Transcript Sex Chromosomes

Midterm
Friday - or - Monday?
Covers Chapters 1,2 and 3.6
If yed, it will mean moving Exam 2 to May 6.
Sex Chromosomes
... ‘X’ and ‘Y’ chromosomes that determine
the sex of an individual in many organisms,
Females: XX
Males: XY
a
A
a
Differential
Region
Differential Region
Paring Region
Paring Region
male
XX: female
a
hemizygous: condition
where gene is present in
only one dose.
male
X Linkage
…the pattern of inheritance resulting from
genes located on the X chromosome.
X-Linked Genes
…refers specifically to genes on the Xchromosome, with no homologs on the Y
chromosome.
P
x
Blue
Female
Gametes
Pink
Male
or
Gametes
or
F1
Blue Female
Blue Male
F1
x
Blue Female
Gametes
or
Blue Male
or
Gametes
or
or
F2
Blue Female
Blue Male
Blue Female
Pink Male
F2
Blue Female
Blue Male
Blue Female
3:1
Pink Male
P
x
Pink
Female
Gametes
Blue
Male
or
Gametes
or
F1
Blue Female
Pink Male
Gametes
or
or
F2
Pink Female
Pink Male
Blue Female
Blue Male
F2
Pink Female
1
Pink Male
Blue Female
Blue Male
1
1
1
Sex Linkage to Ponder
• Female is homozygous recessive X-linked gene,
– what percentage of male offspring will express?
– what percentage of female offspring will express if,
• mate is hemizygous for the recessive allele?
• mate is hemizygous for the dominant allele?
• Repeat at home with female heterozygous Xlinked gene!
Sex-Linked vs. Autosomal
• autosomal chromosome: non-sex linked
chromosome,
• autosomal gene: a gene on an autosomal
chromosome,
• autosomes segregate identically in
reciprocal crosses.
X-Linked Recessive Traits
Characteristics
• Many more males than females show the
phenotype,
– female must have both parents carrying the allele,
– male only needs a mother with the allele,
• Few (or none) of the offspring of affected males
show the disorder,
– all of his daughters are carriers,
• half of the sons born to these daughters are carriers.
X-Linked Dominant
• Affected males married to unaffected females pass the
phenotype to their daughters, but not to their sons,
• Heterozygous females married to unaffected males pass the
phenotype on to half their sons and daughters,
• Homozygous dominant females pass the phenotype on to
all their sons and daughters,
Autosomal Dominant
• Phenotypes appear in every generation,
• Affected males and females pass the
phenotype to equal proportions of their sons
and daughters.
Pedigree for Very Rare Trait
1/2
1/2
Recessive?
---> Yes!
X-Linked?
Autosomal?
---> Yes!
1/2 x 1/2 x 1/2
? = 1/8
Linkage
• Genes located on the same chromosome
segregate together and are said to be linked.
Independent Assortment
A
A
a
B
B
b
A
b
2n = 4
a
B
a
b
Meiosis Prophase I
cont.
A a A a
B B b b
Chiasmata and crossing over
2n = 1
Meiosis
No Cross Over
A
a
B
b
Parent Cell
A
A
a
a
B
B
b
b
Daughter Cells Have Parental Chromosomes
2n = 1
Meiosis
With Cross Over
A
a
B
b
Parent Cell
A
A
a
a
B
b
B
b
Daughter Cells Have Recombinant Chromosomes
Gamate Formation in F1 Dihybrids
P: GGWW x ggww
F1 Genotype:
GgWw
G
g
W
w
alleles
GW
Gw
gW
gw
gametes
.25
.25
.25
.25
probability
How do you test for
assortment of alleles?
F1: GgWw
GW
Gw
gW
gw
.25
.25
.25
.25
Test Cross
• phenotypes of the offspring indicate the
genotype of the gametes produced by the
parent in question.
Test Cross
GgWw x ggww
GW (.25)
gw (1)
GgWw (.25)
Gw (.25)
gw (1)
G gww (.25)
gW (.25)
gw (1)
ggWw (.25)
gw (.25)
gw (1)
ggww (.25)
Test Cross
GgWw x ggww
GW (.25)
gw (1)
GgWw (.25)
P
Gw (.25)
gw (1)
Ggww (.25)
R
gW (.25)
gw (1)
ggWw (.25)
R
gw (.25)
gw (1)
ggww (.25)
P
parental types GgWw and gwgw
recombinant types Ggww and ggWw
P: GGWW and ggww
Recombination Frequency
…or Linkage Ratio: the percentage of
recombinant types,
– if 50%, then the genes are not linked,
– if less than 50%, then linkage is observed.
Linkage
• Genes located on the same chromosome do
not recombine,
– unless crossing over occurs,
• The recombination frequency gives an
estimate of the distance between the genes.
Recombination Frequencies
• Genes that are adjacent have a recombination frequency
near 0%,
• Genes that are very far apart on a chromosome have a
linkage ratio of 50%,
• The relative distance between linked genes influences the
amount of recombination observed.
Dihybrid Cross
P
yellow/round
GGWW
green/wrinkled
x
GW
F1
ggww
gw
GgWw
phenotype
genotype
gametes
genotype
Questions
• Don’t rely on the answers in the back of the book
to solve your problems…
– Don’t just solve them, but understand the principles
needed to solve them.
• More practice questions are online. No answers
are available, i.e. confirm your own answers
a
b
f
c
e
d