Biology 2970 Exam #1

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Mendel's First Law

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Law of Segregation
Each trait is controlled by particulate factors that occur in pairs. During gamete formation, the members of a pair segregate from one another so that each gamete receives just one. The double number is restored upon fertilization

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results of a monohybrid cross

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1:2:1 ratio

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results of a Yy x yy

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1:1

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Test cross

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cross to pure breeding recessive

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gene

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name for one of mendel's factors (general name)

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Allele

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alternate form of a gene

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gamete

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sex cell....sperm or egg; pollen or ova

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zygote

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formed by the fusion of two gametes

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homozygote

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carries identical alleles of a gene

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heterozygote

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carries different alleles of a gene

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genotype

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genetic makeup; description of alleles carried by an individual

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phenotype

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character determine by genotype

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Results of a dihybrid cross

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9:3:3:1

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What Does a Dihybrid Cross tell us?

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That genes behave independently from one another (leads to law of segregation)

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Mendel's Second Law

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Law of Independent Assortment: segregation of each gene pair during gamete formation independent of all others

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Results of a trihybrid cross

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27:9:9:9:3:3:3:1

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Extensions of Mendelian Principles: 4 o'clock plant

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red x white --> pink
pink x --> 1/4 red, 1/2 pink, 1/4 white
* heterozygote...neither allele is dominant

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Extensions of Mendelian Principles: Rabbit Color

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*Homozygote: black (C), himalayan (ch), albino (c)
*Heterozygote: back (Cch), black (Cc), himalayan (chc)

**can tell that these alleles are on the same gene by going a test cross
Cch x cc --> 1 black, 1 himalayan
Cc x cc --> 1 black, 1 albino
chc x cc --> 1 himalayan, 1 albino

brings out the homozygote phenotypes from the heterozygotes

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Extensions of Mendel's Principles

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1. Dominance is not always seen (4 o'clock plant)
2. Multiple alleles possible (himalayan rabbits)
3. Several genes can affect the same character (Chicken color)
4. One gene can affect more than one character (pleiotropy, i.e. White cats)
5. Some alleles are lethal when homozygous (i.e. yellow mice)
6. Alleles can have variable effects on phenotypes (penetrance, expressivity i.e. polydactyl)

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Extensions of Mendel's Principles: Several Genes can Affect Same Character

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*Chickens
C - colored; cc - white; O - colored; oo - white

CCoo x ccOO (all white) --> CcOc (colored) x --> 9 colored: 7 white

Actual biochemical pathway
colorless 1 --> (C) colorless 2 --> (O) pigment

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Epistasis

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*masking the effects of one gene by an allele on another
* gene whose allele is epistatic encodes an enzyme that works upstream in the pathway
- distorted Mendelian Ratios...think 2 or more genes

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Pure Breeding

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Homozygous

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Extensions of Mendel's Principles: One Gene Can Affect More than One Character

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*Pleiotropy - production of superficially unrelated effects by a single allele
i.e. white cats are often deaf, Marfan's syndrome (eye and heart defects)

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Extensions of Mendel's Principles: Some Alleles Are Lethal When Homozygous

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- Discovered by Cuenot in mice
- could not establish a pure breeding line
- expected a 1/4, 1/2, 1/4 ratio when test crossed a yellow mouse with a gray one, but would get a 1:1 of yellow to gray because they homozygous yellow would die
* 1/3 of genes have this property because they are essential genes

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Extensions of Mendel's Principles: Alleles Can Have Variable Effects of Phenotypes

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*penetrance - % of individuals carrying an alleles that are affected by it [polydactly = 100% penetrance]
* expressivity - degree to which phenotype is altered in individuals that are effected [lobster claw deformity = 100%]

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Probability

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proportion of times the event is expected to occur in repeated trials

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Addition Rule

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P (of A or B) = P(A) + P(B)
*only true if A and B are mutually exclusive events
*true only if events cannot occur at the same time

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Multiplication Rule

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P(A) followed by B P(A) * P(B)
- true only if A and B are independent outcomes [outcome of one event has no influence on outcome of another]

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To quantify how well results fit the expectation

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1. assume the model is correct
2. Calculate probability of obtaining data that fit the model AS POORLY as those obtained OR WORSE

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Things to Remember about the Chi-Squared Test

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1. Always use raw numbers
2. P value obtained is an approximation
3. P value means that a certain % of the time you would get the same results or worse

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Discoveries of Early Genetics

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1. Fertilization involved fusion of single sperm and egg
2. Although sperm and egg are grossly different in size, they contribute equally to inheritance
3. Sperm contributes nucleus and little else to the egg

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Germ Cells

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Give rise to gametes
- meiosis only occurs in germ cells

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Somatic Cells

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all other cells in the body besides germ cells

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Interphase

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growth phase
- nuclei stain very brightly with basic dyes
- chromatin meshwork of fibers in the nucleus (color holds due to acidity)

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Prophase

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chromatin begins to condense into units called chromosomes

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Chromosome

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1. only occur in morphologically identical pairs called
2. Chromosome types present are constant within a species
*by definition, each chromosome only has one centromere

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Karyotype

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chromosome sets as seen at the end of prophase

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End of Prophase

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- nuclear envelope breaks down
- spindle fiber breaks down [made of microtubules called tubulin]

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Metaphase

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- chromosomes attach to spindle fibers by their centromeres
- line up with spindle at the equator

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Anaphase

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- centromeres all divide at the same time
- dragged to opposite poles by spindle fibers (V-shaped)

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Telophase

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- spindle breaks down
- nuclear envelopes reform
- cell divides
---> interphase [dna replication]

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Harlequin Chromosomes

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*consistent with semiconservatively replicating chromosomes
- Giensa and fluorescent only stains half substituted chromosomes
- add 5-bromouracil at a replacement for thyamine and it is inserted into new strands. These half substituted new strands catch the dye

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Somatic/germ cells have...

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2 sets of HOMOLOGS said to be diploid

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Gamete cells have...

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only one set of HOMOLOGS, said to be haploids

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Meiosis

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- Reduces chromosomes from 2n to n
- Called the reductional division

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Prophase I

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homologs are physically associated with one another

* bivalent - physical association to one another

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Metaphase I

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- metaphase I plate
- can tell they are being stretched
- semi V shape, but still associated in bivalents

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Anaphase I

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- Being pulled to either end by spindle fibers
- V shape
- Chromosomes

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Meiosis II

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- essentially a mitotic division with no interphase between

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Prophase II

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Chromatin condenses

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Anaphase II

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Initiated by centromere division

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Meiosis/Mitosis Analog - Each Trait Controlled by a Pair of Factors

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Chromosomes occurs in pairs in somatic cells

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Meiosis/Mitosis Analog - Monomers of a Pair Segregate During Gamete Formation

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Homologs Segregate in Meiosis I

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Meiosis/Mitosis Analog - Double Number of Factors Restored Upon Fertilization

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Two n gametes fuse to make a 2n zygote

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Chromosome Theory

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genes are on chromosomes (Walter Sutton)

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Heterogametic

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produces two different chromosome types of gametes (XY)

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Homogametic

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All gametes are the same chomosomally (XX)

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Sex Chromosome

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a chromosome that differ in number between the sexes

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Crisscross inheritance

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Sons look like mothers and daughters look like fathers

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Matroclinous/Patroclinaus Exceptions

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Violations of Crisscross Inheritance
- like mother or like father in phenotype
- Arise due to non-disjunction
*fly example
- OX were red eyed males
- XwXwY were white eyed females

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Incomplete Dominance

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1:2:1

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Dominant Epistasis

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13:3

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Recessive Epistasis

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9:3:4

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Binomial Formula

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If the probability of event A is p and the probability of event B is q (p+q=1), then the probability that in n trials event A occurs s times and event B occurs t time (S+t=n)

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Sex Chromosome

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chromosome that differs in number in the two sexes

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Autosome

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chromosome other than a sex chromosome

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Sex linked

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located on a sex chromosome

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Hemizygous

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present in only one copy

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Turners Syndrome

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XO (1/5000) - females with underdeveloped sexual characteristics, sterile, tend to be short and webbing in skin of neck, mentally normal
* occurs at a lower rate due to spontaneous abortions of zygotes

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Kleinfelter's Syndrome

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XXY [1/1,000] - feminized males, sterile, tall/long limbs, small degree of mental retardation

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XXX females

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1/700 at birth - normal females, tall, usually fairly normal, doesn't have a name...NOT considered a syndrome

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Y plays a major role in sex determination

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XO vs. XY
XX vs. XXY

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X has a minor role in feminization

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XO vs. XX
XXY vs. XY

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Development of Males

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*testis secretes androgens (hormones) that cause the ret of the embryo to develop as male
*primary sex determination - controlled by Y --> gonad
* Secondary sex determination - sex of genitalia controlled by hormones
* Tertiary Sex Determination - events of puberty - controlled by hormones

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XXX females

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1/700 at birth - normal females, tall, usually fairly normal, doesn't have a name...NOT considered a syndrome

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Y plays a major role in sex determination

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XO vs. XY
XX vs. XXY

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X has a minor role in feminization

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XO vs. XX
XXY vs. XY

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Development of Males

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*testis secretes androgens (hormones) that cause the ret of the embryo to develop as male
*primary sex determination - controlled by Y --> gonad
* Secondary sex determination - sex of genitalia controlled by hormones
* Tertiary Sex Determination - events of puberty - controlled by hormones

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Testicular Feminization (tfm)

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Tfm+ encodes the receptor for these androgens
Tfm - no receptor made
i.e. XY, Tfm - ...primary sex determination is fine, but the secondary sex determination is messed up. Phenotypically female with testes

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Testes Determining Factor (Tdf)

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Also know Sex Determining Region (SRY)
* single gene that encodes a protein 204 amino acids in length. It is a "transcription factor", regulatory protein
i.e. XY female - results from mutation in SRY gene (SRY-)
i.e. XX male - SRY gene is moved and translocated to an X or an autosome during meiosis

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propositus

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aka - proband
* an individual that brought attention to the pedigree

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Different kinds of Genetic Diseases

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1. x- linked recessive
2. x-linked dominant
3. autosomal recessive
4. autosomal dominant
5. Y linked gene (rare)

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Primary Sex Determination

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Sry portion of the Y chromosom

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chromomere

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localized condensation

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chromatid

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backbone is single DNA molecule

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Centimorgan

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1% recombination

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homogametic

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only one chromosomal type of gamete

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recombination frequency = 20%

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map distance >20 map units

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pleiotropy

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production of superficially unrelated effects by a single allele

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bivalent

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formed in zygotene

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DNA replication

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absent in interphase between meiosis I and meiosis II

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diplotene

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chiasmata become visable

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5-bromouracil

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analog of thymine

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PD=NPD

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genes on nonhomologous chromosomes

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Sex of chickens

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ZZ - inviable
ZW - female
WW - male