Download X-Linked Characteristics

LECTURE
CONNECTIONS
4 | Sex Determination and Sex-Linked
Characteristics
© 2009 W. H. Freeman and Company
Concept Check 3
What was the genotype of few live F1 red-eyed males
obtained by Bridges when he crossed a white-eyed
female with a red-eyed male?
a.
b.
c.
d.
X+
XwX+Y
X+Y
X+X+Y
X-Linked Characteristics
• The human eye actually detects only three colors: blue
(chromosome 7); red, and green (both at the X
chromosome).
• The human brain mixes the signals from different cone
cells to create the wide spectrum of colors that we
perceive.
• X-linked color blindness in humans: recessive
characteristic
X+ allele for normal color vision
Xc allele for red-green color blindness
Betty has normal vision but her mother is color blind.
Bill is color blind. If Betty and Bill marry and have a child
together, what is the probability that the child will be color
blind? A carrier? Normal?
Concept Check 4
Hemophilia (reduced blood clotting) is an X-linked
recessive disease in humans. A woman with
hemophilia mates with a man who exhibits normal
blood clotting.
1)
What is the probability that their child will have
hemophilia?
2)
What is the probability that their male child will have
hemophilia?
X-Linked Characteristics
•
The presence of different numbers of X chromosomes
in males and females presents a special problem in
development.
•
Dosage compensation: the amount of protein produced
by X-linked genes
Fruit flies: males double the activity of the genes on the X
chromosomes
C. elegans: both X chromosomes in females are half active
Mammals: one X chromosome is randomly inactivated in
females
Female cell
Which X chromosome
is inactivated is
random!!!
Male cell
Females are mosaic for the
expression on X-linked
genes.
• Tortoiseshell cats: X+X0
• Black cats: X+X+
• Orange cats: X0X0
4.2 Sex-Linked Characteristics Are Determined
by Genes on the Sex Chromosomes
• Z-linked characteristics
• Y-linked characteristics:
- are presented only in males;
- all male offspring of a male with a Y-linked thait will
display the trait.