Transcript r sex

THE CHROMOSOMAL BASIS OF
INHERITANCE
9.17 Chromosome behavior accounts for Mendel’s
principles
• Genes are located on chromosomes
– Their behavior during meiosis accounts for
inheritance patterns
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9.4 Homologous chromosomes bear the two alleles
for each characteristic
• Alternative forms of a gene (alleles) reside at
the same locus on homologous chromosomes
GENE LOCI
P
P
a
a
B
DOMINANT
allele
b
RECESSIVE
allele
GENOTYPE:
PP
aa
HOMOZYGOUS
for the
dominant allele
HOMOZYGOUS
for the
recessive allele
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Bb
HETEROZYGOUS
Figure 9.4
• The chromosomal basis of Mendel’s principles
Figure 9.17
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9.18 Genes on the same chromosome tend to be
inherited together
• Certain genes are linked
– They tend to be inherited together because they
reside close together on the same chromosome
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Figure 9.18
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9.19 Crossing over produces new combinations of
alleles
• This produces gametes with recombinant
chromosomes
• The fruit fly Drosophila melanogaster was used
in the first experiments to demonstrate the
effects of crossing over
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A
B
a
b
a
B
A B
a
b
Tetrad
A
b
Crossing over
Gametes
Figure 9.19A, B
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Figure 9.19C
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9.20 Geneticists use crossover data to map genes
• Crossing over is more likely to occur between
genes that are farther apart
– Recombination frequencies can be used to map
the relative positions of genes on chromosomes
Chromosome
g
c
l
17%
9%
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9.5%
Figure 9.20B
SEX CHROMOSOMES AND SEX-LINKED
GENES
9.21 Chromosomes determine sex in many species
• A human male has one X chromosome and one
Y chromosome
• A human female has two X chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
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(male)
(female)
Parents’
diploid
cells
X
Y
Male
Sperm
Egg
Offspring
(diploid)
Figure 9.21A
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• Other systems of sex determination exist in
other animals and plants
– The X-O system
– The Z-W system
– Chromosome number
Figure 9.21B-D
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9.22 Sex-linked genes exhibit a unique pattern of
inheritance
• All genes on the sex chromosomes are said to be
sex-linked
– In many organisms, the X chromosome carries
many genes unrelated to sex
– Fruit fly eye
color is a
sex-linked
characteristic
Figure 9.22A
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– Their inheritance pattern reflects the fact that
males have one X chromosome and females
have two
– These figures illustrate inheritance patterns for
white eye color (r) in the fruit fly, an X-linked
recessive trait
Female
XRXR
Male
Xr Y
XR
Female
XRXr
Xr
XRXr
Male
XRY
XRY
Xr
XRXR
XrXR
XRY
XrY
R = red-eye allele
r = white-eye allele
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Male
XRXr
XR
XR
Y
Female
XrY
Xr
XR
Y
Xr
XRXr
Xr Xr
Y
XRY
XrY
Figure 9.22B-D
9.23 Connection: Sex-linked disorders affect
mostly males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
Figure 9.23A
– A male receives a single X-linked allele from his
mother, and will have the disorder, while a
female has to receive the allele from both
parents to be affected
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• A high incidence of hemophilia has plagued the
royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
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