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Unformatted text preview: Mendel and Meiosis II Particulate Inheritance Meiosis
s Implications for diversity of gametes 3Segregation No Blending! Fig. 13.7 Thanks! Meiosis
diversity of gametes
3Principle s Implications for Thanks again! of Independent Assortment
million possible combinations for humans! See Fig. 13.8 38.4 Meiosis s But wait .... there's more!!!
xRecombination Fig. 12.9 Meiosis
s Implications for diversity of gametes
3Segregation 3Independent Now we're talkin'! assortment 3Recombination Meiosis
s Implications for diversity of gametes -Random fertilization Mendel and Meiosis II Figure 13.2 Inheritance
s More terminology: s Trait
x Characteristics of an organism - e.g. eye color, hair color, seed shape, etc
x The genetic make-up of the organism BB, bb e.g. s Phenotype
x The discernible features of an organism blue eyes, round seeds, red hair
s Relationship between genotype and phenotype is sometimes complicated. Inheritance
s s There are different forms of each gene
x The variants are called alleles Each individual has two alleles per gene
x One from mother, one from father x Homozygous - both alleles are the same 3 RR or rr x Heterozygous - alleles differ 3 Rr
s s s The allele that is expressed = Dominant (uppercase) The allele that is not expressed = recessive (lowercase) Incompletely expressed as phenotype = Incomplete Dominance
x Special Heterozygous condition Inheritance
s You know more than any 19th century scientist did about the mechanism of inheritance. s Modern understanding of inheritance begins with Mendel: x Looked at 7 selected traits of garden peas. Inheritance paternal R Gene for flower color maternal r Meiosis:
One traitHeterozygous alleles Rr Diploid (2n) RR rr Duplication RR 1st division rr 2nd division R R r r Haploid gametes (n) Mendel's breeding experiments
s Cross- pollinate 2 truebreeding plants RR rr R r Mendel's breeding experiments
RR rr s Results: all Rr
R r Rr s Allow the F1 offspring to self-pollinate Mendel's breeding experiments
RR rr s Resulting F2 generation:
x purple x white x 3:1 phenotypic R Rr r Rr x Rr =
RR Rr Rr rr ratio
Phenotypes: Mendel's Laws of Inheritance:
s 1. Law of Segregation: x Alleles segregate into different gametes at meiosis xLeads to variation from parent to offspring Segregation:
The two genes of each pair of homologous chromosomes are separated from each other during meiosis, so they end up in different gametes. Law of Segregation = Monohybrid Cross
s The Punnett square
x Shows all possible combinations of alleles in offspring
s Monohybrid cross
x Inheritance of a single character x Yields 3:1 ratio in F2 Monohybrid Cross: Punnett Square
s Cross 2 heterozygous parents with free (unattached) earlobes. s Determine phenotypic ratio & genotypic ratio Genotypic Ratio 1 EE 2 Ee 1:2:1 1 ee What is the chance of a child having unattached earlobes? 75% Segregation of alleles is a chance event
s Probability = the chance event will occur (% or fraction) s P = # events that can produce a given outcome / total # of possible outcomes
x Probabilities of all possible outcomes for an event must add up to 1 Mendelian genetics reflect the laws of probability
s Multiplicative law: Chance of 2 or more events occurring together is product of their chance of occurring separately.
s Additive law: Chance of an event that can occur in 2 or more independent ways is sum of all individual chances. Chance of EE = ? Chance of Ee = ? Chance of ee = ? Chance of eE = ? EE + Ee + eE = 3/4 What is chance of a child with unattached earlobes? Homozygote or Heterozygote?
s How can you tell if an organism expressing the dominant phenotype is homo- or heterozygous?
x BB or Bb? The Testcross s Cross with bb
x If homozygous = All Bb phenotype = all black x If heterozygous = black ; brown= 1:1 ratio 2. Law of Independent Assortment
s For genes on different chromosomes, alleles will segregate independently. All combinations are possible. Law of Independent Assortment
x Dependent assortment??
3 Genes s Two traits: are on same chromosome 3 Only two types of gametes can form 3 Y stuck to R, y stuck to r 3 YR, yr 3 Phenotypic ratio of offspring 3:1 3 Not seen in crosses... See Fig. 13.5b Law of Independent Assortment
x Independent assortment
3 Genes s Dihybrid cross are on different chromosomes 3 Four types of gametes can form 3 Y not stuck to R; y not stuck to r 3 YR, Yr, yR, yr 3 Phenotypic ratio of offspring 9:3:3:1 See Fig.13.5a Law of Independent Assortment
TtGg x TtGg Summary
s Mitosis and Meiosis II are very similar processes, however ...
x Daughter cells in mitosis are identical x Daughter cells in meiosis are not
3 Segregation, Independent Asst., Recombination s Darwin's dilemma regarding inheritance was largely solved by Mendel
3 Variation is preserved by Segregation 3 Variety in gametes is created by Independent Assortment
s Recombination adds still more variety Summary
s Meiosis produces variable gametes due to
x Independent assortment, Recombination x Results in four distinct gametes from one cell s Modern understanding of inheritance begins with Mendel (Darwin would have loved this) s Study figure 13.5 (p. 273)
x Follow the alleles on the chromosomes x Set up a Punnett square to make predictions about F2 x Talk to your TA if you have trouble!! s s Work genetics problems at the end of Chapter 13 Summary
x Genes, chromosomes, loci, Haploid/diploid, homologous chromosomes= homologues, allele, chromatids, tetrads, chiasmata, synapsis, crossing over = recombination, cytokinesis, segregation, independent assortment, character, trait, genotype/ phenotype, homozygous/heterozygous, dominant/recessive/incompletely dominant, Punnett square, test cross, monohybrid/dihybrid cross
s Mitosis vs. Meiosis (steps & outcomes) = hair = eye = nose Try this: Homologous Chromosome Alleles
1.What is the dominant allele for eye color? 2. What 2 alleles were inherited for eye color? 3. What is the genotype for eye color? 4. What color are the eyes? 5. What is the phenotype for hair color? 6. What is the genotype for hair color? 7. What is/are the Homozygous trait(s)? 8. What is/are the Heterozygous traits(s)? 9. What is the shape of the nose? Blonde (h) Blue (b) Blonde (h) Brown (B) Convex (C) Straight (c) s Dan and Kim are going to have a baby. Kim has dimples in her cheeks (a dominant trait), while Dan does not. s What are the chances Dan and Kim's baby will have dimples? ...
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This note was uploaded on 05/11/2010 for the course BIOLOGY 100 taught by Professor Richard during the Spring '10 term at George Mason.
- Spring '10