Color blindness is a sex-linked trait in humans

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Color blindness is a sex-linked trait in humans

• In humans, the X chromosome has about 5,000 genes
  while the Y chromosome has only 30 genes
• Females are XX, males are XY
• There is no locus on the Y chromosome for the
  color vision/color blindness trait
• Color blindness is far more prevalent in males
  because there is not a second allele to mask the
  recessive allele that causes color blindness

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Hemophilia
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Which of a man's grandparents could not be the
source of any of the genes on his Y-chromosome?
A. Father's Mother. B.
Mother's Father. C. Father's Father.
D. Mother's Mother, Mother's Father, and
Father's Mother. E. Mother's Mother.
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Pedigree analysis
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Nondisjunction
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Turners Syndrome
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Klinefelter Syndrome
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Sex determination in Drosophila

• Although males have an X and a Y chromosome, the
  Y chromosome does not determine the sex of a
  fruit fly
• Rather, the number of X chromosomes determines
  the sex
• Fruitflies with two X chromosomes become female
• Fruitflies with one X chromosome become male
• Through nondisjunction, a fruitfly with only an X
  chromosome but no Y will become male a fruitfly
  with two X chromosomes and a Y chromosome will
  become female

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Sex Determination in Birds

• Female is heterogametic sex

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Sex determination in honeybees

• Diploid individuals are females
• Haploid individuals are males

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In some reptiles, sex determination is
environmental

• Some species of turtles
• Above 32 oC, females produced
• Below 28 oC, males produced
• Between 28 and 32 oC, mixture of males and
  females
• However, snapping turtle eggs incubated either
  below 20 oC or above 30 oC are female males
  produced predominantly in between temperatures
• Note that although environmental factors trigger
  the sexual developmental pathway, the pathways
  themselves are under genetic control

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Figure 10.15a,b
Linkage hypothesis
Morgans test of the hypothesis
Female
Male
Parental generation
Red eyes
White eyes
w
w
w
w
X
wy/ wy
wy / Y
Yellow body
Gray body
y
y
y
y
Female
Male
Meiosis I
F1 generation
X
w
w
w
w
wy/ Y
wy/ wy
y
y
y
y
Phenotype
Genotype
Number
F2 generation males
Meiosis II
4292
wy /Y
w
w
w
w
wy /Y
4605
Gametes
y
y
y
y
wy /Y
86
86
Novel genotypes
wy /Y
44
wy
wy
Hypothesis When two loci occur on one
chromosome, meiosis results in two, rather than
four, types of gametes because independent
assortment does not occur.
Results There are four kinds of gametes rather
than two due to occurrence of two recombinant
genotypes.
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Figure 10.15c
Crossing over provides an explanation
w
w
w
w
y
y
y
y
Crossing over during meiosis I
w
w
w
w
y
y
y
y
Meiosis II
W
w
w
w
Gametes
y
y
y
y
wy
wy
wy
wy
Recombinant chromosomes
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Figure 10.16
Gene 1
Crossing over rarely occurs between adjacent
loci, so recombinations are rare.
0 Yellow body
0 Yellow body
0 Yellow body
Gene 2
1.4 White eyes
1.4 White eyes
Gene 3
Gene 4
Map units
Gene 5
Gene 6
Gene 7
20 Cut wings
Crossing over almost always occurs
between distant loci, so recombinations are
frequent.
Gene 8
Gene 9
Gene 10
Gene 11
Gene 12
The physical distance between loci determines the
frequency of crossing over.
Frequency of crossing can be used to map physical
between loci.
Linkage map
Chromosomes are composed of genes