unit 8: genetics & heredity
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Unit 8: Genetics & Heredity Unit 9: Human Genetic Disorders Ch. 26: Inheritance of Traits & Ch. 27: Human Genetics. Unit 8: Genetics & Heredity. What is genetics? the study of heredity passing of traits from parents to offspring. Chromosomes in Cells. Remember… body cells are diploid - PowerPoint PPT PresentationTRANSCRIPT
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Unit 8: Genetics & HeredityUnit 9: Human Genetic Disorders
Ch. 26: Inheritance of Traits& Ch. 27: Human Genetics
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• What is genetics?– the study of heredity
• passing of traits from parents to offspring
Unit 8: Genetics & Heredity
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Chromosomes in Cells• Remember…
– body cells are diploid• 2 of each
chromosome– 1 from mom & 1 from
dad
– gametes (sperm & eggs) are haploid• 1 of each
chromosome– Why?
» So zygote gets right # of chromosomes…
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• Why is your combination of genes unique?– Chance… don’t know
which sperm will fertilize which egg…
• get ½ of your chromosomes from mom & ½ from dad
• meiosis (formation of gametes)
– crossing-over during prophase 1
– alignment of chromosomes during metaphase 1
Genes
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Genes & Alleles• What is a “gene”?
– section of chromosome that codes for a specific protein
• & determines a specific trait (ex. hair color, eye color, ear shape, etc.)
– genes are paired on homologous chromosomes• different forms of genes for the
same trait are called “alleles”
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Dominant & Recessive Alleles• Each parent contributes 1 allele (form of gene)
for trait & can be dominant or recessive
– What is a dominant allele?• allele that prevents expression of (“masks”/“hides”)
recessive trait
– What is a recessive allele?• allele whose trait can be seen only when the organism is
pure (homozygous) for that trait
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Dominant & Recessive Alleles• How are alleles
represented?– with letters
• usually the first letter of the dominant trait
– If the same letter is used for dominant & recessive, how do we know which allele is which?
» CAPITAL = DOMINANT
» lowercase = recessive
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Allele Combinations• What does “homozygous” mean?
– both alleles are the same• homozygous (pure) dominant (ex. AA)• homozygous (pure) recessive (ex. aa)
• What does “heterozygous” mean?– both alleles are different
• heterozygous (hybrid) (ex. Aa)
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Genotype vs. Phenotype• What is “genotype”?
– organism’s actual genetic “code”/make-up (alleles)
• What does the genotype do?– codes for protein that causes
trait (phenotype)
• How do we represent an organism’s genotype?– 2 letters (one for each allele)
• one from mom & one from dad– ex. PP, Pp, pp
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Genotype vs. Phenotype• What is “phenotype”?
– the outward (physical) expression of the genotype (trait we “see”)
• What actually causes the “phenotype” (trait) we see?– the protein that is produced
(due to the organism’s genotype “code”/alleles)
• How do we represent an organism’s phenotype?– usually an adjective
• ex. purple, white, tall, short, etc.
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Genotype is Expressed as a Phenotype• Ex. Let P = purple & p = white
– homozygous (pure) dominant• genotype PP• phenotype = purple
– homozygous (pure) recessive• genotype pp• phenotype = white
– heterozygous (hybrid)• genotype Pp• phenotype = purple
– dominant trait “masks/hides” recessive trait
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PP
Pp
pp
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• What are Punnett Squares?– a way to predict the results of crosses (mating)
• letters outside represent possible alleles in gametes of each parent
– top = one parent & side = other parent
• letters inside boxes represent possible allele combinations (genotypes) in offspring (& phenotypes)
– can be used to determine probability and ratios
Predicting Traits in Offspring
BB Bb
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Making a Punnett Square• Parents are Tt & tt genotypes…
– So… Tt x tt is our cross (mating)
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Passing Traits to Offspring & Probability• What is probability?
– chance an event will occur
– What is the chance of getting heads? tails?
• ½
– If you flip two coins, of getting 2 heads? 2 tails?
• ½ x ½ = 1/4
– What is the chance of a couple having a boy? a girl?
• 1/2
– of having five girls?• ½ x ½ x ½ x ½ x ½ = 1/32
– or ( ½ )5 = 1/32
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Passing Traits to Offspring & Ratios
• What is a “genotypic ratio”?– probable ratio of
genotypes (alleles) in offspring of a given cross• Ex. If cross Pp & Pp
– 1PP : 2Pp : 1 pp
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Passing Traits to Offspring & Ratios
• What is a “phenotypic ratio”?– probable ratio of
phenotypes (traits) in offspring of a given cross
– resulting from the genotypes of the offspring
• Ex. If cross Pp & Pp• 3 purple : 1 white
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Passing Traits to Offspring & Ratios• What is an
“expected ratio”?– ratio we expect to get
based on probability (Punnett Square)
• What is an “observed ratio”?– ratio we actually get
• Why would these be different?– fertilization is random– some embryos die
during early stages
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Gregor Mendel• Father of Genetics
– 1822-1884
• studied garden pea plants– 7 different traits with
clearly different forms• tried to determine how t
hey were passed from parent to offspring
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Mendel’s Experiments• What happened when Mendel mated
a pure purple parent (PP) & a pure white parent (pp)?–all offspring had:
• purple phenotype
• heterozygous (hybrid) genotype
–Pp
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• What happened when Mendel let the heterozygous (hybrid) offspring from his first experiment self-pollinate?– So… Pp x Pp
• new offspring weren’t all purple…
Mendel’s Experiments
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Mendel’s Principle of Dominance• What did Mendel notice
from his experiments?– that one form dominates
over the other• …dominant trait prevents
the expression of the recessive trait
– What trait was dominant in these plants?
» PUPRLE = dominant– What trait was recessive?
» white = recessive
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Dominant/Recessive is Not Always the Method of Inheritance
• Traits are not always as clearly defined as the 7 pea plant traits Mendel studied.– examples of non-dominant/recessive
inheritance• sex determination• sex-linked traits• codominance• multiple alleles
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Sex Determination• How many chromosomes do
humans have (in body cells)?– 46… 23 pairs
• pairs 1 – 22 = autosomes (body chromosomes)
• 23rd pair determines gender = sex chromosomes
– XX = female
– XY = male
• Which parent’s chromosomes determines if the offspring will be a boy or girl???? Why?– Dad’s b/c he is the only one that
can give a Y; mom always gives X.
What is the probability of having a son? A daughter?
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Sex-linked Inheritance• X & Y chromosomes not fully homologous.
Why?– X is bigger & carries more genes
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Sex-linked Inheritance• How many alleles will a male have for
traits carried only on the X chromosome?– 1 b/c only have 1 X chromosome (Y doesn’t
have allele)• What is this called?
– X-linked or sex-linked» Ex. eye color in fruit flies, hemophilia in humans,
colorblindness in humans
– X-linked traits & disorders are more common in males. Why???• b/c female has XX, more likely she will have a
copy of dominant allele… males = XY… can only get dominant allele on X (& only have 1 X)
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• How do we make predictions made using Punnett squares for sex-linked traits?– Consider the sex chromosome (X/Y) & allele for the trait
it carries (“exponent”) TOGETHER as a unit…• ex. XG (= X w/ dominant allele), Xg (= X w/ recessive allele), Y (= Y
w/ NO allele)
• What if a female is heterozygous (XGXg)?– she does not show the trait/have the disorder, but is a carrier
• & can pass gene to offspring
• Can a male be a carrier?– No, b/c only has one X chromosome
w/ allele… so either has it or doesn’t
Sex-linked Inheritance
XG Xg
XG XG XG Xg
XG Y Xg Y
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• Drosophila (fruit fly) eye color is sex-linked– What are the sex, genotype, & phenotype of each
offspring? Are there any carriers for the white eye gene?
• Left picture: 2 females with red eyes = XRXr (carrier white eye gene) & 2 males with white eyes = XrY
• Right picture: female w/ red eyes = XRXR, female w/ red eyes = XRXr (carrier white eye gene), male w/ red eyes = XRY, & male w/ white eyes = XrY
Sex-linked Inheritance
XR Y
XR XR XR Y
XR Xr Xr Y
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Multiple Alleles & Codominance• What is meant by
multiple alleles?– more than 2 different
forms of an allele exist• but individual still has
just 2
– Ex. human blood types• (3) multiple alleles
– A (IB)– B (IB)– o (i)
•How many possible genotypes are there?
•How many phenotypes?
•Can you spot the blood type that is the result of codominance?
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Multiple Alleles & Codominance• What is meant by
codominance?– both alleles are
“expressed” equally
– Ex. human blood types also exhibit codominance (as well as multiple alleles)• A & B are codominant and
are “expressed” equally– A = B (codominant)– o (recessive)
» So… (A = B) > o
•How many possible genotypes are there?
•How many phenotypes?
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Unit 9: Human Genetic Disorders
• What causes genetic disorders?– DNA mutation (usually recessive) or chromosome
abnormalities (in # or structure) that cause the production of abnormal proteins
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• How can we group genetic disorders?1. autosomal recessive disorders (*most genetic disorders)
• allele is recessive & found on a chromosome from pairs 1 – 22 (autosomes or body chromosomes)
– cystic fibrosis, sickle-cell anemia, Tay-Sachs disease
2. autosomal dominant disorders• allele is dominant & found on a chromosome from pairs 1 – 22
(autosomes or body chromosomes)– Huntington’s Disease
3. sex-linked disorders• allele (which is usually recessive) is found on the 23rd pair of
chromosomes (sex chromosomes)… Usually on the X chromosome– hemophilia, color blindness
4. chromosomal abnormality disorders• result from errors in chromosome # or structure
– Down Syndrome (trisomy 21), Klinefelter’s Syndrome (XXY)
Human Genetic Disorders
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Autosomal Recessive Disorders• What genotype(s) must a
person have to be affected?– homozygous recessive (gg)
• cystic fibrosis• sickle-cell anemia• Tay-Sachs Disease
• Can someone be a carrier? Why/why not?– yes
• b/c if heterozygous (Gg), person carries the gene, but isn’t affected
– due to having the “normal” dominant gene
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Autosomal Dominant Disorders• What genotype(s) must a
person have to be affected?– can be homozygous (GG) or
heterozygous (Gg) b/c allele is dominant
• Huntington’s Disease
• Can someone be a carrier? Why/why not?– No
• b/c even if person is heterozygous (Gg), person will have disorder
– due to dominant “disease” gene blocking “normal” recessive gene
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Sex-linked Disorders• Remember from earlier… hemophilia is
X-linked & recessive– What are the possible genotypes &
phenotypes? Can someone be a carrier?• XHXH = normal female• XHXh = carrier female (but not affected) • XhXh = female w/ hemophilia• XHY = normal male• XhY = male w/ hemophilia
– Why can’t a male be a carrier?• b/c only has one X chromosome
w/ allele… so either has it or doesn’t
– Ex. mom = carrier & dad = normal:• Make a Punnett square.
– genotypic ratio?
– phenotypic ratio?
1 XHXH : 1 XHXh : 1 XHY : 1 XhY
1 normal female: 1 carrier female : 1 normal male: 1 hemophiliac male
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• Remember from earlier… colorblindness is X-linked recessive– What are the possible genotypes &
phenotypes? Can someone be a carrier?• XCXC = normal female• XCXc = carrier female (but not affected) • XcXc = colorblind female• XCY = normal male• XcY = colorblind male
– In this Punnett square, what are the genotypes & phenotypes of the parents?
• father:– genotype = XCY & phenotype = colorblind
• mother:– genotype = XCXc & phenotype = carrier
Sex-linked Disorders
Ishiharatest forred-greencolor-blindness
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Chromosomal Abnormalities in Number
• What causes an abnormal number of chromosomes?– non-disjunction
• failure of paired chromosomes to separate during meiosis 1 or meiosis 2
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Disorders Due to Abnormal Chromosome #• What is Down Syndrome (trisomy 21)?
– when person has 3 copies of chromosome # 21
• What is Klinefelter’s Syndrome?– a sex-chromosome disorder in which males have extra copy
of X chromosome• XXY (or 47, XXY b/c 47 total chromosomes)
• What causes Down Syndrome (trisomy 21) & Klinefelter’s Syndrome?– non-disjunction
• failure of paired chromosomes to separate during meiosis 1 or meiosis 2
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Chromosomal Abnormalities in Structure
• What is causes structural abnormalities in chromosomes?− pieces are
added, deleted, inverted, or translocated
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Review & Animations
• Vocab interactive– http://nortonbooks.com/college/biology/animations/ch10a02.htm
• Crosses– http://www.sonefe.org/online-biyoloji-dersleri/grade-12/monohybrid-
cross/
• Drag & drop genetics– http://www.zerobio.com/drag_gr11/mono.htm
• Various– http://www.abpischools.org.uk/page/modules/genome/dna4.cfm?
coSiteNavigation_allTopic=1
• Genetic disorders– http://www.humanillnesses.com/original/Gas-Hep/Genetic-Diseases.html