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Heredity
Vocabulary
Heredity
Big Ideas
Heredity and Reproduction
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Understand and explain that every organism
requires a set of instructions that specifies its
traits, that this hereditary information (DNA)
contains genes located in the chromosomes of
each cell, and that heredity is the passage of
these instructions from one generation to
another.
Heredity
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Is the passing of physical characteristics, or
traits, from parents to offspring
Genetics is the study of heredity
Parents are referred to as the P generation
The first offspring are referred to as F1
The offspring of the F1 generation are the F2
generation
Note: F stands for “filial”, or son
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Gregor Mendel, a monk living in the mid
nineteenth century, discovered the principles
of heredity through his experiments breeding
pea plants and noticing which traits were
passed from parent to offspring
Mendel’s work was unknown during his
lifetime, but was rediscovered during the
early 1900s
Mendel is known as the Father of Genetics
Heredity
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Different forms of the same gene are called
alleles
During fertilization, sperm from the male
parent and eggs from the female parent join
to form new offspring
An organism’s traits are controlled by the
alleles it receives from its parents
Heredity
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In the system of complete dominance, alleles can be
dominant or recessive
A dominant allele’s traits always show up in the offspring
A recessive allele’s traits is hidden whenever the
dominant allele is present
Heterozygous means that the two inherited alleles are
different. An organism with heterozygous traits is called a
hybrid
Homozygous means that the two inherited alleles are the
same (either both dominant or both recessive). An
organism that is homozygous is called a purebred
Heredity
Dominant versus recessive traits in humans
Trait
eye coloring
Dominant
brown eyes
Recessive
grey, green, hazel, blue
hair
dark hair
non-red hair
curly hair
widow's peak
blonde, light, red
red hair
straight hair
normal hairline
facial features
dimples
freckles
no dimples
no freckles
Heredity
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Using Punnett squares to predict
inherited traits
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A Punnett square shows how dominant
and recessive traits combine
Capital letters stand for dominant traits
Lowercase letters stand for recessive traits
Example: Brown eyes (B) are dominant
over blue eyes (b)
Heredity
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Using Punnett squares
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If one parent is a hybrid
brown-eyed (Bb), and
the other parent is a
pure blue-eyed (bb), this
is how the Punnett
square would look.
First, write the alleles of
one parent on the side
and the other on the top
of the square
b
B
b
b
Heredity
Using Punnett squares
– Rewrite each allele
straight across or
straight down
– By convention, capital
letters go first
– Remember, B (brown
eyes) dominates b
(blue eyes). What you
will see is the
offspring’s genotype,
or actual genes. Let’s
see what the eye
colors are
Mother’s traits
Father’s traits
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b
b
B
Bb
Bb
b
bb
bb
Heredity
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Using Punnett squares
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Remember, B (brown
eyes) dominates b (blue
eyes). So, in this
Punnett square, the
possible offspring (on
average) will be 50%
brown and 50% blue.
The actual appearance
of the organism is its
phenotype.
b
b
B
Bb
Brown
Bb
Brown
b
bb
Blue
bb
Blue
Heredity
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Using Punnett squares
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Let’s try again with two
parents who are both
hybrids for eye color
First, write the alleles of
on parent on the side
and the other on the top
of the square
B
B
b
b
Heredity
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Using Punnett squares
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Rewrite each allele
straight across or
straight down
By convention, capital
letters go first
Remember, B (brown
eyes) dominates b (blue
eyes).
Now lets see what the
eye colors are
B
b
B
BB
Bb
b
Bb
bb
Heredity
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Using Punnett squares
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Remember, in humans,
B (brown eyes)
dominates b (blue eyes).
So the possible
offspring (on average)
will be 75% brown and
25% blue.
b
b
B
BB
Brown
Bb
Brown
B
Bb
Brown
bb
Blue
Heredity
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Using Punnett squares
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Another way to look at
this is that the offspring,
on average, will be 25%
homozygous dominant
(BB), 25% homozygous
recessive (bb) and 50%
heterozygous (Bb)
b
b
B
BB
Brown
Bb
Brown
B
Bb
Brown
bb
Blue
Heredity
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Using Punnett squares
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Let’s try some Punnett
square problems
Write them on your own
paper
In humans, brown eyes (B)
is dominant over blue eyes
(b). One parent is pure
brown eyes, and the other
parent is pure blue eyes,
create the Punnett square
to determine the average
of each eye color
Heredity
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Using Punnett squares
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A brown-eyed man
marries a blue-eyed
woman and they have
three children, two of
whom are brown-eyed
and one of whom is blueeyed. Draw the Punnett
square that illustrates
this marriage.
Heredity
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Using Punnett squares
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In fruit flies, gray body
color is dominant (G),
and ebony body color is
recessive (g). What
happens when a pure
gray fruit fly has offspring
with an pure ebony fruit
fly?
Heredity
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Using Punnett squares
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In a species, tall (T) is
dominant and short (t) is
recessive. If a pure
short parent (tt) mates
with a tall parent (TT or
Tt), what would the
percentages of tall and
short offspring be?
You will need to draw
two Punnett squares to
answer this one
Heredity
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Using Punnett squares
– There is a second system
you need to know, called
co- dominance
– In this system, more than
two alleles exist.
– For example, with blood
types, there are A alleles,
B alleles and b alleles
(recessive)
Heredity
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Using Punnett squares
– “Could a man with type B
blood and a woman with
type AB produce a child
with type O blood?"
– The woman must be AB,
and the man may be
either BB or Bb. Draw the
two Punnett squares and
see if any of the offspring
could have type O (bb).
Note: the only
possible outcomes for
blood type are Bb
(Type B), bb (Type
O), Ab (Type A) and
AB (Type AB)
Heredity
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Using Punnett squares
– There is a third system
you need to know, called
incomplete dominance
– In this system, neither
trait dominates, the
offspring’s traits are a
mixture of both.
– For example, red ( R) and
white (W) would combine
to form pink (RW)
R
R
W
RW
RW
W
RW
RW
Chromosomes and Inheritence
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Chromosomes exist in pairs
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In the early 1900s, Walter Sutton, an American
geneticist, looked at chromosomes in grasshoppers
His hypothesis was that chromosomes are the key
to understanding how traits are passed from
parents to offspring
Sutton discovered that sex cells in grasshoppers
(sperm and egg cells) only had half the number of
chromosomes as regular body cells
Chromosomes and Inheritence
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Chromosomes exist in pairs
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In an organism’s body cells, chromosomes exist in
pairs
In an organism’s sex cells, chromosomes exist
alone
These sex cells are called sperm in males, ova or
egg cells in females
During fertilization, chromosomes in sperm and egg
cells merge to form a cell with chromosomes in
pairs
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One half of each pair came from the male parent
One half of each pair came from the female parent
Chromosomes and Inheritence
Chromosomes and Inheritance
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How do sex cells form?
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Meiosis is the process through which sex cells are
formed
During meiosis, chromosome pairs separate into
two cells
The sex cells that form later have only half as many
chromosomes as the other cells in the organism
Note: females have two X chromosomes
Note: males have one X and one Y chromosome
Chromosomes and Inheritance
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Before meiosis
Chromosomes and Inheritance
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After meiosis
Sex cells
Sexual versus asexual reproduction
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Reproduction
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During sexual reproduction, two parents
contribute different DNA
This results in a new organism with traits of both
parents
Sex cells are formed through the process of
meiosis
Sexual versus asexual reproduction
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Reproduction
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During asexual reproduction, one parent
produces an identical offspring
New cells are created through the process of
mitosis
Sexual versus asexual reproduction
Advantages
Disadvantages
Asexual
Can quickly
produce many
offspring
No variation in
offspring, may not
survive in a new
environment
Sexual
Variation in
offspring, some
may survive if
environment
changes
Finding mate,
waiting for
offspring to
develop
Classwork 1 - Heredity
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Different forms of the same gene are called
The study of heredity
The “Father of Genetics”
Define dominant allele
Define recessive allele
Define Heterozygous
Define Homozygous
Classwork 1 – Heredity (continued)
8. In __________ dominance, neither trait dominates,
and the offspring’s traits are a mixture of both.
9. In __________ dominance, dominant triats mask
recessive traits
10. In __________, more than two alleles are present
Human Genetics
Pedigree: A family tree that shows the presence
or absence of a trait according to family
relationships over several generations
Human Genetics
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Humans inherit 23 pairs of chromosomes (46 total): 23
from Mom and 23 from Dad
There is no noticeable genetic differences between
different “races”
There is no gene for alcoholism. There IS a gene that,
when not properly functioning, gives a greatly increased
likelihood that the person will develop alcoholism IF
certain circumstances occur (like the person drinks
alcohol)
– A dysfunction on the gene BRAC1 causes one type of
breast cancer. That does not mean that everyone with
BRAC1 is destined to have breast cancer. Also, there
are plenty of people without any dysfunction on
BRAC1 that have breast cancer.
Human Genetics
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Some human diseases are caused by genetic
disorders
– Hutchinson's Disease (dominant)
– Sickle Cell (recessive)
– Cystic Fibrosis (recessive)
Some diseases are X-linked (only found on the X
chromosome)
– Hemophilia
Some diseases are caused by having an extra
chromosome
– Down’s syndrome (extra chromosome #21)
Sickle Cell Disease
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An altered hemoglobin
protein (red blood cells)
allows the red blood cells
to change shape when
under pressure or stress
(like during exercise).
This recessive disorder
has remained in constant
levels in the population
because it makes the
person immune to
malaria
Sickle Cell Disease
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An altered hemoglobin
protein A homozygous
person for the disease
frequently dies younger
than normal.
A heterozygous person
for the disease can lead a
normal life and be
immune to malaria; the
heterozygous person has
both sickle shaped red
blood cells and round
shaped red blood cells.
What type of dominance
is shown?
X-linked diseases
If the defective gene only occurs
on the x chromosome then it is
called an x-linked disease. In this
case, women may be unaffected
carriers while all men will be
affected (as they only have one x
chromosome). In pedigrees,
these conditions are passed from
mother to son.
-Ex. Color blindness and male
pattern baldness (not diseases
but still x-linked)
X-linked diseases
An example of an x-linked
disease is Hemophilia. In this
condition, the person affected
has platelets that do not
function normally. When cut,
the hemophiliac will bleed
without clotting. These
individuals used to always die
young but new modern
technology has allowed them
to live longer lives.
About 1/6000 people in
America has hemophilia
Classwork 2 – Human Genetics
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Define pedigree
Humans inherit __ pairs of chromosomes, half from
each parent
This gene causes one type of breast cancer
This genetic disorder affects red blood cells
In this x-linked disease, the person affected has
platelets that do not function normally, which
means they may bleed to death without clotting.
Even though her great-great grandmother Queen
Victoria was a carrier of hemophilia, the children of
this queen do not have the genetic disorder
Genetics