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Genetics 380 Exam 1 Review Sheet, Study notes of Genetics

Concepts are sorted by lecture and images are included where necessary for clarity. Information is taken from both the lectures and the textbook.

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2020/2021

Available from 12/27/2021

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Exam 1 Master Sheet
Lecture 1
Pre-Mendel: sperm and egg contain essences that mix upon conception(pangenesis) to form a new pattern
Peas as model organism: cheap, easy to grow, self-fertilizing, can be cross pollinated
Mendel’s Success: used true-breeding strains, used contrasting characteristics, quantified his results
Conclusions: male and female parents contribute equally(reciprocal crosses gave same results) (no blending)
Dominant vs. Recessive Traits - dominant displayed in F1, recessive trait only in F2
Particulate inheritance - traits are determined by discrete units inherited thru generations - 2 particles in genotype,
but only one is passed on
Steps for Determining results of a hybrid cross.
1. Determine gametes for parents
2. Write gametes and frequency along the side of the table
3. Combine gametes + translate genotype into phenotype
Mendel’s First Law: Principle of Segregation - members of a pair of allele segregate into separate gametes
Gametes form in equal frequencies - homozygote makes 1 type, heterozygote makes two. Subsequent fertilization
is random. Phenotype does not always reveal genotype - Dominant Phenotype can be homo-or heterozygous
Testcross/backcross to determine genotype - cross individual with dominant phenotype to homozygous recessive
Lecture 2 Mitosis and Meiosis
Chromosome - physical structure that carries genes - double-stranded DNA, information arranged linearly
Eukaryotes: contains DNA, RNA and proteins. Dispersed/chromatin for most of the cell cycle - bundles in division
Eukaryotic chromosomes have a distinct shape and size, 1 centromere and 2 telomeres
Distinguished by size and location of centromere relative to telomeres (acro, meta, submeta, telo - centric)
Chromosome # per cell differs among species, but most eukaryotes have 2 copies per somatic cell (2N/diploid #)
2 copies of each chromosome in a diploid = homologous (same length, same centromere, same gene placement)
Must be copied and separated equally during each stage of division
Mitosis: 1 cell -> 2 cells that are genetically identical. Creates somatic cells
Meiosis 1 cell -> 4 variable daughter cells. ½ number of chromosomes in somatic cells. Creates gametes
The Cell Cycle - stages that 1 cell goes through as it grows and divides
G0 - non-dividing phase - cell is stable and at constant size
G1 - 1st gap or growth phase - proteins needed for cell division are synthesized
S - Synthesis - all chromosomes are duplicated
G2 - 2nd gap or growth phase - damaged DNA must be repaired
Mitosis - cell division
Phases of Mitosis:
Prophase: chromosomes condense, mitotic spindles form, nuclear envelope breaks down
Metaphase: microtubules from spindle pole attach to each chromosome at the centromere
- chromosomes moved to metaphase plate in the center of the cell
Anaphase - proteins holding sister chromatid centromeres together are degraded, sister chromatids disjoin
- Sister chromatids move to opposite spindle poles’
Telophase: nuclear envelope reforms across each daughter nucleus, chromosomes uncoil, spindle disappears
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Exam 1 Master Sheet

Lecture 1

Pre-Mendel: sperm and egg contain essences that mix upon conception(pangenesis) to form a new pattern Peas as model organism: cheap, easy to grow, self-fertilizing, can be cross pollinated Mendel’s Success: used true-breeding strains, used contrasting characteristics, quantified his results Conclusions: male and female parents contribute equally(reciprocal crosses gave same results) (no blending) Dominant vs. Recessive Traits - dominant displayed in F1, recessive trait only in F Particulate inheritance - traits are determined by discrete units inherited thru generations - 2 particles in genotype, but only one is passed on Steps for Determining results of a hybrid cross.

  1. Determine gametes for parents
  2. Write gametes and frequency along the side of the table
  3. Combine gametes + translate genotype into phenotype Mendel’s First Law: Principle of Segregation - members of a pair of allele segregate into separate gametes Gametes form in equal frequencies - homozygote makes 1 type, heterozygote makes two. Subsequent fertilization is random. Phenotype does not always reveal genotype - Dominant Phenotype can be homo-or heterozygous Testcross/backcross to determine genotype - cross individual with dominant phenotype to homozygous recessive

Lecture 2 Mitosis and Meiosis

Chromosome - physical structure that carries genes - double-stranded DNA, information arranged linearly Eukaryotes: contains DNA, RNA and proteins. Dispersed/chromatin for most of the cell cycle - bundles in division Eukaryotic chromosomes have a distinct shape and size, 1 centromere and 2 telomeres Distinguished by size and location of centromere relative to telomeres (acro, meta, submeta, telo - centric) Chromosome # per cell differs among species, but most eukaryotes have 2 copies per somatic cell (2N/diploid #) 2 copies of each chromosome in a diploid = homologous (same length, same centromere, same gene placement) Must be copied and separated equally during each stage of division Mitosis : 1 cell -> 2 cells that are genetically identical. Creates somatic cells Meiosis 1 cell -> 4 variable daughter cells. ½ number of chromosomes in somatic cells. Creates gametes The Cell Cycle - stages that 1 cell goes through as it grows and divides G 0 - non-dividing phase - cell is stable and at constant size G 1 - 1st gap or growth phase - proteins needed for cell division are synthesized S - Synthesis - all chromosomes are duplicated G 2 - 2nd gap or growth phase - damaged DNA must be repaired Mitosis - cell division Phases of Mitosis : Prophase: chromosomes condense, mitotic spindles form, nuclear envelope breaks down Metaphase: microtubules from spindle pole attach to each chromosome at the centromere

  • chromosomes moved to metaphase plate in the center of the cell Anaphase - proteins holding sister chromatid centromeres together are degraded, sister chromatids disjoin
  • Sister chromatids move to opposite spindle poles’ Telophase: nuclear envelope reforms across each daughter nucleus, chromosomes uncoil, spindle disappears

Meiosis : gamete formation. 4 haploids from 1 diploid cell. 2 divisions (meiosis 1 and 2, reduction and equational) Cell goes through s phase before meiosis Phases of Meiosis Early Prophase 1: chromosomes condense, homologs pair/synapse, crossing over may happen at chiasmata, 5 Stages of Propahse 1: describe appearance of chromosomes as they condense, pair and recombine

  1. Leptotene “thin thread” condensation begins
  2. Zygotene - “paired thread” synapsis begins
  3. Pachytene - “thick thread” condensation continues, recombination,
  4. Diplotene - “double thread” synapsis breaks down, chiasmata keep homologs joined
  5. Diakinesis “moving apart” DNA synthesis happens during homologous recombination - crossing over between homolgous chromosomes One sister chromatid from each homolog participates in a single crossover event Late Prophase 1: chromosomes condense, homologs pair, crossing over(or not), spindle, envelope breakdown Metaphase 1: each pair of homologs takes up a position on the metaphase plate (homologous pairs align) Anaphase 1: members of homologous pairs disjoin and move to opposite poles (principle of segregation) Telophase 1 - nuclear envelope reforms, interkinesis happens, producing 2 haploid (½ number of chromosomes) Prophase 2 - chromosomes condense, spindle forms, nuclear envelope breaks down Metaphase 2 - individual chromosomes position in equatorial plate Anaphase 2 - sister chromatids are pulled to opposite poles - chromosome count in cell doubled Telophase 2 - nuclear envelope reforms, cytokinesis happens Lecture 5 - Non-Mendelian Ratioes Conditions for Mendel’s Experiments
  6. All traits being considered are unlinked, each gene assorts independently, each gene codes for 1 trait
  7. Every gene has 2 alleles that specify a unique phenotype - 1 allele is dominant over the other
  8. Genotype determines phenotype Mendelian Ratios: Monohybrid Cross 3:1 Dihybrid Cross 9:3:3:1 - why can expression differ from these ratios? Causes involving a single locus: - different types of dominance, multiple alleles, lethal alleles Causes involving multiple loci - incomplete penetrance, variable expressivity Complete dominance - phenotype of heterozygote is phenotype of a homozygote (3:1, 9:3:3:1) Incomplete dominance - intermediate phenotype in heterozygote RR(red), Rr(pink), rr(white) (1:2:1) Codominance - heterozygote has phenotypes of both homozygotes (blood types A, B, AB) Multiple alleles - any individual only carries 2 alleles - may form a dominance series Lethal Alleles - found in essential genes, dominant/recessive, Dominant trait displayed in heterozygote (homozygote does not survive development) (2:1) Penetrance - proportion of individuals w a particular phenotype displaying the expected phenotype
  • Incomplete - less than 100% of population displays the trait (give appearance of skipping generations, may preserve lethal alleles, varying genetic backgrounds Expressivity - phenotype varies among individuals with the same genotype (variation in severity), varying genetic backgrounds and environmental effects. Polydactly is an example of incomplete penetrance and variable expressivity.

Lecture 4 - Sex Influence and Pedigrees

Studying heredity in humans is challenging:

  • Can’t test in humans through a cross, difficult to determine mendelian ratios
  • Deduce modes of inheritance in a trait using pedigrees to visualize how the trait is expressed in families Typically making a best guess - selecting the mode that makes the most sense based on available data Dominant vs/recessive - complete dominance, 1 parent always displays shows phenotype Autosomal vs sex-linked? - both sexes are affected equally if autosomal Do not assume individuals marrying into the family are carries Points to keep in mind
  • Incomplete penetrance and variable expressivity can make a dominant trait appear to skip generations Incomplete penetrance(disease allele is present but is not expressed) Variable Expressivity(disease allele is expressed differently) Sex influenced and sex-limited traits can be passed down by an unaffected individual
  • always autosomal, expression dependent on sex of carrier Geneticists typically look at many families and multiple generations to understand the mode of inheritance Sex influences on heredity: Sex-linked trait - genes on the sex chromosome. Sex influenced - autosomal, but expressed more in 1 sex. Sex limited - autosomal gene limited to the expression of one sex. Genetic maternal effect - nuclear genotype of the maternal parent. Cytoplasmic genes - inherited from 1 parent Genomic imprinting - genes whose expression is affected by the sex of the parent Genetic maternal effect - genotype inherited from both parents, but phenotype reflects mother’s genotype Cytoplasmic Inheritance - genes in chloroplasts and mitochondria
  • mitochondria express different phenotypes when segregated during cell division
  • Mother to offspring b/c egg contributes the majority of the cytoplasm
  • Reciprocal crosses give different results and extensive phenotypic variation Genomic Imprinting - whether inherited from mom or dad. Different results for reciprocal crosses, epigenetics

Lecture 6 - Non - Mendelian Ratios Causes involving multiple loci/environment - environmental modification/gene interaction Causes involving sex-determination - sex linkage, sex limited traits, sex influenced traits Environmental modification - genotype alone does not determine phenotype, environmental conditions can affect it

  • Himalayan rabbits(variable based on body temperature) and freckles(exposure to sunlight) Gene interaction: genes at 2+ loci influence a trait - non allelic, can lead to…. (Deviation from 9:3:3:1)
  • New and unexpected phenotypes from crosses (pea, walnut, rose, and single phenotypes in chicken combs)
  • Epistasis(alleles at 1 locus mask expression at another locus) - eye color in fruit flies
  • complementation (crossing 2 recessive mutants yields wild-type progeny).complementation test (mutant cross) Identifying gene interaction: ratio is a variation of 9:3:3:1, a single trait is influenced by more than 1 gene Sex chromosomes are differently shaped/heteromorphic - act homologous in meiosis due to pseudoautosomal regions X-linked, Y linked, Z linked, W Linked Humans - XX/XY - Y confirms maleness (sry - sex determining region on the Y) - males heterogametic ZZ/ZW - females are heterogametic - reciprocal crosses give different results In fruit flies, the ratio of X-chromosomes to autosomes gives the sex. x<0.5=metamale, .5=male, .5-1 intersex, 1 = female, x>1 = metafemale. (y chromosome required for fertility) T.H. Morgan and Fruit Fly Eyes - initially thought white eyes in female are lethal…. White eyed females viable… reciprocal cross not the same… sex linked. Dosage compensation corrects for potential imbalance in X-linked gene products in females vs males Females have 2 copies - homozygous or heterozygous. Males are hemizygous - traits are always expressed X-inactivation - random, early in development - all daughter cells have the same x inactivated (mosaicism) (Barr body) Sex-influenced - more common in 1 sex. autosomal, male/female affects dominant/recessive (goatbeard) Sex limited - autosomal, zero penetrance in 1 sex - related to secondary sex characteristics Lecture 7 - Gene Linkage in Eukaryotes 23 chromosomes, 20,000 genes. Can’t all assort independently - genes that don’t assort independently = linked. Linkage - deviation from independent assortment favoring parental gametes vs recombinant(not possible in parents) Physical linkage - proximity to each other on chromosome, measured in base pairs/bp Statistical linkage - how physical linkage is observed, experimental data, map units/centimorgans Complete linkage - so close that they never assort independently Testcross in linkage - homozygous recessive crossed to individual with known genotype to show if gametes show deviation from independent assortment Complete Linkage (f(recomb) = 0). Independent Assortment = f(parent) = f(recomb) Incomplete linkage = f(parental) > f(recomb) > 0. Crossing over - physical exchange of chromosomal material between 2 homolgous chromosomes Recombination between 2 genes on homologous chromosomes at 2 different loci (1mu - 1%RF = 1centimorgan) Distance between genes based on detecting linkage and estimating recombination frequency Can also show order of genes based on triple testcross experiment - recombination among 3 genes, 2 single crossovers and a double crossover