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Chapter 4
Heredity and Genes
4.1 Basic
Principles of
Heredity
Heredity
• The passing of traits from one generation to
the next.
• Trait: Characteristic, such as eye color or nose
shape.
• Genetics: The field of biology related to
studying heredity.
• Gametes: sex cells
– Sperm: male gamete
– Ovum: female gamete
• Fertilization: the process by which the male
gamete unites with the female gamete.
• Zygote: The cell formed after the male and
female gamete unite.
Monohybrid Cross
• A cross between two parents that differ only
in one trait.
• Hybrid: The offspring of parents that have
different forms of a trait.
Dominant and Recessive Traits
• Every trait is controlled by two factors.
• Dominant trait: Trait that is expressed
whenever at least one factor for that form of
the characteristic is present.
• Recessive trait: Trait that is not seen if a factor
for the dominant form of the characteristic is
present.
Alleles
• Gene: The “factors” described earlier. A trait
that can be inherited.
• Alleles: Different forms of a gene.
Law of Dominance
• Each organism receives two alleles for a trait
(one from each parent).
• The dominant trait appears if one or both
alleles are for that trait.
• The recessive trait appears ONLY if both alleles
are for that recessive trait.
Law of Segregation
• Gametes randomly receive one of the parent’s
alleles for a trait.
Genotype
• The combination of alleles in an organism.
– Homozygous: The two alleles for the trait are the
same.
– Heterozygous: The two alleles for the trait are
different.
Phenotype
• The trait that the organism displays.
• Incomplete dominance: The offspring show a
blend of their parents’ traits.
• Codominance: Both traits are displayed in the
organism.
4.2 Punnett Squares
A.K.A “The Alien Lesson”
• Punnett square: A chart used to predict the
possible genotypes of the offspring of a cross
between two parent organisms.
• A capital letter is used to represent a
DOMINANT allele, and a lower-case letter is
used to represent a recessive allele.
Remember that homozygous
means that the two alleles are the
same, and heterozygous means
that the two alleles are different.
Our first example involves alien skin
color.
• Blue skin color is dominant, and expressed
with a capital B.
• Green skin color is recessive, and expressed
with a lower-case b.
• The first parent is a homozygous, blue-skinned
alien. Her genotype will be _______.
• The second parent is a homozygous, greenskinned alien. His genotype will be ______.
Our first example involves alien skin
color.
• Blue skin color is dominant, and expressed
with a capital B.
• Green skin color is recessive, and expressed
with a lower-case b.
• The first parent is a homozygous, blue-skinned
alien. Her genotype will be BB.
• The second parent is a homozygous, greenskinned alien. His genotype will be bb.
A Punnett Square for one trait is a box
divided into four squares:
We write the genotype for one parent across the top,
and the other along the left side (Law of Segregation).
It doesn’t matter which parent you write where.
B
b
b
B
Now we start the cross. In each box, write the letter
from above and the letter from the side. It is
customary to write capital letter first in a heterozygote.
B
b
b
Bb
B
Continue to fill in the other three boxes of the Punnett
square.
b
b
B
B
Bb
Bb
Bb
Bb
The genotypes (combinations of alleles) from this cross
are all Bb. All of the offspring (4/4 = 1) can be expected
to have this genotype.
b
b
B
B
Bb
Bb
Bb
Bb
The phenotypes (expressed traits) from this cross are
all blue skin. All of the offspring (4/4 = 1) can be
expected to have this phenotype.
b
b
B
B
Bb
Bb
Bb
Bb
Now, let’s cross two heterozygous
blue-skinned aliens.
• The genotype for each of these aliens is
_______.
Now, let’s cross two heterozygous
blue-skinned aliens.
• The genotype for each of these aliens is
Bb.
Now let’s write the alleles for each parent along the top
and the side of the Punnett’s Square.
B
B
b
b
And now we fill in the four boxes.
B
b
B
b
BB
Bb
Bb
bb
The possible genotypes of this cross are BB (1/4), Bb
(2/4 = 1/2), and bb (1/4).
B
b
B
b
BB
Bb
Bb
bb
The possible phenotypes of this cross are blue skin
(3/4) and green skin (1/4).
B
b
B
BB
blue
Bb
blue
b
Bb
blue
bb
green
Alien Eye Number
• The three-eyes trait is dominant, represented
with a capital E.
• The two-eyes trait is recessive, represented
with a lowercase e.
• Genotype for homozygous two-eyed is _____.
• Genotype for heterozygous three-eyed is
______.
Alien Eye Number
• The three-eyes trait is dominant, represented
with a capital E.
• The two-eyes trait is recessive, represented
with a lowercase e.
• Genotype for homozygous two-eyed is ee.
• Genotype for heterozygous three-eyed is
Ee.
Now let’s write the alleles for each parent along the top
and the side of the Punnett’s Square.
e
E
e
e
And now fill in the boxes.
E
e
e
e
Ee
Ee
ee
ee
The possible genotypes for this cross are Ee (2/4 = 1/2)
and ee (2/4 = 1/2).
E
e
e
e
Ee
Ee
ee
ee
The possible phenotypes for this cross are three-eyed
(2/4 = 1/2) and two-eyed (2/4 = 1/2).
E
e
e
e
Ee
three-eyed
Ee
three-eyed
ee
two-eyed
ee
two-eyed
Dihybrid cross
• A cross that looks at two different traits.
• Generally, traits segregate independently, so
for two traits, there are four different
combinations of alleles that the parent can
pass on to its offspring.
• A heterozygous blue-skinned, heterozygous
three-eyed alien’s genotype is BbEe.
• The possible combinations of alleles from this
parent are:
– BE
– Be
– bE
– be
To do a dihybrid cross, we need a
Punnett’s square divided into 16
boxes.
Each possible allele combination goes
across the top and along the side.
BE
BE
Be
bE
be
Be
bE
be
And now we fill in the boxes.
BE
Be
bE
be
BE
BBEE
BBEe
BbEE
BbEe
Be
BBEe
BBee
BbEe
Bbee
bE
BbEE
BbEe
bbEE
bbEe
be
BbEe
Bbee
bbEe
bbee
Possible Genotypes:
BBEE
BBEe
BBee
BbEE
BbEe
Bbee
bbEE
bbEe
bbee
1/ 16
2/16 = 1/8
1/16
2/16 = 1/8
4/16 = 1/4
2/16 = 1/8
1/16
2/16 = 1/8
1/16
Phenotypes:
BE
Be
bE
BE
BBEE
Blue, 3
BBEe
Blue, 3
BbEE
Blue, 3
BbEe
Blue, 3
Be
BBEe
Blue, 3
BBee
Blue, 2
BbEe
Blue, 3
Bbee
Blue, 2
bE
BbEE
Blue, 3
BbEe
Blue, 3
bbEE
Green, 3
bbEe
Green, 3
BbEe
Blue, 3
Bbee
Blue, 2
bbEe
Green, 3
bbee
Green, 2
be
be
Possible phenotypes:
Blue skin and three eyes: 9/16
Blue skin and two eyes: 3/16
Green skin and three eyes: 3/16
Green skin and two eyes: 1/16
What is the phenotype for Dermed,
our Intro to Health Care alien?
What is the phenotype for Dermed,
our Intro to Health Care alien?
He is a
green-skinned,
three-eyed
alien!
What are the possible genotypes for
Dermed’s skin color?
What are the possible genotypes for
Dermed’s skin color?
He has green
skin, which is a
recessive trait.
Therefore, the
only possible
genotype is bb.
What are the possible genotypes for
Dermed’s eye number?
He has three
eyes, which is a
dominant trait.
Therefore, the
possible
genotypes are
EE or Ee.
4.3 Chromosomes
• Chromosome: The structure that contains
genes.
• Made of DNA (and some proteins to help it
hold shape).
• Found in the nucleus of eukaryotic cells.
• Chromosomes look like thin threads until the
cell is almost ready to divide.
• Just before cell division, the chromosome coils
itself into an X-shape.
Chromatid: Each side of Xshaped chromosome.
Sister
chromatids
Sister chromatids: The pair
of chromatids that make
up one chromosome. They
are exact copies of each
other (so that when the
cell divides, each new cell
gets a copy).
Centromere: The point at
which the two sister
chromatids are held
together.
• In organisms that have two parents (that is,
undergo sexual reproduction), the
chromosomes occur in pairs (one from male
parent, and one from female parent.)
• Humans have 23 PAIRS of chromosomes (46
total chromosomes).
• Homologous chromosomes: The two
chromosomes that form each pair.
Human chromosomes
Mitosis
• Just before cell division occurs, the cells make
a copy of each of their chromosomes and
makes extra organelles and cytoplasm.
• Mitosis: The process by which the nucleus of
a cell divides.
Mitosis has four stages:
• Prophase: Chromosomes group together;
nuclear envelope disappears.
• Metaphase: Chromosomes line up along
center of the cell.
• Anaphase: Centromeres split; sister
chromatids move to opposite ends of the cell.
• Telophase: Cell membrane pinches in at the
center of the cell; nuclear envelope reappears
around the two groups of chromosomes.
1. Prophase
3. Anaphase
2. Metaphase
4. Telophase
• After mitosis is complete, the cell undergoes
cytokinesis: the complete splitting of the cell
into two identical daughter cells, each with a
complete set of chromosomes.
Meiosis
• Remember that during sexual reproduction, a
male gamete (sperm) unites with a female
gamete (ovum) to create the zygote, which
forms into the new organism.
Meiosis
• The new organism must have the same
number of chromosomes as its parent
organisms.
• If two regular body cells joined together to
form the new organism, the new organism
would have twice the number of
chromosomes as the parents.
Meiosis
• Meiosis is the process by which gametes,
which have HALF the number of
chromosomes as the rest of the body’s cells,
are formed.
Meiosis
• The process of meiosis is very similar to
mitosis, but the cycle is repeated.
• During meiosis I, the cell divides into two cells,
each with the original number of
chromosomes.
• During meiosis II, the two cells formed in
meiosis I divide again, creating a total of 4
cells, each with half of the number of
chromosomes as the original cell.
Meiosis I
Meiosis II
Determining Gender
• Sex chromosomes: The pair of homologous
chromosomes that determines the sex of the
organism.
• Autosomes: The other pairs of homologous
chromosomes, that determine all traits other
than sex.
• Females have two X chromosomes (XX). They
pass on only X chromosomes to their
offspring.
• Males have one X and one Y chromosome
(XY). They can pass on either an X
chromosome or a Y chromosome to their
offspring.
– Therefore, the male determines the sex of his
offspring!
Sex-linked traits
• Some traits, other than sex related traits, are
carried on the sex chromosomes (especially
the X chromosome since it’s much bigger).
• Example: color blindness and hemophilia.
• And this causes calico coloring in cats.
4.4 The Role of DNA
• DNA is made of smaller units called
nucleotides.
• A nucleotide is made of a phosphate, a sugar,
and a nitrogenous base.
– Nitrogenous bases:
•
•
•
•
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
DNA is shaped like a double helix (a
twisted ladder).
• The sides of the ladder are made of
alternating sugar and phosphate molecules.
• The rungs are made of bound nitrogenous
bases.
– Adenine and thymine always bind together.
– Cytosine and guanine always bind together.
Alternating sugar and
phosphate molecules
Cytosine
Guanine
Adenine
Thymine
DNA replication
• In order for the two daughter cells to receive
full copies of the parent cell’s genetic
information, the DNA must be copied
(replicated).
The process of DNA replication
• The DNA strands separate (the bonds between
the two bases that make up each rung
separate) – like “unzipping.”
• New bases attach to each strand.
• Two identical DNA molecules are formed
(each with one strand of old DNA and one
strand of new DNA).
Mutation
• Any change in the DNA sequence.
• Depending on the change, it may have little or
no affect, or it may be harmful.
Genetic code
• The order of the bases is the genetic code.
The code determines how the proteins are
made.
RNA
• RNA carries the information from the DNA in
the nucleus to the ribosomes in the cytoplasm
so that proteins can be made.
DNA and RNA are very similar, except:
Strands:
Sugar:
Bases:
DNA
RNA
Doublestranded
Deoxyribose
G. C, A, T
Singlestranded
Ribose
G, C, A,
URACIL (U)
Protein Synthesis
• THE DNA SENTENCE LAB!