Exam 2 Flashcards
principle of segregation (Mendel’s first law)
- Mendel’s first law
- separation of a gene pair during gamete formation
alleles
- alternate forms of a gene
- “different flavors”
homozygous
-organisms with identical alleles for a gene
heterozygous
-organisms with different alleles for a gene
dihybrid cross
- crossing parents with different traits
- SSYY (smooth yellow) x ssyy (wrinkled green)
- 9:3:3:1
principle of independent assortment
- Mendel’s second law
- explains inheritance of two traits
- ex. 9:3:3:1 ratio in F2
locus
-specific site on chromosome that each gene is located on
incomplete dominance
- expression of a phenotype that is intermediate to those of the parents
- cross between 2 homologous dominant
- ex. pink snapdragons (R1R2) made by dominant red (R1R1) and white (R2R2)
dominance
- clearly defined dominant and recessive alleles and their corresponding phenotypes
- RR (red) x rr (white) makes Rr (red)
codominance
- full expression of both alleles is seen heterozygous
- ex. blood type comes from codominant alleles of one I gene (IA, IB, and IO)
- ex. red and white parents make red and white spotted flower (not pink)
exceptions to Mendel’s principles
- ex. lethal yellow gene in mce
- Yy yellow mice mate to make YY (dead), Yy (yellow), yy (nonyellow)
- yellow gene is recessive for death, but dom for yellow coat color
sex chromosomes provide _____ path for embryo development that guides it towards a phenotypic sex
-genetic
phenotypic sex vs. chromosomal sex
- phenotypes arise during embryonic development (may be different than chromosomal sex)
- phenotypes can be opposite sex
- can be intermediate of 2 sexes
- can have characteristics and genitalia of both sexes
- ex. fingerprints arise in womb
several levels that determine sex
- chromosomal sex (XX vs. XY)
- gonadal sex (ovaries vs. testes)
- phenotypic sex (women built to give birth)
1st step in sex determination
- chromosomal sex
- occurs at fertilization
- XX or XY
- 50% chance of either
2nd step of sex determination
- gonadal sex
- if XY, SRY gene on Y chromosome signals gonad to develop into testes
- if XX, lack of SRY gene
3rd step of sex determination
- phenotypic sex
- male: testosterone is converted to DHT which forms genitalia
- female: lack of DHT prevents development of external genitalia
how many babies are born with phenotypic/ gonadal sex that is different than their chromosomal sex?
-1 in 2000 births
androgen insensitivity
- mutation in the X-linked gene for the androgen receptor causes XY males to become phenotypic females
- ex. Caster Semenya Olympic runner
pseudo hermaphroditism disorders
- result in individuals with both male and female structures, but at different times in their lives
- testosterone cat convert to DHT so XY don’t form testis/ appear female
- born with vagina with large clit, develops into testicles ad penis during puberty
sex chromosomes
- X and Y chromosomes
- carry many genes that can be identified by unique inheritance pattern
- X and Y have different patterns of inertance bc they carry different genes
who is affected by x-linked recessive disorders more?
-males (XY)
hemizygous
- one chromosome pair rather than 2
- all genes on the X chromosomes of males bc they only have one X
- cant be heterozygous or homozygous for X-linked genes
sex chromosomes unique pattern of inheritance
- males only give X to daughter and Y to son
- hetero females have 50% chances of passing X-linked recessive traits to male offspring
autosomal dominant
- does not skip generations
- affects both sexes equally
- when one parent is affected the other one is not
x-linked dominant
- dominant= every generation
- males pass ONLY to daughters (not sons)
- heterozygous affected females pass trait equally to sons and daughters
- on average, twice as many daughters as sons affected
hypophosphatemia
- x-linked dominant trait
- low phosphate in blood causes bowleggedness (like in dwarfs)
autosomal recessive
- skips at least 1 generation
- only if received two copies (one from each parent)
x-linked recessive
- recessive= skips at least one generation
- phenotypic expression more common in males
- affected males receive mutant allele from mom and pass to all daughters (not sons)
- hemizygous males and homo females affected
- daughters of male carriers are usually unaffected but sons have 50% chance
color blindness (red and green) and white eye color in fruit flies
- x-linked recessive trait
- affects 8% of males in US
dosage compensation
- mechanism that regulates the expression of sex-linked gene products
- men and women produce same amount of X-gene products
- allows certain x-linked recessive disorders to be expressed in heterozygous females
- one inactive chromosome in females called barr body
bar bodies
- inactive, coiled up x chromosomes in cells
- women can only express one X
- wrinkled piece of paper that can be reopened later and passed on, just never used in that cell
lyon hypothesis of how dosage compensation works
- in heterozygous females, both alleles are active- but not in the same cell
- permanent
- random (either mom or dad’s X)
- in somatic cells only (all cells other than repro)
- inactivation of x chromosome happens early in development
mosaicism
- condition where cells in same organism express different genes
- ex. tortoiseshell cat (only female)
ex. blaschko’s lines
in order for mosaicism to occur…
- XX chromosomes
- heterozygous on X cell
- dosage compensation has to occur
y-linked genes
- traits are passed directly from father to son
- unique and uncommon
incomplete penetrance
-when genotypes don’t always produce the expected phenotype
penetrance
- % of individuals having a particular genotype expressing the expected phenotype
- if 38/42 people are polydactyly… 90% penetrance
- relates to expressivity
T/F: presence of a gene guarantees it will be expressed
false
variation in phenotypic expression is caused by
- age
- genetic interactions
- interactions with environment
gene interaction
- alleles of same gene exhibit independent assortment…. but do not act independently in their phenotypic expression
- give perfect 9:3:3:1 or 3:1 ratio
epistasis
- gene interaction but with different genes (not the same one)
- one gene masks affect of another gene at a different locus
- ratio is off, 9:7
- ex. balding gene doesn’t allow you to see what color hair offspring has
sex-influenced traits
- expressed in both sexes, but expressed differently in males and females
- ex. pattern baldness is autosomal dom in males and recessive in females
- related to x-linked male receptor gene (from mom)
sex limited traits
- inherited by both sexes, but normally expressed only in one sex phenotypically
- ex. DMD/ muscular dystrophy only affects boys
imprinting
-phenomenon where gene expression depends on if paternally or maternally inherited
complex trait
- determined by several gene pairs, nongenetic factors, and environmental interactions
- ex. height in humans
discontinuous characteristics
- only 2 phenotypes
- tall or short pea plants
continuous characteristics
- distribution of phenotypic characters from one extreme to another in overlapping fashion
- ex. human height
polygenic traits
- controlled by 2 or more gene pairs
- typically have bell shaped curve
- ex. eye color
multifactorial traits
- polygenic (controlled by 2 or more gene pairs) AND show interactions with environment (genes + nutrition = height)
- ex. obesity
larger number of loci controlling a trait means ___________.
- more loci= more phenotypic classes
- more phenotypes means less phenotypic difference between classes
- ex. eye color (lots of colors, little difference between classes)
principle of segregation
- diploid (2n) organisms have two alleles at each locus on homologous chromosomes that separate in meiosis
- one allele goes to each gamete
independent assortment
-during the process of separation, both alleles at locus act independently of alleles at other loci
recombination
- sorting of alleles into new combinations
- result of independent separation of loci
linked genes
- genes located close together on the same chromosome
- belong to same linkage group
- usually inherited together/ don’t assort independently (skews mendelian ratios)
- crossing over prevents this
crossing over
- occurs in prophase 1 of meiosis
- prevents linked genes from assorting together
- results in recombination (genetic reshuffling)
- follows ratios
writing linked genes
-must write linked genes on specific alleles like they are arranged on homologous chromosome
A B a b
———- X ————
A B a b
cis configuration / coupling
-arrangement occurs when wild-type alleles are on one chromosome and mutant alleles on the other
repulsion/ trans configuration
-occurs when wild-type allele and mutant allele of dif gene are found on same chromosome
complete linkage
- different genes are located very close to one another on same chromosome
- all progeny are parental
- ex. dwarf and mottled tomato plants
how to test for linkage
- testcross with heterozygous for both characteristics
- if F2 resembles original traits they are nonrecombinant progeny (aka linked)
- ex. (wont produce Md or mD in crosses)
incomplete linkage
- genes that exhibit crossing over
- most progeny are parental
- produce recombinant progeny (different allele combos than parents)
recombination frequency
- % recombinant progeny produced in a cross (50% max)
- # recombinant progeny / total # progeny X100
- lead to genetic maps by Hunt
physical maps
-using exact base pairs btwn loci, more refined, came with new technology
genetic maps
- based off of one another, recombination theories
- uses approximations
- measured in m.u. or cM
- underestimate
