Transcript Human Genetics and Pedigrees

Human Genetics and
Pedigrees
14.1 Human Heredity
Human Chromosomes
 Karyotype: a picture of chromosomes,
paired, and put in order of increasing
size.
 Sex chromosomes: determine the sex
of the organism.
 Females have XX
 Males have XY
 Autosomes: the remaining
chromosomes; carry all other traits.
Human Chromosomes, cont.
 Human egg cells can only carry an X.
 Sperm cells can carry either an X or a
Y.
 This ensures that just about half of
the zygotes will be 46, XX and half
will be 46, XY.
Human Traits
 Pedigree: shows the relationships within a
family, can be used to help.
 Analyze these to infer the genotypes of family
members.
 Environmental influences
 Genes may be denied a proper environment in
which to reach full expression.
 However, these same genes can, in a proper
environment, achieve full potential in a later
generation.
Blood Group Genes
 Most common are the ABO blood
groups and the Rh blood groups.
 Rh – either positive or negative.
 Positive is dominant.
 If Rh+Rh+ or Rh+Rh- the blood type will be
Rh+
 The ABO blood group is more
complicated. There are three alleles for
this gene, IA, IB, i.
Recessive Alleles
 the presence of a normal,
functioning gene is revealed only
when an abnormal or
nonfunctioning allele affects the
phenotype.
 One of the first genetic disorders
to be understood this way was
phenylketonuria or PKU.
 Lack the enzyme that is needed to
break down phenylalanine.
 Phenylalanine is an amino acid
found in milk and other foods.
 Autosomal recessive allele found on
chromosome 12.
Dominant Alleles
 If you have a dominant allele for a
genetic disorder, it will be expressed.
 Ex: dwarfism
Codominant Alleles
 Ex. Sickle cell anemia
Assignment
 Copy the chart on pg. 345 of the
autosomal disorders in humans.
14.2 Human Chromosomes
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Human Genes and Chromosomes
Sex-Linked Genes
X-Chromosome Inactivation
Chromosomal Disorders
Human Chromosomes and Genes
 Genes located close together on the
same chromosome are linked,
meaning that they tend to be
inherited together.
Sex-linked Genes
 Sex-linked genes: genes located on
the x and y chromosomes.
 There are several sex-linked genetic
disorders.
 Colorblindness
 Hemophilia
 Duchenne Muscular Dystrophy
Colorblindness
 Colorblindness: the inability to distinguish certain colors.
 Located on the X chromosome
 Males have just one X chromosome. Thus, all X-linked
alleles are expressed in males, even if they are
recessive.
 In order for a recessive allele to be expressed in females,
there must be two copies of the allele, one on each of
the two X chromosomes.
 Means more common in males than in females.
Hemophilia
 Hemophilia: a
disorder in which a
protein necessary for
normal blood clotting
is not produced.
 People can bleed out
from minor cuts and
scrapes.
 Can be treated by
injections of this
protein.
Duchenne Muscular Dystrophy
 Results in the progressive weakening
and loss of skeletal muscle.
X-Chromosome Inactivation
 In females, one of the X
chromosomes is randomly “turned
off”
 Forms a dense region in the nucleus
called a Barr body.
Chromosomal Disorders
 Nondisjunction: when homologous
chromosomes fail to separate during
meiosis.
 Causes abnormal numbers of
chromosomes in gametes, and a
disorder of chromosome numbers
may result
Down Syndrome
 If two copies of an autosomal chromosome
fail to separate during meiosis, an
individual may be born with three copies of
a chromosome.
 This is known as a trisomy.
 Three copies of chromosome number 21.
 Produces mild to severe mental retardation
and characterized by an increased
susceptibility to many diseases and a
higher frequency of some birth defects.
Sex Chromosome Disorders
 Turner’s Syndrome: female only inherits
one X chromosome  sterile, sex organs do
not develop. (X)
 Klinefelter’s: males  extra X chromosome
interferes with meiosis and usually prevents
these individuals from reproducing (XXY)
 No baby has ever been born without an X
chromosome showing that it is vital to
embryo development.
Sex-linked practice problem
 If a hemophiliac mother is crossed
with a normal father, what percentage
of their children will be hemophiliacs?