteristics?
• Before Mendel’s experiment: this was the theory (blending hypothesis) – Ex. Tall x Short = Medium Mendel’s Conclusion– Offspring have two genes for each trait (one gene
from each gamete)
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Mendel’s Hypotheses:Hypothesis #1
• For each inherited trait, an individual has two copies of the gene – One gene from each parent
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Mendel’s Hypotheses:Hypothesis #2
• There are alternative versions of genes
• Alleles = different versions of genes; an individual receives one allele from each parent
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Mendel’s Hypotheses:Hypothesis #3
• When two different alleles occur together, one may be completely expressed, while the other may have no observable effect on the organism’s appearance.
• Dominant = the expressed form of a trait
• Recessive = the trait which is not expressed when the dominant form of the trait is present
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Mendel’s Hypotheses: Hypothesis #4
• When gametes are formed, the alleles for each gene in an individual separate independently of one another. – Gametes = one
allele for each inherited trait
– Fertilization – each gamete contributes one allele
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Representing Alleles
• Letters are often used to represent alleles
• Dominant Alleles = capitalize the first letter of the trait – Purple flowers = P
• Recessive Alleles = first letter of the dominant trait, in lowercase – White flowers = p
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Combinations of Alleles• Homozygous = if two alleles of a particular
gene present in an individual are the same
• Heterozygous = if two alleles of a particular gene present in an individual are different
• Example: yellow peas (Y) are dominant to green peas (y) – Homozygous for yellow peas = YY– Heterozygous for yellow peas = Yy
yy YY Yy
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Heterozygous Alleles - Ff• Only the dominant allele
is expressed – Recessive allele is
present but not expressed
• Example: Freckles – Freckles, F = Dominant
allele – No Freckles, f =
Recessive allele – Individuals who are
heterozygous for freckles (Ff) have freckles
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Genotype vs. Phenotype• Genotype = the set
of alleles that an individual has – Uppercase letter is
always written first
• Phenotype = the physical appearance of a trait (determined by which alleles are present)
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Laws of Heredity: The Law of Segregation
• Law of Segregation= the two alleles for a trait segregate (separate) when gametes are formed
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Laws of Heredity:The Law of Independent Assortment
• Law of Independent Assortment = the alleles of different genes separate independently of one another during gamete formation– Example: alleles for plant height separate independently of
the alleles for flower color – Applies to:
• Genes on different chromosomes• Genes that are far apart on the same chromosome
http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html
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Punnett Squares & Probabilities
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Punnett Squares: Predicting Expected Results in Crosses
• Punnett Square = a diagram that predicts the expected outcome of a genetic cross by considering all possible combinations of gametes in the cross
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Punnett Squares• Possible gametes from one parent are written along the top of
the square• Possible gametes from the other parent are written along the
left side of the square • Letters inside the boxes = possible genotypes of the offspring
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Monohybrid Crosses
• Homozygous for yellow seeds (YY) x Homozygous for green seeds (yy) =– Only yellow
heterozygous offspring (Yy)
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Monohybrid Crosses • Heterozygous for
yellow seeds (Yy) x heterozygous for yellow seeds (Yy) =– ¼ YY (Homozygous
dominant) – 2/4 Yy
(Heterozygous) – ¼ yy (Homozygous
recessive)
• 1 YY: 2 Yy: 1 yy genotypic ratio
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Let’s Solve a Punnett Square!!!
•Inflated pod shape is DOMINANT
• Constricted pod shape is RECESSIVE
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Step #1: Choose a Letter to Represent your Alleles
• Inflated pea pod = I (dominant)
• Constricted pea pod = i (recessive)
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Step #2: Write the Genotypes of the Parents
• Parent #1 = Ii = Heterozygous Inflated
• Parent #2 = Ii = Heterozygous Inflated
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Step #3: Determine the Possible Gametes
• Parent #1 = Ii = I or i
• Parent #2 = Ii = I or i
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Step #4: Enter the possible gametes at the top and side of the
Punnett Square
I i
I
i
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Step #5: Write the alleles from the gametes in the appropriate boxes
I i
I
i
II Ii
Ii ii
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Step #6: Determine the phenotypes of the offspring
I i
I
i
II Ii
Ii ii
Inflated Pods
Inflated Pods Constricted Pods
Inflated Pods
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Step #7: Determine the genotype and phenotype ratios
I i
I
i
II Ii
Ii ii
Inflated Pods
Inflated Pods Constricted Pods
Inflated Pods
Genotype Ratio: 1 II : 2 Ii : 1 ii
Phenotype Ratio: 3 Inflated : 1 Constricted
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Probabilities Can Also Predict the Expected Results of Crosses
• Probability = the likelihood that a specific event will occur – Words – 1 out of 1– Decimal – 1– Percentage – 100%– Fraction – 1/1
Probability = number of one kind of possible outcome ÷ total number of all possible outcomes
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Probability in Pea Plants: Gamete
• Parent = Yy– Can either contribute a yellow allele (Y) or a
green allele (y)
• ½ chance that the gamete will have Y
• ½ chance that the gamete will have y
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Probability of the Outcome of a Cross• Consider the offspring of two parents who
are heterozygous for freckles:– Mom = Ff
• Possible gametes from mom = F or f• ½ probability of mom contributing F• ½ probability of mom contributing f
– Dad = Ff• Possible gametes from dad = F or f• ½ probability of dad contributing F • ½ probability of dad contributing f
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Probability of the Outcome of a Cross
– Mom F + Dad F = FF (1/2) x (1/2) = ¼ probability of FF (freckles)
– Mom F + Dad f = Ff (1/2) x (1/2) = ¼ probability of Ff (freckles)
– Mom f + Dad F = Ff (1/2) x (1/2) = ¼ probability of Ff (freckles)
– Mom f + Dad f = ff (1/2) x (1/2) = ¼ probability of ff (no freckles)
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Probability of the Outcome of a Cross
• Genotype Probabilities:– ¼ FF (freckles)– ¼ Ff + ¼ Ff = ½ Ff (freckles)– ¼ ff (no freckles)
• Phenotype Probabilities: – ¼ FF + ½ Ff = ¾ freckles– ¼ ff = ¼ no freckles
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Pedigrees
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Studying Pedigrees: How are traits inherited???
• Pedigree= a family history that shows how a trait is inherited over several generations
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Why are pedigrees helpful/useful?
• Pedigrees are helpful if couples are concerned that they might be carriers of genetic disorders
Common Genetic Disorders: Angelman SyndromeCanavan DiseaseCharcot-Marie-Tooth DiseaseCri du Chat SyndromeDuchenne Muscular DystrophyFragile X SyndromeGilbert's SyndromeJoubert SyndromeKlinefelter SyndromeKrabbe DiseaseLesch–Nyhan SyndromeMyotonic DystrophyNeurofibromatosisNoonan SyndromePelizaeus-Merzbacher DiseasePhenylketonuriaPorphyriaPrader-Willi SyndromeRetinoblastomaRubinstein-Taybi SyndromeSpina bifidiaSmith-Magenis SyndromeStickler SyndromeTurner SyndromeVariegate PorphyriaVon Hippel-Lindau SyndromeWilson's DiseaseWolf-Hirschhorn SyndromeXXXX SyndromeYY Syndrome
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Pedigree Symbols
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Pedigree Numbers
• Roman numerals (I, II, III, IV) represent = Generations
•Regular numbers (1,2,3,4) represent = Individuals in each generation
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Pedigree Symbols – Male and Female
= Normal Male
= Normal Female
= Male with trait
= Female with trait
Horizontal line between a male and female indicates
MATING/MARRIAGE
Branching vertical lines point to OFFSPRING
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Autosomal vs. Sex-linked Traits
• Autosomal Trait = appears in both sexes equally, alleles appear on the autosomal chromosomes
• Sex-linked Trait = a trait whose allele is located on the X chromosome– Appears mostly in males – Mostly recessive – Female only exhibits the condition if she
inherits two recessive alleles
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Human Chromosomes
• Humans have 46 chromosomes: – 1 pair of sex chromosomes (X and Y)– 22 pairs of autosomes
• Females have 2 X chromsomes (XX).
• Males have an X chromsome and a Y chromosome (XY)
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Karyotypes• A Karyotype is a test to identify and
evaluate the size, shape, and number of chromosomes in a sample of body cells.
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Dominant vs. Recessive Trait
• Autosomal Dominant Traits = each individual with the trait will have a parent with the trait
• Autosomal Recessive Traits = an individual with the trait can have one, two, or neither parent who exhibit the trait
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Recessive Disorder: Albinism• Albinism = a genetic disorder in which the
body is unable to produce an enzyme necessary for the production of melanin (dark color to hair, skin, scales, eyes, and feathers)
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Genetic Disorders = Carriers • Carriers = individuals
who are heterozygous for a recessive inherited disorder, but do not show symptoms of the disorder – Can pass the recessive
allele for the disorder to their offspring
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Red-Green Color Blindness:A Sex-Linked Recessive Disorder
• X-linked recessive disorder
• Among Caucasian individuals:– 8% of males – 0.5% of females
• Difficulty distinguishing between shades of green and red
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Red-Green Color Blindness:A Sex-Linked Recessive Disorder
• Unaffected female = XRXR
• Affected female = XrXr
• Carrier female = XRXr
• Unaffected male = XRY• Affected male = XrY
Males have the disorder more often than females because they only have one X chromosome.
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Red-Green Color Blindness:A Sex-Linked Disorder
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Heterozygous vs. Homozygous
• Homozygous Dominant or Heterozygous individuals = phenotype will show the dominant characteristic
• Homozygous Recessive individuals = phenotype will show the recessive characteristic
***Heterozygous carriers of a recessive mutation will not show the mutation, can produce children who are homozygous for the recessive allele
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Let’s look at a pedigree for polydactyly: a dominant trait
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Let’s look at a pedigree for phenylketonuria (PKU): a recessive
disorder
The trait skips a generation!!
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Complex Patterns of Inheritance
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Traits Influenced by Several Genes• Polygenic Trait = when several
genes influence a trait – Genes can be:
• Scattered along the same chromosome
• Located on different chromosomes
– Independent Assortment = many different combinations in offspring
• Polygenic traits = – Eye color, height, weight, hair
and skin color
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Polygenic Trait – Skin Color
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Intermediate Traits • Incomplete Dominance =
an individual displays a trait that is intermediate between the two parents
• Examples: – Red snapdragon + White
snapdragon = Pink snapdragon offspring
– Curly hair + Straight hair = Wavy haired offspring
• Neither allele is dominant to the other
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Pink Snapdragon - Heterozygous
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Traits With Two Forms Displayed At The Same Time
• Codominance = when two dominant alleles are expressed at the same time, both forms of the trait are displayed
• Example: – Red Horse + White Horse =
Roan Horse
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Roan Horse - Heterozygous
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Traits Controlled By Genes With Three or More Alleles
• Multiple Alleles = Genes with three or more alleles
• Example: ABO blood groups in humans– IA and IB = Dominant to i;
• A & B are two carbohydrates on the surface of red blood cells
– i = Recessive – When IA and IB are present together =
Codominant
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Traits Controlled By Genes With Three or More Alleles
• An individual can only have two of the possible alleles for the gene
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Genotypes of Each Blood Type
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Traits Influenced By The Environment
• Phenotype is affected by environmental conditions
• Hydrangeas (flowers) range from blue to pink based upon the pH of the soil– Acidic soil = blue
flowers – Basic soil = pink
flowers
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Traits Influenced By The Environment• Siamese Cat
– Fur on ears, nose, paws, and tail is darker than the rest of the body
– Dark fur at locations which are cooler than normal body temperature
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Traits Influenced by the Environment: Identical Twins
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Human Examples: Traits Influenced By The Environment
• Height – What can influence
height besides genes?
• Skin Color
• Human Personality
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Traits Caused By Mutation
• Damaged genes/genes which are copied incorrectly – result in faulty proteins
• Mutations are RARE
• Inherited Mutations cause Genetic Disorders
• Many mutations are carried in recessive alleles – Carrier = heterozygous individual
• Carry the recessive allele but do not exhibit the disorder
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Sickle Cell Anemia • Recessive
• Defective hemoglobin – Red Blood Cells – Binds and transports
oxygen
• Sickle-Cell Shape – Rupture-prone– Clotting in blood vessels
• Heterozygote Advantage = protection from malaria
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Hemophilia • Recessive
• Sex-linked – X chromosome – More males afflicted
than females
• Impairs blood clotting
• English royal family
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Hemophilia: The Royal Family
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Huntington’s Disease • Dominant • First symptoms
appear in thirties or forties: – Mild forgetfulness – Irritability
• Long-term symptoms: – Loss of muscle control– Chorea (physical
spasms) – Severe Mental Illness – Death
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Detecting and Treating Genetic Disorders
• Genetic Counseling = a form of medical guidance that informs people about genetic problems that could affect them or their offspring
• Phenylketonuria (PKU) – Lacks enzyme to convert phenylalanine into tyrosine – Can cause mental retardation – Early intervention involves low-phenylalanine diet
• Gene Therapy = replacing defective genes with healthy ones