Download Notes For Genetics!! File

Unit 3: Genetic Continuity
Chapters 16, 17, & 18
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Pages 522 - 639 (MHR)
Mendelian Genetics
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Traits - distinguishing characteristics or easily observed features of an individual
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Heredity - is the passing of genetic traits (e.g. hair or eye colour) from one generation to
the next. It results in similarities between family members.
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Genetics - is the branch of biology that studies how traits are passed from generation to
generation.
Genetics in history
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believed sperm contained fully formed embryo that grew inside womb of female (fig 16.2,
pg 526)
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others believed theory of blended inheritance.
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traits from each parent blended together in offspring
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E.g. tall + short = medium
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couldn’t explain appearance or disappearance of distinct traits from one generation
to another
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The Austrian monk Gregor Mendel had other ideas...
Gregor Mendel (1822-1884) pg 527
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first scientist to correctly interpret patterns of inheritance
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experimented with pea plants over 8 years (1853-1861) to study inheritance
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Pea plants were a good choice because they:
1. Have seven distinct traits, each of which has only 2 possible forms
e.g. tall vs short, smooth vs wrinkled
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2. Can self-pollinate or cross-pollinate
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self-pollination - eggs from one plant are fertilized by pollen of the
same plant
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cross-pollination - eggs from one plant are fertilized by pollen from
another plant
3. Grow and reproduce quickly (unlike lab rats)
To prevent self-pollination Mendel eliminated either the male or female reproductive
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structures of the flower
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to cross pollinate he brushed pollen from one plant onto the female part of another
plant.
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cross-pollination results in offspring from 2 different plants (offspring from selfpollination is from one plant only)
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cross-pollination allowed Mendel to mate plants with different traits (characteristics)
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to ensure validity of his experiments, Mendel first bred purebred plants
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took 2 tall plants and bred them together.
seeds produced were planted and grew.
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some were tall, others short.
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tall plants were selected and 2 bred together again
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repeated ‘til only tall plants produced
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then called purebred i.e. they descended from ancestors of a distinct type (like
our own Newfoundland Dog or Labrador Retriever who are purebreds!!)
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created purebreds for all seven traits
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called the purebred plants P generation or parent generation
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then the parent plants with different characteristics of the same trait were
crossed (Fig 16.5, pg 529)
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e.g. tall with short (height)
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green with yellow (colour)
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“characteristics (trait)
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offspring of this cross were the first filial generation or F1 generation
also called hybrid because they contained both characteristics for a particular
trait (even if only one characteristic was apparent)
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(here we are only crossing one trait, height. We can therefore say that this was a
monohybrid cross)
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100% of the F1 group were tall (i.e. no blending!!) the short characteristic seemed
to disappear!
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Mendel concluded that the trait for being tall must be dominant, and short must
be recessive
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dominant - the characteristic is always expressed/apparent
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recessive - the characteristic is present but not expressed unless there are
only recessive characteristics present
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repeated these experiments many times and always same results. Sooo... he
developed his principle of dominance i.e. when contrasting traits are crossed, the
offspring express only the dominant trait
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the next part of his experiment, the F1 generation self-pollinated, which produced
the second filial generation (F2 generation)
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repeated this test many times and every trial gave the same results:
3 /4 of the F2 offspring were tall and 1/4 were short (fig 16.5, pg 529)
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from the F2 offspring he developed his law of segregation or 1st law of heredity
which stated that “traits are determined by pairs of factors from each parent.
These factors separate during gamete formation, giving each offspring only one
factor from each parent” (Fig 16.7, pg 530)
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we now call his “factors” genes (part of the chromosome that controls expression
of traits)
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genes can occur in different forms i.e. alleles (dominant or recessive)
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Mendel’s factors are sometimes called unit characters because they are inherited
independent of other traits
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his theory is called unit theory from his discovery that genes are inherited as
independent units.
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if we use letters to represent the different alleles, a purebred tall plant would be
TT (2 uppercase T’s). It is homozygous (2 alleles are the same)
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a purebred short would be tt, also homozygous (both alleles are the same)
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the F1 generation were all heterozygous, i.e. the tall parent donated a T and the
short parent donated a t, so the result is Tt (2 alleles are different) (fig 16.6, pg
530)
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to represent Mendel’s experiments symbolically:
Purebred
Purebred
Tall
Short
TT
tt P generation


Tt
F1 generation
(100% tall)
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(come back to this business of crosses after probability)
PROBABILITY (pg 531)
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“the chance or likelihood of a particular outcome”
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usually expressed as a ratio
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important to genetics because enables us to predict expected outcomes of genetic
crosses
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can use product rule or Punnett squares
Product rule
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the chances of independent events occurring at the same time is equal to the product of
the chances of these events occurring separately
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e.g. What is the likelihood of getting 2 heads if we flip 2coins?
-The chance of getting one head is ½
-The chance of getting another head is ½
-The chance of getting both heads at same time is
½X ½= 1/4
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Rule of independent events: chance has no memory i.e. if you toss 2 heads in a row, the
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likelihood of tossing another head is still ½.
however the chances of flipping 3 heads in a row is ½ x ½ x ½= 1/8
genetic e.g. - 2 heterozygous tall plants (Tt) are crossed
½ the gametes are T
½ the gametes are t
T might combine with another T or a t.
Punnett Squares (pg 532)
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visual representation of possible offspring allele combinations
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used to calculate chance (probability) of inheriting a particular trait
shows all possible gamete combinations from set of parents (egg/sperm)
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dominant allele - capital letter!! (e.g. T)
recessive allele - lowercase letter!! (e.g. t)
Example:
P generation
TT x
tt
Tall plant
T
T
t
Tt
Tt
t
Tt
Tt
Short Plant
TT

tt 
F1 generation
100% of offspring is tall
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Punnett sq’s give predictions as to possible outcomes from a given set of alleles
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can use to determine genotype and/or phenotype
- genotype - genetic makeup of an organism
- phenotype - appearance of trait in organism
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E.g. F1
phenotype is
F1
Tt
T
t
T
TT
Tt
t
Tt
tt
generation has genotype Tt but
tall
cross (self pollination):
Tt
} F2 generation
3 tall:1 short
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Complete dominance - the type of inheritance where both heterozygotes and dominant
homozygotes have the same phenotype.
e.g. Both Tt and TT are tall. The “tall allele” is dominant
Practice problems 1,2, & 3 page 533 (MHR text)
Test Cross (pg 533-534 MHR)
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How can we tell if a plant showing a dominant phenotype is heterozygous or homozygous?
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e.g. how can we tell if a tall plant is TT or Tt?
Or if a yellow pea plant is YY or Yy?
Perform a test cross!! (the crossing of the unknown individual with a homozygous
recessive individual)
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e.g. we cross the unknown tall plant (either TT or Tt) with a known short plant
(know tt).
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If unknown plant is TT ... all offspring will have genotype Tt and will be tall
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(phenotype).
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If unknown plant is Tt ... ½ offspring will be tall and ½ will be short i.e. ½ will be Tt
and ½ tt.
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Do the Punnett Square to see why (see fig 16.10, pg 534 for another example)
Dihybrid Cross (pg 536 MHR)
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Pea plants have many traits (colour, height, shape etc.)
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Are they inherited together or separately? Does pea shape influence pea colour?
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Mendel bred 2 more types of purebred plants
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homozygous dominant for both shape and colour i.e. round (RR) and yellow (YY)
seeds
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homozygous recessive for both shape and colour i.e. wrinkled (rr) and green (yy)
seeds
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then he performed a dihybrid cross which is a cross of two plants that differ in two
traits i.e. one is round & yellow and the other is wrinkled and green. (Fig 16.12 pg 536)
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F1 generation - all round & yellow (all heterozygous: RrYy)
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F2 generation -9 round :3 round : 3 wrinkled : 1 wrinkled
yellow green
yellow
green
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9:3:3:1 made sense if alleles for different traits were inherited separately i.e. alleles for
colour not connected to alleles for shape
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Mendel’s 2nd law of inheritance or the law of independent assortment states that
inheritance of alleles for one trait does not affect the inheritance of alleles for another
trait.
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offspring may have characteristics that were not present in the parents
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Punnett square: all possible gamete combinations for one parent go across top and all
possible gamete combinations for the other parent go down the side.
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e.g. F1
RrYy x
Possible
are RY, Ry,
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
Rryy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
cross
RrYy.
combos
rY, ry
RrYy
RrYy
R - round; r - wrinkled
Y - yellow; y - green
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Beyond Mendel’s Laws (pg 541-543)
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Mendel’s pea plants demonstrated complete dominance.
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Other organisms show different patterns of inheritance
Incomplete dominance (fig 16.15 pg 541)
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Occurs when neither of the alleles present is dominant
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A blending of the traits occurs
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Many examples in plants including snapdragons, and Japanese four-o’-clock flowers
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e.g. red or white snapdragon flowers are homozygous whereas pink ones are heterozygous
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Here RR = red, R’R’ = white, RR’ = pink
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Two red alleles are necessary to produce a red flower i.e. RR
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If only 1 R, not enough pigment to be red. Therefore blends with white and appears pink.
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Still follows Mendel’s laws of segregation and independent assortment.
Offspring don’t show Mendelian ratio because neither allele is dominant
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Fig 16.15 pg 541 shows P, F1 and F2 generations.
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P generation
F1 generation
F2 generation
RR x R’R’
All pink (all RR’)
1 red RR: 2 pink RR’: 1 white R’R’
RR’ x RR’
R
R’
R
RR
RR’
R’
RR’
R’R’
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Co-dominance (fig 16.16 pg541)
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both alleles for a trait are dominant i.e. both alleles of a gene are expressed at the same
time in a heterozygous individual
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e.g. feather colour in chickens black - BB, white - WW
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cross black rooster with white hen
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blend? Get grey feathers?
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If only black dominant, would get all black offspring.
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No. Get 1 chicken with some white and some black feathers. Results in a checkered
pattern. See fig 16.16 pg541
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also seen in horses. Cross a red horse with a white horse and get a roan horse (red and
white hair)
Multiple Alleles (pg 542-543)
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when a gene has more than 2 alleles for a given trait
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e.g. human blood types: 3 alleles - A, B, and O.
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table 16.1 pg 542 blood types & genotypes
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people have 2 of the 3 alleles: IA, IB, or i
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IA and IB are dominant over i
IA and IB are co-dominant when both present
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IAi or IA IA result in blood type A (hetero- or homozygous)
IBi or IB IB result in blood type B (hetero- or homozygous)
IAIB results in blood type AB (heterozygous)
ii results in blood type O (homozygous)
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skin colour, eye colour is controlled by even an greater number of alleles
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continuous variation that can’t be split into easily defined categories
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e.g. cross a heterozygous person with type A blood with an individual heterozygous for
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type B blood. What are the possible offspring phenotypes?
IA
i
IB
I AI B
IBi
i
I Ai
ii
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ii is the genotype for type O
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IAIB is the genotype for type AB
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IBi is the genotype for type B
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IAi is the genotype for type A
Modern Ideas
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Mendel said each trait has 2 ‘factors’ which separate during gamete formation
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went unnoticed until 1900's when better technology showed cellular processes &
chromosomes
The Chromosome Theory of Inheritance (pg 545-546)
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Walter Sutton and Theodor Boveri (scientists) studied meiosis
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saw chromosome behaviour during meiosis was like that of Mendel’s factors (fig 16.19
pg546)
they made 3 key observations
- chromosomes occur in pairs which segregate during meiosis
- align independently of one another
- inheritance of one doesn’t affect other chromosome being found in the gamete
formed basis for chromosome theory of inheritance
- genes are on chromosomes
- it is the segregation and independent assortment of chromosomes that accounts
for patterns of inheritance
Morgan’s discoveries
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Thomas Morgan - 1910 studied eye colour in fruit flies
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crossed 2 red-eyed parents - got white-eyed male
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principle of dominance??
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crossed a red-eyed female offspring of the white-eyed male with normal red-eyed male
- all female offspring had red eyes
- ½ male offspring had red eyes
- ½ male offspring had white eyes
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eye colour linked to sex
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concluded gene for eye colour located on X chromosome
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1st time gene linked to specific chromosome
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found that some genes didn’t follow law of independent assortment, tended to be
inherited together.
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genes located on same chromosome usually don’t segregate.
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this rule doesn’t always apply
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linked genes do sometimes segregate
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this is due to gene-chromosome theory - genes exist at specific sites in a linear fashion
along chromosomes.
Pairs of homologous chromosomes separate during gamete formation, and form two
gametes.
Each gamete contains a separate allele for each trait.
During fertilization, chromosomes from one gamete will combine with another
gamete.
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crossing over can occur during cell division (fig 16.20 pg 546) see “crossing over”
overhead
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genes very close on chromosome - inherited together
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genes separated on chromosome - likely to be separated by crossing over
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more distance between genes - more likely cross over will occur
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crossing over breaks gene linkages and creates variation
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restate Mendel’s law of independent assortment:
if crossing over doesn’t occur, genes on different chromosomes will assort independently
while genes on the same chromosome will be inherited together.
Sex Determination and Chromosomes
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chromosomes in cells from males and females are identical except one pair: the sex
chromosomes (pair 23)
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other 22 pairs of chromosomes called autosomes
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sex chromosomes determine sex of organism
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females - two X-chromosomes as 23rd pair
males one X- and one Y- chromosome as 23rd pair
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female gametes only have X-chromosomes when separated in meiosis
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male gametes produce both X and Y chromosomes
cross male and female
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X
Y
X
XX
XY
X
XX
XY
half offspring - female (XX);
half - male (XY)
could say male responsible for determining sex of offspring
Sex-Linked Inheritance (pg 546-547)
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Some inherited traits depend on the sex of the parent carrying the trait
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Why??
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The trait is carried on the sex chromosome
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Sex linked inheritance - the transfer of genes on the X or Y chromosome from one
generation to the next
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a gene only on the X chromosome is called X-linked
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a gene only on the Y chromosome is called Y-linked
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most of the known sex-linked genes are X-linked, very few are Y-linked. Maybe because
Y-chromosome is much smaller.
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Morgan hypothesized that the gene for eye colour in fruit flies is on the X chromosome
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said white eyed trait was recessive
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to get white eyed female both parents would need to donate an allele for white eyes
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fig 16.22 pg 547. Punnett square of XrY x XRXR
- XrY - white eyed male, and XRXR - red eyed female
Xr
Y
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XR
XRXr
XRY
XR
XRXr
XRY
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male donates Y chromosome to all male offspring therefore doesn’t donate allele
for eye colour
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female must donate the X chromosome to all male offspring
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female offspring get one X from father, and one of the mother’s X chromosomes
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all female offspring are heterozygous and red-eyed XRXr and all male offspring red
eyed XRY
red eyed male XRY with a heterozygous red eyed female XRXr
XR
Y
XR
XRXR
XRY
Xr
XRXr
Xr Y
Sex linked traits and Punnett squares
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Same as autosomal trait squares except:
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alleles on X if X-linked and none on Y chromosome
e.g. Xr Y and XRXR
when writing the genotypes and phenotype ratios we ALWAYS include the sex in
the ratio.
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Remember autosomal inheritance usually involves pairs of genes, gender is irrelevant
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sex-linked inheritance involves pairs of genes on the X chromosomes in the female and a
single gene in the male
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sex linked defects/disorders are more common in males than females
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males only need one recessive allele for trait to be seen
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females need two recessive alleles to display trait, or could be a carrier if heterozygous.
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red-green colour blindness, hemophilia, muscular dystrophy are all connected to sex
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more common in males
X-Linked Recessive Inheritance (pg 558-559)
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Colour blindness fig 16.36, pg 559
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8% men, 0.4% women red-green colour blind
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both alleles for “seeing” red & green on X chromosome
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if one allele absent, can’t distinguish between colours
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why more in men??
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defective allele is recessive condition so
woman must have 2 recessive alleles (XbXb) one from a colour blind father, and one
from her mother
man only needs to inherit one recessive allele from mother XbY
woman can have normal vision but be a carrier XBXb
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XBXB
Normal
vision
females 
males 
XBY
Normal
Vision
or
or
XBXb
or
carrier
(Normal vision)
XbXb
Colour
blind
XbY
Colour
Blind
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for a female to be colour blind BOTH chromosomes must have the gene for colour
blindness
2.
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Hemophilia
recessive, sex (X-linked) disorder
blood doesn’t clot properly
XHXH or
XHXh or
XHY all normal phenotype
Xh Xh or
Xh Y
hemophilia
3. Muscular dystrophy
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progressive wasting of skeletal muscle
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carried on X chromosome
Karyotypes (pg 559)
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useful in diagnosing genetic disorders
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illustration of chromosomes arranged in homologous pairs
chromosomes stained, pictures enlarged, arranged in pairs according to size,
banding, etc.
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humans have 46 chromosomes in somatic cells: 44 are autosomes and 2 are sex
chromosomes
Do Core Lab #6: Karyotype Lab
Polygenic Traits
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also ‘multiple gene inheritance’
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traits determined by a number of different contributing genes present at different
locations
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expression depends on sum of influences of all the genes
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skin & eye colour, susceptibility to cardiovascular disease, athletic ability, ear length in
corn.
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