Sex linkage refers to traits related to an individual's sex chromosomes. Genes on the X or Y chromosome are sex-linked and their inheritance differs between males and females. Females have two X chromosomes and can be homozygous or heterozygous for X-linked genes, while males only have one X chromosome and are hemizygous. The most common type is X-linked recessive inheritance where defective genes on the X chromosome cause traits mostly in males.

2. Sex linkage is the phenotypic expression of an
allele related to the chromosomal sex of the
individual. This mode of inheritance is in
contrast to the inheritance of traits on autosomal
chromosomes, where both sexes have the same
probability of inheritance.

3. In mammals, XY develops testicles, which secrete male sex
hormones, and the fetus develops into a male. An XX fetus
develops into a female. Thus sperm can be either X or Y;
ova are always X.
Sex linked inheritance involves genes located on either the
X or the Y chromosome. Females can be homozygous or
heterozygous for genes carried on the X chromosome; males
can only be hemizygous.

4. X-linked recessive:
The most common type of sex-linked inheritance involves
genes on the X chromosome, which behave more or less as
recessives.
Females, having two X chromosomes, have a good chance
of having the normal gene on one of the two. Males,
however, have only one copy of the X chromosome - and the
Y chromosome does not carry many of the same genes as the
X, so there is no normal gene to counter the defective X.
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5. An example of this type of inheritance is color blindness in
human beings. Using lower case letters for affected, we
have
Affected male: xY Color blind
Non-affected males XY Normal color
vision
Affected female xx Color blind
Carrier female xX Normal color vision

6. THE POSSIBLE MATINGS:
xY to xx (both parents affected) xx females and xY males, all offspring affected.
xY to Xx (affected father, carrier mother) half the females will be xX and carriers, half will be xx
and affected. Half the males will be XY and clear, half will be xY and affected.
xY to XX (affected father, clear mother) all male ofspring XY clear, all daughters Xx carriers.
Note that the daughters of an affected male are obligate carriers or affected. The unaffected sons
of an affected male cannot carry the problem.
XY to xx (father clear, mother affected) xY males (affected) and xX daughters (carriers.)
XY to Xx (father clear, mother carrier) half the males affected (xY) and half clear (XY); half
females clear (XX) and half carriers (Xx)
XY to XX (father and mother both genetic clears) all offspring clear.
Note that all female offspring of affected males are obligate carriers (if not affected.) Likewise,
any female who has an affected son is a carrier. Non-affected sons of affected fathers are
genetically clear.
This type of inheritance may be complicated by the sublethal effect of some X-linked genes.
Hemophilia A in many mammals (including dogs and people) is a severe bleeding disorder
inherited just like the color-blindness above. Many affected individuals will die before breeding,
but for those who are kept alive and bred for other outstanding traits, non-affected sons will not
have or produce the disease. All daughters, however, will be carriers.

7. X-Linked Dominant Inheritance
A male or female child of a mother affected with an
X-Linked dominant trait has a 50% chance of inheriting
the mutation and thus being affected with the disorder.
All female children of an affected father will be
affected (daughters possess their fathers' X-
chromosome). No male children of an affected father will
be affected (sons do not inherit their fathers' X-
chromosome).

8. Y-linked inheritance:
The Y chromosome in most species is very short with very
few genes other than those that determine maleness. Y-linked
inheritance would show sons the same as their fathers, with no
effect from the mother or in daughters.
In humans, hairy ears appear to be inherited through the Y
chromosome. Padgett does not list any known problem in dogs
as being Y-linked.
♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂♂

9. XX The symbol used for females
•DD/Dd= normal
XY The symbol used for males (dominant trait)
•dd= diabetic
(recessive trait)
XD XD Homozygous normal female
XD X d Heterozygous normal female (carrier)
HOMOGAMETIC =
having same sex
chromosomes
HETEROGAMETIC =
Xd Xd Homozygous affected female
having different sex
chromosomes
XD Y Normal male HEMIZYGOUS = is the
condition with one kind
Xd Y Affected male

10. Ex. 1 A Heterozygous normal female marries a
diabetic male.
XD Xd x
Xd Y
XD Y XD Xd Xd Xd Xd Y
1 : 1 : 1 : 1
Ex. 2 A homozygous diabetic female marries a
normal male.
Xd Xd x XD Y
XD Xd Xd Y XD Xd Xd Y
2 : 2 ► both phenotypic ratio and genotypic ratio
► all of their son will be diabetic

11. Sex-Linked Inheritance Problem Set
The study of inheritance of genes located on sex
chromosomes were pioneered by T. H. Morgan and
his students at the beginning of the 20th century.
Although Morgan studied fruit flies, the same genetic
principles apply to humans. Since males and females
differ in their sex chromosomes, inheritance patterns
for X-chromosome linked genes vary between the
sexes.

12. Problem 1: Crossing a white-eyed
female and red-eyed male fly
In a cross between a white-eyed
female fruit fly and red-eyed male,
O % of the female offspring will
have white eyes.
Problem 2: Another white-eyed
female x red-eyed male fly cross
All of the females will have red
eyes; all of the males will have
white eyes.
All of the females are red-eyed and
heterozygous. All of the males are
white-eyed and hemizygous.

13. Genotypes and phenotypes of parents
The female parent must be
homozygous because she
has the recessive white-
eyed phonotype.
The male parent is
hemizygous, red-eyed.

14. Genotypes and phenotypes of offspring
All of the females eggs will
contain an X chromosome with
the white-eye mutation.
The sperm will contain either a
normal X chromosome or a Y
chromosome.

15. Summary
We use a Punnett Square to predict the outcome of this
cross Female offspring receive an X chromosome from both
the sperm and egg. All females receive the dominant, red-eyed
allele from their fathers and the recessive, white-eyed allele
from their mothers.