Post on 19-Dec-2015
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
X-Linked Genes
…refers specifically to genes on the X-chromosome, with no homologs on the Y chromosome.
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 X-linked 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 TraitsCharacteristics
• 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.
Recessive? ---> Yes!
Pedigree for Very Rare Trait
Autosomal?X-Linked? ---> Yes!
1/2
1/2
1/2 x 1/2 x ? 1/2 = 1/8
MeiosisNo Cross Over
Parent Cell
Daughter Cells Have Parental Chromosomes
A a
B b
A
B
a
b
a
b
A
B
2n = 1
MeiosisWith Cross Over
Parent Cell
Daughter Cells Have Recombinant Chromosomes
A a
B b
A
B
A
b
a
b
a
B
2n = 1
Gamate Formation in F1 Dihybrids P: GGWW x ggww
G g W w
GW Gw gW gw
alleles
gametes
probability.25 .25 .25 .25
F1 Genotype: GgWw
Test Cross
• phenotypes of the offspring indicate the genotype of the gametes produced by the parent in question.
Test CrossGgWw x ggww
GW (.25)
Gw (.25)
gW (.25)
gw (.25)
gw (1)
gw (1)
gw (1)
gw (1)
G gww (.25)
GgWw (.25)
ggWw (.25)
ggww (.25)
Test CrossGgWw x ggww
GW (.25)
Gw (.25)
gW (.25)
gw (.25)
gw (1)
gw (1)
gw (1)
gw (1)
Ggww (.25)
GgWw (.25)
ggWw (.25)
ggww (.25)
P
P
parental types GgWw and gwgw P: GGWW and ggww
R
R
recombinant types 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
yellow/round green/wrinkled
GGWW x ggww
GW gw
GgWw
phenotype
genotype
gametes
genotype
P
F1
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