Genetics Exam 3

Dr. sweet

tetrads

at the pachytene stage, the homologous chromosomes form these; also called bivalents

synaptonemal complex

zipper-like proteins connecting tetrads

crossing over

recombination by ___

autosome

chromosome pairs 1-22

sex chromosome

chromosome pair 23

allosome

another name for sex chromosome

heteromorphic

one chromosome is smaller and one is larger

heterogametic sex

the sex with both heteromorphic chromosomes

homogametic sex

the sex with homomorphic chromosomes

heterogametic sex chromosomes

which type of sex chromosome has very little recombination occur?

pseudoautosomal regions (PAR)

where does recombination occur in heterogametic sex chromosomes?

y-linked genes

genes on the y chromosome

SRY gene

sex determining region of the y; determines the sex in XY systems

testis determining factor (TDF)

sry gene codes for this transcription factor; controls the expression of other genes coded on autosomes

males can be XX if the SRY gene is present; females can be XY if the SRY gene is absent

examples of exceptions to the XX and XY rule

errors in recombination, deletions, and mutations in autosomes

reasons for exceptions to the XX and XY rule

hemizygous

y-linked genes are this; only ever one copy of the y chromosome

through male lineage

how are y chromosomes passed down?

haplotypes

group of genes or sites inherited together from a single parent

mitochondrial haplotypes to look at haplogroups

what genetic marker could we use from maternal lineage?

y adam

most recent male that is the common male ancestor of all living humans (>200kya)

mitochondrial eve

most recent female that is the common female ancestor of all living humans (180kya)

human X chromosome

females have 2 copies and inherited from either father or mother; males have 1 copy inherited from their mother

x-inactivation

one copy of the x chromosome in females randomly inactivates by compacting one chromosome into heterochromatin

mosaicism in x-linked traits

what can x-inactivation lead to?

lyonization

what is another term for x-inactivation?

part of x-linked genes are still active

why is turner syndrome a thing if one of the x-chromosomes gets inactivated anyway?

Nettie Stevens; mealworms

who first discovered x and y chromosomes? what organism did she find them in?

thomas hunt morgan

hypothesized about sex-linked traits; based his work on color on the eye of fruit flies

w+

allele for red eyes; wild type

w

allele for white eyes; mutant type

females had all red eyes, but males were 50/50; concluded that gene for eye color was a sex-linked trait; XW+ or XW

what did thomas hunt morgan find out when concluding his fruit fly experiment?

color blindness

x-linked trait, tends to affect males; affect opsin genes

OPN1LW

red cones; affected by color blindness

OPN1MW

green cones; affected by color blindness

male pattern baldness

x-linked; likely partly responsible in many cases

hemophilia

blood does not clot well; x-linked trait

inbreeding

reason for hemophilia in european royal families

ZW/ZZ system; females are the heterogametic sex

sex determining system of birds, lepidopterans, some reptiles, and some crustaceans

X0 system;; males are X0

sex determining system of insects, and arachnids

haplo-diploid

sex determining system of hymenopterans; males are typically haploid from an unfertilized egg

paternal genome elimination

sex determining system in many types of arthropods; males inherit but do not pass on their father’s genetic material

5 sets of X and Y

sex determining system of monotreme mammals; X and Y are not comparable to the X and Y in therian mammals; do not have SRY gene

temperature during development; can still have X/Y or Z/W chromosomes

sex determining system of some reptiles

muller’s ratchet

process through which, in the absence of recombination, and accumulation of irreversible deleterious mutations result

gregor mendel experiments

experiments crossing pea plants and counting the phenotypic ratios of the offspring

phenotype

physical trait of an organism

law of dominance, law of segregation, law or, law of independent assortment

mendel’s three principles of inheritance

law of segregation

alleles segregate into separate gametes during meiosis, only one allele per gamete

law of independent assortment

alleles at 2 different genes sort into gametes independently of one another

unlinked genes

assumption for dihybrid crosses

dihybrid cross

cross between two individuals with two different traitd

9:3:3:1

phenotypic ratio for a dihybrid cross

polygenic trait

more than 1 gene is responsible for the trait

human height, eye color, and skin color

examples of polygenic traits

incomplete dominance

heterozygotes express an intermediate trait

carl correns

who experimented with incomplete dominance using 4 o’clock plants?

overdominance

heterozygotes have greater reproductive success than either the homozygote genotypes, also known as heterozygote advantage

codominance

heterozygote expresses phenotypes from both alleles

rhesus (Rh) factor

type of protein on the outside of your red blood cells

second pregnancy; the mother to be Rh- and the fetus to be Rh+; mother produces antibodies that can attack the fetus’ blood cells

what happens with incompatibilities of Rh factor?

ABO and Rh factors

what makes up blood type?

O-

universal donor

AB+

universal recipient

thelytoky

female birth; virgin birth

automixis

fusion of 2 gametes in a single individual

apomixis

generation of seeds without meiosis and fertilization (in plants); female clones

arrkenotoky

form of parthenogenesis where males develop from unfertilized eggs (and are haploid, or in some cases homozygous for sex determining gene)

pseudo-arrhenotoky

males inherit but do not pass on their father’s genetic material; functionally equivalent to haplo-diploidy

meiotic drive

unequal segregation of chromosomes or alleles that allow for the overrepresentation of the element

monkeyflowers, stalk-eyed flies, neurospora fungi

examples of meiotic drive

lethal alleles

allele that can cause the death of an individual; often but not always recessive

Achondroplasia

type of dwarfism; homozygous dominant is lethal; around 80% are from a novel mutation in the FGFR3 gene

chiasmata

what do homologous chromosomes form during the pachytene stage

homologous chromosomes

what do pairs of chromosomes exchange genetic material through?

diplotene

during what stage does the synaptonemal complex begin to disappear?

sister chromatid exchange

recombination between sister chromatids; does not result in a new combination of alleles

genetic recombination

recombination does result in new combination of alleles

holliday model of recombination

described by Robin Holliday in fungi; original description involved single-stranded break

single-stranded break

homologous chromatids are nicked at the same location

holliday junctions

strands to the left of the nicks invade corresponding chromatids and form what?

2 heteroduplex regions

what does branch migration create?

nonhomologous end joining

broken ends are pieced back together; DNA polymerase fills in the gap and phosphodiester bonds connected with ligase; may result in a small deletion in the region

The double-stranded break repair model

strand invasion and formation of D-loop; resolution of Holliday junctions results in either non-recombinant or recombinant chromosomes; formation of double holliday junctions

synthesis-dependent strand annealing (SDSA) model

formation of a single holliday junction; release from HJ; DNA polymerase fills the gaps; results in not true recombination

RecBCD

three proteins that find and/or cause DSB

RecA

promotes strand invasion in single-stranded DNA

single stranded binding proteins (SSB)

prevents strand degradation

RuvABC

protein complex that promotes branch migration in Holliday junctions

gene conversion

one allele is converted to the allele on the homologous chromosome

gene conversion by mismatch pair

corrects mismatches in heteroduplex regions, which can lead to gene conversion; also scenarios with repair without gene conversion

gene conversion by gap year

DNA polymerase uses the allele from the homologous chromosome as a template to fill in a gap

G-C

gene conversion tends to be ___ biased

mutation process; can lead to a false signal of positive selection; GC biased gene conversion can happen irrespective of the fitness of the organism

why does gene conversion tend to be GC-biased

unequal crossing over

between non-equivalent sequences, can lead to deletion and/or duplications

inversion

can be caused by crossing over; pericentric and paracentric

pericentric

includes centromere

paracentric

does not include centromere

translocation

parts of different chromosomes rearrange; can be caused by non-homologous recombination

robertsonian translocation

extreme version of translocation; entire arm that fuses to another chromosome; down syndrome can be caused by this