Biology Ch. 9 notes “Genetics” Mendel’s Laws

Biology
Ch. 9 notes
“Genetics”
Mendel’s Laws
9.1 Describe pangenesis theory and the blending
hypothesis. Explain why both ideas are now rejected.
Pangenesis:
 The ancient Greek physician, Hippocrates (460-370 b.c.),
said
 particles called pangenes travel from each part of an
organism’s body to the eggs or sperm and are then
passed to the next generation
 moreover, changes that occur in the body during life are
passed on.
Blending:
 Early 19th century
 Offspring inherit traits from both parents.
 Hereditary materials contributed, mix in forming the
offspring
 Like blue and yellow make green
 Remain inseparable like green
Rejected:
 Gametes are not made up of particles of body cells.
 Changes in body cells do not affect gametes.
 Does not explain how traits missing in one generation
show up in the next.
9.2 Explain why Mendel’s decision to work with peas was
a good choice.
Gregor Mendel
 1860’s
 Austrian Monk
 University of Vienna
 Physics, math (incl. statistics), chemistry
 Published a paper in 1866
o Parents pass on discrete heritable factors
(genes)
o Heritable Factors retain individuality generation to
generation.
o You can mix colored marbles but they don’t blend
their colors.
o Paper discovered 50 years later, simultaneously, by two
researchers working independently who each came to the
same conclusions as Mendel
 Success:
o experimental approach
o choice of organism
o choice of characters used
o used statistics
Garden Pea
 short generation times (two generations / growing season)
 produced large numbers of offspring from each mating
 came in many readily distinguishable varieties
 crosses could be strictly controlled due to flower anatomy
 typically self-fertilize due to closed petals around pistil/anther
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Terms
characteristic : trait ::
gene : allele ::
height : short
9.2 Define and distinguish between true-breeding
organisms, hybrids, P generation, F1, F2.
true-breeding: sexual reproduction produces offspring with
inherited traits identical to those of the parents. Homozygous,
RR or rr.
hybrids: The offspring of two parents that differ in one or
more inherited traits; heterozygous, (Rr).
P generation: Mendel’s true-breeding parental generation
P = RR x rr
F1 generation: First Filial (family): the result of Mendel’s
Parental cross, will always be a hybrid.
P = RR x rr
F1 = all Rr
F2 generation: Second Filial: the result of crossing two F1’s.
P = RR x rr
F1 = all Rr
F2 = 1RR: 2Rr: 1rr
Rr
meiosis
2nn
Rr
R
r
R
RR
Rr
r
Rr
rr
9.3 Define and distinguish between the following pairs of
terms:
genotype: genetic make-up, use letters, RR
phenotype: expressed, physical traits, use words: red
dominant allele: the allele that determines appearance, use
capital letters.
recessive allele: the allele that is not expressed in the
presence of a dominant allele (Rr). This allele is only
expressed in the absence of the dominant allele, rr = white,
use lower case letters.
heterozygous: having two different alleles for a gene: Rr
homozygous: having one kind of allele for a gene: RR or rr.
monohybrid cross: Following the cross of just one trait at a
time.
Punnett square: A box format used to show the different
possible combinations of alleles in offspring.
Reginald Punnett explains Mendel's Law of Segregation and
Punnett Squares
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9.3 Explain how Mendel’s Law of Segregation describes
the inheritance of a single characteristic.
A sperm or egg carries only one allele for each
inherited character because allele pairs separate
(segregate) from each other during the production of
gametes.
This explains how a trait can disappear in one generation and
reappear in the next generation. Blending of alleles does not
occur and each allele retains its ability to encode it’s trait. (see
Activity online)
9.4 Describe the genetic relationship between
homologous chromosomes.
They carry genes for the same characteristic at the same loci
on the chromosomes. The genes may be different alleles.
One homologue comes from one parent and the other
homologue comes from the other parent.
9.5 Explain how Mendel’s law of independent assortment
applies to a dihybrid cross. see BioFlix online
9.5 Illustrate this law with examples from Labrador
retrievers.
9.5 Illustrate Independent Assortment on your own with
examples from Mendel’s work with peas. Use the
information on p.155.
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9.6 Explain how a testcross is performed to determine the
genotype of an organism.
If the genotype of an organism with a dominant characteristic is
unknown, cross it with an individual that has the recessive trait.
It will be homozygous for the trait.
A cross with a heterozygote will reveal the recessive trait in ½
of the offspring. If no recessives are created (after many
offspring are produced), the unknown genotype is homozygous
dominant.
9.7 Explain how and when the rule of multiplication and
the rule of addition can be used to determine the
probability of an event.
 The probability scale ranges from 0 to 1.
 An event that is certain to occur has a probability of 1.
 An event that is certain not to occur = 0.
 The probability of all events must add to 1.
 examples:
 tossing a coin and getting heads = ½
 tossing a coin and getting tails = ½
 Drawing a jack of diamonds = 1/52
 Drawing any other card = 51/52
o Each toss of a coin is independent of the next toss.
o If two coins are tossed simultaneously, they are
independent.
o A compound event is the probability of two independent
events coming up with the same result.
Rule of multiplication:
 To figure probability of compound events.
 Example: What is the probability of getting both heads.
 ½x½=¼
 By multiplying fractions you are saying that it is less likely
to happen because the number gets smaller.
Rule of Addition:
 If there is more than one way to get a combination, use
the rule of addition.
 Example: What is the probability of getting one head and
one tail? (Hint: you can toss head/tail OR tail/head and
get the combination.)
 (½ x ½) + (½ x ½) = ½
 ¼+¼=½
 By adding together the chances of each coin coming up
one way and then adding it to the probability that they will
come up the other way, you are saying that it is more likely
to happen because adding fraction makes a bigger
number.
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On your own:
A plant of genotype AABbCC is crossed with an AaBbCc plant.
What is the probability of an offspring having the genotype
AABBCC?
What is the probability of an offspring having the genotype
AaBbCC
How about: AABbcc?
9.7 Explain why Mendel was wise to use large sample
sizes in his studies.
Statistics requires that a sample size that is large enough to
avoid individual anomalies from changing the results must be
used.
A= 1 x ½ = ½
B= ½ x ½ = ¼
C= 1 x ½ = ½
½ x ¼ x ½ = 1/16
9.8 Explain how family pedigrees can help determine the
inheritance of many human traits.
Because we can’t control human matings, geneticists must
analyze the results of matings that have already occurred. The
diagram of this information is called a family tree or pedigree.
AaBbCC
Symbols are used in a pedigree.
normal male =

affected male = 
½ x ½ x ½ = 1/8
normal female = 
affected female = 
AABbcc
mating/marriage =  
1x½=½
(½ x ½) + (½ x ½) = ½
0x½=0
1x½=½
(½ x ½) + (½ x ½) = ½
½x1=½
offspring of a mating in order of birth L to R
 
½x½x0=0
    
Activity: Making and Interpreting Pedigree Studies
9.9 Explain how recessive and dominant disorders are
inherited. Provide examples of each. table 9.9 p.163
Recessive Disorders
 Most human genetic disorders are recessive.
 Most recessive who have recessive disorders are born to
normal parents who are both heterozygous or carriers of
the recessive allele. What is P?
 Most common fatal genetic disease in the US is cystic
fibrosis. (CF).
o Carried by 1/25 people of European ancestry.
o Excessive excretion of mucus from lungs,
pancreas, other organs.
o Interferes with breathing.
 Disorders not evenly distributed due to prolonged
geographic isolation.
 Example:
o Martha’s vineyard (early inhabitants isolated
there)
o led to frequent marriages between close
relatives.
o Frequency of an allele that causes deafness was
high
o deafness allele was rarely transmitted to
outsiders.
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Dominant Disorders
 Caused by one dominant allele.
 Examples:
o Polydactyly
o Webbed fingers and toes
o achondroplasia: one form of dwarfism
o Huntington’s disease: symptoms appear after
reproductive years. P = ½ that offspring will
inherit.
9.10 Fetal Testing
AMNIOCENTESIS
CHORIONIC
VILLUS
SAMPLING
(CVS)
ULTRASOUND
IMAGING
DESCRIPTION
HEALTH
RISKS
ADVANTAGES
DISADVANTAGES
14-20 wks gestation,
Needle inserted to withdraw
10ml of amniotic fluid which
surrounds the baby.
Karyotype after several weeks.
Maternal bleeding
Miscarriage
Premature birth
Complication rate=1%
Very early in gestation
Chromosomal abnormalities
Identified ex. trisomies
Other biochemical tests can
be performed with cells
to ID Tay-Sachs for example.
Must wait
several weeks
for results
8-10 weeks gestation
Sample extracted from placenta
through vagina/cervix
Karyotype in 24 hours
Maternal bleeding
Miscarriage
Premature birth
Complication rate=2%
Not as invasive as amnio
Earlier in gestation
Much quicker results
Uses sound waves to produce
an image of fetus,
looks like a shadow
None known
noninvasive
9.10 Explain the ethical dilemmas created by advances in
technology.
 Can give valuable information to carriers of genetic
disease.
 If confidentiality is breached:
o will carriers be stigmatized?
o Possibly denied health coverage?
o Life insurance?
o Denied employment?

Information can be difficult to cope with and strain
marriages.
o What should be done about the information?
o Termination?
o time to prepare emotionally, medically,
financially.
End of Problem Set I
9.11 Incomplete Dominance: When the appearance of the F1
offspring falls between the phenotypes of the parents.
 Example: Four o’clock flowers
o RR = red
o rr = white
o Rr = pink
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9.12 Multiple alleles: Three alleles per gene present in the
population (Two per individual)
Example: ABO blood groups
Carbohydrates on the surface of RBC’s are recognized by the
immune system as “self” or “non-self.” If the carbs are
recognized as “non-self” the immune system attacks with
antibodies.
type A type B type AB type O
AA
BB
AB
OO
AO
BO
If you mix the following blood types together, which ones are
OK and which ones will be attacked (shaded)? Type AA is
done for you.
donor AA AO BB BO AB OO
Which blood type is
AA
OK OK
OK
the universal donor?
AO
BB
Which blood type is
BO
the universal recipient?
AB
OO
9.12 Codominance: When both alleles are fully
expressed.
Which blood type shown above represents codominance?
In some animals (horses, cattle, hogs) red hair color is codominant
to white. This coat color is called roan. What would you expect to
find if you looked at the animal’s coat up close?
9.13 Pleiotropy: A gene affecting multiple characters.
Example: Sickle-cell anemia
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9.13 Explain how the sickle-cell allele can be adaptive.
Sickle-cell Anemia
 Due to an allele that makes abnormal hemoglobin
 HH normal, Hh = usually normal, hh = suffer
 Abnormal hemoglobins tend to link together and crystallize
 Crystallization causes RBC to form sickle shape
 Can cause severe anemia and weakness
 Blood flow limited, causes severe pain, damage to organs
 Low oxygen makes it worse
 Kills 100,000 people a year globally
Sickle-Cell Disease is the most common inherited disorder
among people of African descent, 1/400 African-Amer., 1/10
AA is a carrier.
Many homozygotes die before they pass on alleles. Why
the high frequency of carriers?
Sickle-Cell most common in tropical Africa.
Malaria, a parasite of RBC’s, also common there.
When parasite enters normal RBC, causes it to sickle.
Body destroys sickled/infected cells.
Hh are resistant to malaria, more HH die from malaria.
Called heterozygote advantage.
9.14 Polygenic inheritance: The additive affects of two or more
genes on a single phenotypic character. (This is the converse of
pleiotropy, in which a single gene affects several characters.)
Whenever a character shows an even gradation between extremes in the
population, it is probably due to polygenic inheritance.
Below each phenotype, write the genotype(s) that would correspond.
Two pairs of genes are (A and a, B and b) are responsible for ear length
in corn. They assort independently. Possible phenotypic results are:
longest
long
medium
short
shortest
From the problem above, what would you get if you crossed longest
with shortest? Show a Punnett square.re.
Answer:
longest x shortest
AABB x aabb
ab
AB
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9.15 Explain why skin color is not sufficiently explained
by polygenic inheritance.
 Skin color is explained by polygenic inheritance.
 It is controlled by at least three genes (6 alleles)
 This makes seven possible phenotypes.
 Can you fit everyone’s skin tone into seven categories?
What makes the
difference?
9.15 Environment: Many characters result from a combination of
heredity and environment.
Examples:
 human skin color affected by exposure to the sun
 human: number of white blood cells affected by altitude, physical
activity, infectious agents.
 human height and weight affected by nutrition
 tree shape: affected by sun, nutrients, wind
End of Problem Set II
9.17 Linked Genes:
Found on the same chromosome. The closer they are on the same
chromosome, the less likely they are to get separated by crossing
over. So, they are usually inherited together.
When following a dihybrid cross in which genes are linked (a.k.a.
“close together on the same chromosome” or “do not assort
independently” you will not FOIL to “get all possible gametes”
because they are linked and will be inherited together.
P = BBRR x bbrr
(In this example, “bigs” are linked and “littles” are linked.
Keep them that way!)
F1 = BbRr
BbRr
BbRr
BR
br
Consider the two extremes.
Bacteria have one main chromosome and all the genes are
carried on it. If the chromosome (and its copy) are sorted
correctly, then ALL of the genes are sorted correctly.
F2 Punnett:
meiosis
2nn
linked
Why would nature link genes on chromosomes if it decreases
genetic variation?
BR
br
BBRR
BbRr
BbRr
bbrr
The opposite extreme would be to have chromosomes no
bigger than a single gene. This would give the greatest
degree of variation.
Most organisms have an intermediate number of genes per
chromosome. Is there a theoretical “ideal” number of
chromosomes?
MUCH less diversity is created when genes are linked.
Find sample problems on Problem Set III.
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Think about “Size and Shape”, the necessary components of
cell division, and the fidelity necessary for life when cells
divide.
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1% crossing over=1 map unit
9.18 How can crossing over frequency be used to make a gene
map of a chromosome?
 The closer they are the less often they cross over.
 The farther apart they are the more likely they are to cross over.
 If they are more than ½ a chromosome distant, they cross over
50% of the time, just as often as if they were on separate
chromosomes. They assort independently.
9.20 Explain how sex is determined in humans.
Presence or absence of Y.
9.20 What is the significance of the SRY gene.
 SRY=Sex-determining Region of Y.
 Triggers testis development.
 Codes for proteins that regulate other genes on the Y
chromosome.
 These genes produce proteins for normal testis development.
 Absence of SRY in the presence of Y?
A-B: 30%
B-C: 10%
A-C: 40%
0
30
40
Answer Under this text box
I I I I I I I I I
A
B
C
1% crossing over=1 map unit
A-D:
B-D:
B-C:
C-D:
5%
15%
5%
10%
Answer Under this text box
0
5
15
20
I I I I I I I I I
A?
D
A?
C
D
What information would you need
to determine the position of A?
A-C = 5%
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9.20 Describe some alternative sex-determination
systems
Insects (Grasshoppers, roaches, etc):
 XO system
 Female: XX
 Male: X
Fish, Butterflies, Birds: eggs determine sex:
 ZW system:
 Female: ZW
 Male: ZZ
Ants and Bees
 No sex chromosomes
 Chromosome #
 Females: diploid (fertilized eggs)
 Males: haploid (unfertilized eggs)
 Called: parthenogenesis
9.21 Sex-linked inheritance (More specifically, X-linked)
 Very much like linked genes
 Linkage to sex chromosomes only
 X usually carries the trait but it is absent on the Y.
9.22 Why do males suffer from sex-linked traits more
often than females.
 Males inherit more often because they don’t have
a second chance (X chromosome) to get a
normal allele.
9.23 Explain how the Y chromosome can be used to trace
human ancestry.
 X and Y are not homologous
 No crossing over during Prophase I
 Males inherit their Y from dad virtually unchanged
 Random changes (neutral mutations) can be
traced back
 All males in a certain line will have the changes
End of Problem Set III
Activity: Inheritance of Sex-Linked Traits
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