Genetics

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Genetics

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The study of genes, how they work together to produce the variation we observe and how genes are transmitted from one generation to another.

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Eugenics

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Manipulation of human reproduction to create people with desirable traits.

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Gene

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Fundamental unit of inheritance; something within an organism that influences its appearance and passed onto progeny

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Genotype

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An organism's genetic composition or makeup

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Phenotype

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What an organism looks like, function of both genotype and environment

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Alleles

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Alternative forms of a gene

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Dominant Allele

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An allele that exhibits its phenotype in heterozygotes

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Recessive Allele

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Phenotype that is only observed in the homozygote

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Dihybrid Cross

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Cross between 2 heterozygotes at a particular loci. 9:3:3:1 ratio

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How to tell if Recombinant

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1. Determine expected based on the null hypothesis
2. Set up a table listing each with observed & expected values.
3. Determine degrees of freedom.
4. Use p table to determine if observed & expected values are due to chance.

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Linkage

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Parental combination of alleles is found more frequently in gametes than the recombinant configuration.

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Parental type

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Configuration of alleles that were inherited from each parent.

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Recombinant type

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Alternative configuration.

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2 Ways For Independent Assortment

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1. Genes are on different chromosomes
2. Genes are so far apart on the same chromosome that there is a crossover between them in every meiosis.

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Interference

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A crossover in one interval on a chromosome decreases the probability of a crossover in an adjacent interval.
*Interference plays an important role in distributing crossovers throughout the genome so that every chromosome gets at least one.

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Incomplete or Partial Dominance

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The phenotype of the heterozygote is intermediate between homozygote.

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Codominance

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Both alleles contribute equally to the phenotype of the heterozygote

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Penetrance

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The proportion of individuals of a given genotype that display the phenotype associated with that genotype

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Maternal Imprinting

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Gene is inactivated during oogenesis
i.e. Angelman Syndrome

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Genomic imprinting

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The expression pattern of a gene depends upon the parent from whom it was inherited

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Epistasis

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When the genotype at one locus masks the phenotype of a second locus

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Gene Pool

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The sum of all the total alleles in the breeding members of a population at a given time

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Genotype Frequency

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Proportion of individuals in a population that carry a particular genotype

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Allele Frequency

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Proportion of gene topics in a population that are given alleles

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Prophase

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Chromatids condense

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Metaphase

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Sister chromatids align with kinetochore pointing to opposite poles of spindle

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Anaphase

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Cohesin destroyed, sisters pulled to opposite poles.

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Cohesins

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Multi-subunit ring structures that bind to DNA and hold sister chromatids together after replication.

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Meiosis

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A specialized cell division that reduces the chromosome number in half to produce gametes for sexual reproduction.

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Rec8

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A meiosis specific subunit whose cleavage is regulated.

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Meiotic prophase

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Sister chromatids form a synaptonemal complex.

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Chromosome architecture during meiotic prophase

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Leptotene, Zygotene, Pachytene, Diplotene

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Bivalent

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Synapsed pair of replicated homologous chromosomes

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Univalent

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Condensed pair of sister chromatids

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Meiotic Anaphase I vs Meiotic Anaphase II

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Anaphase I is when the arm cohesion is released, but Anaphase II is when the centromere cohesion is removed. These two events allow sister chromatids to segregate.

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Chromosome architecture during meiotic prophase

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Leptotene, Zygotene, Pachytene, Diplotene

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Autosomes

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Chromosomes that are the same in both sexes

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Sex Chromosomes

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Chromosome pair that differ between sexes

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Homogametic

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Gametes are the same with regard to a particular chromosome

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Heterogametic

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Gametes are different with regard to a particular chromosome

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Sex linkage

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Different patterns of inheritance are observed in reciprocal crosses due to genes present in the sex chromosomes

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SRY gene

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Gene located on the Y chromosome which is sufficient to make a male

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Pseudoautosomal Region (PAR)

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Region of homology shared by mammalian X and Y chromosomes where cross overs occur during meiosis.

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Nondisjunction

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The failure of homologous chromosomes to separate normally during nuclear division.

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Autosomal Dominant Characteristics

ex) Huntington's Chorea

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1. Should occur in men & women with equal frequency
2. Should appear in every generation
3. 50% of affected offspring's children should be affected.

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Autosomal Recessive Characteristics

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1. Should occur in men and women with equal frequency
2. The appearance of the trait often skips a generation
3. An affected child can be born to unaffected parents

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Pedigree Analysis

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Study of human crosses

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X Linked Dominant

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1. Occurs in men and women
2. May appear in every generation
3. NEVER see father to son inheritance
4. All daughters of an affected man will be affected.

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X Linked Recessive

ex) Hemophilia

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*1. Shows up preferentially in males but women can be affected too
2. NEVER see father to son inheritance
3. Affected daughters should have affected fathers.

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Inbreeding

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The breeding of related individuals within an isolated group

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Carrier

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An individual who is heterozygous for a recessive mutation and doesn't exhibit phenotype but can transmit mutant allele to offspring.

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Rare Trait Assumptions

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1. People marrying into the family are assumed to be homozygous dominant for pedigrees that exhibit recessive inheritance.
2. Affected people are assumed to be heterozygous for dominant traits.

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Sex Influenced Traits


ex) Pattern Balding

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Are encoded by autosomal genes but the traits appear preferentially in one sex or another, because of the different hormonal environment provided by that sex.

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Quantitative Traits

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Follow a continuous distribution (in contrast with discrete traits)

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Polygenic Traits

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Affected by variation at many loci

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Multifactorial

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Affected by environment as well as genetic variation

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Quantitative Inheritance

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Alleles make additive contributions to phenotype

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Heritability Estimates

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-Are properties of populations, not individuals.
-Depend on levels of environmental and genetic variation for the specific population under study

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Complex Traits

ex) schizophrenia, heart disease.

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-Often less quantifiable than metric traits, or not fully penetrant
-Often heterogenous (similar phenotypes with different causes)

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Linkage

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Parental combination of alleles is found more frequently in gametes than the recombinant configuration

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Parental Type

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Configuration of alleles that were inherited from each parent

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Recombinant Type

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Alternative configuration

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2 Ways for Independent Assortment

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1. Genes are on different chromosomes
2. Genes are far apart on same chromosome

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Prototrophy

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Cells that are able to grow at minimal medium because they can synthesize their own nutrients

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Auxotrophy

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Cells that require the addition of a specific nutrient to the medium to grow

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If crossing over occurs BEFORE chromosome duplication...

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Only 2 types of tetrads

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If crossing over occurs AFTER chromosome duplication...

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4 types of tetrads

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PD

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Tetrads without recombination

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NPD

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Tetrads with 1 recombination

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Tetratype

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Tetrad containing 4 genotypes; 2 parental and 2 recombinant.

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For linked genes, a single crossover produces a ___ tetrad. A 4 strand double crossover leads to an ____.

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1) Tetratype
2) NPD

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What are yellow twin spots on fruit flies?

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Mitotic recombination - which happens to repair the cell when DNA damage occurs.

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Mosaic

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When patches of cells are homozygous for mutation in a heterozygous organism

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Loss of Heterozygosity

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If mutant allele becomes homozygous for a tumor suppressor gene.

ex) If people are heterozygous for Rb, they are predisposed to tumors.

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What are some examples of aneuploidy?

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Nullosomy, Monosomy, Double Monosomy, Trisomy, Tetrasomy