Download Ch 7 Mendel Powerpoint

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)
Compared to:
Sex Chromosomes: control the
development of sexual characteristics
– 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. Does
not have
disorder
Dominant allele
disorders are rare.
Huntington’s disease is
an example of a disease
caused by a dominant
allele.
Has
disorder
(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
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
How are sex-linked genes expressed
differently in phenotypes of males & females?
• Males have an XY genotype; therefore...
– All of a male’s sex-linked
genes are expressed.
– Males have no second
copies of sex-linked
genes to mask the
effects of another allele.
Therefore, you will see
sex linked genetic
disorders more often in
males than females.
– 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 sex-linked genetic
disorders?
All sex-linked genes are expressed in males,
even recessive. Females have a backup X
chromosome which can mask genetic disorders.
Page 203
Can you explain what
is going on in this
picture?
Why is there a color
variation between the
female & male calico
cats?
The female calico cats have two chromosomes
with different alleles for fur color. Both alleles
are expressed in a random pattern. The male
cat has only one X chromosome, and it’s allele
for fur color is expressed across the entire
body.
Xº = Orange fur Xº = Black fur allele
• 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.
XC
XC
Y
XCXC = normal female
Xc
XCXC
XCXc
XCY
XcY
XCXc = female, normal vision
(carrier)
XCY = normal vision male
XcY = color blind male
Neither female are colorblind – 1 female is a
carrier however
1 male born normal & 1 male born colorblind
• Some traits are neither totally dominant nor totally
recessive.
• Incomplete dominance - when neither gene/allele is
completely dominant nor completely recessive to the
other
- Heterozygous phenotype is intermediate between the
two homozygous phenotypes
– Example: White flowers and red flowers produce
pink flowers
Codominance
When both alleles of a gene are expressed- neither is dominant or recessive
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.
(b/c could quickly and cheaply grow new generations)
• Morgan found that linked traits are on the
same chromosome. Traits can be inherited
as a group.
• Chromosomes, not genes, assort
independently during meiosis.
Mutant
Wild type
Common
phenotype
Different
phenotype
Famous Scientist
Mendel said:
Genes assort
independently of
one another.
Punnett & Bateson
said: (invented the
punnett square)
Suggested some
genes are linked
together
Morgan said:
Chromosomes, not
genes assort
independently.
Sturtevant (Morgan’s
student) said:
frequency of crossovers was related to
the distance bw genes.
• Linked genes are not inherited together
every time.
•
•
Gene linkage: the tendency: for genes located close together on a
chromosome to be inherited together.
Chromosomes exchange homologous genes during meiosis.
Genes
Close
together
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. How frequently a cross over will
occur is related to the distance bw the 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
Study Guide worksheet page 44
Linkage Map 7.3
Cross-Over
Frequencies:
A-B 20%
B-C 5%
A
D
B
C
0
7
20
25
A-C 25%
A-D 7%
D-B 13% 20-7=13
D-C 18% 25-7=18
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 used 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.
How can you
tell if a
chromosome
is a autosome
or a sex
chromosome?
If the same #
of male &
female have
the phenotype
then the gene
is most likely
autosome.
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. Shows
changes in chromosomes. Ex: if you have an extra
chromosome
Two methods
used to study
human
chromosomes:
1.Pedigrees
2. Karyotypes
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