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Two copies of each autosomal gene affect
phenotype (physical).
• Mendel studied
autosomal gene traits,
like hair texture.
Autosome –
chromosome with genes
not related to sex of
organism (body cells)
– Carrier – has an allele for a trait or disease that is not
expressed.
– Carrier does not have disease symptoms but can
pass it on to offspring.
Dominant allele
disorders are rare.
Huntington’s disease is
an example of a disease
caused by a dominant
allele.
(dominant)
• Genes on sex chromosomes are called sex-linked genes.
.
– Y chromosome - male characteristics .
– X chromosome - genes affects many traits.
Males can pass on X or Y
Females only pass on X
Who determines the sex of the
offspring?
Father – he can provide an X or
Y chromosome
Egg
X
X
XX
XX
X
X
Body Cell
X
XX
XX
XY
XY
X
XY
Body Cell
Y
Y
Sperm
1female:1male
• Males have an XY genotype.
– All of a male’s
sex-linked genes
are expressed.
– Males have no
second copies of
sex-linked genes
– Y chromosome is
much smaller
• Females have an XX genotype.
X chromosome inactivation -randomly “turns off”
one X chromosome.
Why are males more likely than females to have genetic disorders?
All sex-linked
genes are
expressed, even
recessive.
Females have a
backup X
chromosome.
• Color blindness is a
problem in which red or
green look like shades
of gray or other colors.
• The gene is carried on
the X chromosome and
is a recessive trait.
XCXC = normal female
XCXc = female, normal vision
(carrier)
XCY = normal vision male
XcY = color blind male
XC
XC
Y
Xc
XCXC
XCXc
XCY
XcY
• Some traits are neither totally dominant nor totally
recessive.
• Incomplete dominance - when neither gene is totally
dominant to the other
- Heterozygous phenotype is intermediate between the
two homozygous phenotypes
– Example: White flowers and red flowers produce
pink flowers
Codominance
Disease in which the
body makes sickleshaped red blood cells.
Sickle-shaped cells don’t
move easily through your
blood vessels. They’re
stiff and sticky and tend
to form clumps and get
stuck in the blood vessels
.
•
Sickle Cell Anemia
• R = Round blood cells
• R’ = Sickle Cells
R
R
R’
RR
RR’
RR’
R’R’
RR = normal blood
RR’ = some sickle cells, some
normal cells
R’R’ = has sickle cell anemia
R’
• Codominant - alleles will both be completely expressed.
– Codominant
alleles are
neither
dominant nor
recessive.
– The ABO blood
types result
from
codominant
alleles.
Example – red and white flower produce a
flower with BOTH colors
• Many genes have more than two alleles.
• Polygenic traits
are produced
by two or more
genes.
Order of dominance:
brown > green > blue.
• Epistatic gene - can interfere with the
expression of all other genes.
Mice have 5
genes that
control fur
color.
2 genes for
general color
1 for shading
1 for spots
1 epistatic
gene for color
that overrules
all other genes
• Phenotype is a combination
of genotype and
environment.
• The sex of sea turtles
depends on both genes
and the environment.
Warm eggs develop into
females
• Height is an example of a
phenotype strongly affected
by the environmental factors
such as early nutrition and
health care.
Gene linkage was explained through fruit flies.
• Morgan found that linked traits are on the
same chromosome. Traits can be inherited
as a group.
• Chromosomes, not genes, assort
independently during meiosis.
Wild type
Mutant
• Linked genes are not inherited together
every time.
• Chromosomes exchange homologous genes during
meiosis.
Linkage maps – map of location of genes on a chromosome.
• The closer together two genes are, the more likely
they will be inherited together.
• Cross-over frequencies are related to distances
between genes.
• Cross-over frequencies can be converted into map units.
– gene A and gene B cross over 6.0
percent of the time
– gene B and gene C
cross over 12.5 percent
of the time
– gene A and gene C cross over 18.5 percent of the
time
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 chromosome carries about 1100 genes while the Y carries about 250
Pedigree - chart for tracing genes in a family.
• Phenotypes are used to infer genotypes on a pedigree.
• Autosomal genes show different patterns on a pedigree
than sex-linked genes.
Widow’s peak: W = widow’s peak
w = non widow’s peak
• If the phenotype is more common in males,
the gene is likely sex-linked.
Colorblindness: M = normal vision m = colorblindness
Several methods help map human chromosomes.
• Karyotype - a picture of all chromosomes in a cell.
XY
• Karyotypes can show changes in chromosomes.
– deletion of part of a chromosome or loss of a
chromosome
– large changes in chromosomes
– extra chromosomes or duplication of part of a
chromosome