Transcript Complex Inheritance and Human Heredity

Complex Inheritance
and Human Heredity
11
The Big Idea
Human inheritance does not
always follow Mendel’s laws.
Main Idea #1
The inheritance of a trait over several generations
can be shown in a pedigree.
Recessive Genetic Disorders
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Remember - recessive traits can be masked by a
dominant trait.
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Therefore, a person who is heterozygous for a
characteristic may be considered a carrier for that trait
since it will not be expressed.
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Even though a carrier is not affected by the trait,
they can pass the trait on to future generations.
In order for the recessive trait to show in the individual,
the person must be homozygous recessive.
Common Recessive Disorders
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Cystic Fibrosis
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Affects the mucus-producing glands, digestive enzymes, and
sweat glands
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Chloride ions are not absorbed into the cells of a person with
cystic fibrosis but are excreted in the sweat
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Without sufficient chloride ions in the cells, a thick mucus is
secreted
The mucus clogs ducts in the pancreas, causes problems with
digestion, and blocks some of the tiny passages in the
respiratory system.
Common Recessive Disorders
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Albinism
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Caused by altered genes, resulting in the absence of melanin
(color pigment) in the hair and eyes
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Common characteristics include: white hair, very pale skin, and
pink pupils
Tay-Sachs Disease
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Caused by the absence of an enzyme responsible for breaking
down fatty acids
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The fats begin accumulating in the brain, destroying brain cells
and mental deterioration.
Dominant Genetic Disorders
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Some genetic disorders are controlled by a dominant allele.
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People who have only one copy of this allele will display the trait.
Only those who are homozygous recessive will not have the
disorder.
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Huntington’s Disease
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Disease that gradually destroys the nervous system.
Symptoms usually begin between the ages of 30 and 50.
Achondroplasia is a genetic condition that causes small body size
and limbs that are comparatively short. This is the most common
form of dwarfism.
Pedigrees
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A pedigree is a diagram that traces the inheritance of a particular
trait through several generations.
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Males are represented by squares, and females by circles.
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One who is affected by the trait will have a symbol that is colored
in, while those that are unaffected by the trait will have an unfilled
symbol.
Main Idea #2
Complex inheritance of traits does not follow
inheritance patterns described by Mendel.
Complex Patterns of Inheritance
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Incomplete Dominance
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The heterozygous phenotype is an intermediate
phenotype between the two homozygous
phenotypes.
Complex Patterns of Inheritance
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Codominance
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Both alleles are expressed in the heterozygous
condition.
Complex Patterns of Inheritance
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Another example of codominance is sickle-cell disease.
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Changes in hemoglobin cause blood cells to
change to a sickle shape.
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People who are heterozygous
for the trait have both normal
and sickle-shaped cells.
Complex Patterns of Inheritance
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Multiple Alleles
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Some traits are controlled by more than 2 alleles for a
particular gene.
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Human blood groups are controlled by 3 alleles - IA, IB, and i.
IA and IB are codominant to each other, and dominant to i.
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There are 4 possible blood types - A (IAIA or IAi), B (IBIB or IBi),
AB (IAIB), and O (ii).
Complex Patterns of Inheritance
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Sex Determination
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Sex chromosomes determine an
individual’s gender.
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The X chromosome is much larger than
the Y chromosome.
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Therefore, it makes sense to assume
that X chromosomes carry more genes
and genetic information than the Y
chromosomes.
Sex-Linkage
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The presence of a gene on a sex chromosome is called
sex linkage.
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Genes found on the X chromosome are called X-linked,
and genes found on the Y chromosome are called Ylinked.
Example of Sex-Linked Trait
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Colorblindness is an X-linked trait, carried by sex chromosomes.
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The allele for colorblindness is represented by XC, while the normal
allele is X.
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A female must have two XC alleles in order to be colorblind,
but if a male carries the XC allele, he will definitely be
colorblind.
Colorblindness
Test
In each of the circles, see if
you can identify a hidden
number. A person with
normal vision will see the
numbers, while a person
who is colorblind will see
only the circle.
Another Example of Sex-Linkage
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Hemophilia is another example of a sex-linked trait.
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This rare disease causes blood to not clot properly.
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Failure of blood to clot may result in death.
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This trait is located on the X chromosome.
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Males are more likely to display this disorder since
they only have to have one copy of the allele.
Polygenic Traits
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Polygenic traits arise from the interaction of multiple
pairs of genes.
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Examples include skin color, height, and body mass.
Environmental Factors
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Sometimes, the environment affects how a trait is
expressed.
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These influences include:
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environmental factors
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diet and exercise
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sunlight and water
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temperature
Twin Studies
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Twin studies can help scientists determine if a
characteristic is caused by genetics or by environmental
conditions (Nature vs. Nurture).
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Traits that appear frequently in identical twins are at
least partially controlled by heredity.
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Traits expressed differently in identical twins are
strongly influenced by environment.
Main Idea #3
Chromosomes can be studied using karyotypes.
Karyotypes
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A karyotype is a picture of a person’s chromosomes.
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Chromosomes are taken from the stage of metaphase and
stained.
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They are then placed in order of decreasing size. The last pair is
the sex chromosome pair.
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Disorders caused by an incorrect number of chromosomes or
malformed chromosomes are the easiest to identify using a
karyotype.
Normal Karyotypes
Telomeres
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Telomere caps consist of
DNA associated with
proteins.
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They serve a protective
function for the structure of
the chromosome.
Nondisjunction
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Homologous chromosomes separate during the final
steps of meiosis to ensure that each developing sperm
or egg receives one copy of each chromosome.
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Nondisjunction is the failure of a pair of homologous
chromosomes to separate properly.
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Resulting sex cells will either have an extra
chromosome (trisomy) or a missing chromosome
(monosomy).
Down Syndrome
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One of the most common disorders
due to nondisjunction is Down
syndrome, also known as trisomy
21.
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Down syndrome occurs when the
person has a third copy of the 21st
chromosome.
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Common features include: short
stature, heart defects, distinctive
facial features, and mental disability.
Disorders Due to Nondisjunction
of Sex Chromosomes
Klinefelter
Syndrome
(XXY)
Jacob’s
Syndrome
(XYY)
Trisomy X
Syndrome
(XXX)
Turner’s
Syndrome
(XO)
Types of Mutations
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Mutations can involve an entire chromosome or a single DNA
nucleotide, and they may take place in any cell.
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Germ-cell mutations occur in an organism’s gametes (germ cells).
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These do not affect that organism, but may be passed on to their
offspring the germ cell becomes fertilized.
Somatic mutations occur in an organism’s body cells and can affect
the organism.
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These mutations cannot be passed on to offspring.
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Some examples include skin cancer and leukemia.
Lethal mutations cause death, usually before birth.
Chromosome Mutations
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Deletion (1)- results in the loss
of a piece of chromosome due
to the breakage of that
chromosome; genetic
information will be lost
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Duplication (2)- results in the
copying of a segment of the
chromosome
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Inversion (3)- a segment of a
chromosome breaks off and
reattaches itself to the
chromosome in a reversed
order
Chromosome Mutations
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Insertion (1) - a segment of
a chromosome breaks off
and reattaches itself to
another homologous
chromosome
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Translocation (2) segments of chromosomes
break off and exchange
places on different
chromosomes
Chromosome Mutations
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Nondisjunction is also a type of chromosome mutation.
It affects the total number of chromosomes instead of
pieces of individual chromosomes.