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Chapter 5
Heredity
Gregor Mendel
• The father of genetics
and heredity
• Famous for his pea
experiments
• Was a monk in a
monastery when he
did his ground
breaking work.
• Died in 1884.
• The genetic experiments Mendel did with pea
plants took him eight years (1856-1863) and he
published his results in 1865. During this time,
Mendel grew over 10,000 pea plants, keeping
track of progeny number and type. Mendel's
work and his Laws of Inheritance were not
appreciated in his time. It wasn't until 1900, after
the rediscovery of his Laws, that his
experimental results were understood.
• Why did it take so long to recognize his work?
Are You Unique?
• In the past ten
thousand years,
there have
billions of
people who
have lived.
• Have they all
been different?
Yes – But Why?
In December 1999, the first human
chromosome was completely
sequenced. Chromosome #22 is
one of the smallest human
chromosomes and has 33.5
million base pairs of DNA.
http://www.dnaftb.org/dnaftb/1/conc
ept/
Concept
Animation
Audio video clip 2
Problem
http://www.dnaftb.org/dnaftb/15/con
cept/
• animation
http://www.dnai.org/a/index.html
• Copying the code
• Reading the code
• Controlling the code
Getting back to Mendel
• Mendelian Genetics
• Mendel was interested
in the way traits were
passed from parents to
offspring. = HEREDITY
• Mendel simplified his
investigation by
studying only one
organism (the garden
pea plant)
Why Pea Plants?
• They grow quickly
• They are usually self pollinating
• They come in many varieties
• Self Pollinating – contains both male and
female reproductive structures – thus
pollen from one flower or plant can fertilize
the eggs of the same flower or the eggs of
another flower on the same plant.
How Did He Do it?
• Studied only one characteristic at a time
(ie. Plant height, or pea color)
• He chose plants with 2 forms of the
characteristic to be studied.
– Short vs. tall
– Smooth peas vs. wrinkled peas
– Purple flowers vs. white flowers
• He always chose true breeding plants –
when self pollinating these always produce
the offspring with the same characteristics.
(SS or ss) or (TT or tt) as the parent –
identical alleles for a trait
• He started cross breeding 2 plants with
different forms of the same trait via crosspollination.
Cross Pollination
1) the anthers on the
stamen of one plant are
removed (so the plant
can not self-pollinate)
2) Pollen from another
plant is used to fertilize
the plant without
anthers.
* This allows Mendel to
control which pollen
fertilizes which plant
Know This Nomenclature
• True Breed Smooth (SS) x True Breed wrinkled (ss) = parents
Cross pollination
• The offspring of 2 true breeds = f1 generation (all Ss but all
smooth)
Self pollination
(Ss) X (Ss)
• The offspring of an f1 generation due to self pollination = f2
generation
How did Mendel follow the scientific
method?
Ask a Question –
How are traits inherited?
Form a Hypothesis –
If traits are inherited then their patterns can be
predicted.
Test the Hypothesis –
Cross true-breeding plants and offspring.
Analyze the Results –
Identify patterns in inherited traits.
Draw Conclusions –
Traits are inherited in predictable patterns.
How he deciphered heredity
• Mendel chose to study
only one characteristic,
such as plant height,
flower color, or seed
shape.
True breeding plants – a
plant that only reproduces
offspring with the same
traits as the parent when it
self-pollinates. (SS or ss)
For example- a tall true
breeding plant will always
produce offspring that are
tall.
• He then cross pollinated
two true breeding plants
with different forms of a
single trait. For
example- a true breeding
tall plant with a true
breeding short plant.
•Cross Breeding – cut the
anthers from the stamens
of one plant (so that it can’t
self-pollinate) then add
pollen to it from another
plant.
• Crossed plants that had true bred
smooth seeds (SS) with those that
had true bred wrinkled seeds (ss).
• The offspring from this first
generation (f1) were all smooth.
• This same thing occurred no matter
what the trait tested – all the f1
generation possessed only one of
the parent’s traits.
• dominant trait – the one and only
trait that shows itself in f1 generation
• recessive trait – the trait that does
not show itself in the f1 generation
Mendel’s first
experiment
Mendel: Experiment 1
Remember – in sexual
reproduction each
parent donates 1 gene
each.
An SS can only donate
“S”
But a Ss can donate
either an “S” or an “s”
Mendel’s second experiment
• Mendel allowed the first
generation (f1) to self
pollinate.
• This time the f1 generation
which was all smooth seeds
(the dominant trait)
produced a next generation
(f2) which possessed some
smooth seeds and some
wrinkled seeds.
• Somehow the recessive
trait showed up again.
• Every fourth seed was
wrinkled.
P parents
F1 offspring
F1 breeding
F2 offspring
Mendel’s Actual Results
1) P = smooth seeds crossed with wrinkled
seeds
F1 = all smooth seeds (so smooth is
dominant and wrinkled is recessive)
F2 = 5,474 smooth seeds and 1,850
wrinkled seeds is a ratio of 2.96 : 1
2) P = green seeds crossed with yellow
seeds
F1 = all yellow seeds (so yellow is
dominant and green is recessive)
F2 = 6,022 yellow seeds and 2,001
green seeds is a ratio of 3.01 : 1
3) P = purple flowers crossed with white
flowers
F1 = all purple flowers (so purple is
dominant and white is recessive)
F2 = 705 purple flowers and 224 white
flowers is a ratio of 3.15 : 1
4) P = constricted pods crossed with
inflated pods
F1 = all inflated pods (so inflated is
dominant and constricted is recessive)
F2 = 882 inflated pods and 299
constricted pods is a ratio of 2.95 : 1
5) P = green pods crossed with yellow
pods
F1 = all green pods (so green is dominant
and yellow is recessive)
F2 = 428 green pods and 152 yellow pods
is a ratio of 2.82 : 1
6) P = terminal flowers crossed with axial
flowers
F1 = all axial flowers (so axial is dominant
and terminal is recessive)
F2 = 651 axial flowers and 207 terminal
flowers is a ratio of 3.14 : 1
7) P = dwarf stem crossed with tall stem
F1 = all tall (so tall is dominant and dwarf
is recessive)
F2 = 787 tall stems and 277 dwarf stems
is a ratio of 2.84 : 1
Let’s Look at the Ratios
Flower Color
Seed Color
Seed Shape
Pod Color
Pod Shape
Flower Position
Plant Height
3.15:1
3.00:1
2.96:1
2.82:1
2.95:1
3.14:1
2.84:1
3:1
(purple:white)
(yellow:green)
(smooth:wrinkled)
(green: yellow)
(inflated:constricted)
(middle:end)
(tall:short)
Math Break (probability)
If Mendel looked at 7 different traits each
with only 2 forms, how many different
plants are possible?
Pea
Shape
Flower
Color
Pea Color
purple
green
Smooth
white
purple
Wrinkled
white
yellow
yellow
green
yellow
green
yellow
green
The Answer Is….
2x2x2x2x2x2x2
Or 2
=
7
128 different combinations
What are the chances of getting a plant that is tall,
with green wrinkled peas in a inflated yellow pod
with purple flowers located at only the ends of
branches (assume equal chance of each individual
trait possibility)?
Mendel’s Brilliant Conclusion
The only way to explain the presence of only one trait
in the f1 generation and the 3:1 ratios in the f2
generation was if each plant had 2 sets of
instructions for each characteristic.
Each parent donates one set of instructions known as
genes
Therefore, every fertilized egg (offspring) would have 2
forms of the same gene for every characteristic. The
2 forms are individually termed an allele. The
combination of the 2 alleles in the offspring is
termed the genotype.
Since we know smooth seeds are dominant as
they show up exclusively in the f1 generation we
will label that gene with a capital S. The recessive
gene we will label as a small s.
Parents
Pure Breed
Wrinkled Seeds
Pure Breed
Smooth Seeds
ss
SS
S gene (or allele)
Offspring (f1)
s gene (or allele)
Ss genotype which makes all smooth
seeds because smooth is dominant
In f1 if there were 4 offspring (or 10,000) their only possible
allele combination is Ss. The allele combination is called
the genotype. Thus 4 Ss which are all smooth seeds.
Second Generation – this generation is allowed to self
fertilize. The possible male genotypes are Ss and the
possible female genotypes are also Ss. Each can send
an S allele or an s allele.
Male Contribution
Smooth Seeds
Female contribution
Smooth Seeds
f1
Ss
Ss
or
or
S gene
s gene
S gene
s gene
Offspring (f2)
SS
Ss
Ss
ss
F2 generation
•
•
•
•
SS
Ss
Ss
ss
Smooth
Smooth
Smooth
What is the Ratio?
3:1
smooth to wrinkled
Wrinkled
What are the percentages?
75% Smooth and 25% Wrinkled
Terminology
• Homozygous – possessing 2 of the same
alleles for a particular trait (SS or ss or TT
or tt)
• Heterozygous – possessing different
alleles for a particular trait (Ss or Tt)
• Homozygous dominant = SS or TT
• Homozygous recessive = ss or tt
The Punnett Square
• The Punnett square is a
simple grid with the all the
possible sperm/male alleles
along one side, all the
possible egg/female alleles
along another side, and all
the possible offspring
genotypes filling the grid.
• Fusion of those two gametes
produces the genotype of the
offspring (zygote) in the
boxes.
For a given trait, dominant traits are symbolized by CAPITAL letters
and recessive traits by lower case letter.. Always use the same letter
for dominant and recessive. DO NOT USE DIFFERENT LETTERS
Therefore all genotypes from true breeding
organisms are represented by 2 similar
alleles –represented by similar letters (pp or
PP, rr or RR, etc.)
The cross between 2 true breeding plants
one purple flowered and one white
flowered would be (where purple is
dominant and white is recessive):
PP x pp
The Punnett Square for PP x pp
p
Flower Color
f1
P
Pp
purple
P
p – termed allele
Pp – termed genotype
Purple – termed phenotype
Pp
Pp
purple
purple
Defn: phenotype – an organism’s inherited appearance
The Punnett Square for Pp x Pp
P
Flower Color
f2
P
p
PP
p
Pp
purple
purple
Pp
pp
purple
white
Punnett Square Practice
1)
What happens in f1 when you cross a Pure Breed Green Pod
(GG) with a Pure Breed Yellow pod (gg)?
2)
What happens in f2 when you allow a green pod genotype of
Gg to self pollinate?
3)
What happens when you cross a Gg green pod with a yellow
pod gg?
4)
The allele for a cleft chin, C, is dominant among humans.
What would be the results from a cross between a
heterozygous woman and a homozygous dominant man?
5)
What about a Cc and a cc combination.
6)
What is the ratio of offspring with a cleft chin to offspring
without a cleft chin in #’s 4 and 5)?
If 2 adults both do not have cleft chins. What are the
genotype and phenotype possibilities for their children?
If a child was born with a cleft chin. What possible genotype
combinations could his parents have had?
7)
8)
Mini-Lab
Assume: Brown eyes are
dominant over blue eyes.
Take masking tape and label both
sides of 2 coins. Label both
coins with a B and a b, to
represent a mixed genotype
for brown eyes.
In this example 2 people
heterozygous for eye color
produce offspring .
1)
2)
Flip each coin 50 times.
Record your results for each
allele from coin 1 and coin 2
and the subsequent
genotype and phenotype of
the offspring.
Flip each coin an additional
40 times and record your
results as above.
flip
1
2
3
…
Coin 1
Allele
Coin 2
Allele
Genotype
Phenotype
• A gene can be defined as
a region of DNA that
controls a hereditary
characteristic. It usually
corresponds to a
sequence used in the
production of a specific
protein or RNA.
• In humans, Genes can be
as short as 1000 base
pairs or as long as several
hundred thousand base
pairs. It can even be
carried by more than one
chromosome.
What are genes?
• Not as simple as Mendel’s
pea plants
The 46 human
chromosomes = 23 pairs
house almost 3 billion
base pairs of DNA that
contains about 30,000 40,000 protein-coding
genes. The coding regions
make up less than 5% of
the genome (the function
of the remaining DNA is
not clear) and some
chromosomes have a
higher density of genes
than others.
Sex Chromosomes
• Females are XX
• Males are XY
• What sex alleles can the female donate?
•X only
• What sex alleles can the male donate?
•X or Y
• Which parent is responsible for the
determination of the gender of the child?
Males Only
seX- linked Traits
•
Sex Linked Genes
A particularly important category of genetic linkage has to do with the
X and Y sex chromosomes. These not only carry the genes that
determine male and female traits but also those for some other
characteristics as well. Genes that are carried by either sex
chromosome are said to be sex linked.
•
Men normally have an X and a Y combination of sex chromosomes
(XY), while women have two X's (XX). Since only men inherit Y
chromosomes, they are the only ones to inherit Y-linked traits. Men
and women can get the X-linked ones since both inherit X
chromosomes.
Sex cell inheritance patterns for
male and female children
X-linked traits that are not related to
feminine body characteristics are
primarily expressed in the observable
characteristics, or phenotype , of
men. This is due to the fact that men
only have one X chromosome.
Subsequently, genes on that
chromosome that do not code for gender
are usually expressed in the male
phenotype even if they are recessive
since there are no corresponding genes
on the Y chromosome to dominate over
them, in most cases.
X-linkage in men
In women, a recessive
allele on one X
chromosome is often
masked in their
phenotype by a
dominant normal allele
on the other.
This explains why
women are frequently
carriers of X-linked
traits but more rarely
have them expressed in
their own phenotypes.
X-linkage in women
There are about 1,000 human X-linked genes. Most of
them code for something other than female anatomical
traits. Some of the non-sex determining X-linked genes
are responsible for abnormal conditions such as:
hemophilia
red-green color blindness
congenital night blindness
high blood pressure
duchene muscular dystrophy
fragile-X syndrome .
Queen Victoria of England was a carrier of the gene
for hemophilia. She passed the harmful allele for
this X-linked trait on to one of her four sons and at
least two of her five daughters. Her son Leopold had
the disease and died at age 30, while her daughters
were only carriers. As a result of marrying into other
European royal families, the princesses Alice and
Beatrice spread hemophilia to Russia, Germany, and
Spain. By the early 20th century, ten of Victoria's
descendents had hemophilia. All of them were men.
Queen Victoria
(1819-1901)
By comparison to the X chromosome, the much
smaller Y chromosome has only about 26 genes and
gene families.
Most of the Y chromosome genes are involved with
essential cell house-keeping activities (16 genes)
and sperm production (9 gene families).
Only one of the Y chromosome genes, the SRY gene,
is responsible for male anatomical traits.
Because the Y chromosome only
experiences recombination with the X
chromosome at the ends (as a result of
crossing-over), the Y chromosome
essentially is reproduced via cloning
from one generation to the next.
This prevents mutant Y
chromosome genes from
being eliminated from male
genetic lines.
Subsequently, most of the
human Y chromosome now
contains genetic junk rather
than genes.
Sex-linked genes are genes on the X- and Ychromosomes. Traits controlled by these
genes are called sex-linked traits. Two sexlinked traits include hemophilia and
colorblindness.
.
Hemophilia is a genetic
disorder in which a
person’s blood clots slowly
or not at all. If a person has
the dominant allele XH, he
or she will have normal
blood. If a person only has
the recessive allele Xh, he
or she will have hemophilia
Red-green colorblindness is also a
genetic disorder. In this disorder, the
person does not see red and green
properly. This person will see green as
gray and red as yellow. If a person has
at least one dominant allele XC, he or
she will not have colorblindness. If a
person has only the recessive allele Xc,
he or she will have colorblindness.
• Questions
• 1) How are the alleles for sex-linked genes passed from parent to
child?
• ______________________________________________________
____________
• ______________________________________________________
____________
• 2) How many X-and Y-chromosomes do males have?
______________________
• 3) How many of each do females have?
__________________________________
• 4) Define the carrier of a trait in terms of alleles.
___________________________
• ______________________________________________________
____________
Yes, someday you
will be just like us.
Questions
•
1) Why are only females carriers for hemophilia? For red-green colorblindness?
•
______________________________________________________________________________
•
______________________________________________________________________________
•
______________________________________________________________________________
•
2) Which of the parents can pass the allele for hemophilia to a son? Explain.
•
______________________________________________________________________________
•
______________________________________________________________________________
•
______________________________________________________________________________
•
3) Which of the parents can pass the allele for hemophilia to a daughter? Explain.
•
______________________________________________________________________________
•
______________________________________________________________________________
•
______________________________________________________________________________
•
4) Explain why in family #3 there are no colorblind children even though one of the parents is
•
colorblind?
•
______________________________________________________________________________
•
______________________________________________________________________________
•
______________________________________________________________________________
•
5) The brother of a woman’s father has hemophilia. Her father does not have hemophilia, but
•
she is concerned that her son might. Could she have passed the allele for hemophilia onto her
•
son? Explain.
•
______________________________________________________________________________
•
______________________________________________________________________________
•
______________________________________________________________________________
•
6) A woman’s father is colorblind. She marries a colorblind man. Might their son be
•
colorblind? What about their daughter? Explain why or why not.
•
______________________________________________________________________________
•
______________________________________________________________________________
•
______________________________________________________________________________
What is Crossing Over
A process in genetics by which the two
chromosomes of a homologous pair
exchange equal segments with each other.
Crossing over occurs in the first division of
meiosis
NOVA Movies
Watch Chapter 1 in total
Watch Chapter 2 until courting
Watch all of chapter 3
Messages in the Genes
(remind about vocabulary)
http://www.pbs.org/wgbh/nova/miracle/program.html#
MEIOSIS
• The process of making sex cells in sexually
reproducing organisms.
• It differs from meiosis in that it results in sex
cells with half the normal number of
chromosomes.
• ie. – in humans there are normally 46
chromosomes but in human sex cells (sperm or
eggs) there are only 23 chromosomes.
• In asexual reproduction – mitosis is used
because one organism splits to become two
organisms. Thus offspring are identical to
parent.
• In sexual reproduction two organism will make
one new one and therefore they must send half
the information each. Thus offspring reflect a
combination of parents genes.
• The matching chromosomes that come from
each parent are called homologous.
Chapter 5 Section 2
Incomplete Dominance
In many ways Gregor Mendel was quite lucky in
discovering his genetic laws. He happened to
use pea plants, which happened to have a
number of easily observable traits that were
determined by just two alleles. And for the traits
he studied in his peas, one allele happened to be
dominant for the trait & the other was a
recessive form. Things aren't always so clearcut & "simple" in the world of genetics, but
luckily for Mendel (& the science world) he
happened to work with an organism whose
genetic make-up was fairly clear-cut & simple.
If Mendel were given a mommy black mouse &
a daddy white mouse & asked what their
offspring would look like, he would've said
that a certain percent would be black & the
others would be white. He would never have
even considered that a white mouse & a
black mouse could produce a GREY
mouse! For Mendel, the phenotype of the
offspring from parents with different
phenotypes always resembled the phenotype
of at least one of the parents. In other words,
Mendel was unaware of the phenomenon of
INCOMPLETE DOMINANCE.
Incomplete Dominance - a cross between
organisms with two different phenotypes
produces offspring with a third phenotype that is
a blending of the parental traits.
I remember Incomplete Dominance in the form of
an example like so:
RED Flower x WHITE Flower ---> PINK Flower
It's like mixing paints, red + white will make
pink. Red doesn't totally block (dominate)
the white, instead there is incomplete
dominance, and we end up with something
in-between.
• We can still use the Punnett Square to solve
problems involving incomplete dominance. The
only difference is that instead of using a capital
letter for the dominant trait & a lowercase letter
for the recessive trait, the letters we use are both
going to be capital (because neither trait
dominates the other) RR x WW or we could use
1 2
1 2
one letter with a superscript F F x F F .
WW x
True breed white
True breed red
pink
pink
pink
pink
• Incomplete Dominance Sample Questions
• 1. A cross between a blue rocket bird & a white rocket bird
produces offspring that are silver. The color of rocket birds is
determined by just two alleles.
a) What are the genotypes of the parent rocket birds in the
original cross?
• Since there are only 2 alleles & three phenotypes (blue, white, &
silver), we must be dealing with incomplete dominance. So the
blue parent is homozygous blue (BB) & the white parent is
homozygous white (WW).
• b) What is/are the genotype(s) of the silver offspring?
• The silver offspring are hybrids (BW), one blue allele & one
white allele, neither one dominating the other. Instead, we get a
blending of blue & white, i.e. silver.
silver x silver =
BW x BW
blue
silver
silver
white
• As you can
see, 25% (1/4)
of the
offspring are
homozygous
white (WW),
25% (1/4) are
homozygous
blue (BB), &
50% (2/4) are
hybrid &
therefore have
the silver
phenotype.
2. The color of fruit for plant "X" is determined by two
alleles. When two plants with orange fruits are
crossed the following phenotypic ratios are present
in the offspring: 25% red fruit, 50% orange fruit, 25%
yellow fruit. What are the genotypes of the parent
orange-fruited plants?
Again, it comes in really handy if you can recognize
right off the bat that we have three phenotypes & just
2 alleles. That means we are dealing with either
incomplete or codominance. Since orange is a blend
of red & yellow, it's incomplete dominance. So the
"in-between" phenotype is the hybrid, orange in this
example. We'll use RR = red, YY = yellow, & our
orange fruits are RY.
Codominance
• First let me point out that the meaning of
the prefix "co-" is "together".
Cooperate = work together. Coexist =
exist together. Cohabitat = habitat
together.
The genetic gist to codominance is pretty
much the same as incomplete
dominance. A hybrid organism shows a
third phenotype --- not the usual
"dominant" one & not the "recessive" one
... but a third, different phenotype. With
incomplete dominance we get a blending
of the dominant & recessive traits so that
the third phenotype is something in the
middle (red x white = pink).
• In COdominance, the "recessive" &
"dominant" traits appear together in the
phenotype of hybrid organisms.
• I remember codominance in the form of an
example like so:
• red x white ---> red & white spotted
• With codominance, a cross between
organisms with two different phenotypes
produces offspring with a third phenotype
in which both of the parental traits appear
together.
• R = allele for
red flowers
W = allele for
white flowers
• red x white --->
red & white
spotted
RR x WW --->
100% RW
A very very common phenotype used in
questions about codominance is roan fur
in cattle. Cattle can be red (RR = all red
hairs), white (WW = all white hairs), or
roan (RW = red & white hairs together). A
good example of codominance.
Another example of codominance is
human blood type AB, in which two types
of protein ("A" & "B") appear together on
the surface of blood cells.
• There are three forms of the gene (alleles) that control
the ABO blood group, which are designated as iA, i B,
and i. You have two alleles (one from your mother and
one from your father), which are referred to as your
genotype. The inheritance of the alleles is codominant, meaning that if the allele is present, it gets
expressed. The following genotypes will yield these
blood types:
• iAiA or iAi - Both genotypes produce the A protein (type
A).
• iBiB or iBi - Both genotypes produce the B protein (type
B).
• iAiB - This genotype produces the A and B protein (type
AB).
• ii - This genotype produces no protein (type O).
So, your blood type does not necessarily tell you exactly
which alleles you have. For example, a person with
blood type A could have either two iA alleles or one iA
allele and one i allele. It is possible for two parents with
the same blood type (A or B) to have a child with type O
blood. Both parents would have to have a mixed
genotype, such as one i allele together with either one iA
or one iB allele.
Sample Questions
1. Predict the phenotypic ratios of offspring when
a homozygous white cow is crossed with a roan
bull.
2. What should the genotypes & phenotypes for
parent cattle be if a farmer wanted only cattle
with red fur?
3. A cross between a black cat & a tan cat
produces a tabby pattern (black & tan fur
together).
a) What pattern of inheritence does this
illustrate?
b) What percent of kittens would have tan fur if a
tabby cat is crossed with a black cat?
Predict the phenotypic ratios of offspring when a
homozygous white cow is crossed with a roan bull
• Step #1 --- recognize that
"roan" is a codominance
trait.
• Homozygous white = WW, &
roan = RW (a hybrid cow).
So our cross is WW x RW &
the punnett square should
look something like what
you see here.
• The results:
2/4 offspring (50%) will be
roan (RW), & 50% will be
white (WW).
2. What should the genotypes & phenotypes for parent
cattle be if a farmer wanted only cattle with red fur?
Well, the only way to have red fur is to be homozygous
red (RR). In order to get that genotype in all the
offspring both parents must be "RR". A parent with
one or more "W" alleles will cause the inheritence of
roan fur in some offspring. Go ahead & work out all
the punnett squares if you don't believe me.
Only RR x RR gives you 100% RR.
RR x RW would produce 50% roan, 50% red,
RW x RW produces 25% red, 50% roan & 25% white,
WW x RW would produce 50% roan, 50% white,
& WW x RR would produce 100% roan (RW).
A cross between a black cat & a tan cat produces a
tabby pattern (black & tan fur together).
3. a) What pattern of inheritence
does this illustrate?
Codominance, two phenotypes
together at the same time.
b) What percent of kittens would
have tan fur if a tabby cat is
crossed with a black cat?
Tabby cats are the hybrids
(because they have both
colors) & a black cat must be
homozygous black.
So the cross for this problem
is BB (black) x BT (tabby). The
p-square is at the right. The
results show that 50% of the
offspring will be BB (black) &
50% will be tabby (BT). So to
answer the question, 0% of
the kittens will be tan.
Chapter 5 - Heredity
Section 2 - Meiosis
• In asexual
reproduction only one
parent is needed for
reproduction – in this
process the copying of
genetic material occurs
through mitosis
• Most single celled
organisms reproduce
in this way.
Insert picture here
Sexual Reproduction
• 2 parent cells join together to form a new
individual
• This type of reproduction must work differently
so that each parent may contribute to the
genetic makeup of the offspring.
In sexual reproduction The parent cells are known
as sex cells.
Sex cells are different from all other ordinary body
cells in that they have only half the usual number
if chromosomes.
Human body cells normally
possess 46 chromosomes
(23 homologous pairs) but
human sex chromosomes
contain only 23
chromosomes
(none of which are paired).
Sperm –
male sex cells
Ova or Eggs – female sex cells
Each sperm and egg has only one of the
chromosomes from each homologous pair.
Science Blooper
• In 1918, a prominent
scientist miscounted
the number of
chromosomes in a
human cell. He
counted 48. For
almost 40 years,
scientists thought
that his was correct.
In fact, it wasn’t until
1956 that
chromosomes were
correctly counted and
found to be only 46.
OOPS
Less is More
• Why is it important that sex cells have half
?
the usual number of chromosomes
• Because when the males sperm and the
females egg join to forma new individual,
each parent donates one-half of a
homologous pair of chromosomes.
Meiosis
• Sex cells are made through a process
termed meiosis.
• Meiosis – produces new sex cells with half
the usual number of chromosomes.
– This process involves the chromosomes
dividing once and the nucleus dividing twice.
Mitosis Revisited
http://www.biology.arizona.edu/c
ell_bio/tutorials/cell_cycle/cells3
.html
Mitosis Demo
•
Macaroni Meiosis vs. Macaroni
Mitosis
Mitosis
1)
Take your 4 pieces of yarn and
create a single circular cell
membrane
2)
Start with 4 chromosomes ( 2
pairs)
3)
Replicate your chromosomes
4)
Attach homologous
chromosomes to each other
5)
Line them up in the middle
6)
Separate the homologous
chromosomes to 2 sides
7)
Take the 4 pieces of yarn close
off to 2 cells
Meiosis
1)
Take your 2 pieces of yarn and
create a single circular cell
membrane
2)
Start with 4 chromosomes
3)
Replicate your chromosomes
4)
Attach homologous
chromosomes to each other
5)
Line them up in the middle
6)
Separate the homologous
chromosomes to 2 sides
7)
Take the 2 pieces of yarn close
off to 2 cells
8)
Now in each of those 2 cells,
line the 4 paired chromosomes
up in the middle
9)
Separate the homologous
pieces in each cell
10) Close off the now 4 cells
ASIDE
In human males , meiosis and sperm production takes
about 9 weeks. This continuous process begins at puberty
and ends at death
In human females, meiosis and egg production begins
before birth. The process stops abruptly prior to
birth and then re-begins at puberty and continues
until menopause. Between puberty and menopause,
one egg per ovary resumes meiosis and finishes its
development. Therefore, the meiosis of a single egg
may take up to 50 years to complete.
• Why does the incidence of birth defects and genetic
disorders rise with age of the mother? Why is the
age of the father less of a factor?
• Answer: age of the sex cell
Guess Who?
If human sex cells are created
by meiosis, how are cat sex
cells created?
Answer: by meowsis
Meiosis explains Mendel
Male or
Female
There are two types of chromsomes:
Autosomes – the 22 pairs of
chromosomes which do not play a
role in sex determination. Each
has a match.
Sex chromosomes – the
chromosomes that carry the genes
that determine whether the
offspring will be male or female.
Male or Female Continued
• In humans:
• X chromosome
carries the genetic
information for
femaleness.
• Y chromosome
carries the genetic
information for
maleness.
• Females are XX
• Males are XY
X
Y
• If females are XX and males are XY which
parent is ultimately responsible for the
determination of the sex of the offspring?
•Answer – the male because only the male
can contribute the different sex chromosome
(females have no Y’s to contribute)
• Recessive inheritance Albinism
• X linked traits ie hemophilia
• Hemophilia and brachydactaly – dominant
inheritance
• Cancer heart disesae – inherited
predisposition – the role of ones
environment – why could George Burns
smoke cigars until he was 99?
Then interphase doubling occurs
NOVA Movies
Watch Chapter 1 in total
Watch Chapter 2 until courting
Watch all of chapter 3
Messages in the Genes
(remind about vocabulary)
http://www.pbs.org/wgbh/nova/miracle/program.html#