Transcript Sex chromosomes

Warm-Up 11-29-10
 List
all of the possible gametes produced
by organisms with the following genotypes.
Separate the gametes with a comma:
AA, bb, AABb, aaBb
 What are the genotypic ratios of the
offspring for the following cross?
Ff x ff
Beyond dominant and recessive alleles

There are some exceptions to Mendel’s
principles.
 Codominanace
Incomplete Dominance
Multiple Alleles

When more than 2 alleles for the same
trait exist within a population.

Each individual organism can only contain 2
alleles (one from mom and one from dad)
although, more than 2 alleles may exist within
the population.
Polygenic Traits

Traits that are determined by
alleles from more than one
gene; these traits usually have
a range of phenotypes.

Ex. Skin color in humans & height
in humans.
Incomplete Dominance

Cases in which there is no dominant
allele. A heterozygous phenotype is
somewhere in between the two
homozygous phenotypes.
Codominance

Both alleles contribute to the phenotype
and can be observed in the offspring.
Sex Determination
There are two types of chromosomes.
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Autosomes: Of the 46 chromosomes, 44 of them
(22 pairs of chromosomes) are called autosomes
(non-sex chromosomes).
Sex chromosomes: The last two chromosomes
are called the sex chromosomes because they
determine the sex of the person. Females have two
X chromosomes (XX) and males have one X and
one Y chromosome (XY).
Sex Linked Traits

Traits that are determined by alleles that are found on
the X or Y chromosome.

The Y chromosome is shorter and does not carry all the same
alleles are the X chromosome.
Sex Linked Traits

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Females are XX and males are XY.
Females can be homozygous or
heterozygous for a trait carried on the X
chromosome, but males (only having 1 X
chromosome) are hemizygous.
Sex Linked Traits

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If they inherit a defective gene from
the parent, then they will exhibit the
trait because they cannot inherit a
second gene to mask it.
Conversely, a healthy male cannot be
“hiding” a bad recessive allele
because they only have one X
chromosome.
Example of a sex-linked Punnett square:

XBXb (heterozygous female with normal vision) crossed to
XBY (hemizygous male with normal vision)
XBY
XB
Y
XB XB
XBY
XB
XB Xb
XB Xb
Xb
XbY
Sex Linked Traits
Karyotypes
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Scientists can analyze an individual’s
chromosomes by taking a picture of cells
during mitosis.
It is easiest to view chromosomes during
mitosis because they are condensed.
Scientists will cut and paste to arrange the
chromosomes in pairs to form a karyotype.
They are arranged from largest (pair #1) to
smallest (pair #22). The last pair (#23) is the
sex chromosomes.
Karyotype
X
Y
Is this karyotype for a male or female?
Male or Female?
X
X
Y
Male
X
Female
Karyotypes

Genetic disorders can be
identified by looking at an
individual’s karyotype.

Ex: Down Syndrome can
be confirmed in an individual
when 3 chromosome 21s
are present in a karyotype.
Pedigree

A pedigree, similar to a family tree, is used to
demonstrate how traits are passed from one
generation to another.
Genetic counselors use pedigrees to follow how
genetic disorders are inherited.
People who are heterozygous for a recessive
genetic disorder (they are unaffected) are called
carriers.
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Sometimes, carriers of traits may be represented as a
half-shaded shape or a shape with a dot in the middle.
female
marriage
male
parents
children
Add to your notes:
Sometimes, but not always, carriers of traits (heterozygotes)
may be represented as a half-shaded shape or a shape with
a dot in the middle.
Blood Typing
In humans, blood type is determined by the
Rh blood group and the ABO blood group.
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The Rh blood group determines if your blood is
positive or negative.
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There are two Rh alleles: the Rh+ allele is dominant and the
Rh- allele is recessive.
Your blood is positive if you are Rh+ /Rh+ or Rh+/Rh-.
Your blood is negative if you are Rh-/Rh-
When determining the ABO blood group
there are three alleles: IA, IB, and i.
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The IA and IB alleles are codominant. The
IA and IB alleles cause expression of
carbohydrate chains called antigens on
surface of red blood cells. They help your
body identify the cells.
The i allele is recessive to the IA and IB
alleles. The i allele produces O blood
because it does not produce any antigens.
Blood Transfusions
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If the blood recipient has never been exposed to an
antigen (A or B) and that antigen enters the body, it
will cause an immune reaction. This can cause
death.
Blood Transfusions

In emergency rooms, if there is not enough time to
figure out the blood type of the patient, which type
of blood will the patient receive Type O blood. This
is because these blood cells have no A or B
antigens. People with Type O blood are called
universal donors.
Blood Transfusions
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People with AB blood can receive any blood type.
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They are considered to be the universal
recipient.
Genetics and the Environment

Characteristics are determined by both
genes and the environment.
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External: While genes will influence the height
of a plant, the amount of water, sun, and other
climate conditions will also affect the height.
Genetics and the Environment
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Internal: There are recent findings that
proteins involved with DNA can turn genes
on or off based on environmental factors.
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Certain chemical exposure can turn genes on
or off (make the traits show up or not) for
generations after exposure, but there are no
changes to the DNA (no mutations).
This new understanding of how genes are
expressed is called epigenetics.