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AP Bio: Thursday 3/17/11 Genetic Patterns of Inheritance. Happy St. Patrick’s Day / Evacuation Day! Homework: PS 16 #1-7 (be ready to discuss these on Monday) Do Now: Get some breakfast, get out your notebooks, get ready to learn. Today’s Goals: - PowerPoint PPT Presentation
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AP Bio: Thursday 3/17/11Genetic Patterns of InheritanceHappy St. Patrick’s Day / Evacuation Day!Homework: PS 16 #1-7 (be ready to discuss
these on Monday)Do Now: Get some breakfast, get out your
notebooks, get ready to learn.Today’s Goals:
Explain how Mendel’s Laws of Inheritance are based on the events of meiosis
Solve genetics problems involving one gene, two genes, multiple alleles, incomplete dominance, sex-linked traits, and epistasis
Agenda: Lecture mixed w/ practice problems
Genetic Patterns of Inheritance
AP BiologyChapters 14-15
Karyotypes
Autosome = Chromosomes
1-22
Sex Chromosome = X or Y
Trisomy 21 (Down Syndrome)
Mendel’s 1st Law:
Law of SegregationAlleles “segregate” into different gametes
during meiosis
a a A A
a Aa A
Monohybrid CrossInvolves one geneCrossing two heterozygotes yields:
A
a
aA Phenotypes:
Ratios:
Mendel’s 2nd Law:
Law of Independent AssortmentAlleles on one set
of chromosomes segregate independently from alleles on other sets of chromosomes.
Results in different allele combinations in different gametes, and in different combinations of traits
Dihybrid CrossInvolves two genes on different chromosomes
First step is always to arrange the gametesCrossing two heterozygotes yields:
RY
ry
RY
Ry
rYRy
ry
rY
Beyond the Dominant/Recessive Paradigm…Codominance
Both alleles fully expressed1:2:1 phenotypic ratioEx: red, white, red/white
spotsIncomplete Dominance
Dominant allele partially expressed
1:2:1 phenotypic ratioEx: red, white, pink
Multiple AllelesMore than two versions
(alleles) of the gene
ABO Blood Types –Show Multiple Alleles AND Co-dominance
Gene I controls ABO Blood Type.
Three possible alleles:IA – makes A antigensIB – makes B antigensi – makes no antigens
Possible genotypes (sets of two alleles):
Beyond the Dominant/Recessive Paradigm…
Polygenic traitsControlled by many genesOften show continuous
variation in phenotypes
Beyond the Dominant/Recessive Paradigm…
EpistasisThe effects of one
gene hide or alter the effects of another gene
9:3:4 phenotypic ratio
Hemophilia in the Royal Family
Sex-Linked Traits
XR – normal visionXr – red-green colorblind (recessive)
XR Xr = carrier female (normal vision)XrY = colorblind male (only needs one recessive allele to be
colorblind)
Controlled by genes on sex chromosomes
X-linked traits – on X chromosomeHemophiliaRed-green
colorblindness
More on X-Linked TraitsTry these sample
problems…Carrier female x normal
male
Normal female x colorblind male
Hairy Ears
Why do only males have them?
Why do only males produce the SRY (testis-determining) protein?
They are Y-linked!
Hairy Ears
If a man with hairy ears marries a woman with non-hairy ears, what is the chance that (a) their daughters will have hairy ears?(b) their sons will have hairy ears?
The end of this lecture.
Pedigree AnalysisGet a worksheet…
Evaluating the Validity of Genetics Experiments… Using Chi-SquaredStatistics! Woot woot!Get a worksheet… enough of this powerpoint
business.
More Chi-Squared PracticeTwo Drosophila genesBody color: gray (G) or black (g)Wing shape: normal (N) or vestigial (n)Cross a double heterozygote with a double
recessiveGgNn x ggnn
Possible phenotypes
Expected Outcomes
Observed Outcomes
Gray/Normal 250 410
Gray/Vestigial 250 90
Black/Normal 250 80
Black/Vestigial
250 420
Linked GenesChi-Squared Test REJECTS our null hypothesis.Independent Assortment of the two genes
doesn’t seem to apply Why?
Conclusion: Two genes on same chromosome (linked)• Recombinant genotypes created by
crossing over between the two genes
• Frequency of recombinant genotypes based on the distance between genes on the chromosome
Finding Distances Between Linked GenesCalculate the recombination frequency –
how frequent are the genotype combinations that AREN’T the same as the parents?
Percentage of recombinants = distance in map units
Practice this with the fruit fly example…
Linkage MapsShow the relative location of genes on a
chromosomeFirst, determine recombination
frequenciesBody color & wing shape = 17% recombinantsBody color & eye color = 7% recombinantsWing shape & eye color = 23% recombinants
1 % recombination frequency = 1 map unit(Can’t be over 50%... why not?)
Create a linkage map (aka genetic map)