Exam #3 Content Flashcards
Molecular Nature of Genes
Nucleotide sequences that encode functional RNA’s (made of DNA)
- Carried on chromosomes
- Gene products (RNA and Proteins) influence phenotypic traits by regulating biochemical pathways
- Nucleotide sequence determines gene function
Gene
- Sequence of nucleotides that encodes a functional RNA
Allele
- One possible alternative forms of a gene
Many different alleles…
Can exist for a single gene
Same Alleles
- If 2 copies of a gene have identical DNA sequences
Different Alleles
- If 2 copies have 1 or more differences in DNA sequences
Locus
Where a gene is located on a chromosomes
- Same genes should be in the same location
- Different alleles of the same gene will be at the same locus
Where do new alleles come from
Mutations
- When a mutation occurs that changes the DNA sequence of a gene, a new allele is created
Haploid
Each individual only has 1 copy of each genes
- Different alleles maybe present in a population but individuals cannot have more than 1 copy of a single allele
Diploid
Each individual carry 2 copies of each gene
- Can carry at most 2 alleles
Homozygous
Both copies of gene are the same in an individual
Heterozygous
Both/two copies are different
- For any single genes: can be heterozygous for 1 gene and homo for another
Wild Type
Allele that is most common in a population
- AKA: normal, functional
- geno and pheno that are found most commonly in nature
Mutant Allele
Another allele that is not the wild type
- contains modified genetic info
- geno and pheno that differ from wild type due to an alteration in the genome
Loss of Function
Any mutation that decrease function of gene (expression, efficiency)
- there are more ways to decrease function than to increase function
Gain of Function
Any mutation that increases function of gene
- less common than loss of function
Dominance
Describes relationship between two genes
Complete Dominance
AKA: Simple dominance
Only 1 phenotype visible
- visible: dominant, not visible: recessive
- shown in mendel’s pea plants
Gene Dosage
How many copies of a gene there are
- the more copies of a gene -> the more gene products when they are expressed
Haplosufficient (more common)
In diploids many genes only require a single copy for wild type function
- 1/2 normal amount of gene product is sufficient enough for gene function
- if haplosufficient individuals carrying 1 functional allele and 1 nonfunctional allele (hetero) it will display wild type pheno
- more common bc most genes are haplosufficient
HaploINsufficient
If 1/2 normal amount of gene product is NOT sufficient for normal function
- dosage sensitive: requires more than 1 copy of function for wild type function
-
Words for genes
- haplosufficient and haploinsufficient
Words for alleles
- dominant and recessive
- loss and gain of function
Loss of Function Summary
Cause by mutations that decrease gene expression or activity
- usually recessive to wild type alleles for haplosufficient
- often dominant over wildtype for haploinsufficient
- more common than gain of function
Gain of Function Summary
Caused by mutations that increase gene expression or activity, or confer a new expression pattern
- often dominant over wild type alleles
- rare
Chromosomal Theory of Inheritance
Pre-dates our understanding of DNA as genetic material
- We know now: Chromosomes carry genes, which are made of DNA sequences
Diploid Cell
Carries two copies of each chromosome
- Humans: 2n=46
Haploid Cell
Carries one copy of each chromosome
- Humans: n=23
What does number of Chromosomes (haploid #) tell us about a species?
A.) species with more chromosomes have more genes
B.) species with more chromosomes have larger body size
C.) species with more chromosomes have greater complexity (more cell types, more sophisticated behavior, etc.)
D.) all of the above
E.) none of the above
E.) none of the above
- haploid/chromosome # has no correlation with complexity of genes (# of genes, organism size etc.)
- chromosome # is usually arbitrary
Chromosome Number and arrangement can change over evolutionary time
Human Chromosome #2 is result of fusion of 2 ancestral ape chromosomes
- human chromo #2 carries same genes as chimps (chromosomes 2a and 2b)
In Eukaryotes…
Mitosis and Meiosis: transmission of genetic material from one generation to the next
Mitosis
Leads to production of two daughter cells
- each w/ same # of chromosomes as parents
2n->2n
- two daughter cells that are identical to each other and to parents
Meiosis
Production of four Gamete cells
- each w/ 1/2 number of chromosomes
-diploid->haploid or 2n->n
- four gamete cells that genetically unique
Cell Division
Essential for growth
- when diploid goes through mitosis: produces two daughter cells w/ complete sets of chromosomes
Cell Cycle
Continuous alteration btwn division and non-division
Phases
- Interphase
- Mitosis
Mitosis phases
Division Phase
1. Prophase
1.5. Prometaphase*
2. Metaphase
3. Anaphase
4. Telophase
*sometimes not considered a phase
Interphase phases
Non-division Phase
-G1
-S Phase
-G2
G0 phase
Not in Cell Cycle (interphase or mitosis)
- when cells exit the cell cycle but can re-enter the cell cycle
Quiescent G0 stage
- not dividing or preparing to divide
S Phase (Interphase)
Cell’s DNA is replicated to produce two identical copies
Mitosis (what happens)
Each Daughter cell gets a complete copy of the cell’s DNA
In eukaryotes DNA is…
Found in the cells nucleus
Interphase Chromatin
Unfolded state of chromatin that DNA is in for most of the cell cycle
Mitotic Chromosomes
DNA is replicated and condensed into chromosomes
- for preparation of cell division
Sister chromatids
The two identical DNA molecules that each mitotic chromosomes contain
- b/c mitosis occurs after DNA replication
Structure of a condensed mitotic chromosome
Centromere
- point of connection btwn sister chromatids (picture in notebook)
- where spindle fibers will attach to the chromosomes during mitosis
- centromere location can vary: can be high or low on the sister chromatids
Chromosomes exist…
In homologous pairs in diploid organisms
Homologous Chromosomes
Can be identifies by their similar size and centromere position
- one homolog is “maternal” the other is “paternal”
- carry the SAME GENES but may carry DIFFERENT ALLELES
DNA Replication…
Produces identical sister chromatids (picture in notebook)
Human Cells prior to Mitosis
Contains 46 chromosomes, 92 chromatids
- each replicated chromosome (46) consists of two sister chromatids (46x2=92)
Before mitosis
- the 2 sister chromatids share a centromere & are considered part of a single chromosome
Interphase (not part of mitosis)
Chromosomes: extended and unfolded, forming chromatin
- DNA replication occurs: S Phase
- cell spends most of its time in interphase
Prophase (pictures in notebook of all steps)
Chromosomes condense
- sister chromatids are already attached at the centromere
- the nuclear envelope breaks down
- centrioles migrate to opposite poles
Metaphase, including Prometaphase
Spindle fibers form
- chromosomes align at the metaphase plate
Anaphase
Centromeres split and sister chromatids separate (called: Disjunction)
- sister chromatids are now called daughter chromosomes
- daughter chromosomes migrate to opposite poles
Telophase
Daughter chromosomes arrive at opposite poles
- Cytokinesis occurs: division of the cytoplasm
- chromosomes decondense and nuclear envelope reforms
Overall: Mitosis
Produces two daughter cells that are genetically identical to the parent cell
Meiosis
Produces: 4 gametes (or spores) w/ 1 haploid set of chromosomes
- reduces chromosome # diploid to haploid
Fertilization
Fusion with two haploid gametes
- restores diploid # of chromosomes in the next generation
- haploid to diploid
Meiosis consists of…
Two consecutive divisions
- meiosis 1 and meiosis 2
Meiosis 1
“Reductional” Division
- homologous chromosome pair: segregate during Anaphase 1
- reduces # of chromosomes by 1/2
Meiosis 2
“Equational” Division
- sister chromatids separate during Anaphase 2
- the # of chromosomes stays the same
- mechanisms are similar to mitosis
DNA replication for meiosis occurs…
During interphase before meiosis 1
- DOES NOT occur before meiosis 2
Prophase 1
Homologous chromosomes pair
- Synapsis: to form tetrads, 4 chromatids align for crossing over (allows for crossing over)
Crossing over: Chiasmata
- form between non-sister chromatids, allows reciprocal exchange of DNA, aka: crossing over
- the actual “crossing” of chromatids, form an “X”
Tetrads
Physical correlation of 4 chromatids
Crossing Over
During Prophase 1
- results in genetic exchange between homologous chromosomes
- increases genetic variation
- reciprocal: same exact section is crossed, down to the base pair
Metaphase 1
Tetrad align at the metaphase plate
- two tetrads align in metaphase 1
Anaphase 1
Tetrads separate and 1 homolog: dyad, from each pair migrate to each pole this is called disjunction
- each chromosome (pair of sister chromatids) is now: dyad
- Sister chromatids DO NOT separate during meiosis
Disjunction
Migration of chromosomes to each pole
Dyad
Physical correlation of 2 chromosomes
- half a tetrad
Telophase 1
Dyads reach opposite poles
- chromosomes DO NOT decondense back into chromatin
Daughter cells either (depends on species)
- enter a short interphase (w/ no DNA rep)
- or proceed directly to meiosis 2
Meiosis 2 (process)
Each cell starts w/ 1 of the original pair of homologous chromosomes
- starts haploid, ends haploid
During:
- sister chromatids in each dyad separate
- daughter chromosomes migrate to opposite poles
Resulting Gametes:
- receive 1 chromosome from each original homologous pair
Key differences between mitosis and meiosis
Chromosome #:
- meiosis: reduced, diploid to haploid
- mitosis: maintained, diploid stays diploid
Produces:
- meiosis: 4 gametes, genetically distinct
- mitosis: 2 daughter cells, genetically identical to each other and parents
Crossing Over:
- meiosis: occurs in Prophase 1
- mitosis: does not occur
Homologous chromosomes:
- meiosis: HC pair and segregate
- mitosis: line up and divide independently
Mitotic Daughter Cells receive…
Both alleles of every gene
- both maternal and paternal homolog
- each end up with 1 copy of each chromosome
- this is how and why daughter cells of mitosis are identical
Meiosis products receive…
each gamete gets either the maternal or paternal homolog, but never both
- get only 1 allele of each gene
Genotype, Phenotype and allele frequency of gametes (Meiosis)
All possible gamete types are formed with equal frequency during meiosis
- tetrads align randomly at metaphase 1: all possible metaphase arrangements will occur with equal frequency
Mendelian (Transmission) Genetics
How are traits (phenotypes) transmitted from parent to offspring
Gergor Mendel (1822-1884)
Father of Modern genetics
- did not use the term “genes” or “genetics”
- was not aware of chromosomes or DNA
Experiments in Plant Hybridization (1866)
Choose garden peas
- studied 7 characteristics
Mendel choose garden peas b/c…
Easy to cultivate
- grow to maturity in 1 season
- true-breeding strains were available
- strains have observable characters w/ distinct forms
- matings could be controlled: self-fertilization or cross-fertilization
Mendel studied 7 characteristics
Seed shape: round/wrinkled
Seed color: yellow/green
Pod shape: full/constricted
Pod color: green/yellow
Flower color: violet/white
Flower position: axial/terminal
Stem height: tall/dwarf
Keys to Mendel’s Success
Focus on one or a small # of traits
- kept accurate and quantitative records
True Breeding
Actually homozygous
- produces identical offspring every time
- what Mendel started with
Mendels Approach
Interbreed distinct strains and observe the appearances of the offspring
1.) remove stamens from purple flowers
2.) transfer pollen from stamen fo white flower to carple of purple flower
3.) pollinated carple mature into pod
4.) plant seeds from pod
5.) examined offspring: all purple flowers were produced
Crosses
Parental generation: P1
-female parent x male parent= first filial
First Filial: F1 generation
P1->F1->F2->F3->F4…
Monohybrid cross
Always starts with true-breeding/homozygous
- crossed to generate F1 that are heterozygous for 1 gene
Mendels observations of first monohybrid cross
F1 hybrids displayed 1 of the parental phenos but not the other
In mendels pea plants violet allele is _____ and white allele is ____
dominat, recessive
NOT: haplosufficient/insufficient bc these words describe GENES NOT ALLELES
Complete Dominance, aka: simple dominace
Phenotype of only 1 allele is visible in a heterozygous
- visible: dominant
- not visible: recessive
- does not tell us which is wild type (most common in nature)
- violet allele exhibits complete dominance over white allele
Mendel’s Monohybrid Cross #2
Crossed his monohybrid F1 together
- recessive pheno reappeared in F2
F2: displayed a 3:1 ratio, purple:white
Mendel Observed discrete inheritance
Discrete inheritance not blending inheritance
- F1 were not intermediates between the 2 parental phenos, and always resembled 1 or the other parent phenos
- recessive traits reappeared unchanged in F2
Discrete Inheritance
only 1 phenotype is shown at a time
Blending inheritance
a combination/intermediate of both phenotypes
Mendel found
Complete dominance for all 7 traits
- F1 always looked like parents
- F2 always shows recessive phenotype
- not sex-linked
How to write genotypes (textbook)
1st letter of recessive traits is used as symbol
- dominant: uppercase
- recessive: lowercase
Mendelian Ratio
3:1 phenotypic ratio
- F2 generation of a monohybrid cross
Mendel’s first three postulated
By observing results of monohybrid crosses Mendel came up with these:
1.) Unit factors occur in pair
- genetic characteristics are controlled by unit factors (genes) existing in pairs
2.) Dominance/Recessiveness
- when 2 unlike unit factors are responsible for a single characteristic, 1 factor is dominant over the other, recessive
3.) Segregation
- during formation of gametes the paired unit factors segregate randomly, so that each gamete receives 1 or the other w/ equal likelihood
Modern correlation of mendel’s 3 postulated
1.) Unit factors occur in pairs
- unit factors = genes
- diploid organisms carry 2 copies of each gene (1 maternal and 1 paternal)
2.) dominance/recessiveness
- in heterozygous the phenotype of 1 allele is often dominant over the other
3.) Segregation
- homologous chromosomes separate during meiosis 1
- each gamete receives either maternal or paternal homolog
The pea (Pisum Sativum) genome
We only know how the complete genome for mendels plants
Mendelian Traits
Trait/phenotype whose inheritance is consistent with mendel’s poslulates
- mendelian pattern of inheritance
Practical Applications of Mendelian Genetics
Can use mendelian principles to
- predict genotype and phenotype of offspring
- calculate probabilities that a specific trait or combination of traits will appear
Punnett Squares
used to predict genotype and phenotype w/ frequencies, predict probabilities
1.) predict all gamete types that can be produced
2.) draw table with same # of rowa as parental gamete types
3.) fill in table by combining gametes in each roe and column
4.) resulting gametic combinations represent progeny genotypes
Typical mendelian ratios for F2 generation of a monohybrid
For traits that exhibit a mendelian pattern of inheritance
- Results from cross btwn 2 monohybrids: geno 1:2:1, pheno 3:1
What to use mendelian principles to…
make predictions about progeny geno and pheno
- sometimes additional info is available which may rule our 1 or more possibilities, altering the probability or an out come
In a standard monohybird cross what is teh probability that an F2 individual will be heterozygous?
A.) 25%, 1/4
B.) 50%, 1/2
C.) 66%, 2/3
D.) 75%, 3/4
E.) 100%, 1/1
B.) 50%, 1/2
In a standard monohybrid cross what is the probability than an F2 individual displaying the dominant phenotype is a heterozygous?
A.) 25%, 1/4
B.) 50%, 1/2
C.) 66%, 2/3
D.) 75%, 3/4
E.) 100%, 1/1
C.) 66%, 2/3
Mendelian Inheritance Pattern rule
1.) if at least 1 parent is homozygous dominant, none of the progeny will exhibit the recessive phenotype
2.) 2 homozygous recessive parents cannot produce an offspring which displays the dominant phenotype or carry dominant allele
3.) individuals displaying the dominant phenotype maybe homozygous dominant or heterozygous
- phenotype alone cannot distinguish them
4.) even if we don’t initially know geno of parent we maybe able to deduce it based on the pheno ratio of their progeny
- testcross
Homozygous dominant and heterozygous are…
Phenotypically indistinguishable
- someone displaying dominant phenotype maybe homozygous dominant or heterozygous (GG or Gg)
- when uncertain write: G- (GG or Gg)
- can use a testcross to determine the genotype
Test Cross
To determine if homozygous dominant or heterozygous
- used because an individual displaying the dominant phenotype can be either homo dom or hete
- if recessive phenotype shows in 1 of the offspring the tested individual must be heterozygous (“carrier”)
Mendel observed the same inheritance pattern in all 7 of his monohybrid cross experiments
Complete dominance in F1 generation
- 3:1 phenotype ratios in F2 generation
Dihybrid Cross
Two pairs of contrasting traits
- mendel’s F1 dihybrids displayed dominant phenos for both traits regardless of traits parents presented
- 9:3:3:1 ratio
Mendelian inheritance with multiple genes
- Principles that govern mendelian inheritance of a single gene also apply considering multiple genes
- punnett squares can be used to predict the inheritance of multiple traits in a single cross
- remember that a key step in building a punnett square is determining what types of gametes will be produced by the parents
Possible gametes
All possible gamete types will be produced with equal frequency
- # of possible gametes depends on: # genes, chromosomes, and alleles
- For now: consider two or more genes to be on different chromosomes (but two or more genes can be on the same or 2 different chromosomes)
Mendel’s Dihybrid Cross
Began with true-breeding lines
- P1 plants: homozygous for 2 genes (seed color and seed shape)
Individuals that are Homozygous can…
Can only produce 1 gamete types
- “true-breeding” lines are can only produce 1 gamete type no matter how many genes or traits are under consideration
Individuals that are heterozygous can…
Can only produce 4 gamete types
- Dihybrid: individual that is heterozygous for 2 genes
9:3:3:1
Mendelian ratio typical of F2 generation or a dihybrid cross
- this ratio reflects independent inheritance of the two traits
9- 9/16 display both dominant phenotypes
3- 3/16 display 1 dominant and 1 recessive
3- 3/16 display 1 dominant and 1 recessive
1- 1/16 display both recessive phenotypes
- expected 3:1 ratio for each individual trait still stands
Dihybrid Cross
Each trait that is inherited independently, so a dihybrid cross is really a combination of 2 monohybrid crosses
The Product Law
Used to predict frequency with which 2 independent events will occur simultaneously
- probability of each plant bearing yellow or green seeds is independent of probability of each plant bearing round or wrinkled seeds
p(yellow, round) = p(yellow) * p(round)
= 3/4 * 3/4= 9/16
Mendel’s 4th postulate
4.) Independent Assortment
- during gamete formation, segregating, pairs of unit factors assort independently of one another
Modern Correlation of 4th Postulate
During metaphase 1, all possible tetrad arrangements will occur w/ equal frequency and each gamete has an equal probability of receiving the maternal or paternal allele for each gene
- when homologous chromosomes separate from each other during anaphase 1
What is the probability that an F2 plant with round yellow seeds has a genotype GgWw?
A.) 1/16
B.) 2/16
C.) 4/16
D.) 9/16
E.) 4/9
E.) 4/9
How can we determine the genotype of a round yellow F2 plant?
Perform a test cross
- displays both dominant phenotypes
- uncertain genotype
Summary of Mendel’s Work
Inheritance is particulate, not blending
- inheritance is controlled by discrete factors (genes)
4 Postulates: gene/traits that follow these exhibit a mendelian pattern of inheritance
Mendel’s final 4 postulates
1.) genes occur in pairs, each individual carries two alleles
2.) heterozygous: 1 allele is dominant and 1 is recessive
3.) paired alleles segregate away from each other during gamete formation
4.) alleles of 2 different genes will assort independently of each other
- genes/traits that follow these rules exhibit mendelian pattern of inheritance
All Mendelian Ratios
monohybrid SELF cross (Aa x Aa)
1:2:1 genotype
3:1 phenotype
monohybrid TEST cross (Aa x aa)
1:1 geno
1:1 pheno
dihybrid SELF cross (AaBb x AaBb)
9:3:3:1 pheno
dihybrid TEST cross (AaBb x aabb)
1:1:1:1 geno
1:1:1:1 pheno
Mendelian principles are the result of:
Molecular nature of genes and alleles
- biochemical roles of gene products (RNA and protein)
- cytological behavior or chromosomes during meiosis
We can use mendelian principles to:
Predict geno and pheno of offspring
- calculate probabilities that a specific trait or combination of traits will appear in a given individual
You should be able to…
Deduce dominance relationships and genotypes from written description
- Mendel crossed pea plants w/ full yellow pea pods to green constricted. All F1 plants had full green (example in notebook lecture 11/2)
Mendelian ratios, gamete formation and predictions
Understanding gamete formation is key to understanding mendelian ratios and making predictions
- how many gamete types can be produced
- what are the possible gamete genotypes
Monohybrid ratios (1:2:1, 3:1) are key to understanding inheritance of multiple traits since individual traits undergo independent segregation
Monohybrids
Produce 2 gamete types
Aa-> A and a
Dihybrids
produce 4 gamete types
AaBb-> AB, Ab, aB, ab
Trihybrids
Produce 8 gamete types
How to find how many gamete types can produce
n= number of genes
2^n= number of gametes produced
- individuals heterozygous for n genes can produce 2^n different gamete types
Mendelian principles apply to any # of traits
Trihybrid and on
- product rule is easier than Punnett squares for large # of traits
Many bacterial and single-celled eukaryotes reproduce by…
Cell division (mitosis)
- some can exchange genetic material through non-sexual means
Some organisms can alternate between asexual and sexual reproduction
- yeast
- chamydomonas
In many diploid eukaryote…
Sexual reproduction is the sole method of producing new individuals
In Sexually reproducing species how are “male” and “female” typically distinguished?
D.) male and females produce gametes of different sizes
In Sexually reproducing species “male” and “female” are typically distinguished by gamete size
“female”: produce larger gametes
“male”: produce smaller gametes
Isogamy
Gametes are morphologically indistinguishable
- often divided into mating types
- yeast: a and alpha
- chlamydomonas: + and -
Anisogamy
Gametes are morphologically distinct
- usually different sizes
Sex Determination in humans
Typical human biological sex determination
- female: 46, XX karyotype
- male: 46, XY karyotype
- there are a few expectations
*Use “male” and “female” to describe primary and secondary sex characteristics (“biological sex”)
*Biological sex is not the same thing as gender or sexual orientation/identity
Biological sex is a…
Spectrum not a binary
“Sex” characteristics in humans are highly variable
Don’t always correlate with each other
- Sex assignment at birth is usually based ONLY on external genital appearance, does not always match sex chromosomes
- natural spectrum of human development includes intersex variations
Sexual Dimorphism
In Anisogamous species male and female often exhibit genetic and morphological differences
Sex Determination
Controls whether and embryo (or individual cell) will display male or female sexual characteristics
Genotypic Sex Determination (GSD)
Sex is determined genetically by the chromosome or gene composition of a cell or organism
Environmental Sex Determination (ESD)
Sex determined by environmental conditions encountered during an organisms development
- temperature, day length, proximity of conspecifics
Sex Chromosome
Chromosome that exhibits sex-specific differences in individuals of the same species
- human X and Y chromosomes
Autosome
Any chromosome that is NOT a sex chromosome
- human chromosome I-22
Homogametic Sex
Sex in which both sex chromosomes are the same
- XX in humans
- in organism with GSD
Heterogametic Sex
Sex which has two different Sex Chromosomes
- XY in humans
- in organisms with GSD
Vertebrates employ many different sex determination systems
Sex determination systems have changed many times during vertebrate evolution
Mammals: GSD
- female: XX, male: XY
Birds: GSD
- females: ZW, male: ZZ
Crocodilians: ESD
- female: low or high temp, male: intermediate temp
Sex Determination in Insects
Drosophila (fruit fly): GSD
- female: XX, male: XY
Butterflies: GSD
- female: ZW, male:ZZ
Bees: Haplodiploidy
- no sex chromosomes
- females: diploid, male: haploid
In human opterans (bees, ants, wasps)
Sex is specified by # of chromosomes\ sets
- female: diploid
- male: haploid
Sex Determination in Drosophila
Y chromosomes is not involved in Sex determination
- ratio of X chromosomes to sets of autosomes determines sex (X to A ratio, X:A)
X= # of X chromosomes
A= # of haploid sets of autosomes (normally, 2)
- Y chromosome is not required for maleness but is required for male fertility
- Drosophila example in notes
The Y Chromsome in Humans
Y chromosome initiates development of mae features in humans (and other mammals)
- XX (46, XX female)
- XY (46, XY male)
Sex-Determining region Y (SRY)
Gene on the Y chromosome that initiates male development
Features of SRY
- Encodes testis determining factors (TDF), key regulator of male-specific gene expression
- loss of function mutations in SRY can promote female development in XY individuals
- translocation of SRY (transfer to a different chromosome) can promote male development in XX individuals
Human sex chromosome variation can result in Differences Sex Development: DSD
Klinefelter Syndrome
- 47, XXY
- trisomy, 2n+1
Turner Syndrome
- 45 XO or 45 X
- monosomy, 2n-1
Human Male Development
SRY gene on the Y chromosome promotes male development in humans
Dosage Compensation
In flies and mammals: male and females have different # of copies (“doses”) of X-linked genes
- males typically have 1 copy (XY)
- females typically have 2 copies (XX)
Dosage Compensation in Flies and Humans
Human:
XX- 1 chromosome inactive, 1 active
XY- single chromosome active
Flies/ Drosophila:
XX- both chromosomes active
XY- single chromosome hyperactive
Dosage compensation in Drosophila
- MSL= Male Specific Lethal
- MLE= maleness
- MOF= males absent on the first
- x-linked genes in males upregulated by the dosage compensation complex (DCC)
Dosage cokmp in mammals
X-chromosome inactivation: all but 1 x chromo is inactivated in each cell of early mammalian embryos
- randomly chooses 1 X to inactivate, each cell has a different random X that is inactivated
The Lyon Hypothesis
Either the maternal or paternal X- chromo is inactivated in each cell
- inactivation occurs randomly in early embryos
- x-inactivation is heritable through mitosis; the same x chromo that is inactive in parent cell will be inactive in daughter cells
- x-inactivation is reset (erased) during meiosis
- x-chromos are inactivated through a reversible epigenetic mechanism
Barr Bodies
Inactivated X-chromos condense into “barr bodies”
- intact x-chromos: highly compressed and transcriptionally repressed, can be seen under a microscope
- All but 1 x-chromo is inactivated no matter how many X’s are present in both sexes
# of barr bodies= # X-chromos - 1, in either sex
Phenotypic Consequences of X-chromo Inactivation
Female heterozygote for x-linked genes can display a “mosaic” phenotype
- example slide #11 (11/7/22)
- tortishell cats; almost always female, tortishell pattern reflects random x-inactivation
X-Chromosome inactivation is a reversible epigenetic process
- inactivated x-chromos are not destroyed or degraded in any way are transcriptionally inactive
- epigenetic: does not involve changes in DNA sequence, it is a change in chromatin state
- x-inactivation is erased during meiosis and the chromosome is passed onto the progeny in its original, active state
Hemizygous (Hemizygote)
If a diploid individual only carries 1 copy of a given gene
- phenotypically identical to homozygous
- In XX/XY species, XY individuals are hemizygous for X-linked genes
Chromosomal basis of x-linked inheritance
X and Y act as a homologous pair during meiosis and gamete formation
Unique aspects of X-linked inheritance
- XY parents typically do not pass their X to their XY offspring, they pass Y instead, X would come from the other parent
- XY need to receive inly 1 recessive allele from 1 parent to display recessive pheno, but XX need to receive 2 recessive alleles to show recessive pheno
- a heterozygous XX carrier for x-linked trait can produce XY offspring who display the recessive pheno regardless of the XY parents x-chromo geno
*X-Linked traits often deviate from Mendelian ratios
X-linked recessive conditions affect males more than females
- because they only need 1 copy of the recessive allele where females would need 2 copies
- red-green colorblindness
Limits of Mendelian inheritance, in mendels experiments
Every trait was controlled b ya single gene
- phenotype was determined solely by the genotype
- every plant with the same geno displayed the same pheno
- this is not always the case: mendel was lucky
Some phenotypes are continuous (non-discrete)…
..and influenced by multiple genes
Sometimes indivudals w/ the same genotype can display different phenotypes
Penetrance
- what percentage of individuals w/ the same genotype will express the phenotype
- some do not display pehnotype
Expressivity
- to what degree is the phenotype expressed in any 1 individual
- display phenotypes in a range of severity
Why does this happen?
- biology is messy
- characteristics can be dependent on more than one gene
example in notes: polydactyly
Factors that influence penetrance and expressivity
Environmental
- temperature, humidity, diet
- siamese cats, temp affects melanin production in fur and can change their colors
Polygeneic (quantitative traits)
some traits are determined by multiple genes acting together
- can show continuous or discontinuous variation
Continuous Variation
- traits are distributed around the average value
Discontinuous Variation
- either one of the other
- plants are either tall or dwarf
Human height
is heritable but also strongly influenced by environmental factors
