Download Chapter 11 and 12 from Campbell Biology 10th Edition By Keshara

Chapter 11 and 12 from Campbell Biology 10th Edition
By Keshara Senanayake
Ms.Reep
Chapter 11 - Mendel and Gene Idea
Chapter 12 - The Chromosomal Basis of Inheritance
Chapter 11 Campbell Biology 10th Edition
“Mendel and the Gene Idea”
 Mendel worked w/ peas because they have a variety  a heritable feature that varies among individuals
is called a character (like flower color)
 each variety for a character is a trait (Purple vs. white)
>Mendel controlled mating between plants  while pea plants self usually fertilize Mendel did stuff
 Mendel tracked characteristics that occurred in distinct alternative forms (purple or white color)  made
sure he started experiments w/ varieties that over many generations of self pollination produced the same
variety as the parent plant  termed true breeding
 Mendel matted two true-breeding plants via hybridization  true-breeding parents are the P generation
>hybrid offspring  F1 generation  allowed F2 hybrids to pollinate created the F1 generation  from F2
Mendel deduced 2 fundamental principals: law of segregation and law of independent assortment
The Law of Segregation
Purple X White  produces all purple in F1  but F1 X F1  white reappear
>Mendel reasoned heritable factor of white is masked  purple is dominant trait and white is a recessive
trait  a 3:1 inheritance pattern in F2
(4) major concepts make up Mendel’s mode
>(1) Alternative versions of genes account for variations in inherited characteristics  alternative versions
of genes are called alleles  related to concept to chromosomes/DNA  each gene is a sequence of
nucleotides at a specific place (locus) along a particular chromosome  DNA at locus can vary slightly in
its nucleotide sequence/info (purple/white alleles are 2 diff DNA sequence variations on flower-color
locus)
>(2) For each character, an organism inherits two copies (2 alleles) of each parents, one from each parent
 each somatic cell in diploid organism has (2) sets of chromosomes (one set from each parent)  (2)
alleles at a particular locus may be identical (as in P generation) or differ (as in F1)
>(3) If two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance;
the other, the recessive allele, has no noticeable effect on the organism’s appearance
>(4) Law of segregation states that two alleles for a heritable character segregate during gamete formation
and end up in two different gametes
>so an egg/sperm gets one of the two alleles that are present in the somatic cells of the organism making
the gamete  this segregation corresponds to the distribution of the (2) members of a pair of homologous
chromosomes to different gametes in meiosis
 if a organism has identical alleles for a characteristic (true breeding) allele is present in all gametes (if
different 50% dominant 50% recessive)
>Punnett square is used to predict allele composition of all offspring resulting from a cross between
individuals of known genetic makeup  capital is dominant (lowercase is recessive)
organism that has a pair of identical alleles for a character is homozygous for the gene controlling that
character  “breed true” because all their gametes contain the same allele  if we cross dominant
homozygote with recessive homozygote every offspring has two different alleles and is said to be
heterozygous for that gene  Heterozygous produce gametes with different alleles (not true breeding)
 observable traits are the phenotype (genetic makeup is the genotype)
 If you have a mysterious pea plant that is purple (can’t tell if PP or Pp) to determine genotype we can
cross this plant w/ a white-flowered plant (pp) which will make only gametes w/ a white flowered p 
allele in gamete contributed by mystery plant determine appearance of the offspring  if all purple it’s PP
but if white/purple appear it is Pp
 cross of Pp X pp cross will have a 1:1 phenotypic ratio  testcross is breeding an organism of unknown
genotype w/ a recessive homozygote to reveal the genotype of that organism
The Law of Independent Assortment
 law of segregation was derived from experiments w/ a single character
>F1 progeny produced in his crosses of true-breeding parents were monohybrids (heterozygous for the one
particular trait being followed)  cross between such heterozygote is a monohybrid cross
>second law of inheritance found by following (2) characteristics (like seed color/seed shape)
>Mendel say yellow (Y) dominant and green (y) recessive  round (R) dominant and (r) wrinkled
recessive
>crossing (2) true breeding pea varieties that differ in both characters  cross w/ YYRR and YyRr
>the F1 plants will be dihybrids  individuals heterozygous for the two characteristics followed in the
cross YyRr
>need to know if the two characters are transmitted from parents to offspring as a package (or
independently)
>In F1 YyRr has both dominant phenotypes (yellow/round) no matter what
>needs to see if F1 plants self-pollinate and produce F2  to see if the hybrids transmit their alleles in the
same combinations as their alleles were inherited from the P generation, then F1 hybrids will produce only
two classes gametes: YR and yr  “dependent assortment” hypothesis says phenotypic ratio is 3:1 like a
monohybrid cross
>alternate hypothesis is that two pairs of alleles segregate independently of each other  states that genes
are packaged into gametes in all possible allelic combination as long as each gamete has one allele for each
gene
>F1 plant (with self pollination) will produce (4) classes of gametes in equal quantities: YR, Yr, yR, and yr
 sperm of four classes fertilize eggs w/ 4 classes so it will form 16 equally probably ways in which alleles
can combine in the F2 generation  result in (4) phenotypic categories w/ ratio of 9:3:3:1  support
hypothesis that the alleles for one gene segregate into gametes independently of the alleles of other genes
Law of independent assortment: two or more genes assort independently of each other pair during gamete
formation  applies only to genes located on different chromosomes not homologous
The Laws of probability govern Mendelian inheritance 
>in independent events one event does not affect the other’s probability of occurring  alleles of one gene
segregate into gametes independently of another gene’s alleles
The Multiplication and Addition Rules Applied to Monohybrids
multiplication rule states that to determine the probability that two or more independent events will occur
together in some specific combination, we multiple the probability of one event by the probability of
another event
>apply to F1 monohybrid cross  w/ seed shape as a heritable character the genotype of F1 plant is Rr 
segregation in a heterozygous plant is like flipping a coin  each egg produced has a ½ chance of carrying
the dominant allele (R) and a ½ chance have a recessive allele (r)  same odds apply to sperm cells
 for a particular F2 plant to have wrinkled seed both sperm and egg must carry the r allele  probability
that an r allele will be present in both gametes at fertilization if found by multiplying ½ (probability in egg
to have r) X ½ (probability in sperm to have r)  so probability F2 plant in having wrinkled seeds is ¼
>to figure out that an F2 plant from a monohydric cross will be heterozygous rather than homozygous
needs a 2nd rule  F1 gametes can combine to produce Rr offspring in two mutually exclusive ways: for
any particular heterozygous F2 plant the dominant alleles can come from the egg or the sperm  according
to the addition rule the probability that any one of two or more mutually exclusive events will occur is
calculated by adding their individual probabilities  multiplication rule gives us the individual probability
that we can now add together  probability for on possible way of obtaining an F2 heterozygote 
dominant allele from the egg and recessive allele from the sperm is 1/5  probability for the other way,
recessive allele from the egg and the dominant allele from the sperm is also ¼  via rule of addition we get
the probability of an F2 heterozygote as ¼ + ¼ = ½.
Solving Complex Genetics Problems with the Rules of Probability
 rules of probability can be used to predict the outcome of crosses involving multiple characteristics 
each allelic pair segregates independently during gametes formation thus a dithered or other multicharacter
cross is equivalent to two or more independent monohybrid crosses occurring simultaneously  can
determine probability of specific genotypes occurring in the F2 generation
>consider this dihybrid cross simply mentioned before between YyRr heterozygotes
>first focus on seed-color character  for monohybrid Yy plant we can deduce that the probabilities of the
offspring genotypes are ¼ YY, ½ Yy, and ¼ for yy.  for seed shape we can deduce ¼ RR, ½ Rr, and ¼ rr
 with these probabilities we can use the multiplication rule to determine the probability of each of the
genotypes in the F2 generation
So:
Probability of YYRR = ¼ (probability of YY) X ¼ (RR) = 1/16
Probability of YyRR = ½ (Yy) X ¼ (RR) = 1/8
>both correspond to larger punnet square
>imagine a cross of two pea varieties in which we track the inheritance of three characters  a cross of
trihybrid w/ purple flowers and yellow, round seeds (heterozygous for all three genes) with a plant with
purple flowers and green, wrinkled seeds (heterozygous for flower color but homozygous recessive for the
other two characteristics)
>w/ Mendel symbols our cross is PpYyRr X Ppyyrr  what fraction of offspring from this cross is
predicted to exhibit the recessive phenotypes for at least two of the three characters?
>first list all genotypes we could get that fulfill this condition: ppyyRr, ppYyrr, Ppyyrr, Ppyyrr, and ppyyrrr
 next calculate the probability for each of these genotypes resulting from our PpYyRr X Ppyyrr cross by
multiplying together the individual probabilities for the allele pairs
>in a cross involving heterozygous and homozygous allele pairs (Yy x yy) the probability of heterozygous
offspring is ½ and the probability of homozygous offspring is ½  we use addition rule to add the
probabilities for all the different genotypes that fulfill the condition of at least two recessive traits:
PpyyRr ¼ (probability of pp) x ½ (yy) X ½ 9Rr) = 1/16
PpYyrr (¼ X ½ X ½) = 1/16
Ppyyrr ½ X ½ X ½ = 2/16
Ppyyrr ¼ X ½ X ½ = 1/16
Ppyyrr ¼ X ½ X ½ = 1/16
Chance of at least (2) recessive traits = 6/16 = 3/8
>faster for solving genetics problems via rules of probability than by filling in punnet squares  rules of
probability give us the chance of various outcomes  large the sample size the closer the outcomes
Inheritance patterns are often more complex than predicted by simple Mendelian Genetics
>extending Mendelian Genetics for a single gene  inheritance of characters determined by a single gene
deviates from simple Mendelian patterns when alleles are not completely dominant or recessive , when a
particular gene has more then (2) alleles or when a single gene produces multiple genotypes
Degrees of Dominance
 in Mendel’s pea experiment F1 offspring always looked like one of the two parental varieties because
one allele in a pair showed complete dominance over the other
some genes neither allele is completely dominant and F1 hybrids have a phenotype somewhere between
those of the two parental varieties  called incomplete dominance  seen when red snapdragons are cross
w/ white snapdragons
 all F1 have pink flowers  this third intermediate phenotype results from flowers of the heterozygotes
having less red pigment than red homozygotes  interbreeding F1 hybrids produces F2 offspring w/ a
phenotypic ratio of one red to two pink to one white (the genotypic and phenotypic ratios for F2 generation
are the same, 1:2:1)
>segregation of red-flower and white-flower alleles in the gametes produced by the pink-flowered plants
confirms that the alleles for flower color are heritable factors that maintain their identity in hybrids
 in co dominance the two alleles can affect the phenotype in separate distinguishable ways  human MN
blood group is determined by co dominant alleles for two specific molecules located on the surface of red
blood cells M and N molecules
>a single gene locus (at which two allelic variations are possible) determines the phenotype of this blood
group  individuals homozygous for the M allele (MN) have red blood cells w/ only M molecules
(homozygous N have only N) but BOTH M and N molecules are present on the red blood cells of
individuals heterozygous for the M and N alleles (MN)
>MN phenotype is not intermediate between M and N phenotypes  BOTH M and N phenotypes are
exhibited by heterozygotes (w/ both molecules present)
The Relationship Between Dominance and Phenotype
note that an allele is dominant because it is seen in the phenotype (not because it studies a recessive
allele) alleles are variations in a gene’s nucleotide sequence  when a dominant allele coexists w/ a
recessive allele in a heterozygote they do not interact
for an character, the observed dominant/recessive relationship of alleles depends on the level at which
>tay sachs provides an example  brain cells of a child w/ tay sachs allele (homozygote) have the disease
 at the organism level the tay sachs allele qualifies as recessive  the activity level of the lipidmetabolizing enzyme in heterozygotes is intermediate between that in an individual homozygous for the
normal allele and in individual w/ tay sachs  intermediate phenotype observed at the biochemical level is
characteristic of incomplete dominance of either allele  the heterozygote condition does not lead to
disease symptoms since ½ of the normal enzyme is sufficient to prevent lipid accumulation in brain  we
find that heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules
 thus at the molecular level the normal allele and the tay sachs are co dominant
>so allele appears to be completely dominant, incompletely dominant, or co dominant depends on the level
in which the phenotype is analyzed
Frequency of Dominant Alleles
 do not assume that a dominant allele is more common  polydactyl (extra toes/fingers) are caused by
the presence of a dominant allele  but 5 digits (finger/toes) allele which is recessive is more common
Multiple Alleles
 ABO blood groups in humans are determined by three alleles of a single gene: I^A, I^B, and I
>each person has two alleles of the three for the blood group gene which determines his/her blood
phenotype: A, B, AB, or O  letters refer to two carbohydrates A and B found on a person’s red blood
cells  carb A (type A) carb B (type B) both (type AB) neither (type O)
Pleiotropy
 most genes have multiple phonotypical effects  pleiotropy
 pleiotropic alleles are responsible for the multiple symptoms associated w/ certain heredity diseases
(cystic fibrosis/sickle-cell disease)
>in garden peas the gene that determines flower color also affects the color of the coating on the outer
surface of the seed (gray or white)  a single gene can affect a number of characteristics in an organism
Extending Mendelian Genetics for two or more gees
 dominance relationships/multiple allles/pleiotropy all do w/ effects of alleles in a single gene 
consider a case where two or more genes determine a particular phenotype
>in first case one gene affects the phenotype of another because the two gene products interact  in second
multiple genes independently affect a single trait
Epistasis  the phenotypic expression of a gene at one locus alters that of a gene at a second locus
>example in Labrador retrievers  black coat color is dominant to brown  B and b will be the two alleles
 for a lab to have brown fur bb “chocolate labs”  a second gene determines whether or not pigment will
be deposited in the hair  dominant allele symbolized by E results in the deposition of either black or
brown pigment depending on the genotype at the first locus  if the lab is homozygous recessive for the
second locus (ee) then coat color is yellow (regardless for the genotype at the black/brown locus)  the
gene for pigment deposition (E/e) is epistatic to the gene that codes for black or brown pigment (B/b)
>if we mate black abs that are heterozygous for both genes (Bb/Ee) although the two genes affect the same
phenotypic character (coat color) they follow the law of independent assortment  represents an F1
dithered cross  as result of epistatis the phenotypic ratio of G2 offspring is 9:3:4 and  other epistatis
produce different rations but are all modified versions of 9:3:3:1
Polygenic inheritance
 for many characteristics (human skin color/height) either/or classification is impossible because the
characters vary in the population in graduations along a continuum  called quantitative characters
>this usually indicated polygenic inheritance  additive effect of two or more genes on a single phenotypic
character (opposite of pleitropy)
>evidence to show skin pigment in humans is controlled by at least (3) separately inherited genes 
consider three genes w/ dark-skin allele for each gene (A, B, or C) contribute one “unit” of darkness to the
phenotype and being incompletely dominant to the other  light skin allele (a,b, and c)  AABBCC is
very dark aabbcc is very light  AaBbCc would have an intermediate shade  AaBbCc and AABbcc
would make same genetic contribution (three units) to skin darkness
>7 skin color phenotypes that could result from a mating between AaBbCc heterozygotes  in large # of
mating the majority of offspring will have intermediate phenotypes
Nature and Nature: the Environmental Impact on Phenotype
 in humans nutrition influences height, sun tanning darkens skin and experience improves performance
on intelligence test
 phenotypic range is broadest for polygenic characters  environment contributes to the quantitative
nature of these characters (seen in the continuous variation of skin color)  geneticists refer to such
characters as multifactorials  meaning that many factors (Both genetic and environmental) collectively
influence phenotype
Integrating a Mendelian View of Heredity and Variation
>important to make the transition from the reductionist emphasis on single genes and phenotypic characters
to the emergent properties of the organism as a whole
 term phenotype can refer to not only a specific character but also to an organism in its entirely
>term genotype can refer to an organism’s entire genetic makeup, not just its alleles for a single genetic
locus  in most cases a gene’s impact on a phenotype is affected by other genes and by the environment
Many human traits follow mendelian patterns of inheritance
Pedigree Analysis
 geneticist assemble information about members of a family into a tree diagram that describes the traits
of parents and children across the generations  called a pedigree
>diagram above shows three-generation pedigree that traces the occurrence of widow’s peak due to
dominant allele W  individuals who lack a widow’s peak allele is homozygous recessive (ww)  two
grandparents must have Ww genotype since some of their offspring are homozygous recessive  offspring
in 2nd generation w/ window’s peak must also be heterozygous (product of Ww x ww)  third generation
sister w/ widows peak can be Ww or WW
for the 2nd one regarding earlobes (same family), f is recessive and F is dominant (results in free
earlobes)
>w/ pedigree you can use medelian inheritance to understand the genotypes shown for the family members
 important application of a pedigree is to help us calculate the probability that a future child will have a
particular genotype and phenotype
 if the couple represented in the second generation decides to have one more child what is the probability
the child will have widow’s peak?  this is like a mendalian F1 monohybrid cross (Ww X Ww) and thus
the probability that a child will inherit a dominant allele and have a widows peak is ¾ (¼ WW + ½ Ww)
 what is the probability the child will have attached earlobes  another monohybrid (Ff X Ff) but we
need to figure out the chance that the offspring will be homozygous recessive (ff)  that probability is ¼
>what is the chance that the child will have a widow’s peak and attached earlobes?  assuming that the
genes for these two characters are on different chromosomes the two pairs of alleles will assort
independently in this dithered cross (WwFf X WwFf)  so we can use the multiplication rule: ¾ (chance
of widow’s peak) X ¼ (chance of attached earlobes) = 3/16 (chance of widow’s peak and attached
earlobes)
 thousands of genetic disorders are known to be inherited as simple recessive traits
The Behavior of recessive alleles  genes code for proteins of specific function
>an allele that causes a genetic disorder (call it allele a)  codes for either a malfunctioning protein or no
protein at all  in the case of recessive disorders, Heterozygotes (Aa) are typically normal in phenotype
since one copy of the normal allele (a) produces a sufficient amount of the specific protein  so a
recessively inherited disorder shows up only in the homozygous individuals (aa) who inherit one recessive
allele from each parent
>carriers are phenotypic ally normal w/ regard to the disorder, heterozygote may transmit the recessive
allele to their offspring
>a mating between two carriers corresponds to Mendelian F1 monohybrids cross  genotypic ratio for the
offspring is 1AA : 2Aa: 1 aa  each child has a ¼ chance of inheriting a double dose of the recessive allele
 three offspring w/ normal phenotype (AA and (2) Aa, which are heterozygous carriers)
 recessive homozygotes could result from Aa x aa and aa X aa  if disorder is lethal before reproductive
age or results in sterility not aa individuals will reproduce
 when disease-causing recessive allele is rare it is unlikely two carriers w/ same harmful allele will meet
 if man and woman are blood relatives the probability of passing on recessive traits increase greatly
(consanguineous mating)  indicted in pedigrees by DOUBLE LINES
 most common lethal disease in U.S is cystic fibrosis  the normal allele for this gene codes for a
membrane protein that functions in the transport of chlorine ions between certain cells and the extra cellular
fluid  these chlorine transport channels are defective or absent in the plasma membrane of children who
inherit two recessive alleles for cystic fibrosis  result is an abnormally high concentration of extra
cellular chloride (causes mucus that coats certain cells to become thicker/stickier)  mucus builds up in
pancreas/lungs/other organs and leads to pleiotropic (multiple) effects (this is AUTOSOMAL
RECESSIVE)
 sickle cell disease is the most common inherited disease among people from African descent
>two sickle cell alleles are necessary for an individual to manifest full-blown sickle-cell disease 
presence of one sickle-cell allele can affect the phenotype  so at the organism level the normal allele is
incompletely dominant to the sickle cell allele
>heterozygote (carriers) have sickle-cell trait are usually healthy  at molecular level the two alleles are co
dominant; both normal/abnormal (sickle-cell) hemoglobin are made in heterozygotes
Dominantly inherited Disorders
 one example is achondroplasia, a form of dwarfism  heterozygous individuals have that phenotype
>dominant alleles that cause a lethal disease are much less common than recessive alleles that have lethal
effects  lethal recessive can pass from one generation to another by heterozygous carriers because the
carriers have normal phenotypes  a lethal dominant allele often cause death because an individual can
mature/reproduce
 late-onset diseases caused by lethal dominant allele may pass on (Huntington’s disease which is
autosomal dominant)
many people are susceptible to diseases that have a multifactoral basis (a genetic component + a
significant environmental influence) (like heart disease)
 genetic counseling is based on mendelian genetics
** from 9th**
 amniocentesis can determine whether a developing fetus has a genetic disease (like tay sachs) 
procedure involves a physical inserting a needle into the uterus and extracting about 10mL of amniotic
fluid
>alternative technique is called chorionic villus sampling (CVS)  a physician inserts a narrow tube
through the cervix into the uterus and suctions out a tiny small of tissue from the placenta  can be tested
 PKU can be detected at birth  cannot metabolize the amino acid phenylalanine
CHAPTER 12 Campbell Biology
“The Chromosomal Basis of Inheritance”
Mendelian Inheritance has its physical basis in the behavior of chromosomes
>chromosomes and genes are both present in pairs in diploid cells; homologous chromosomes separate and
alleles segregate during the process of meiosis; and fertilization restores the paired condition for both others
independently
>according to the chromosome theory of inheritance Mendelian genes have specific loci (positions) along
chromosomes and it is the chromosomes that undergo segregation and independent assortment
>figure on the next page shows the behavior of homologous chromosomes during meiosis for the
segregation of the alleles at each genetic locus to different gametes.
>also shows behavior of no homologous chromosomes can account for independent assortment of the
alleles for two or more genes located on different chromosomes
Morgan’s experimental evidence showed that chromosomes are the location of Mendel’s heritable factors
>Morgan used Drosophila Melanogaster (type of fruit fly) (insect that feeds on the fungi growing on fruit)
>they have four chromosomes (3 pairs of autosomes and 1 pair of sex chromosomes)
>most commonly observed in natural populations such as red eyes in Drosophila is called the wild type 
alternatives are called mutant phenotypes, like white eyes
>Morgan and his students invented notation for symbolizing alleles in Drosophila  white eyes is w while
the superscript + identifies a wild-type trait: w+ (red eyes)
>human genes are usually written in capitals (HD for Huntington’s)
Correlating behavior of a Gene’s Alleles with behavior of a chromosome pair
 Morgan mated white eyes male fly with red eye female and all F1 offspring had red eyes (suggested
wild type is dominant)  when Morgan bred f1 together he saw classical 3:1 phenotypic ratio among F2 
saw that white eye trait showed up only in males (all F2 females at red eyes and ½ males had red eyes and
½ males had white eyes)  said eye color was sex linked
(diagram mentioned from
before)
 Note that female sly has TWO X chromosomes and male fly has a X and a Y  Morgan suggested that
the gene involved in his white eye mutation was exclusively on the X chromosome w/ no corresponding
allele present on the Y chromosome
>reasoning: for a male a single copy of the mutant allele would confer white eyes (male has only one X) so
no wild type allele (w+) is present to mask recessive allele  female could have white eyes only if both her
X chromosomes carried the recessive mutant allele (w)  impossible for F2 females because all F1 fathers
had red eyes
>finding of the correlation between a particular trait and an individual’s sex provided support for
chromosome theory of inheritance (specific gene is carried on a specific chromosome)
Sex-linked genes exhibit unique patterns of inheritance
human males have X and Y  Y is smaller than X  2 X’s becomes a female
>short segments at either end of the Y chromosome are the only regions that are homologous w/
corresponding regions of the X  homologous regions allow the X and Y chromosomes in males to pair
and behave like homologous chromosome during meiosis in the testes
>in testes/ovaries two sex chromosomes segregate during meiosis  each egg receives one X chromosome
>½ the sperm cells a male produces receives an X chromosome and ½ receive a Y chromosome  if sperm
w/ X fertilizes a cell the zygote is a XX (Female)  mammalian sex determine is chance (50-50)
>in Drosophila males are XY but sex depends on the ratio between the # of X chromosomes and the # of
auto some sets (not based purely on presence of Y)
>SRY is a gene on the Y chromosome required for the development of testes, named “Sex-determiningregion”  absence of SRY the gonads develop into ovaries
>a gene located on either sex chromosome is called a sex-linked gene  those located on the Y
chromosome are called Y-linked genes
>human X chromosome contains X-linked genes  males and females inherit a umber of X chromosomes
leads to a pattern of inheritance different from that produced by genes located on auto some
Inheritance of X-linked Genes
 most Y-linked genes determine sex  X chromosome have genes for many characters unrelated to sex
>fathers pass X-linked alleles to all daughters but none to their sons  mothers can pass X-linked alleles to
both sons and daughters (bellow figure shows inheritance pattern of X-linked color blindness)
>if x-linked trait is due to a recessive allele a female will express the phenotype only if she is homozygous
for that allele  because males have only one locus we use the term hemizygous
 any male receiving the recessive allele from his mother will express the trait  so far more males than
females have X-linked recessive disorders
 colorblindness is X-linked and a female can get two doses of X (color-blind father and a mate who is a
carrier)  Duchenne muscular dystrophy is a muscle degenerative disorder that is also X-linked (recessive)
>Hemophilia is an X-linked recessive disorder regarding problem with blood clotting
X inactivation in female mammals  females inherit two X  note most of one X chromosome in each
cell in female mammals becomes inactivated during early embryonic development (so it does not have 2x
the number of X-linked genes)  so cells of females and males have the same effective dose (one copy) of
most X-linked genes
>inactive X in each cell of a female condenses into a compact object  “Barr body” which lies inside of
the nuclear envelope
>most of the genes of the X chromosome that forms the Barr body are not expressed  bar-body
chromosomes in the ovaries are reactivated in the cells that gives rise to eggs so every female gamete has
an active X
>selection of which X chromosomes will form Barr body occurs randomly and independently in each
embryonic cell present at the time of X inactivation  so females consist of a mosaic of two types of cells
1) those with the active X derived from their mother 2) those with the active X derived from the mother 
after an X chromosome is inactive in a particular cell all mitotic descendants of the cell have the same
inactive X
 if a female is heterozygous for a sex-linked trait ½ her cells will express one allele and others will
express the alternate allele
 mosaicsm results in the mottled coloration of a tortoiseshell cat (diagram below)
 inactivation of an X chromosome involves modification of DNA and histone proteins w/ it  attaching
a methyl group (-CH3) to one of the nitrogenous base of DNA nucleotides  certain regions in X
chromosome are involved in inactivation  (2) regions, one on each X associate briefly w/ each other on
each X chromosome at an early stage then one of the genes, XIST (X-inactive specific transcript) becomes
active only on the chromosome that will become the Barr body  multiple copies of RNA produce of this
gene attach to the X chromosome on which they are made  interaction of RNA w/ chromosome initiate X
inactivation and RNA produces of other genes nearby regulate the process
Linked genes tend to be inherited together because they are located near each other on the same
chromosome
 chromosome have many genes  genes located near each other on the same chromosome tend to be
inherited together  called linked genes (sex-linked = single gene on sex chromosome / linked genes = two
or more genes on the same chromosome that are inherited together)
How Linkage Affects Inheritance
 we can see linkage affecting inheritance of two different characters in the Drosophila experiments
 like body color and wing size (each w/ different phenotypes)  wild-type flies have gray
bodies/normal-sized wings  mutants have black bodies/smaller “vestigial” wings (neither on sex
chromosome)  Morgan did crosses shown in figure below  generate F1 dithered flies and second a test
cross  concluded body color/wing size are inherited together in certain parental combinations since genes
for these characters are near each other on the same chromosome
 above shows both combinations of traits not seen in P generation were also produced in Morgan’s
experiments (showing both traits are not always linked)  need to explore genetic recombination
(production of offspring with combinations of traits that differ from those found in either P generation
parent)
Genetic Recombination and Linkage
meiosis/random fertilization generate genetic variation in offspring of sexually reproducing organism
due to independent assortment of chromosomes and crossing over in Meiosis I
Recombination of Unlinked Genes: Independent Assortment of Chromosomes
 in a cross with a yellow-round pea that is heterozygous for both seed color/seed shape (dithered YyRr)
w/ a plant w/ green-wrinkled seeds (homozygous for both, yyrr) will produce YyRr, yyrr, Yyrr, yyRr
>note ½ (YyRr and yyrr) match either parental phenotype and are called parental type  two nonparental
phenotypes are called recombinant types (recombinants)  if 50% of all offspring are recombinants it is
said that there is a 50% frequency of recombination  50% frequency of recombination in these testcrosses
is observed for any two genes that are located on different chromosomes (unlinked)  basis of unlinked
genes is the random orientation of homologous chromosomes at metaphase I of meiosis (leads to
independent assortment of two unlinked genes)
Recombination of Linked Genes: Crossing Over
 as we saw in the figure above (15.9) most of the offspring of the test cross for body color/wing size had
parental phenotypes  suggest the two genes were on the same chromosome (since occurrence of parental
types w/ a frequency greater than 50% indicated genes are linked  17% offspring were recombinants) 
some process must occasionally break physical connection between specific genes on same chromosome 
crossing over  accounts for the recombination of linked genes  occurs while replicated homologous
chromosomes are paired during prophase of meiosis I  proteins help in exchange of corresponding
segments of maternal/paternal chromatids  end portions of 2 nonsister chromatids trade places each time
as crossover occurs
 we can observe how crossing over in dihybrid females fly results in recombinant eggs and then
recombinant offspring in the testcross  most eggs had chromosome w/ either b+ vg+ or b vg parents
genotype for body color and wing size  some eggs had recombinant chromosome (b+ vg or b v g+) 
fertilization of various eggs by homozygous recessive sperm (b vg) produced offspring population w/ 17%
exhibited a no parental recombinant phenotype (showing combinations of alleles not seen before in P)
New Combinations of Alleles: Variation for Natural Selection
 meiosis contributes to the variation in offspring  each pair of homologous chromosomes line up
independently of other pairs during metaphase I and cross before that in prophase I (mix/match
maternal/paternal homologs)  allele of genes as shown w/ Mendel lead to variation also
>we can now see that recombinant chromosomes resulting from crossing over may bring alleles together in
new combinations and subsequent events of meiosis distribute to gametes the recombinant chromosomes in
a multitude of combinations  random fertilization furthers the variant allele combination
>gives the raw materials for natural selection to work  if traits conferred by particular combination of
alleles are better suited for a given environment organism w/ those genotypes will thrive/leave more
offspring  letting the continuation of their genetic complements  the interplay between environment
and genotype will determine which genetic combination persist over time
Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry
 discovery of linked genes and recombination due to crossing over led to the construction of the genetic
map, an ordered list of genetic loci along a particular chromosome  led by Sturtevant, he hypothesized
the % of recombinant offspring, the recombination frequency, calculated previously depends on the
distance between genes on a chromosome  assumed crossing over is a random event w/ chance of
crossing over approximately equal at all points along a chromosome
 genetic map based on recombination frequencies is called a linkage map
 above figure shows Sturtevant’s linkage map of (3) genes  body color (b), wing size (vg) and cinnabar
(cn) [affects eye color]
>cinnabar eyes is a mutant phenotype which is a brighter red than wild-type  recombination frequency
between cn and b and between cn and b is 9%; between cn and vg is 9.5% and between b and vg 17% 
shows that crossovers between cn and b and between cn and vg are ½ are frequent as crossovers between b
and vg  only map w/ cn about midway between b and vg is consistent w/ these data  Sturtevant
expressed distance between genes in map units; one map unit = 1% recombinant frequency
some genes on a chromosome are so far away from each other that crossover between each other is
certain  observed frequency of recombination involving (2) such genes can have a maximum value of
50%
>a result indistinguishable from that for genes on different chromosomes  the physical connection
between genes on the same chromosome is not reflected in the results of genetic crosses  despite being
on the same chromosome (physically connected)  genes are genetically unlinked  alleles of such genes
assort independently like their on different chromosomes
>genes located far apart on a chromosome are mapped by adding the recombination frequencies from
crosses involving closer pairs of genes lying between the two distant genes
 genes clustered into groups that are linked genes are Linkage groups
>because linkage map is based on recombination frequencies it is an approximate picture of a chromosome
 frequency of crossing over is not really uniform over the length of the chromosome and therefore map
units do not correspond to actual physical distances  linkage maps portrays the order of the genes along a
chromosome (but not the precise locations of those genes)
>geneticist construct cytogenetic maps of chromosomes which locate genes w/ respect to chromosomal
features and these maps display the physical distances between gene loci in DNA nucleotides
>comparing linkage maps and physical map w/ a cytogenetic map of the same chromosome we find that
the linear order of the genes is identical in all maps (but not the spacing between genes)
Alterations of chromosome number or structure cause some genetic disorders
 phenotype of an organism can be affected by small-scale changes involving individual genes  random
mutations can lead to new alleles
 large-scale chromosomal changes can affect an organism’s phenotype  physical/chemical
problems/errors during meiosis can damage chromosomes in major ways  can lead to
miscarriages/individuals born w/ thee types of genetic defects exhibit various developmental disorders 
plants may tolerate such genetic defects better than animals do
Abnormal Chromosomal Number
 an occasional problem occurs when meiotic spindle distributes chromosomes to daughter cells
incorrectly  no disjunction in which the members of a pair of homologous chromosomes do not 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 gametes receives no copy
<if either of the aberrant gametes unites w/ a normal one at fertilization the zygote will have an abnormal
number of a particular chromosomes (known as aneuploidy)
 fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing
chromosome in the zygote (lead to 2n-1 chromosome)  the aneuploid zygote is monosomic for that
chromosome
if a chromosome is present in triplicate in the zygote (lead to 2n+1 chromosome) the aneuploid cell is
trisomic for that chromosome
 mitosis will then transmit the anomaly to all embryonic cells  if organism survives it has a set of traits
caused by the abnormal dose of the genes associated with the extra or missing chromosome (down
syndrome is trisomy)
non disjunction also occur during mitosis  if an error takes place early in embryonic development then
the aneuploid condition is passed along by mitosis to a large # of cell
 some organisms have more than two complete chromosome sets in all somatic cells  term for this
chromosomal alteration is polyploidy
>triploidy (3N) tetraploidy (4N) are 3 and 4 chromosomal sets respectively
>a triploid may arise by the fertilization of an abnormal diploid egg produced by no disjunction of all its
chromosomes  tetrapolidy can come from the failure of the 2n zygote to divide after replicating its
chromosomes  subsequent normal mitotic divisions would then produce a 4n zygote
 polyploidy is common in plants (bananas = 3n / wheat = 6n / strawberries = 8n)
Alterations of Chromosome Structure
errors in meiosis or damaging agents (radiations) can cause breakage of chromosomes and lead to (4)
types of changes in chromosome structure
 deletion is when a chromosomal fragment is lost  affect chromosome is missing certain genes 
“deleted” fragment may become attached as an extra segment to a sister chromatid making a duplication
 a detached fragment can attach to a nonsister chromatid of a homologous chromosome  although the
“duplicated” segments might not be identical because the homologs could carry different alleles of certain
genes
>a chromosomal fragment may also reattach to the original chromosome but in the reverse orientation
producing an inversion
 4th type results of chromosomal breakage is for the fragment to join a nonhomologous chromosome 
“translocation”
>deletions/duplications are common in meiosis  during crossing over there are unequal-sized DNA
exchanges  products involve one chromosome w/ deletion and one chromosome w/ duplication
>diploid embryo that is homozygous for a large deletion (or has single X chromosome w/ large deletion)
may have a lethal condition
 in reciprocal translocations (where segments are exchanged between no homologous chromosomes) and
in inversions the balance of genes is not abnormal (all genes are present in normal doses) 
translocations/inversions can alter the phenotype because a gene’s expression can be influenced by its
location w/ neighboring genes
Human Disorders Due to Chromosomal Alterations
 syndrome is a characteristic of the type of aneuploidy
Down Syndrome  result of an extra chromosome 21
Aneuploidy of Sex Chromosomes
>appear to upset the genetic balance less than autosomes  Y carries few genes and extra copies of X
become inactivates as Barr bodies ins somatic cells
 an extra X in males producing XXY cause Klinefelter syndrome (man is sterile)  an extra Y, XYY
these males are generally normal
 females with XXX (heh) are healthy but are at risk for learning disabilities  monosomy X (Turner’s
syndrome) is X0 (only viable monosomy in humans)  they are sterile
Disorders Caused by Structurally Altered Chromosomes
 deletions in human chromosomes even in heterozygous state cause severe problems  cri du chat
results from a specific deletion in chromosome 5
>chromosomal transportation have been implicated in certain cancers such as chronic myelogenous
leukemia
#LAST BIO STUDY SHEET FOR 1st TERM
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