Download Biology Passage 2 - HCC Learning Web

Genetics and Evolution
1. Introduction to Genetics
a. Overview and Sex Linkage
b. Genotype vs. Phenotype
2. Meiosis
a. Overview
b. Process
c. Dondisjunction
3. Mendelian Genetics
a. Hereditary Laws
b. Test Cross, Punnett Square and Rules of Probability
c. Applications
4. Linkage
a. Linkage
b. Recombination
c. Applicatiom
5. Pedigrees
a. Overview
b. Pedigree Analysis
6. Population Genetics
a. Overview
b. Hardy-Weinberg Equilibrium
c. Application
7. Evolution by Natural Selection
a. Overview
b. Modes of Natural Selection
8. Species and Speciation
a. Species
b. Speciation
9. Taxonomy
a. Binomial Classification
b. Anatomical Axes
Introduction to Genetics
A. Overview and Sex Linkage
1. Genetics (defined): describes trait inheritance through generations
Inheritance is validated by DNA as the genetic material
2. Gene
a. (defined): portion of DNA encoding for a particular product
not necessarily a protein product (i.e. tRNA has function)
b. if protein, usually encodes for a particular trait
c. distributed along chromosome
d. locus: specific location of the gene on a chromosome (map)
3. Genome
a. 23 different chromosomes (haploid-n)
b. Each cell has 2 copies of each chromosome (46 total) (diploid-2n)
1. called homologous chromosomes
2. similar but NOT identical in genetic content
a. one from mom; one from dad
b. have 2 copies of the same gene (alleles)
1. gene can have many alleles in population – polymorphism
2. but we only carry 2 (one allele on each chromosome)
rd
c. 23 Chromosome is the Sex Chromosome
1. either X or Y
a. females have 2 copies of X
b. males have 1 copy of X; 1 copy of Y
50% chance of being a male during reproduction
2. encodes traits besides gender
a. termed sex-linked traits
b. follow a unique pattern of inheritance
1. men are hemizygous for X-linked traits
Have only 1 X chromosome so express the 1 allele!
2. example: hemophilia. X linked (recessive allele)
a. males express recessive allele (clot factor)
b. females have 2 X alleles so rarely express; carriers!
3. Y linked traits are very rare; only expressed only in males
B. Genotype vs. Phenotype
1. Genotype (defined): DNA sequence of the 2 alleles a person carries
2. Phenotype (defined): Physical expression of the genotype
3. Classical Dominance:
a. Straightforward: Dominant vs. Recessive alleles
example: Flower petal color: R (dominant) - Red; r (recessive) - White
1. homozygous dominant: RR - 2 identical dominant alleles at locus
2. heterozygous: Rr - 2 different alleles (dom and rec) at a locus
3. homozygous recessive: rr - 2 identical recessive alleles at locus
b. Can not tell the genotype from the phenotype (exactly)
Homozygous dominant and heterozygous have same phenotype
4. Non-Classical Dominance:
a. Complicated pattern of inheritance
1. Incomplete: blended phenotypic mixture of both alleles
example: Rr flower would appear pink
so represent dominant as R and recessive as W allele
2. Co-dominance: no recessive alleles; all dominant
example: blood groups (genes a, b, i); (phenotypes A, B, AB, O)
3. Epistasis: gene expression depends on another genes expression
example: curly hair expression is ‘trumped’ by baldness gene
4. Pleiotrophy: many genes effect one phenotype
example: pigmentation of skin is controlled by at least 3 genes
Meiosis
A. Overview
1. Mitosis produces two identical daughter cells from the parent cell
a. genetically: parent (2n)  2 daughters (2n each)
2. Meiosis:
a. produces four different daughter cells (gametes) from the parent cell
1. genetically: parent (2n)  4 daughters (n each)
b. process takes place in sex organs with special cells
1. males: testes – spermatogonia become spermatozoa
2. females: ovaries – oogonia become ova
Meiosis facilitates genetic variation by recombination (crossing over)*
and the random segregation* of homologous chromosomes
B. Process
1. DNA duplication
a. Homologous chromosomes duplicate; produce 2 pair of sister chromatids
2. Prophase I and Metaphase I (Meiosis I)
a. prophase I – homologous chromosomes synapse; form tetrads*
1. genetic recombination (crossover) occurs – exchange material
b. metaphase I – homologous chromosomes align on plate
1. align across from each other, but in random arrangement to asters
3. Anaphase I, Telophase I and Cytokinesis I (Meiosis I)
a. anaphase I – homologous chromosomes randomly segregate to poles*
4. Prophase II, Metaphase II, Anaphase II, Telophase II (Meiosis II)
a. anaphase II – sister chromatids randomly segregate to poles*
C. Nondisjunction
(defined) – failure of homologous chromosomes (anaphase I) or sister chromatids
(anaphase II) to separate and properly segregate during meiosis, producing gametes
with an abnormal number of chromosomes (aneuploidy).
Mendelian Genetics
A. Hereditary Laws
1. Principle of Segregation (deals with 1 gene)
a. (defined) when 2 genetic alleles separate, only one allele is contributed per
individual to the next generation
b. takes place during Anaphase I of Meiosis
2. Law of Independent Assortment (deals with >1 gene)
a. (defined) the alleles of one gene separates into gametes independently of the
alleles of another gene
b. takes place during Anaphase I and Anaphase II of Meiosis
c. the genes must not be linked on the same chromosome
B. Test Cross, Punnett Squares and Rules of Probability
1. Test Cross uses mating as a genetic tool to experimentally determine genotypes by
observing the phenotypes of filial progeny (F) from a cross of parents (P)
follows Mendelian Hereditary Law (Great – we can predict outcome!)
2. Punnett Squares theoretically predict the results of a cross between two parents
3. Rules of Probability also determine the probability of an outcome in a test cross
a. Rule of Multiplication: calculates probability of 2 independent events happening
PA + PB = (PA)(PB)
b. Rule of Addition: calculates probability of either of 2 events happening
PA or PB = PA + PB - [(PA)(PB)]
C. Applications
1. Four Monohybrid (One Gene) Crosses to know
2. Dihybrid (2 Gene) Cross where both parents are heterozygous for the traits
Dominant Allele:
G = Green Color
W = Wrinkled Shape
Recessive Allele:
g = Yellow Color
w = Smooth Shape
3. Good God!; a Trihybrid (3 Gene) Cross (Application of the rules of probability)
A woman heterozygous for brown eyes (blue is recessive) and brown hair (blond is
recessive) marries a man with the same genotype. What is the probability they will
have a son with brown hair and brown eyes who is able to have blue-eyed children?
Two choices to answer this question:
1. Be a masochist and do a punnet square
2. Be smart and use the Rules of Probability! (I choose #2)
So… Genotypemom: XX/Bb/Ww
Genotypedad: XY/Bb/Ww
What did the question ask? (multiply individual probabilities)
1. Have a son… [50% probability (XX, XX, XY, XY) – ½]
2. With brown hair… [75% probability (BB, Bb, Bb, bb) – ¾]
3. With brown eyes but can have child with blue eyes…
[50% probability (WW,Ww, Ww, ww) – ½]
So… P = (½)(¾)(½) = 3/16
Linkage
A. Linkage
1. (defined) 2 or more different genes located on the same chromosome
2. called ‘linked’ genes because they do not Independently Assort in Meiosis
B. Recombination (Crossover) of homologous chromosomes
1. Facilitates independent assortment of genes in Meiosis
2. Recombination likelihood (or frequency) increases with gene distance on the
Chromosome, so we can use it to ‘map’ genes on a chromosome
3. Recombination frequency = number of recombinants/total number of offspring
C. Application
A pure breeding tall green plant, which is Dominant, is crossed with a pure
breeding short yellow plant (which is recessive). A plant from this cross (F1) is self
pollinated (mated with itself) to produce an F2 generation. The F2 generation
phenotypes: 30 Tall Green, 9 Short Yellow, 2 Tall Yellow, 1 Short Green. How do
we know the genes are linked and if so, what is the recombination frequency?
Dominant Allele:
T = tall
G = green
Recessive Allele:
t = short
g = yellow
Pedigrees
A. Overview
1. can’t perform controlled genetic crosses on humans, but we need to know the
inheritance of a trait
2. Pedigree (defined): Chart depicting trait inheritance
a. Males (square); Females (circle)
b. Mating: connected by horizontal line
c. Offspring: connected by vertical line
d. Affliction: indicated by shading
B. Pedigree Analysis
a. Dominant or Recessive Allele causes the trait?
Recessive traits commonly skip generations…Dominant traits do not.
b. Autosomal or Sex chromosome linked gene?
Sex linkage is more prevalent in Men due to hemizygosity…
c. If Sex chromosome, X or Y?
Y linkage is apparent in all blood related males…(father to son only)
d. Check for Mitochondrial Inheritance.
Affected females have all affected children.
Affected males do not pass trait on.
Population Genetics
A. Overview
1. Population Genetics: describes trait inheritance in populations over time
Relevant to natural selection and evolution
2. population (defined): members of a species that sexually reproduce with each other
3. gene pool (defined): sum total of genetic information in a population
4. allele frequency (defined): percentage of an allele per total alleles in a population
B. Hardy-Weinberg Equilibrium
1. (defined): allele frequency will remain constant in a gene pool over time
a. Meiosis does not affect allele frequency in gametes, just combinations!
b. equilibrium is reached after one breeding cycle in a population
2. Depends on Assumptions: (impossible to meet!)
a. No mutation
b. No migration
c. No natural selection
d. Random Mating
e. Large Breeding population
3. Mathematical derivation:
D+d=1
where: D (dominant allele frequency in population)
d (recessive allele frequency in population)
D2 + 2Dd + d2 = 1
where: D2 (DD genotype frequency observed)
2Dd (Dg genotype frequency observed)
d2 (dd genotype frequency observed)
C. Application
In a certain randomly-mating population, the frequency of the allele causing an
autosomal dominant disease is 0.1. What would be the frequency of affected
individuals in this population?
D + d = 1; if D = 0.1, d = 0.9
D + 2Dd + d2 =1; so (0.1)2 + 2(0.1)(0.9) = .19 or 19%
2
Evolution by Natural Selection
A. Overview
1. Evolution (defined): changes in life on Earth
2. Natural Selection (defined): interaction between organisms and environment
causing differential reproduction of different phenotypes thereby altering the gene
pool.
a. can not introduce genetic diversity (new alleles); only alters allele frequency
3. Fitness (defined): how successful an animal is in passing on its genes to future
generations
4. Sources of Genetic diversity:
a. New alleles – Mutation
b. New combination of existing alleles – Sexual Reproduction
B. Modes of Natural Selection
1. Directional: an extreme trait is eliminated – extreme height
2. Divergent: an average trait is eliminated – medium sized stature
3. Stabilizing: all extreme traits are eliminated – very tall or very short
4. Artificial: unnatural selective force applied – pollution
5. Sexual: ritualistic mating – behavioral acts (dancing)
6. Kin: sociological forces – arranged events
Species and Speciation
A. Species
1. (defined): populations capable of sexual reproduction with each other
2. Reproductive Isolation
a. Prezygotic Barrier: prevents hybrid zygote formation
b. Postzygotic Barrier: prevents development, survival, reproduction of hybrid
B. Speciation
1. (defined): creation of a new species
2. Cladogenesis (defined): branching of speciation
3. Homologous or Analogous structures
Physical features of 2 species dependent on ancestry
4. Convergent Evolution: unrelated lineages acquire same biological trait
5. Divergent Evoluton: related lineages acquire different traits
6. Parallel Evolution: related lineages acquire same biological trait
Taxonomy
A. Binomial Classification (based on observable characteristics)
a. Domain – Eukarya
b. Kindom – Animalia
c. Phylum – Chordata (Vertebrata)
d. Class – Mammalia
e. Order – Primates
f. Family – Hominidae
g. Genus – Homo
h. Species – sapiens [we are Homo sapiens]
B. Anatomical Axes