Human Heredity
7.1 Chromosomes and Phenotype
Two copies of each autosomal gene affect phenotype.
• Mendel studied autosomal
gene traits, like hair texture.
7.2
Complex Patterns of Inheritance
The student is expected to:
6F predict possible outcomes
of various genetic combinations
such as monohybrid crosses,
dihybrid crosses and
non-Mendelian inheritance
TEKS 6F
Beyond dominant and recessive alleles
• There are some exceptions to Mendel’s
principles. Luckily, none of these
exceptions are exhibited in pea plants.
• If so, Mendel would not have been able to
figure out inheritance.
• Some alleles are neither dominant nor
recessive.
– Incomplete Dominance
– Codominance
Incomplete dominance:
• situation in which one allele is not
completely dominant over another; the
phenotype is a “blending” of the two alleles
– Example: In some plants, when a truebreeding plant with red flowers is crossed with
a true-breeding plant with white flowers, pink
flowers are produced. Neither red nor white is
dominant over the other.
7.2
Complex Patterns of Inheritance
Phenotype can depend on
interactions of alleles.
TEKS 6F
• In incomplete dominance, neither allele is
completely dominant nor completely recessive.
– Heterozygous phenotype is intermediate
between the two homozygous phenotypes
– Homozygous parental phenotypes not seen in F1
offspring
• Consider this
Punnett square:
Codominance:
•
situation in which both alleles of a gene
contribute to the phenotype of the organism;
both alleles are expressed but NOT blended
–
Example: In cows, the allele for red fur is
codominant with the allele for white fur.
Heterozygous cows carrying one red and one white
allele have spotted fur, known as roan.
7.2
Complex Patterns of Inheritance
TEKS 6F
• Codominant alleles will both be completely
expressed.
– Codominant
alleles are neither
dominant nor
recessive.
– The ABO blood
types result from
codominant
alleles.
• Many genes have more than two alleles.
• Consider this
Punnett square:
• Many traits are controlled by multiple
alleles or multiple genes.
– Multiple alleles (more than 2 choices)
– Polygenic (multiple genes control a single
trait)
Multiple alleles:
•
the case where three or more alleles of
the same gene exist. Remember, an
organism will have only two of these
alleles (one from mom and one from
dad).
– Examples: Coat color in rabbits, blood type
in humans
Multiple alleles:
Polygenic traits:
•
traits that are determined by alleles from
more than one gene; these traits usually
have a range of phenotypes
– Examples: skin color in humans, height in
humans
7.2
Complex Patterns of Inheritance
Many genes may interact to
produce one trait.
• Polygenic traits
are produced by
two or more
genes.
Order of dominance:
brown > green > blue.
TEKS 6F
Human chromosomes
1. Most of our cells contain 23 pairs of
chromosomes, for a total of 46 chromosomes.
a. These cells are called somatic cells or body
cells.
b. Two exceptions are the gametes (sex cells),
sperm and eggs, which have one copy of each
chromosome. Sperm and egg cells each have
23 chromosomes.
Human chromosomes
2. There are two types of chromosomes.
a. Autosomes: Of the 46 chromosomes, 44 of them
(22 pairs of chromosomes) are called autosomes
(non-sex chromosomes).
b. 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).
4. Scientists can analyze chromosomes by taking
a picture of cells during mitosis. It is easiest to
view chromosomes during mitosis because
they are condensed. From a picture of
chromosomes, scientists can 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
Male or Female?
X
X
Y
Male
X
Female
Karyotype Analysis
Too many = Trisomy ___
Too few = monosomy ____
If there is a Y it is male, no matter how many
X’s
Only X it is a female
Human Traits
1. A pedigree is similar to a family tree- both are
used to show relationships in a family.
2. Pedigrees can be used to demonstrate how traits
are passed from one generation to another.
3. Genetic counselors use pedigrees to follow how
genetic disorders are inherited.
4. People who are heterozygous for a recessive
genetic disorder (they are unaffected) are called
carriers.
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.
Human Genes: Blood typing
In humans, blood type is determined by the Rh blood
group and the ABO blood group.
1.
The Rh blood group determines if your blood is positive
or negative.
a. There are two Rh alleles: the Rh+ allele is dominant
to the Rh- allele.
b. Your blood is positive if you are Rh+ /Rh+ or Rh+/Rh-.
Your blood is negative if you are Rh-/Rh-
2. The ABO group is more complicated.
There are three alleles: IA, IB, and i.
a. 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.
b. The i allele is recessive to the other two
alleles. The i allele O does not produce
antigens.
c. The ABO blood group is important in blood
transfusions.
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.
Donors
A
A
Recipients
i.
B
AB
O
B
AB
O
= cannot transfuse;
immune reaction
(clumping of cells)
= OK to transfuse
ii. In emergency rooms when there isn’t time to figure out
the blood type of the patient, which type of blood will the
patient receive? Type O because these blood cells have
no A or B antigens. People with Type O blood are called
universal donors.
iii. Who is the universal recipient that can receive blood
from any donor? Type AB.
Human Chromosomes
Most genetic disorders are caused by mutations on
autosomes, or non-sex chromosomes.
Examples include:
1. Autosomal recessive disorders: albinism, cystic
fibrosis, Tay-Sachs disease
2. Autosomal dominant disorders: most common form
of dwarfism (achondroplasia) and
Hypercholesterolemia (high cholesterol)
3. Codominant disorders: sickle-cell disease
Albinism
Achondroplasia
Some genetic
disorders are caused
be genes on the sex
chromosomes.
1. Most of these genes
are on the X
chromosome because
the Y chromosome is
very small and has few
genes. The genes on
X chromosome are
different from the
genes on the Y
chromosome.
2. Because females are XX they have two copies of
the genes on the X chromosome. For sex-linked
traits, females can be homozygous dominant,
heterozygous, or homozygous recessive.
7.1 Chromosomes and Phenotype
Males and females can differ in sex-linked traits.
• Genes on sex chromosomes are called sex-linked genes.
– Y chromosome genes in mammals are responsible for
male characteristics.
– X chromosome genes in mammals affect many traits.
7.1 Chromosomes and Phenotype
• Male mammals have an XY genotype.
– All of a male’s sexlinked genes are
expressed.
– Males have no
second copies of
sex-linked genes.
7.1 Chromosomes and Phenotype
•
Female mammals have an XX genotype.
– Expression of sex-linked genes is similar to
autosomal genes in females.
– X chromosome inactivation randomly “turns off”
one X chromosome.
3. Because males are XY they have only one copy of the
genes on the X chromosome; this is called hemizygous.
In males, only one recessive allele on the X chromosome
is necessary for the recessive phenotype to be expressed
because there is not another allele for this gene on the Y
chromosome. Some sex-linked (also known as X-linked)
genetic conditions include:
a. Color blindness- the inability to distinguish certain colors
b. Hemophilia- missing protein important for blood clotting
c. Duchenne Muscular Dystrophy- progressive weakening of
skeletal muscles
Colorblindness- Test A
Everyone should be able to see a circle, star, and square in the demonstration picture.
Colorblindness- Test B
Colorblind individuals should see the yellow square.
Color normal individuals should see the yellow square and a "faint" brown circle.
Colorblindness- Test C
Colorblind individuals should see nothing.
Color normal individuals should see a "faint" brown boat.
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
Xb Y
Chromosomal Disorders
Remember that meiosis is the reductional cell division that
divides one diploid cell to produce four haploid gametes (sex
cells, sperm or egg). Normally gametes have one copy of
each chromosome.
1. Sometimes chromosomes might not separate properly during
meiosis; this is called nondisjunction.
2. If nondisjunction occurs, abnormal numbers of chromosomes
(usually one is missing or there is an extra copy of one) are
found in gametes and disorders of chromosomal numbers
may result.
gametes
3.
Trisomy: Some chromosomal disorders are caused by
having three copies of one chromosome. This is
called trisomy. In trisomies, the gamete of one parent
donated two of one type of chromosome to the child
and the gamete of the other parent donated one
chromosome (like normal).
4.
Monosomy: Chromosomal disorders characterized by
missing one chromosome are called monosomies. In
monosomies, the gamete of one parent donated one
chromosome and the other did not donate any.
Some examples of chromosomal disorders
resulting from nondisjunction:
1. Down syndrome- Trisomy 21
2. Klinefelter’s syndrome- XXY (male)
3. Turner’s syndrome- XO (female)
Add to your notes:
As long there is at least one Y chromsome, the
karyotype is male.
Human Genome Project
•
The human genome project sequenced the human
genome. Now the code is being analyzed and
scientists are finding genes for many traits and
genetic disorders. In gene therapy, a gene that
has been mutated and does not work properly is
replaced by a normal, working copy of the gene.
Gene therapy is a work in progress.