Do Now
Answer the questions below in your notebook/binder:
Grasshoppers are diploid and have a total of 24 chromosomes
in each of their somatic cells.
a. How many chromosomes are in a grasshopper egg cell?
b. How many chromosomes are in a grasshopper zygote?
Meiosis and Mendelian Genetics
Vocabulary
Heredity: the passing on of traits genetically
from one generation to another.
Genetics: the scientific study of heredity
Trait: a specific characteristic that varies from
one individual to another
Probability: the chance that a particular event
will happen
Vocabulary
Punnett square: a diagram that shows the gene
combinations that might occur when two
parents make offspring
Homozygous: having two identical alleles for a
particular trait (aa or AA)
Heterozygous: having two different alleles for a
particular trait (Aa)
Vocabulary
Phenotype: the physical characteristics of an
organism (such as brown hair, freckles, a big
nose)
Genotype: the genetic make-up of an organism
(AA, Aa, or aa)
Dominant: a gene that can cover up the
phenotype of a recessive allele
Recessive: a gene whose trait can only be seen if
there are no dominant alleles present
Meiosis Review:
Recall that during meiosis, homologous
chromosomes pair up in the middle of
the cell to form tetrads and cross over,
leading to new combinations of genes on
each chromatid.
Meiosis Review:
The four
chromatids from
the tetrad each
end up in a
different gamete
Check for Understanding
This diagram shows
how chromosomes
separate during
meiosis.
Did these
chromosomes cross
over? How can you
tell?
Gregor Mendel
Gregor Mendel was an Austrian monk who
studied the traits of the pea plants in his garden.
He is often
considered
“The Father of
Genetics.” His work
was very important
to the understanding
of genetics.
Mendelian Genetics
Mendelian genetics only works with traits that
are controlled by one gene. The presence of
different alleles of that gene determines the
Suppose this gene is the gene for a dimpled
trait.
chin. A dimpled chin is a trait that is only
controlled by one gene, meaning that there is
one location (loci) on this homologous pair of
chromosomes that is for the dimpled chin gene.
There are no other genes anywhere, on any
chromosome, that control the dimpled chin
trait.
Some traits, like height and skin color, are
controlled by many genes. For these traits,
There are several genes with different loci.
The genes may even be on different chromosomes
Homozygous and heterozygous
describe the genotype of an organism
An organism that is homozygous for a trait has two copies of the same allele. If two
dominant alleles are present, the organism is homozygous dominant for that trait.
If two recessive alleles are present, the organism is homozygous recessive
(PP or aa )
An organism that is heterozygous
for a trait has two different alleles,
and only the dominant allele is
expressed in the phenotype.
(Bb)
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Examples
Dracula has a widow’s peak,
which is a dominant trait.
We know Dracula must have
at least one dominant
allele for a widow’s peak (W).
Dracula’s genotype could be
homozygous dominant (WW)
or heterozygous (Ww)
If Dracula had no widow’s peak,
he would be homozygous
recessive (ww)
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This is my friend Jace.
Jace has naturally
straight hair, which is
a recessive trait.
We know Jace’s genotype is homozygous recessive
for straight hair (tt)
A person with curly hair (the dominant trait) would
have the genotype TT or Tt
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Check for Understanding
Having a dimple (or a
cleft) in your chin is a
dominant trait. The
recessive trait is
having a smooth or
straight chin.
What are the possible
genotypes for
Eminem’s chin? Use
the letter D for the
dominant allele and d
for the recessive allele.
Recall: Chromosomes
Sex chromosomes are the X and Y chromosomes
that determine an individual’s sex
Autosomes are the rest of an individual’s
chromosomes
Humans have 23 pairs of chromosomes:
One pair of sex chromosomes
And 22 pairs of autosomes
Human Chromosomes
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Sex-Linked Traits
Alleles for sex-linked traits can
be located on either
the X or Y chromosome.
Most genes for sex-linked traits are located
on the X chromosome (X-linked), simply because
the X chromosome has many more genes than
the Y chromosome
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Remember when we watched
Lorenzo’s Oil?!?
Lorenzo Odone had a disease
called ALD
(adrenoleukodystrophy). This
gene that causes this disease is
on the X chromosome. Even
though he got the gene from his
mother, his mother did not have
the disease because she has two
X chromosomes, so her good
gene on her other X
chromosome was enough to
keep her healthy. Because
males have XY for their sex
chromosomes, they will be
affected by genes on the X
chromosome.
Example: Colorblindness
Example: colorblindness is an X-linked, recessive
trait. Since females have two X chromosomes, a
female must have two alleles for colorblindness to
be colorblind (one on each X chromosome).
A female with only one allele for colorblindness
is called a carrier, because she will not express
the colorblind phenotype, but she may
pass the colorblind allele to her offspring.
Since males only have one X chromosome,
they will be colorblind if they have the allele
for colorblindness
-The genotype of a male affected
with colorblindness would be XCY
-The genotype of a carrier
female would be XCX
-The genotype of female affected
With colorblindness would be XCXC
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X-Linked Traits
X-linked recessive traits are called recessive
because the other, normal allele on the second X
chromosome in females can mask the trait.
So a female with two X chromosomes must have
two copies of the allele to express the X-linked
recessive phenotype
The Y chromosome has different genes than the
X chromosome, so it cannot mask the allele on
the X chromosome
The X and Y chromosomes are not
homologous
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Check for Understanding
If a woman who is colorblind has a son with a
man who is not colorblind, what is the
probability that the son will be colorblind?
If a colorblind woman has a daughter with a
colorblind man, what is the probability that the
daughter will be colorblind?
Punnett Squares
A Punnett square is a diagram that shows the
gene combinations that might occur when two
parents make offspring
The genotype of one parent
is listed across the top,
and the genotype of the other
parent is listed down the left
side.
The four boxes inside
represent the possible
genotypes of the offspring
Let’s fill in this Punnett Square
together
Let’s say the trait we are looking
at is for height in pea plants. T
represents the dominant tall
allele, and t represents the
recessive short allele.
What are the genotypes of the
parents in this Punnett square?
What is the probability of having
tall offspring?
What is the probability of having
short offspring?
X-Linked Punnett Square
Hemophilia is an Xlinked genetic disease. It
causes blood to clot
improperly, so people
with the disease bleed
longer than people
without the disease.
In this Punnett square,
we are crossing a carrier
female with a male who
has hemophilia.
Let’s do the first Punnett Square on
your Punnett Square Worksheet
together
Note that eye color is really more
complicated than this. This is a
simplified example.
1. B= Brown eyes b= blue eyes
Mom= Bb Dad= BB
What are the eye color possibilities
if they chose to have children?
Be sure that you include the ratios
of the genotypes (and the words
used to describe those alleles) and
phenotypes of the characteristics.
True-breeding: a plant that always produces
offspring with identical traits when it self-pollinates
Hybrid: the offspring of plant parents with
different traits
Segregation: separation
Independent assortment: the fact that genes for
different traits end up in gametes independently of
other traits. For example, your genes for hair color
do not influence what genes you will get for your
eye color.
Mendelian Genetics Continued: The
Laws of Segregation and Independent
assortment
Mendel’s Law of Segregation
Check for Understanding
Mendel’s Law of Independent
Assortment
Mendel’s Law of Independent
Assortment
okay
Check for Understanding