Download sex-linked gene

recessive
Sickle-Cell Disease
recessive
• Sickle-cell disease affects one out of 400
African-Americans
• The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in
red blood cells
• Symptoms include physical weakness, pain,
organ damage, and even paralysis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-UN2
Degree of dominance
Complete dominance
of one allele
Example
Description
Heterozygous phenotype
PP
same as that of homozygous dominant
Pp
Incomplete dominance Heterozygous phenotype
intermediate between
of either allele
the two homozygous
phenotypes
C RC R
Codominance
Heterozygotes: Both
phenotypes expressed
C RC W C WC W
IAIB
Multiple alleles
In the whole population, ABO blood group alleles
some genes have more
IA , IB , i
than two alleles
Pleiotropy
One gene is able to
affect multiple
phenotypic characters
Sickle-cell disease
polydactyly
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive alleles
• For example, one baby out of 400 in the United
States is born with extra fingers or toes
Photos: eHand.com (the electronic textbook for hand surgery)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Blood Typing Problem
Concept 15.2: Sex-linked genes exhibit unique
patterns of inheritance
In humans and some other animals, there is a
chromosomal basis of sex determination
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Females are XX, and males are XY
Each ovum contains an X chromosome, while a
sperm may contain either an X or a Y
chromosome
Other animals have different methods of sex
determination
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance of Sex-Linked Genes
The sex chromosomes have genes for many
characters unrelated to sex
A gene located on either sex chromosome is
called a sex-linked gene
In humans, sex-linked usually refers to a gene on
the larger X chromosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sex-linked genes follow specific patterns of
inheritance
For a recessive sex-linked trait to be expressed
A female needs two copies of the allele
A male needs only one copy of the allele
Sex-linked recessive disorders are much more
common in males than in females
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 15-5
X
Y
Barr body – an inactivated X chromosome
in each of a female’s diploid cells.
Correlating Behavior of a Gene’s Alleles with
Behavior of a Chromosome Pair
In one experiment, Morgan mated male flies with white eyes
(mutant) with female flies with red eyes (wild type)
The F1 generation all had red eyes
The F2 generation showed the 3:1 red:white eye ratio,
but only males had white eyes
Morgan determined that the white-eyed mutant allele must
be located on the X chromosome
Morgan’s finding supported the chromosome theory of
inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Chromosomal Basis of Sex
In humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome
Only the ends of the Y chromosome have
regions that are homologous with the X
chromosome
The SRY gene on the Y chromosome codes for
the development of testes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 26.14b
Sex-linked recessive
Sex-linked recessive
pingrybiology.pbwiki.com
www.kelleycom.com
Hemophilia is a rare, inherited bleeding disorder in which
blood doesn’t clot normally. People with hemophilia may
bleed for a longer time than others after an injury. Individuals
with the disorder may bleed internally, especially in knees,
ankles, and elbows. This bleeding can damage organs or
tissues and, sometimes, be fatal.
The European Royal Family
Photo: www.freewebs.com/georgeromanov/Foto%201913.jpg
Color Blindness
some forms are
Sex-linked
Fig. 26.18