Transcript Chapter 7: Extending Mendelian Genetics

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KEY CONCEPT: The chromosomes on which genes
are located can affect the expression of traits.
Gene expression is often related to whether a gene is
located on an autosome or on a sex chromosome.
All humans have 46 chromosomes
• 23 pairs
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44 are autosomes (“normal” traits)
• Pairs 1-22
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2 are sex chromosomes (X or Y)
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XX = female
XY = male
Sex chromosomes determine an organism’s sex
Autosomes are the other chromosomes that DO NOT play a direct
role in sex determination.
• Mendel studied autosomal
gene traits, like hair texture.
• Mendel’s rules of inheritance
apply to autosomal genetic
disorders.
– A heterozygote for a recessive
disorder is a carrier.
– Disorders caused by dominant
alleles are uncommon.
• protein cystic fibrosis transmembrane
conductance regulator (CFTR). This
protein is required to regulate the
components of sweat, digestive fluids, and
mucus.
• Leads to thick, viscous secretions
(dominant)
Males and females can differ in sex-linked traits.
 Genes on sex chromosomes are called sexlinked genes.
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• Y chromosome genes in mammals are responsible for
male characteristics.
• X chromosome genes
in mammals affect many traits.
• X chromosomes carry some
genes that Y does not.
•Y is not large enough to carry
them
•Cause a difference of
inheritance between males and
females
 Male
mammals
have an XY
genotype.
– All of a male’s sex-
linked genes are
expressed.
– Males have no
second copies of sexlinked genes.
 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.
XO =Orange Fur
Xo = Black fur
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The inactive X chromosome is silenced by it being
packaged in such a way that it has a transcriptionally
inactive structure called heterochromatin (silencing of
genes)
X-inactivation prevents them from having twice as many X
chromosome gene products (RNA or proteins) as males,
which only possess a single copy of the X chromosome.
• Abnormal amounts of gene products can be related to oncogenes
which cause cancer.
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The choice of which X chromosome will be inactivated is
random in placental mammals such as humans, but once an
X chromosome is inactivated it will remain inactive
throughout the lifetime of the cell and its descendants in the
organism.
• Most of the sex linked genes are found only on
the X chromosome
• Very few are found on the Y chromosome
• Cause difference in male & female inheritance
 Males will be affected or unaffected only
 Females will be affected, unaffected, or a carrier for
these traits.
 Found
on the X chromosomes: red-green
color blindness, muscular dystrophy,
baldness (male pattern baldness),
hemophilia
 Found on the Y: SRY gene (maleness
gene)
• Many genes that code for proteins are found on
the Y gene
Notice male chance is
50% and female
chance is 50%
Female carriers can pass on a
trait without showing that trait
themselves
 KEY
CONCEPT: A combination of methods
is used to study human genetics.
 Human genetics follows the patterns seen in
other organisms.
 The basic principles of genetics are the
same in all sexually reproducing organisms.
• Inheritance of many human traits is complex.
• Single-gene traits are important in understanding
human genetics.
 Females
can carry sex-linked genetic
disorders.
 Males (XY) express all of their sex linked
genes.
 Expression of the disorder depends on
which parent carries the allele and the
sex of the child.
X
Y
A
pedigree is a
chart for tracing
genes in a family.
 What
you need to
know in order to
read a pedigree
chart.
 Phenotypes
are used to infer genotypes
on a pedigree.
 Autosomal genes show different patterns
on a pedigree than sex-linked genes.
 If
the phenotype is more common in
males, the gene is likely sex-linked.
 KEY
CONCEPT: Phenotype is affected by
many different factors.
 The rules are not always followed!
 There are some exceptions
 Phenotype can depend on interactions of
alleles.
 In
incomplete dominance- heterozygous
genes have a phenotype that is between the
dominant and recessive traits
• Black + White = gray
• Also called intermediate inheritance
• Heterozygous phenotype is intermediate between the
two homozygous phenotypes
• Homozygous parental phenotypes not seen in F1
offspring
 Face
shape
 Ear size
 Type of hair (curly, wavy. Straight)
 Nose size
 Mouth size
 Codominant
alleles will both
be completely
expressed (both
dominant).
– Codominant alleles
are neither dominant
nor recessive.
– The ABO blood
types result from
codominant alleles.
 Multiple Alleles
• More than 2 alleles
may exist for a trait
• Blood Type – A, B, O,
& AB types
 IA, IB, i (3 alleles)
 Type AB
 DO NOT mask over
each other
 Many
genes may interact to produce one
trait.
 Polygenic Inheritance – when two or more
genes affect a trait
• Examples: height, skin color, eye color
• Multiple genes contribute to the trait
• AaBbCc – trait using three sets of genes
• Some traits can be affected by dozens of genes
Order of dominance:
brown > green > blue.
In humans, eye color is the result of polygenic
inheritance. At the present, three gene pairs
controlling human eye color are known. Two of
the gene pairs occur on chromosome pair 15
and one occurs on chromosome pair 19. The
bey 2 gene, on chromosome 15, has a brown
and a blue allele. A second gene, located on
chromosome 19 (the gey gene) has a blue and
a green allele. A third gene, bey 1, located on
chromosome 15, is a central brown eye color
gene.
The brown allele is always dominant over the blue allele
so even if a person is heterozygous (one brown and
one blue allele) for the bey 2 gene on chromosome 15
the brown allele will be expressed. The gey gene also
has two alleles, one green and one blue. The green
allele is dominant to the blue allele on either
chromosome but is recessive to the brown allele on
chromosome 15. This means that there is a dominance
order among the two gene pairs. If a person has a
brown allele on chromosome 15 and all other alleles
are blue or green the person will have brown eyes. If
there is a green allele on chromosome 19 and the rest
of the alleles are blue, eye color will be green. Blue
eyes will occur only if all four alleles are for blue
eyes.
 In
complete heterochromia, one iris is a
different color from the other. In partial
heterochromia or sectoral heterochromia,
part of one iris is a different color from its
remainder.
 Most cases of heterochromia are hereditary,
caused by a disease or syndrome, or due to
an injury.
 The pigment is produced in a specialized
group of cells known as melanocytes.
Not Enough Scientific Evidence to support
 It is possible that there is a gene variation that can cause
purple eyes, lighter skin and allows people to remain
healthy so they live longer, but there is not necessarily
enough evidence that this specific condition exists today.
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The traits of Alexandria’s Genesis appear to be too
widespread to be part of a single gene, which means they
would have to stem from several unrelated genes. Some of
the traits associated with this disorder are physiologically
impossible.
• One could not fail to produce waste because toxins would build up in
the body, causing death.
• Fair skin that resists burning is also impossible because the only way
to protect the skin is by the presence of melanin that makes the skin
darker.
• Primarily responsible for skin color, also for eye
color, and hair color.
• The difference in skin color between lightly and
darkly pigmented individuals is due not to the
number (quantity) of melanocytes in their skin,
but to the melanocytes' level of activity
• Tyrosinase is required for melanocytes to
produce melanin from the amino acid tyrosine.
Eumelanin: There are two types are recognized:
black and brown. Black melanin is darker than
brown. A small amount of black eumelanin in the
absence of other pigments causes grey hair. A small
amount of brown eumelanin in the absence of other
pigments causes yellow (blond) color hair.
 Pheomelanin: imparts a pink to red hue and, thus, is
found in particularly large quantities in red hair.
Pheomelanin is particularly concentrated in the lips
and nipples.
 How they differ from each other is their chemical
structure.
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Albinism results from inheritance of recessive gene alleles and is
known to affect all vertebrates, including humans.
Albinos lack an enzyme called tyrosinase.
 The
environment interacts with genotype.
• Phenotype is a combination
of genotype and environment.
•The sex of sea turtles
depends on both genes
and the environment
(temperature dependent- sex
determination)
•Occurs in embryonic stage. Males are
produced at lower temperatures than
females.
•Height is an example of a
phenotype strongly affected
by the environment.