Transcript Karyotypes and Sex-Linked Traits

Karyotypes and
Sex-Linked Traits
Karyotype
 Karyotype – a picture of
chromosomes
 Autosomes: the first 22 homologous
pairs of chromosomes.
 Autosomes are the same for both males
and females.
 Sex chromosomes: determines the
sex of the individual.
 The sex chromosomes are the 23rd pair
of chromosomes.
Male of Female?
Female: XX
Male: XY
Determining Gender
 Which parent determines the sex of an
offspring – DAD
 Why?
All moms have the genotype XX. When
egg cells are made, they will all carry a
single X chromosome.
All dads have the genotype XY. When
sperm cells are made, 50% will have an
X chromosome and 50% will have a Y
chromosome.
 Therefore, males and females are born in
roughly a 50:50 ratio
Diagnosing Disorders from
Karyotypes
 Some disorders can be diagnosed by looking at a
person’s karyotype
 Most are caused by nondisjuction during meiosis
 Nondisjunction – failure of chromosomes to
separate during meiosis
 Disorders:
 Down Syndrome (Trisomy 21) – individual has 3
of the 21st chromosome instead of 2
 Turner Syndrome – female has only one X for
her sex chromosome
 Klinefelter’s Syndrome – male has or more
extra X chromosome
Sex-Linked Traits
 SEX-LINKED TRAITS: those traits that
are controlled by genes on the X or Y
chromosomes.
 Most sex-linked traits are on the X
chromosome.
 The Y chromosome is much smaller than
the X chromosome and only contains a
few genes.
 Ex:
Hemophelia
Colorblindness
Example
 In humans, hemophilia is a sex-linked trait. Having
hemophilia is recessive (XH) to being normal (X). The
heterozygous female is called a carrier. Cross a carrier
female with a normal male.
XHX
XH
x
XY
X
Genotype Ratio:
X
Phenotype Ratio:
Y
Example
 Cross a carrier female with a male with hemophilia.
XHX
XH
x
XHY
X
Genotype Ratio:
XH
Y
Phenotype Ratio:
Example
 In humans, red-green colorblindness is a sex-linked
trait. People with red-green colorblindness can not tell
the difference between red and green. Colorblindness is
the result of a recessive allele. Cross a female with
colorblindness with a male with normal vision.
XCXC
XC
x
XY
XC
Genotype Ratio:
X
Phenotype Ratio:
Y
Why are sex-linked traits more common
in males than in females?
 Because a male only has to inherit ONE recessive
allele in order to get a sex-linked trait and a female
has to inherit TWO recessive alleles in order to acquire
the sex-linked trait. It is easier to inherit one recessive
allele than two. If the female only inherits one
recessive allele, then they are a carrier but have the
normal phenotype.
 X’s are dominant to Y’s.
If female gets a “bad” from one parent, she could
still get a “good” X, and become a carrier (has the
trait but isn’t expressed). The only way for the
female to express the trait is to inherit two “bad” X’s.
If a male gets a “bad” X, the only other sex
chromosome is a Y. The “bad” X is dominant to the
Y, therefore, the male will express the trait.