Genetics

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3 branches of Genetics

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1. Transmission
2. Molecular
3. Population

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Transmission Genetics

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Study of how traits are inherited and expressed by individuals from one generation to the next

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Molecular Genetics

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Study of molecules that encode and control expression of traits at cellular level

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Population Genetics

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Study of how alleles are distributed within and among populations and how these distribution and frequency patterns change over time

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Biparental inheritance

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One set of chromosomes is from maternal parent and one from paternal parent

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Karyotype

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The complement of a eukaryote chromosome

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Cell Cycle (4 Phases)

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1. G1: Gap Phase 1
2. S: DNA Replication Phase
3. G2: Gap Phase 2
4. M: Mitosis and cell division

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4 Stages of Mitosis

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Prophase
Metaphase
Anaphase
Telophase

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Product of Mitosis

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2 identical daughter cells

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Prophase

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DNA condenses into chromosomes, breaking of nuclear envelope

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Metaphase

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Chromosomes align on spindle

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Anaphase

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Spindle fibers contract and separate chromatids

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Telophase

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Chromatids unravel, envelope forms

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Meiosis results in...

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half the # of chromosomes and genetic recombination

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Chiasmata

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Location of crossing-over on chromatid

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Prophase I

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DNA condenses. Crossing-over happens as pairs match up.

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Metaphase I

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Homologous pairs line up on equatorial plane

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Anaphase I

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Chromosomes (not chromatids) seperate

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Telophase I

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1 chromosome from each pair clusters at poles, unravels, etc.

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Meiosis II

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Same process as mitosis, just results in 4 different daughter cells rather than 2 identical

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What results in increased genetic variation and when does it happen?

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Crossing-over (prophase I)
Independent Assortment (metaphase I)

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What is the ploidy of mitosis and meiosis

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Mitosis: 2n entire time
Meiosis: 2n, but only n after Anaphase I (needs to be n b/c fused with other n during fertilization)

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Assumptions of Mendelian dihybrid cross (4)

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1. 2 clearly distinguishable phenotypic traits
2. Each trait is controlled at a single loci
3. Only 2 traits at each locus (R or r)
4. No interaction btw the two loci

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Setting up a dihybrid cross

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1. FOIL! Done to determine possible parental alleles
2. Punnet Square

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Probability: and/or rules

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And=multiplication
or=addition
(of prop of each event)

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How do you tell if outcome follows Mendelian assumptions?

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Use chi-square test:
1. Determine observed and expected (based on ratios)
2. (observed-expected)^2 / expected
3. Add up the values
4. Determine degress of freedom (n-1)
5. Look for #3 based on #4 in a chi-square distribution table. If less than .05, then follows Mendelian

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Synthenic loci

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Physically located on the same chromosome

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How does loci distance influence recombination?

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The closer they are, the less likely they will be separated during crossing-over.

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When are loci considered linked?

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If the rate of recombination is less than 50%

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CentiMorgans

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genetic distance btw loci

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How to calculated centiMorgans

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Add the recombinants (least occuring combos), divide by total #, multiply by 100

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What are the 6 ways Mendelian ratios are not met? (forms of alleles)

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1. Lethal alleles
2. Incomplete dominance
3. Variable penetrance and expressivity
4. Sex linkage
5. Multiple alleles
6. Epistasis

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Lethal Alleles

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When one combination results in death of offspring (2:1 phenotypic ratio)

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Incomplete Dominance

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Heterozygous offspring doesn't have same phenotype of either parent due to interaction of different alleles at the SAME loci (1:2:1 ratio)

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Variable Penetrance

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Trait is not expressed at all in some individuals while others with same genotype express trait
ex: 90% of ppl with Rb rb have retinoblastoma, therefore is 90% penetrant.

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Variable Expressivity

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Trait is expressed with different intensities in different individuals with same genotype. ALWAYS expressed, unlike variable penetrance.

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Sex-Influenced Traits

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Expressed differently in male and females

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Sex-Limited Traits

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Expression of phenotype is limited to only one sex. Ex: female can have same genotype but not express trait

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Sex-Linked Traits

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Trait is coded on X or Y chromosome

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Multiple Alleles

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Several Alleles at same loci
Ex: Blood type= I^a I^b or I^o

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Epistasis and the two types

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Two or more loci interact to control a phenotypic trait
1. Complementary gene action: necessary for expression of another allele
2. Hypostatic gene action:surpresses action of a differentl allele at another locus

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Cytoplasmic inheritance

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Inheritance of DNA via mitochondria or chloroplast from ovum/ovule. Only transmitted via mother (mitochondrial DNA)

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Broad sense heritability

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H^2=Variation genotype/variation population

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Narrow sense heritability

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h^2=Variation allele/variation population

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Response

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R=h^2 x S
(s=selection differentail)

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Bulls weighs 1351 lbs and a cow weighting 1121 were selected from a heard w/ a mean weight of 1095.7 lbs. Two offsprings weighed 1139 and 1201. What is realizied heritability for live weight? (h^2)

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R: (1139+1201)/2 - 1095.7 = 74.3
S: (1351+1121)/2 - 1095.7 = 140.3
R/s: 74.3/140.3 = 0.529

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How to calculate realized heritability

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R/S
R: mean of offsrping - average
S=mean of selected (parents) - average