Mendel and Heredity
Chapter 8
8.1 The Origins of Genetics

Heredity is the passing
of characters from
parents to offspring
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Used throughout history
to alter crops and
domestic animals
Gregor Johann Mendel –
Austrian Monk


Used pea plants and bred
different varieties
Developed rules to
accurately predict
patterns of heredity
Why Peas?

2 characters have clearly different
forms

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Male and female reproductive
parts are in same flower
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Character = inherited characteristic
(color)
Trait = single form of character
(purple)
Can control fertilization
Flower can fertilize itself (selffertilization) or can cross pollen from 1
plant to another (cross-pollination)
Peas are small, grow easily,
mature quickly, and produces
many seeds so results obtained
quickly
Traits Expressed as Simple Ratios

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Mendel started by looking at 1 characteristic
(monohybrid), such as color, with 1 pair of
contrasting traits, purple or white flowers
Only allowed plants to self-pollinate for many
generations
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True-breeding – all offspring show only 1 trait
Parental (P) generation
Cross pollinated 2 P generation plants with
contrasting traits
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Offspring called filial (F1) generation
Counted numbers of each trait

Allowed F1 generation to self pollinate
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Offspring called F2 generation
Each characterized and counted
Mendel’s Results

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F1 showed only 1 form of character other had
disappeared
When F1 self pollinates other trait reappears in
some of F2
Found ratio of traits to be 3 to 1

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3 white flowers to 1 purple flower
Same ratio found for any trait he studied
8.2 Mendel’s Theory

We used to think offspring were blend of
traits

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Tall x short = medium
Mendel’s experiments showed us this is
not entirely true
Mendel’s Hypothesis


There are 2 copies of a gene, one from each
parent, for each inherited characteristic
There are different versions of genes called
“alleles”

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Tall or short
When both versions are present one may be
dominant (completely expressed) and the other
may be recessive (not expressed when
dominant is present)
When you form gametes, alleles separate
independently so only one allele in each gamete
Mendel’s Finding in Modern Terms

Use letters to show alleles
Capitol = dominant (T, P, Y, etc…)
 Lower case = recessive (t, p, y, etc…)
 Homozygous = letters are same

 Homozygous
dominant = TT, PP
 Homozygous recessive = tt, pp

Heterozygous = letters are different
 Tt,
Pp
 Only dominant allele is expressed

Genotype = set of alleles
What you actually have
 TT, Tt, or tt
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Phenotype = what is expressed
How it looks
 Tall, Tall, or Short

Mendel’s Laws of Heredity

Law of Segregation
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2 alleles for a character segregate when gametes are
formed
Behavior of chromosomes during meiosis
Law of Independent Assortment

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1 character does not affect another
Alleles of different genes separate independently of
on another
Now know this only applies to genes located on
different chromosomes or that are far apart on same
chromosome
8.3 Studying Heredity:
Punnett Squares


Breeders want
certain
characteristics
when they breed
(cross) animals
Horticulturists
produce plants
with specific
characteristics

Punnett Square
Used to predict
outcomes
 Shows all
possible
combinations of
gametes
 Put 1st parents
genotype on top
 Put 2nd parents
genotype on side
 Do the cross

TT
Tt
Tt
tt
• So in Mendel’s F1
generation, a pure Tall
plant bred with a pure
short plant can only
give 1 kind of offspring
due to dominance of
tall allele
Determining Unknown Genotypes
How do you know if a tall plant is
homozygous or heterozygous? They both
look tall
 Can do a Test Cross

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If dominant phenotype is shown with unknown
genotype, cross it with homozygous recessive
Test Cross Results

If unknown is
homozygous
dominant, all
offspring of test
cross will have
dominant trait
Test Cross Results

If unknown is
heterozygous,
offspring of test
cross will have 2
dominant and 2
recessive
phenotypes
Can use probability calculations to predict
results of genetic crosses
 Probability is the likelihood a specific event
will occur
 Probability = # of 1 kind of possible
outcome divided by total number of
possible outcomes
 We will express these as fractions
 Chance a coin will come up heads

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1 head / 2 sides = ½
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DD = ¼
Dd = 2/4 or ½
dd = ¼
Dihybrid Cross
Uses a Punnett Square to determine
outcomes of 2 traits at one time
 Example: Surface and Color

Surface:
 Color:
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RR, Rr, or rr
YY, Yy, yy
round or wrinkled
yellow or green
What are the possible combinations?

RY, Ry, rY, ry

So if you have 2 purebred homozygous
parents RRYY and rryy and you mate
them, what do you get?

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All offspring will be RrYy
What if you have F1 breed?
Make a Punnett Square of possible gametes
for each parent
 What possible combos can parents offer?

 Do
you remember FOIL?
 RY, Ry, rY, ry
You never have to count the results
of a dihybrid cross between
heterozygotes!
9 with both dominant traits
 3 with first dominant and second recessive
 3 with first recessive and second dominant
 1 with both recessive traits
 So 9:3:3:1

Inheritance of Traits

Pedigree

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Family history that shows how a trait is inherited over
several generations
Helpful in tracking genetic disorders
Carrier – have allele for trait but show no
symptoms
Things You Can Find From A Pedigree

Autosomal or SexLinked?
If autosomal it will
be equal in both
sexes
 If sex linked
generally only
found in males

Y
linked

X
Hairy ear rims
linked

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Color-blindness
Hemophilia

Dominant or recessive
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Autosomal Dominant –
every individual with
condition will have parent
with condition
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Achondroplasia – type of
dwarfism
Huntington’s Disease –
brain degenerates
Autosomal Recessive – 1,
2, or no parents with
condition
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Cystic fibrosis
Sickle cell anemia
Albinism

Heterozygous or
Homozygous
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Autosomal
homozygous dominant
or heterozygous
phenotype will show
dominant allele
Homozygous
recessive will show
recessive allele
2 heterozygous of
recessive allele don’t
show condition but can
have children that do
8.4 Complex Patterns of Heredity

Complex Control
Most of the time characters display much
more complex patterns than simple dominantrecessive patterns
 Characters can be influenced by several
genes

Polygenic Inheritance

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Several genes affect a character
These genes may be scattered along same
chromosome or on different chromosomes
Determining the effect of any one gene is difficult
Crossing over and independent assortment
create many different offspring combos
Eye color, height, weight, hair, intelligence, and
skin color
Usually gives a range of expression
Polygenic Inheritance
Intermediate Characters

Incomplete dominance
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Phenotype that is intermediate between 2 parents,
neither is completely dominant

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White x red = pink
Straight hair x curly hair = wavy hair
Multiple Alleles
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Characters controlled by genes with 3+
alleles
Humans have ABO blood types
 IA, IB, i
 Letters A and B refer to carbohydrates on
surface of red blood cells
 i has neither carbohydrate
 IA and IB are dominant over I, but not over
each other (codominant)
 Still only 2 possibilities in a person

Blood Types
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2 forms are displayed
at the same time
Codominance – both
expressed, not
blended
IAIB both expressed
ii = Type O
Characters Influenced by Environment

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Plants may change color
based on pH of soil
Arctic fox
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Siamese cats
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Summer – enzymes produce
pigments for darker fur
Winter – no enzymes, no
pigments to darken fur
Dark fur in cooler parts
Humans
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Height related to nutrition
Skin color based on sun
exposure
Twins are genetically identical,
any difference is due to
environment
Genetic Disorders
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Proteins encoded by genes must function
precisely for normal development and function
Genes may be damaged or copied wrong
causing faulty proteins
Mutation = changes in genetic material

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Rare because cells try to correct errors
Harmful effects produced by inherited mutations

Many carried by recessive alleles
Sickle Cell Anemia
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Recessive genetic
disorder
Mutated allele produces
defective form of
hemoglobin causing red
blood cells (rbc) to be
misshapen
These rupture easily
causing less O2 to be
carried and may get stuck
and cut off blood supply
Recessive allele protects
heterozygous individuals
from malaria

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Parasites in sickle rbc die
Normal rbc still transport
oxygen
Cystic Fibrosis

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Most common fatal,
hereditary, recessive
disorder in Caucasions
1 in 25 has at least 1
copy of defective gene
that makes a protein
needed to move chloride
in and out of cells
Mucus clogs organs
1 in 2,500 homozygous
for cystic fibrosis
No cure
Hemophilia
Impairs bloods ability to clot
 Sex-linked
 Dozen+ genes code for clotting proteins
 1 mutation on X chromosome causes
Hemophilia A
 Males only get 1 X chromosome

Huntington’s Disease
dominant allele on autosome
 1st symptoms - mild forgetfulness and
irritability in 30’s and 40’s
 Eventually lose muscle control, spasms,
severe mental illness, and death

Treating Genetic Disorders
Most can’t be cured
 Genetic Counseling – tells of possible
genetic problems with offspring, may be
treated if early enough

Phenylketonuria (PKU)
Lack enzyme that converts amino acid
phenylalanine into tyrosine so it builds up
in the body and causes severe mental
retardation
 Can be placed on phenylalenic diet

Gene Therapy

Replace defective genes with normal ones
Isolate copy of gene
 Put working copy into a virus
 Virus infects and puts gene in
 Infected cells are cured
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Still trying to get this to work