Chapter 15 - Inheritance Flashcards
what are genes?
segments of DNA located along chromosomes
different genes are on different chromosomes
how are chromosomes and genes present?
chromosomes and genes are both present in pairs in diploid cells
homologous chromosomes separate and alleles segregate during the process of meiosis - after meiosis, fertilization restores the paired condition for both chromosomes and genes
what is the law of independent assortment?
alleles of genes on non-homologous chromosomes assort independently during gamete formation
the random arrangement of chromosome pairs at metaphase I accounts for independent assortment of the alleles for two or more genes located on different homolog pairs
what is the law of segregation?
the two alleles for each gene separate during gamete formation
separation of homologs during anaphase I accounts for the segregation of the two alleles of a gene into separate gametes
what is the chromosome theory of inheritance?
according to this theory, Mendelian genes have specific loci (positions) along chromosomes and it’s the chromosomes that undergo segregation and independent assortment
where does Mendelian inheritance have its physical basis
in the behavior of chromosomes
which organism did Morgan use to prove that chromosomes are indeed the location of Mendel’s heritable factors?
fruit fly: drosophila melangaster
advantages of using the fruit fly
- a single mating will provide hundreds of offspring
- it only has four pairs of chromosomes: 3 pairs of autosomes and 1 pair of sex chromosomes
what is the wild type?
the phenotype for a character most commonly observed in natural populations, such as red eyes in Drosophila
what are mutant phenotypes?
traits that are alternatives to the wild type, such as white eyes in Drosophila
this is because they’re due to alleles assumed to have originated as changes or mutations in the wild-type allele
what identifies a wild type genotype?
for a given character in flies, the gene takes its symbol from the first mutant discovered –> the allele for white eyes in Drosophila is symbolized by w
a superscript (+) identifies the allele for the wild-type trait
what was Morgan’s experiment? what were the results?
1) he mates a white eyed male fly with a red eyes female
2) all the F1 offspring had red eyes (wild-type allele is dominant)
3) when he bred the F1 flies to each other, he got the classical 3:1 phenotypic ratio among F2 generation BUT there’s a plot twist, the white-eye trait showed up only in males - all the F2 females had red eyes while half the males had red, and half had white
conclusion: fly’s eye color is liked to its sex –> furthermore, since only males in the F2 generation had white eyes, Morgan deduced that this eye color gene is located on the X chromosome and that there is no corresponding locus on the Y
what did Morgan’s experiment support?
**provided support for the chromosome theory of inheritance: namely, that a specific gene is carried on a specific chromosome
also indicated that genes located on a sex chromosome exhibit unique inheritance patterns
what are the different sex systems?
mammals: XY
grasshoppers, cockroaches, insects: females XX, males X
birds, fish insects: females ZW, males ZZ
halpo-diploid: no sex chromosomes in most species of bees/ants –> females develop from fertilized eggs are are diploid, males develop from unfertilized eggs and are haploid
which chromosome is bigger in a human?
the Y chromosomes is much smaller than an X
short segments on either end of the Y chromosome are the only regions that are homologous with regions of the X which allow the X and Y to pair and behave like homologs during meiosis –> The X and Y chromosomes share small regions of DNA homology that allow them to pair (synapse) and undergo crossing over at meiosis in males
what is SRY
a gene for sex-determining region of Y
in the absence of SRY, the gonads develop into ovaries
Y directs the male pathway –> The SRY gene on Y directs testis development –> Testis produce sex hormones that determine maleness
what is a sex-linked gene?
a gene located on either sex chromosome; those located on the Y chromosome are called Y-liked genes
Y-linked genes
Y chromosome is passed along virtually intact from a father to all his sons
because there are so few Y-liked genes, very few disorders are transferred from father to son on the Y chromosome
what are X-liked genes
the human X chromosome contains approximately 1,100 genes which are called X-liked genes
fathers pass X-liked alleles to all of their daughters but to none of their sons
inheritance of X-liked genes
if an X-liked trait is due to a recessive allele, a female will express the phenotype only if she is homozygous for that allele (XnXn)(if XNXn then she’ll only be a carrier)
any male receiving the recessive allele from his mother will express the trait because his genotype will be XnY (males have more X-liked recessive disorders than females)
color blindness is an X linked gene
what is Duchenne muscular dystrophy?
if affects about 1/3500 males in the US
it’s characterized by a progressive weakening of the muscles and loss of coordination
the disorder is traced to the absence of a key muscle protein called dystrophin and have mapped the gene for this protein to a specific locus on the X chromosome
what is hemophilia?
an X-liked recessive disorder defined by the absence of one or more of the proteins required for blood clotting - when a person with hemophilia is injured, bleeding is prolonged because a firm clot is slow to form
what are examples of X-linked disorders?
- color blindness
- duchenne muscular dystrophy
- hemophilia
X inactivation in female mammals
- almost all of one X chromosome in each cell in female mammals becomes inactivated during embryonic development = males and females have the same dose of most X-liked genes
what is a Barr body?
the inactive X in each cell of a female condenses into a compact object called a Barr body which lies along the inside of the nuclear envelope
most genes of the X chromosomes that form the Barr body aren’t expressed –> Barr body chromosomes are reactivated in the cells that give rise to eggs so that every female egg has an active X after meiosis
which X chromosome will form the Barr body?
it’s completely random and independent in each embryonic cell
as a consequence, females consist of a mosaic of two types of cells: those with the activated X derived from the father and those with the active X from the mother (ex. calico cat fur color)
after an X chromosome is inactivated in a particular cell, all mitotic descendants of that cell have the same inactive X
what does inactivation of an X chromosome involve?
- modification of the DNA and proteins bound to it, called histones (including attachment of methyl groups to DNA nucleotides
a particular region of each X chromosome contains several genes involved in inactivation - the two regions, one on each X chromosome, associates briefly with each other in each cell at an early stage in development –> then one of the genes called XIST becomes active only on the chromosome that will become the Barr body
gene locations and inheritance relationship
genes located near each other on the same chromosome tend to be inherited together in genetic crosses because they’re physically linked on the same dsDNA molecule; such genes are said to be genetically linked and are called linked genes
linked genes are attached to the same centromere
all genes on the same chromosome are part of the same linkage group!
how does linkage between genes affect the inheritance of two different characters?
wild-type flies are grey with normal wings
mutant flies are black with small wings (vestigial)
mutant alleles are recessive to the wild type alleles and neither gene is on a sex chromosome
1) P generation: b+b+v+v+ x bbvv
2) F1: dihybrids b+bv+v (wild type phenotype)
3) mated wild type F1 dihybrid females with homozygous recessive males: b+bv+v x bbvv (to reveal genotype of the eggs made by the female)
4) the resulting flies had a much higher proportion of the combinations of traits seen in the P generation than would be expected if the two genes assorted independently
conclusion: body color and wing size are usually inherited together in specific (parental) combination because the genes for these characters are near each other on the same chromosome
however, there were also combinations of traits not seen in the P generation (non parental phenotypes) which suggests that body color and wing size alleles aren’t always linked genetically
what is genetic recombination?
the production of offspring with combinations of traits that differ from those found in either P generation parent
some offspring have combinations of traits that do not match those of either parent
what’s the difference between linked genes and sex-linked genes?
linked genes are two or more genes on the same chromosome that tend to be inherited together
sex-linked gene is a single gene on a sex chromosome
what are parental types?
the offspring that are expected to inherit a phenotype that matches either of the parental phenotypes
what are recombinants?
non-parental phenotypes are also found among the offspring -since they have new combinations of seed shape and color
when 50% of all all offspring are recombinants, geneticists say there’s a 50% frequency of recombination - this frequency of recombination in such cross tests is observed for any two genes that are located on different chromosomes and thus cannot be linked
when do you know if genes are linked?
when the occurrence of parental types has a frequency greater than 50%, it indicates that the genes are linked
this suggests that some processes must occasionally break the physical connection between specific alleles of genes on the same chromosome
what is crossing over? what does it cause?
it accounts for the recombination of linked genes
causes recombination
when does crossing over occur?
it occurs while replicated homologous chromosomes are paired during prophase of meiosis I, a set of proteins orchestrates and exchange of corresponding segments of one maternal and one paternal chromatid
in effect, the end portions of two non sister chromatids trade places each time crossover occurs
what is the recombination frequency based off of?
the percentage of recombinant offspring, the recombination frequency, depends on the distance between genes on a chromosome
crossing over is a random event with the chance of crossing over approximately equal at all points along a chromosome
the farther apart two genes are the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
what’s a linkage map?
a genetic map based on recombination frequencies is called a linkage map
a linkage map is based on the assumption that the probability of a crossover between two genetic loci is proportional to the distance separating them
linkage map does portray the order of genes along a chromosome but doesn’t portray the precise location of those genes
what is nondisjunction?
ideally the exotic spindle distributes chromosomes to daughter cells without error but there is an occasional mishap called a nondisjunction in which the members of a pair of homologous chromosomes don’t move apart properly during meiosis I or sister chromatids fail to separate during meiosis II
one gamete receives two of the same type of chromosome and another gamete receives no copy
what is aneuploidy?
if either of the aberrant gametes unites with a normal one at fertilization, the zygote will also have an abnormal number of a particular chromosome
what is monosomic?
fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the zygote (2n-1 chromosomes)
the aneuploid zygote is said to be monosomic for that chromosome
what is trisomic?
if a chromosome is present in triplicate in the zygote, so that the cell has 2n+1 chromosomes, the aneuploid cell is trios for that chromosome
down syndrome
what is polyploidy?
when an organism has more than two complete chromosome sets in all their somatic cells (3n or 4n)
fairly common in plants
how might triploid and tetraploid cells arise?
a tripod cell may arise by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes
tetraploidy could result form eh failure of a 2n zygote to divide after replicating its chromosomes
what are the four changes in chromosome structure? what causes them?
errors in meiosis or damaging agents such as radiation can cause breakage of a chromosome
1) deletion
2) duplication
3) inversion
4) translocation
what is deletion?
occurs when a chromosomal fragment is lost so then the affected chromosome is missing certain genes (if the centromere is deleted then the entire chromosome will be lost)
what is duplication?
the “deleted” fragment might become attached as an extra segment to a sister chromatid
OR
a detached fragment could attach to a non sister chromatid of a homologous chromosome –> in that case duplicated segments might not be identical because the homologs could carry different alleles of certain genes
what is inversion?
a chromosomal fragment may also reattach to the original chromosome but in the reverse direction
what is translocation?
a translocation moves a segment from one chromosome to an non-homologous chromosome (switch chunks)
in a reciprocal translocation, the most common type, non homologous chromosomes exchange fragments
what kinds of alterations in chromosome structure are most common during meiosis?
deletions and duplications
in crossing over, nonsister chromatids sometimes exchange unequal sized segments of DNA so one chromosome has a delation and the other a duplication
what is down syndrome?
it’s the result of an extra chromosome 21 so that each body cell has a total of 47 chromosomes
the cells are trisomic for chromosome 21
what is genomic imprinting? when does it occur?
a variation in phenotype that depends on whether an allele is inherited from the male or female parent
In each generation, the old imprints are erased in gamete producing cells
Re-imprinted according to the sex of the individual
most imprinted genes are on autosomes
occurs during gamete formation
distance and recombinants
If two genes are on different chromosomes, under the law of independent assortment the testcross should produce a 1:1:1:1 phenotypic ratio. A maximum of 50% of all offspring are recombinants.
If two genes are physically very close together on the same chromosome very tightly linked*, we should expect to see a 1:1:0:0 ratio with only parental phenotypes among offspring.
what is genetic linkage?
when offspring have the same combinations of alleles as one or the other parent
if there’s a linkage of genes the offpsring will show predominantly parental combination
What is meant by 17% recombination frequency?
It means that among every 100 gametes produced by the heterozygous parent, 17 contain recombinant chromosomes produced by crossover.
And, the other 83 have parental chromosomes that are not products of crossing-over.
what is XYY syndrome?
1 of every 1,000 males
Normal sex development
Phenotypically “normal”
Mischaracterized as “supermales”
what is XXX syndrome?
1 in 1,000 live births
Phenotypically normal (maybe slightly taller)
At risk of learning disabilities
what is XXY syndrome?
(Klinefelter Syndrome) 1 in 500-1,000 live births Sterile Enlarged breasts and other female body characteristics
what is XO syndrome?
The only viable monosomy
Among the most common genetic disorders
1 in 2,500 female births
Sterile, short stature, wide shoulders, low-set ears, webbed neck and cognitive deficits
One less or more copy of a chromosome (2n-1 or 2n+1) can create problems for an individual organism largely because:
many biological functions are due to multiple gene products working together in a correct ratio
what is the philadelphia chromosome?
translocation
Chronic Myelogenous Leukemia (CML) arises due to a mitotic reciprocal translocation
between Chromosomes #9 and 22 in precursor cells to white blood cells
The easily recognized shortened
chromosome #22 is called the
Philadelphia chromosome
organelles and mendelian inheritance
some genes are located in organelles in the cytoplasm like mitochondria and chloroplasts - they are outside the nucleus so these genes are called extranuclear genes
Organelle genes do not follow Mendelian inheritance
Sperm contributes little more than DNA, while the cytoplasm of the egg is full of mitochondria
