Genetics Exam 1

true-breeding

• used in parental generation because it allowed for results to be reproduced because parent plants in every experiment would always have the same genotype

blended inheritance theory

• explain what physical traits are passed from parent to progeny

• progeny should have intermediate phenotype (a mix) somewhere between that of both parents

• example

  • a cross between a small dog and a large dog would yield all medium sized dogs

reciprocal cross

• addressing that the male gamete or parent who creates male gamete, contributes to it more somehow to the phenotype of the progeny than the female parent does

• this experiment suggests that each parent is equally contributing to the progeny and the phenotype the same amount

• swap which parent is carrying which gene

monohybrid cross

• h = always crossing 2 homozygous parents to get heterozygous offspring in F₁ generation

• m = looking at one trait

• used to examine the inheritance of a single trait over a generation

• 3:1 ratio

dominant trait

• dominant trait is displayed in F1 generation when starting with true breeding parents

recessive trait

• displayed only in the F2 generation

  • small number of individuals in F2 generation

theory of particulate inheritance

• traits are determined by discrete units that are inherited intact through generations

  • inherited from on generation to the next, in other words

• each individual has 2 particles in genotype, but they pass only 1 particle on to progeny and that way you can produce progeny that also has two particles

• every parent has 2 particles which give rise to a phenotype and each parent passes just one of those particles onto the progeny

principle of segregation

• when a parent makes gametes, those 2 particles must segregate or separate from each other so that only one particle is present in the gamete

  • a gamete from each parent comes together to create the progeny

• sum: the idea that these particles, these genes have to separate from each other and then we create 2 different gametes. gametes can form in equal frequency

  • every individual has 2 particles

• must separate so each parent can create 1 particle or one copy of every gene

  • happening during anaphase 1 of meiosis

• homologous chromosomes separating from each other during gamete formation

test cross

• taking unknown individual (individual with dominant phenotype) and crossing it to a homozygous recessive individual

back cross

• depends on genotype and type of parent and F1 generation but not always true

chromosome

• physical structure that carries genes

• double-stranded DNA molecule containing genetic information (genes) arranged in a linear sequence

• in eukaryotes, contains DNA, RNA, and proteins

• in dispersed state (called chromatin) throughout nucleus during most of cell cycle

• coils up and condenses during MEIOSIS and MITOSIS

  • cells is getting ready to divide, DNA begins to condense and starts to form a linear structure

centromere

• constricts region that happened somewhere a long to the length of chromosome

• no genes in this region

• contains highly repetitive DNA and it provides a binding site for the proteins that are going to be important when the chromosome has to move during mitosis or meiosis

telomere

• at the ends, no genes in region → just highly repetitive DNA

metacentric

• centromere in center of length of chromosomes

sub-metacentric

• centromere is slightly toward one end of the chromosome

acrocentric

• centromere is moving closer to the end of the chromosome

• p arm

  • shorter arm

• q arm

  • longer arm

telocentric

• centromere is essentially all the way at one end

ploidy

• number of sets of chromosomes

somatic cells

• gametes have half as many chromosomes as __

homologous chromosomes

• same length

• same centromere placement

• same genes

• not necessarily same alleles

• not identical

• 2 copies

  • same length centromere placement but will no be identical because they wont have the same alleles

    • form of gene

• both will be the same chromosomes but they will not necessarily be identical

mitosis

• parent cell (2n) → two daughter cells, each 2n

• summary

  • 1 division

  • 1 cells → 2 cells

    • identical daughter bodies

  • creates somatic cells

meiosis

• parent cell (2n) → four daughter cells, each n

• summary

  • 2 divisions

  • 1 cells → 4 cells (non identical)

  • creates gametes

  • a single cell creates 4 non-identical gametes

cell cycle

• G₀: non-dividing phase

• G₁: 1st gap or growth phase

• S: DNA synthesis

• G2: 2nd gap or growth phase

• M: mitosis phase

• note

  • G₁, S, G₂ = INTERPHASe

    • DNA is diffuse through the whole cell

G₀

• non-dividing phase

• cell is in stable state at constant size

G₁ phase

• 1st gap or “growth” phase

• proteins needed for cell divisions are made

• making the proteins it needs so it can

S phase

• DNA synthesis or being made

• all chromosomes are duplicated

• we are copying all the chromosomes in the cell

• necessary because when the cell divides 2 daughter cells are created and each one has to have the same number of chromosomes

• starting cell must DOUBLE the number of chromosomes before division happens, otherwise it would be impossible to create those 2 equal selves

• cells MUST go through this phase (chromosomes are replicated) before mitosis

G₂ phase

• 2nd gap or “growth” phase

• the cell is rechecking all of the DNA that just got synthesized to make sure it’s okay and then preparing to go through and do division

M phase

• mitosis phase

• cell division

• DNA is going to start to condense and form the linear chromosomes

interphase mitosis

• DNA is diffuse during this phase (G₁, S, G₂) in nucleus

counting chromosomes

• count number of centromeres

counting chromatids/DNA molecules

• count the telomeres on one end

prophase mitosis

• chromosomes condense

• mitotic spindles form (outside of nucleus)

• nuclear envelop breaks down

• mitotic spindles are important for attaching to the chromosomes, holding the chromosomes in the center, and then ultimately for pulling chromatids apart from each other

• 4 chromosomes and 8 chromatids or DNA molecules

metaphase mitosis

• microtubules from spindle pole attach to each chromosome at centromere

• chromosomes are moved to metaphase plate in center of cell

• = ORGANIZATIONAL PHASE

• chromosomes are lining up at the center of the cell on the metaphase plate

• they are held together by the microtubules that are extending from the spindle pulls on either side of the cell

• they are lining up in one line down the middle

• microtubules are extending and attaching to the centromeres

  • what is holding all the chromosomes together

anaphase

• ACTION

  • sister chromatids are going to pull apart from each other

• proteins holding sister chromatid centromeres together are degrades

• sister chromatids separate (disjoin)

• sister chromatids move to opposite spindle poles

• problem that can occur

  • nondisjunction

    • for failure of the chromatids to separate from each other

• chromosome number in cell has now DOUBLED, still one cell but more chromosomes present (more centeromeres)

telophase

• nuclear envelope reforms around each daughter nucleus

• chromosomes uncoil or relax

• spindle disappears

• then cytokinesis (cytoplasm divides)

  • at the same time essentially

• result = 2 GENETICALLY IDENTICAL CELLS

  • clones and identical to the original cell

meiosis

• process that leads to gamete formation

• 4 haploid cells are produced from 1 diploid cell

  • 4 unique cell that has 1 copy of every chromosome

• 2 divisions

• remember: cell goes through S phase (chromosomes replicate) before meiosis

  • go through G₁, S, G₂ before we start dividing

  • every chromosome consists of 2 chromatids

• mendels theory of particulate inhertiance

  • every parent has 2 particles and will pass one particle onto their progeny

    • progeny then has 2 particles

    • particles = chromosomes

    • every parent has 2 copies of each chromosome and will pass 1 copy of chromosome to their progeny

  • gametes are the cell that contain one copy of every particle

    • this is the process of creating those gametes

prophase 1

• chromosomes condense

• homologous chromosomes pair (synapse)

  • homologous find each other

• crossing over may happen (recombination)

• when homologous find each other and pair up, we can have crossing over or recombination

• diploid

leptotene

• “thin thread”

• condensation begins of chromosomes

• chromosomes begin to condense and become more tightly packed

zygotene

• “paired thread”

• synapsis begins

  • homologous find each other and line up

  • red copy find blue copy and will align

pachytene

• “thick thread”

• condensation continues

• recombination = equal exchange of chromosomal material between two homologous

  • linear structure is appearing to be thicker as the chromosomes condense more and more DNA condenses more

• has the synaptonemal complex

  • where recombination is going to happen

diplotene

• “double thread”

• synapsis breaks down

• chiasmata keep homologs joined

• contains the

  • chiasmata

    • places where crossing over happened

  • bivalent or tetrad

    • refers to the fact we have 2 homologs

    • if we have homologs pair lined up with each other, there are 4 sister chromatids

      • 4 sister chromatids = 2 chromatids from one homolog and one from the other

• summary

  • synapsis breaks down

  • homologous are coming apart from each other but they are going to be held together by protein complexes that are present along the length of the chromosome everywhere where recombination happens

diakinesis

• “moving apart”

• homologous pairs are held next to each other but there are places where there is crossing over

  • equal exchange of chromosomal material

homologous recombination

• DNA synthesis happens during this

  • process of cutting, destroying some DNA and synthesizing more DNA has to happen during this whole event

• crossing over between homologous chromosomes

• one sister chromatid from each homolog participates in a single cross-over event

prophase 1

• chromosomes condense

• homologous chromosomes pair (synapse)

• crossing over happens (or not)

  • happens along the length of every chromosome as the homologous piar

  • random event, will not always happen and will not always happen in the same place every time

metaphase 1

• each pair of homologs takes up a position on metaphase plate

  • homologous pairs lining up

• homologous pairs align here (not like in mitosis)

• orientation of each pair of homologs with respect to all other is random (independent assortment)

  • aligns with mendels second law: the law of independent assortment

anaphase 1

• members of homologous pairs disjoin and move to opposite poles

  • each member of the homologous pair is getting pulled to one side or the other side of the cell

• sister chromatids are still attached but homologous pairs are disjoining

• Mendel’s Principle of Segregation

  • separating of the particles, the genes on the chromosome and they are separating and segregating from each other so that we can create gametes that will only have one particle

telophase 1

• nuclear envelope reforms

• interkinesis happens (cytokinesis between meiosis 1 and 2)

  • dividing up cytoplasm to create 2 separate cells

• results in 2 haploid cells

  • each cell here has only one copy of every chromosome

  • the reduction division

    • reduced number of chromosomes in each cell by half

• total number of chromosomes in each cell is half of starting number

prophase 2

• chromosomes condense

• spindle forms

• nuclear envelope breaks down

• similar to mitosis’s

metaphase 2

• individual chromosomes position in equatorial plate

• looks like mitosis except that each of these cells has half as many chromosomes

anaphase 2

• ACTION = pulling part of sister chromatids

• sister chromatids are pulled to opposite poles

• chromosome number in cell has now doubled

telophase 2

• nuclear envelop reforms

• cytokinesis happens

• 2 cells → 4 cells

meiosis 2 products

• results = 4 UNIQUE HAPLOID gametes

• n = chromosomes

  • n = 2

• unique because recombination has occured

• everyone has one copy of each chromosome but its different because of recombination

principle of independent assortment

• when pairs of homologous chromosomes line up on the metaphase plate during metaphase 1 of meiosis, the way one pair lines up has no impact on the way any other pair of chromosome lines up

• when we form gametes, pairs of homologous chromosomes will align independently of one another at the metaphase plate

• genes on different chromosomes move independently during meiosis during gamete formation

dihybrid cross

• allows us to look at the inheritance of two traits over several generations

• 9:3:3:1

• same results as the other one

• demonstrates that inheritance of 2 traits are independent from each other

probability is used

• what are the chances of pulling out an individual that has certain traits

• what is the frequency of that particular trait within my group

• what are the chances an event will occur?

  • event could be choosing an individual of some given trait

• not asking for all the possibilities

addition rule

• probability that either 1 of 2 MUTUALLY EXCLUSIVE events happens is the sum of their independent probabilities

• ALWAYS make sure that the events are mutually exclusive in order to do this type of problem

• P (this) OR P (that) → ADD

• example

  • what is the probability of being a sophomore or a senior?

mutually exclusive

• one of the events happens then the other event cannot happen

• one event PREVENTS the other from happening

• examples

  • turning left and turning right

  • eating a snack and taking a nap

multiplication rule

• probability that 2 INDEPENDENT events happen simultaneously or in a particular order is the product of their independent probabilities

• P (this) AND P (that) → multiply

• if you can restate the question to be an AND statement, multiply

• example

  • what is the probability of being a female senior

  • reworded: what is the probability of being a female and a senior

independent events

• have no influence on each other so the outcome of one event is not going to impact any other event

binomial expansion

• used to calculate P of several events when they can happen in any order

• n = total number of events

• s = total number of 1st type of event

• t = total number of 2nd type of event

• a = P (1st event)

• b = P (2nd event)

• to use this equation, you must be given multiple independent events that can happen in any order

chi-square test

• compares observed values (experimental results) with expected values (calculated based on given information and current understanding of the system)

• we determine p-value

p-value >0.05

• do not reject H₀

• highly likely the difference is due by chance

p-value <0.05

• reject H₀ (if the p is low, reject that Ho!)

• not likely that the difference from observed and expected values is due by chance, something else is at play

• unlikely that is it due by chance

best guess

• mode of inheritance where we make the fewest number of assumptions for individual genotypes as we are going through and analyzing

consanguinity

• often a clue that trait is autosomal recessive

• its mating between related persons

dominant trait in pedigree

• 1 parent always shows phenotype in every generation

recessive trait in pedigree

• skips generations

autosomal trait in pedigree

• will see equal numbers of both sexes

• both sexes are affected equally

incomplete penetrance

• individual has disease allele but does not show trait

• can make a dominant trait appear to skip generations

variable expressivity

• individuals with disease allele express trait differently

• can make a dominant trait appear to skip generations

sex-influenced/sex-limited traits

• expression of disease allele depends upon sex of carrier

• can be passed on by an individual who does not express trait themsevles

genetic maternal effect

• nuclear genotype of the maternal parent

• the progeny gets genes for the trait in question from both parents, but the phenoptype that the progeny expresses is not determined by their own genotype but instead are determined by the genotype of their mom

• moms genotype dictates phenotype of the progeny

• often due to function of proteins very early in development (before the embryo starts synthesizing its own proteins, we are still using proteins from mom)

• proteins

  • function soon after fertilization

  • stored in egg cytoplasm

  • made by mom, so based on her genotype

• example

  • shell coiling in snails

cytoplasmic inheritance

• cytoplasmic genes, which are usually inherited entirely from only one parent

• not all genes in a cell reside in the nucleus

  • genes that are present in mitochondria, which are cytoplasmic organelles

• chloroplasts and mitochondria are cytoplasmic organelles that contain genetic material

• mitochondria

  • in humans, contains 37 genes

  • thousands mitochondria per cell

  • up to 10 copies of mtDNA in each

• in humans, a trait that displays cytoplasmic inheritance is encoded by genes found in the mitochondria

• during cell division, mitochondria randomly segregate into progeny cells

• in 1 cell, different mitochondria could have distinct mutations that affect overall phenotype

  • different cells will have different numbers of mutant mitochondria

• from mother to offspring because egg provides cytoplasm

  • traits are expressed in both males and females

  • traits show lots of variation

    • different number’s of mitochondria’s that have mutations

• sperm: gives paternal set of chromosomes

• egg: gives maternal set of chromosomes and mitochondrial DNA

• characteristcs

  • present in males and females

  • usually inherited from one parent, usually the maternal parent

  • reciprocal crosses give different results (just like sex-linked traits)

  • exhibit extensive phenotypic variation, even within a single family

genomic imprinting

• genes whose expression is affected by the sex of the transmitting parent

• gene expression affected by whether gene comes from mom or dad

• autosomal traits will show different results for reciprocal crosses even though they are autosomal traits

• one example of epigenetics

  • phenotype results from genotype and chemical modifications on DNA that regulate gene expression

• progeny gets genes from both mom and dad but the copy from one parent is turned off whereas the copy from the other parent is turned on

• mistakes in this in humans:

  • unusually large at birth; enlarged tongue, liver, kidneys; increased susceptibility to certain cancers; non-mendelian inheritance

  • chimeric children can result if proper imprinting does not happen in a population of cells during development

  • if both copies of IGF-2 are silenced, Silver-Russel syndrome results (unusually small body)

• example

  • Igf2 gene

    • encodes protein that promote growth (embryonic growth)

    • dad’s copy is expressed

    • mom’s copy is silenced

conditions that were true for Mendel’s experience

• all traits being considered are unlinked

  • each gene assorts independently

  • one gene codes for one trait

• every gene has 2 alleles

  • each allele specifies a unique phenotype

  • 1 allele is completely dominant over the other

    • F₁ generation would always look like one of their parents

      • not necessarily true for all traits

• genotype determines phenotype

  • if an individual has a certain gene, that gene would always be expressed and you would see the resulting phenotypes

complete dominance

• phenotype of heterozygote looks like phenotype of homozygote

• in other words, the F₁ generation resembled one parent (phenotype)

incomplete/partial dominance

• phenotype of heterozygote is intermediate between phenotypes of different homozygotes

• use R1 and R2 when you don’t know dominance

• phenotypic ratio = genotypic ratio

• 1:2:1 PHENOTYPE ratio

• 1:2:1 GENOTYPE ratio

codominance

• heterozygote has/shows phenotypes of both homozygotes

• 1 gene

  • codes for a sugar transferase enzyme (adds sugar molecule onto H antigen)

  • H antigen is on the surface of all red blood cells

• I^A codes for A transferase

• I^B codes for B transferase

• i codes for nonfunctional transferase

  • no sugar at that position

  • type O blood

• I^A > i

• I^B > i

allelic series

• any gene may exist in many forms within a population

• each individual member of the population only carries two alleles

• alleles may form a dominance series

• dominance series

  • properties of an allele in relation to another allele

• by looking at different phenotypes of various heterozygous individuals, its possible to sometimes come up with a dominance series which displays the relationship of different alleles with each other

lethal alleles

• found in essential genes

• can be dominant or recessive

• first one studied were recessive

• conclusion

  • first generation must be heterozygous

    • only way we can observe two different phenotype in progeny

  • __ must be dominant allele

    • know this because we see this phenotype in the heterozygote

• to determine dominance

  • look for heterozygote individual and see what phenotype they display

• 2:1 ratio (deviation from Mendelian ratio)

pleiotropic

• single gene influences multiple phenotypes

penetrance

• proportion of individuals with a particular genotype that display the expected phenotype

incomplete penetrance

• less than 100% of individuals with a given genotype show the expected phenotype

  • may give a trait the appearance of skipping generations

  • can allow lethal alleles to persist in a populations

    • mechanism: individual carrying the allele showing the lethality phenotype but have the potential to pass it on

  • due to carrying genetic backgrounds and environmental effects

    • genetic background

      • all genes within an individuals genome and rest of DNA that regulates expression of those genes

• if 75% penetrance means is you looked at heterozygous flies, 75 of them would show lobe and 25 of them would have normal eyes even though all 100 of them are carrying the lobe allele

• example

  • polydactyly

    • too many digits dominant allele incompletely penetrance

      • individual could have that allele (heterozygous) and have perfectly normals hands and feet

        • could pass allele to progeny and the progeny can show the trait

          • gives the appearance of skipping generations

variable expressivity

• phenotype varies among organisms with identical genotypes

• variation in the severity of a trait among individuals in population

• due to varying genetic backgrounds and environmental effects

• example

  • polydactyly

    • more fingers than normal but they look normal

    • others will show a more subtle phenotype e.g. just a little knob or bump rather than a whole extra digit

environmental modification

• for many traits, genotype alone does no specify phenotype

• environmental conditions can affect phenotype

• example

  • temperature

    • affects fur color in Himalayan rabbits

      • oveall body temp is too high

        • no melanin

        • white fur

      • temp at extremities is lower

        • melanin made

        • black fur

    • light

      • stimulates growth in plants

      • tanning/freckling in humans

    • diet

      • especially important during development

      • can affect height, weight, brain development/function

gene interactions

• genes at 2 or more loci influence the same trait

  • non-allelic

• several genes functioning to influence the same trait → must be on different loci

• can lead to

  • novel/unexpected phenotypes from crosses

    • often due to cellular function of gene products when they work in a single biochemical pathway

    • happens when you have several gene products that work together in one biochemical pathway

  • epistasis

    • allele(s) at one locus mask effect of genotype at 2^nd locus

  • complementation

    • crossing two recessive mutants generates wild-type progeny

• the examples we considered in class will deal with only two genes, so a deviation from 9:3:3:1 Mendelian ratio in F2 is an indictor (studying 2 genes in a dihybrid cross)

complementation

• crossing two recessive mutants generates wild-type progeny

• if this happens, the wild-type progeny is going to be shown

• 2 genes with this if complete

• 1 gene if this fails

• wildtype phenotype displayed in progeny from cross of two recessive mutant parents

  • cross mutant with a mutant and see wild type in the next generation

• test indicates we are dealing with RECESSIVE traits

• preform:

  • cross a mutant with a mutant

    • if all progeny are the mutant phenotype, then mutations in a strain are in the same gene

      • get mutant result when crossing mutant x mutant means they are mutant in the same gene

      • mutant genes are allelic (at the same locus)

    • if all progeny are the wt phenotype, then mutations in the strain are on different genes

      • mutant genes are NOT allelic

        • when crossing mutant x mutant and saw all wt progeny, the mutations must be on different genes

• we look at the F1 generation

• a single trait is specified by more than one gene

• example

  • we have 3 strains of a flower:

    • strain 500 is purple (wt color)

    • strain 501 is white (mutant strain in NJ)

    • strain 502 is white (mutant strain in HI)

  • in both strain 501 and 502, white color is recessive trait

    • confirm by crossing to a homozygous wt strain

      • look at this trait expressed in heterozygous progeny

        • how we figure out dominance

novel/unexpected phenotypes from crosses

• part of gene interaction

• often due to cellular function of gene products when they work in a single biochemical pathway

• happens when you have several gene products that work together in one biochemical pathway

• we look at the F2 generation

• a single trait is specified by more than one gene

• example

  • inheritance of comb shape in chickens

    • multiple genes affect a single trait

      • genes at both R and P loci influence comb shape

    • novel phenotypes observed

      • dominant alleles at both loci → walnut

      • recessive alleles at both loci → single (novel phenotype)

epistasis

• allele(s) at one locus mask effect of genotype at 2^nd locus

• genotype at one locus masks gene expression at another locus

• dominance and this

  • the effects of a gene at one locus masks the effects of a gene at a different locus

• how do we recognize epistasis?

  • look for modification of the 9:3:3:1 ratio in the F2 generation, indicates that I am dealing with 2 genes and that we have gene interaction

• example

  • two genes involved in eye color

    • p = purple; p+ = wt

      • purple is recessive to wt (red) color

    • s = suppressor; s+= wt (non-suppressing)

      • recessive s allele masks effects of genotype at p locus

    • if s+_, then p locus dictates phenotype

    • if ss, then p locus is suppressed and phenotype is red (wt)

sex chromosomes

• heteromorphic

  • different shapes

• different shapes

• act homologous during meiosis

  • in a male during gamete formation, the X and Y chromosomes have to find each other and line up at the metaphase plate

• have pseudoautosomal regions that allow chromosomes to pair during prophase

  • genetic recombination can occur between X and Y here

  • a region that will allow those 2 chromosomes to pair with each other

  • at the telomeres

sex determination in humans

• XX/XY system

• presence of Y chromosome confers maleness

• SRY (sex determining region on the Y) gene

  • found close to the telomere on the short air of Y chromosome

• females are homogametic (XX)

• males are heterogametic (XY)

ZZ/ZW system

• found in birds, butterflies, fish, some reptiles

• males are homogametic (ZZ)

  • same gametes

  • males when they go through meiosis will create gametes that are the same

    • will have one copy of every autosome and one copy of a Z sex chromosome

• females are heterogametic (ZW)

  • different

  • ½ gametes have Z chromosomes and other ½ have W chromosome

sex determination in fruit flies

• ratio of # X chromosomes: # sets of autosomes

  • 1 → female

  • 0.5 → male

• sterile males

  • laking the Y chromosome, still develop phenotypically as a male

• metafemale:

  • XXX; AA

    • 1.5

  • XXXY; AA

    • 1.5

  • XXXX; AAA

    • 1.3

  • 1.5, 1.3

• metamale

  • XO; AAA

    • 0.33

• intersex

  • XX; AAA

    • 0.67

• Y chromosome is required for male fertility, but not for sex determination

nondisjunction

• creates gametes that have an incorrect number of chromosomes

• an incomplete set or more than one complete set

dosage compensation

• corrects for potential imbalance in amount of X-linked gene products present in cells of males vs females

• humans have mechanism for this compensation mechanism where expression off of the x-chromosome is dialed down in females

• necessary because of unequal number of sex chromosomes in males and females

  • females have 2 copies of X-linked genes

    • can be homozygous or heterozygous for X-linked alleles

  • males have 1 copy of X-linked genes

    • hemizygous for x-linked alleles because they only have one copy of the x allele so there is not opportunity for masking any alleles

    • recessive alleles cannot be masked

    • display X-linked recessive traits more often than females

• done in humans (and other mammals) by inactivating gene expression on one X chromosome

X-inactivation

• is random

  • is the decision about which X-chromosome gets inactivated that the part that is random

  • roughly half of the cells are going to inactivate dads X chromosome and about half of the cells will inactivate moms

• happens early in development

• results in formation of Barr body (the inactivated X)

  • every one of those cells in our little ball, one of our X’s are inactivated, could be from mom or dad

• leads to MOSAICISM in females, which is seen in fur patterns of tortoiseshell cats

  • roughly half of every females cells should have moms cell x-inactivated

• random early on in life where one chromosome is going to become very, very, very tightly packed so that little to no gene expression happens off of that X-chromosome

• after x-inactivation, the cells continue to divide and every time they divide and create 2 daughter cells, those daughter cells are going to have the same X-chromosome inactivated as their starting cell → something that gets passed on through a mosaicism event

• example

  • fertilization event, sperm and egg meet, create one diploid cell that is a zygote and that cell divides and goes through mitosis to create 2 daughter cells, those cells go through mitosis and so on, that is the development of our embryo

mosaicism

• ~50% of cells in body should have each X chromosome inactivated

• in heterozygote for X-linked recessive trait, enough cells expressing “normal” X in every tissue → normal phenotype

• if X-inactivation is not random, mild phenotypes may be observed

  • larger % of cells in body that are expressing one X over the other, might observe X-linked recessive trait

sex-influenced traits

• expressed in both males and females, but which allele is dominant is affected or influenced by the sex of the individual

• traits which are more common in 1 sex

• genes are autosomal NOT sex-linked

• being male or female affects whether allele is dominant or recessive

  • environment of the male or female body that dictates whether an allele will be dominant or recessive

• nomenclature is with numbers because we see a different dominance relationship in males vs females

sex-limited traits

• expression is limited to one sex

  • not expressed at all in one sex

• extreme examples of sex-influenced traits

• genes are autosomal not sex-linked

• allele has 0 penetrance in 1 sex

• traits are often related to secondary sex characteristics

• expression is limited to males, females allele is silenced, regardless to phenotype

• if a female is carrying a trait that is limited to males, then it can look like that trait is skipping generation where in fact it is a female carrying the allele but not expressing it