chromosomes become visible as long, thin, slender threads.
chromosomes continue to contract during leptotene and throughout the entire prophase.
small areas of thickening (chromomeres) develop along each chromosome, giving it the appearance of a necklace of beads.
PROPHASE I: ZYGOTENE
active pairing of the threads makes it apparent that the chromosome complement of the meiocyte is in fact to complete chromosome sets.
Thus, each chromosome has a pair partner, and the two become progressively paired, or synapsed, along their lengths (homologous chromosomes).
In between the synapsis of both of the homologous chromosomes a synaptonemal complex is formed which mediates the synapsis.
PROPHASE I: PACHYTENE
chromosomes are thick and fully synapsed.
number of homologous pairs of chromosomes in the nucleus is equal to the number n.
recombinantion happens while nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions.
the beadlike chromomeres align precisely in the paired homologs, producing a distinctive pattern for each pair called tetrad.
PROPHASE I: DIPLOTENE
each chromosome is seen to have become a pair of sister chromatids.
the synapsed structure now consists of a bundle of four homologous chromatids.
pairing between homologs is less tight, they start to repel each other and start to separate.
replication of DNA becomes visible in this stage.
physical evidence of the crossover and recombination between non sister chromatids is present, named chiasmata.
PROPHASE I: DIAKINESIS
long, filamentous chromosome threads of the interphase have been replaced for more compact units that are more maneuverable.
this allow for further separation and motion of the homolog chromosomes and the terminalization of chiasmata.
normal prophase events continue.
thread-like structure found in the nucleus of eukaryotic cells that carries the genetic information.
FEATURES OF CHROMOSOMES
primary constriction -- centromere
second constriction -- nucleolar organizer region (NOR)
tertiary constriction -- satellite DNA
PRIMARY CONSTRICTION: CENTROMERE
defines the shape or type of chromosome based on the presence and relative length of each arm on either side of the centromere.
SECONDARY CONSTRICTION: NUCLEOLAR ORGANIZER REGION (NOR)
one in each chromatid.
stains positive with silver.
not associated with centromere.
site of ribosomal RNA production (nucleus)
TERTIARY CONSTRICTION: SATELLITE DNA
occur in small sections that are highly repetitive in nature.
NOT associated with the previous two types.
specific structure that characterizes the end of the chromosome.
needed to prevent attack by nucleases.
formed by repeated sequences of TTAGGG.
centromere located in the center with two equal length arms
centromere located near the center with two unequal length arms
centromere located very nearly at the end of the visible arm
centromere located near the end with an obvious long and short arm
main sets of chromosomes.
contain the genome of the organism.
required for a normal individual.
small, extra chromosomes found in some species.
no obvious functions.
gained or lost without obvious effects.
chromosomes or portions of the chromosomes that are greatly extended during interphase.
regions of DNA that contain the coding region of the gene.
constitutive: regions of DNA that are always tightly condensed. blocks of the karyotype around centromere.
chromosomes or part of chromosomes that undergo condensation only during certain stages of development or in certain tissues.
study of the movement and composition of chromosomes
different stains or treatment that result in differential staining of the chromosome into specific linear pattern.
R-bands: reverses the pattern of G-bands.
show constitutive heterochromatin.
fluorescent based G-band pattern.
shows NOR regions.
SEX DETERMINATION PARENT
parent that in a cross is responsible of assigning the offspring's sex due to the type of sex determination system the individuals belong to.
mammals: male is the sex determination parent.
birds: female is the sex determination parent.
XX: XO SYSTEM
present in insect orders.
female: homogametic X chromosomes. (XX)
male: with only one X chromosome. (XO)
differ in number of chromosome present, female 2n will be even and male 2n will be odd.
sex determining parent: male.
location of the important determining factor: the absence of a second sex chromosome will determine maleness, the presence of two X chromosomes will determine femaleness.
XX: XY SYSTEM
present in mammals.
female: homogametic X chromosomes.
male: heterogametic XY chromosomes.
the Y chromosome is morphologically different from the X. generally smaller and contains a lot of heterochromatin.
sex determining parent: male.
location of the important determining factor: the presence of the Y chromosome determines maleness.
homogametic: producing only one kind of gamete.
heterogametic: producing two (or more) different types of gametes.
ZZ: ZW SYSTEM
present in birds.
female: heterogametic ZW chromosomes.
male: homogametic ZZ chromosomes.
the Z is very large and the W is very small and mostly heterochromatin.
sex determining parent: female.
location of the important determining factor: the presence of the W chromosome determines femaleness.
MULTIPLE SEX CHROMOSOME SYSTEM
may need more than one pair of chromosomes to determine the sex.
sex determining parent:
location of the important determining factor:
NEO XY SYSTEM
XO---> X1X2 neoY
very complex event.
like a short term multiple sex chromosome system.
occurs in populations not in individuals.
involves a reciprocal translocation between the X chromosome and an autosome (an exchange of information).
BALANCED SEX DETERMINATION
all individuals carry the genes for both sexes.
location of the determining factors:
- female determiners reside on the X.
- male determiners reside on the autosomes.
sex determining parent: sex is not determine by a parent, but by the ratio of X chromosomes vs autosomes.
male 1:2 ratio.
female 2:2 ratio.
HAPLODIPLOID SEX DETERMINATION
system based on a single gene with 9 different alleles.
female: heterozygous individuals. (2n)
male: hemizygous individuals. (1n)
homozygous individuals become weak, sickly males that are sterile and die off.
Diploid females can lay either fertilized (2n) or unfertilized (1n) eggs.
refers to a gene which is present in a single dose or a single copy rather than the normal two copies in a diploid.
GENIC SEX DETERMINATION
sex can be determined by the alleles present as one (or more) gene (s).
corn is monoecious (has both sexes) but can be made dioecious by substitution at either of two genes.
NONGENETIC SEX DETERMINATION
environmental sex determination.
some stimuli form the environment initiates the development of one sex over the other.
for example, temperature dependent sex determination in some turtles
eggs incubated at 25C all male.
eggs incubated at 30C all female.
eggs incubated at 27.5C 50-50.
small dark staining body in the nucleus of female cells that's not present in male cells.
helpful in identifying the genetic female condition.
there's always one less Barr body than the X chromosomes in the individual.
Lyon and Russel theory.
only one X chromosome is active in interphase with the other X chromosome inactive and condensed (facultative heterochromatin)
the Barr body is this heterochromatized X chromosome.
only one active X chromosome is needed for the cell metabolism in female cells.
all additional X chromosomes become heteropyknotic.
is a matter of chance which X chromosome becomes inactive, but it's that X chromosome which will be inactive in all the subsequent cells arising from the original one.
the gene that expresses the inactivation of that second X chromosome is located near the centromere in the long arm of the X chromosome.
SEX DETERMINATION IN MAMMALS
sex is determined by the distribution of X and Y chromosome in the zygote, but the whole Y is not needed to produce a male.
TDF: testicular determining factor; responsible for the development of testis instead of ovaries (default system).
all the characteristics of sex are dependent on factors produced by the gonads.
SEX DETERMINATION THEORY
in theory only one gene on the Y chromosome is required to start the pathway of testis determination.
SEX DETERMINATION GENE
there's this small region of the Y that has been shown to be sex determining, called SRY (sex related Y).implicated due to: location, highly conserved sequence, timing of activity and DNA binding region.
SEX CHROMOSOME ABNORMALITIES
gynandomorphs: sexual mosaics.
this happens when some parts of the individual express male and other parts express female.
karyotype of 44+XXY 2n=47.
Barr body positive.
phenotype: male, under developed genitalia, sparse body hair/beard, some breast development, below normal IQ, often tall with long limbs, generally sterile.
karyotype of 44+X 2n=45.
Barr body negative.
phenotype: female, under developed breasts, webbing of neck, no menstruation, skeletal abnormalities, short stature, below normal IQ, generally sterile.
TRIPLO X SYNDROME
karyotype of 44+XXX 2n=47.
Barr body double positive.
Phenotype: appear as a fairly normal female, may be sterile or fertile.
karyotype of 44+XYY 2n=47.
Barr body negative.
phenotype: "super males", often tend to be tall, overly aggressive, below normal IQ, usually fertile.
TESTICULAR FEMINIZATION (TFM)
a recessive allele occurring very rarely in male births.
chromosomally normal males 44+XY.
phenotypically appear as normal females: testis present (but undescended), sterile, androgene (testosterone) produced.
due to the abscence of receptor sites for the androgens (male hormones).
DO NOT directly influence the process of sex determination.
DO, however, affect the development of the secondary sex characteristics.
SEX INFLUENCED DOMINANCE
different pattern of dominance is observed in heterozygotes of the two sexes.
due to the different influences and interaction with the characteristic sex hormones.
SEX LIMITED EXPRESSION
the sex hormone can function as a limiting factor in the expression of some genes.
uniform expression in one sex, but if you transfer the same genotype to the opposite sex a different phenotype is produced.
examples: milk production, restricted to females; and beard shape, restricted to males.
occurs because there are genes on the X chromosome other than sex determiners.
physically linked together on the X chromosome, so must be inherited as a single unit.
x-linked dominant: female shows it.
x-linked recessive: male shows it and females are carriers.
genes that are located on the same chromosome.
Y LINKED GENES
residing in the Y chromosome.
never appear in females and are passed from father to son only.
CLASSIC SEX LINKED PATTERN
expression in a male.
passed to all of his daughters, but not expressed. passed to none of the sons.
passed into half of the males and females of the next generation with expression in the males due to hemizygous state (not females).
called the CRISS-CROSS PATTERN.
group of genes that occur on a chromosome. linked through their physical association.
24 linkage groups are expected in mammals (22 autosomal, the X chromosome and the Y chromosome).
parental type: expected phenotypic classes based on the initial distribution of the alleles.
recombinant type: unexpected phenotypic classes based on the initial distribution of the alleles.
The genetic exchange involved is due to the crossing over (genetic recombination) in meiosis I, prophase I; (pachytene).
DIFFERENT DEGREES OF LINKAGE
tight linkage: genes are close together and tend to be inherited together.
loose linkage: genes are relatively far apart and may be inherited together or may be involved in cross over.
complete linkage: genes are so close together that no crossing over is ever noted.
start with a simple dihybrid test cross. [+a+b x aabb]*
if they ARE NOT linked expect a 1:1:1:1 ratio
if they ARE linked expect more parental types and fewer recombinant types.
*a test cross is useful due to hemizygous expression.
FREQUENCY OF RECOMBINATION
the frequency of recombination is dependent on how close the genes are to one another.
very close together: few or no recombinant types.
far apart: observe numerous recombinant types.
if two genes on a chromosome are so far apart that there is a cross-over in virtually every cell during meiosis.
thus equal number of parental and recombinants would be recover.
so, linkage can NOT always be distinguished from independent assortment.
1 map unit = 1 percent recombination.
P: parental line, initial distribution.
RP: reconstituted parental line, looks like the initial distribution but took place during a cross over.
R: recombinant line, not the initial distribution.
measure of the actual occurrence of double cross overs vs the theoretically expected.
degree to which a cross over in one area reduces the probability of a cross over in the same general area.
LINKAGE IN HUMANS
very difficult due to low number of offspring from a single mating combined with the rarity of crossing over between tightly linked genes.
euploid variation: addition or loss of entire SETS of chromosomes.
aneuploid variation: loss or gain of an individual chromosome.
structural variation: variation in shape or form within individual chromosomes.
haploid: cells that contain the gametic number of chromosomes. (1n)
monoploid: contain the lowest haploid number in a polyploid set. a single set of chromosomes or one complete genome. (1X) behaves as diploid.
diploid: condition of having two copies of the complete genome. normal number on somatic cells. (2n)
polyploid: condition where three or more complete sets of the genome.
hypoploid: less than the normal number of chromosomes.
hyperploid: more than the normal number of chromosomes.
2n-1: monosomic. (-1 copy)
2n-2: nullisomic. (loss both copies of the same autosome; loss of a pair)
2n+1: trisomic. (+1 copy)
2n+2: tetrasomic. (gain both copies of the same autosome; gain of a pair)
influence of the genetic material depends on the quantity or the number of gene copies, as well as the presence or absence of specific alleles.
1. all changes from the diploid state change the genetic balance and the size of the nuclei and cells.
2. changes from diploid to haploid open the organism to the deleterious effects of mutation through the hemizygous state.
normal part of most life cycles.
somatic haploidy is rare.
in abnormal haploidy the negative effects are very severe and survival depends on the level of ploidy.
fusion of a nonreduced gamete (2n) with a normal gamete (1n) to produced a triploid (3n).
meiotic problems occur due to the presence of three homologous chromosomes.
few balanced or normal gametes are produced.
female triploids tend to produce gametes that tend to be normal.
male triploids have much more sever effects on the resulting gametes.
very important phenomenon in higher plants.
grasses and angiosperms are polyploids.
RARE in higher animals due to:
1. delicately balanced mechanisms of sex determination.
2. greater developmental complexity is more easily disturbed.
3. unisexuality of higher animals (makes it hard to effectively nothing to mate with).
TYPES OF POLYPLOIDS
autopolyploids: organism with multiple copies of the genome of a single biological species. AAAA.
allopolyploid: organism with multiple copies of the genome of two or more distinct biological species. AABB.
MEIOSIS IN POLYPLOIDS
meiosis in autopolyploids gets very messy due to formation of multivalents.
polyploids are often highly sterile, but it is based on genetic imbalance rather than the segregational pattern.
ORIGINS OF POLYPLOIDS
1. zygotic doubling: doubling of the chromosome number in the zygote.
2. meristematic doubling: doubling of the chromosomes inthe development of the gamete tissue producing tissue.
3. geametic nonreducation: failure of meiosis I or meiosis II.
tend to do better than autopolyploids due to pairing in meiosis which reduces the multivalent formation.
used in agriculture.
FURTHER CONCEPTS OF POLYPLOIDY
amphidiploid line: all of the polyploids that arise as a result of the hybridization of two or more biological species. basic starting point.
genomic allopolyploidy: classic allopolyploid with little or no heterosynaptic pairing; all bivalents. genome mantains identity.
segmental allopolyploidy: segments of the two genomes are very similar and thus some pairing/multivalents can be formed.
autoallopolyploidy: an individual with both autopolyploid and allopolyploid characteristics. multiple copies of the genome of one species plus multiple copies of the genome of another species. AAAABB.
primary nondisjunctions: failure of meiosis I. all gametes are unbalanced; worse than 2dary nondisjunctions.
secondary nondisjunctions: failure of meiosis II. chance of normal offspring.