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Sex Linked Traits Notes Handout by cuiliqing


									Sex-Linked Traits Notes

Remember from the past that ______ chromosomes determine the sex of an individual.

Sex          Sex Chromosome                                     Diagram
Female       Two X chromosomes                                  XX
Male         One X chromosome and One Y chromosome              XY

Sex Determination in Humans:

What is the phenotype percentage? _______________

What is the genotype percentage? _______________

Sex Linked traits: are traits for which genes are on the X chromosome, occurs in humans.

Females will have a total of ________ allele(s) for a particular gene carried on the X chromosome.

Males will have a total of _________ allele(s) for a particular gene carried on the X chromosome.

Female                                                                  Male

Types of Inheritance:

X-linked Dominant: dominant allele carried on one or both of the X-chromosomes will cause expression
of the phenotype
X-linked Recessive: recessive alleles must be present on both X-chromosomes for a woman in order for
the phenotype to be expressed, whereas a man only needs one recessive allele to express the

Note: A carrier is an individual who is heterozygous for a recessive trait. Although the trait does not
appear in the phenotype, the heterozygous individual “carries” the allele

Sex-Linked Inheritance Problem:

Color blindness is a sex-linked recessive disorder. Suppose a heterozygous female (carrier) is crossed
with a normal male. What would be the genotype and phenotype proportions of the F1 generation?

Phenotype: ______       normal females               Genotype:

             ______     normal male

             ______     colour blind male

Pedigree Notes: A pedigree is a family tree that allows you to follow the inheritance of a single gene
Sex Linked Inheritance and Genetic Disorders

X-linked hypophosphatemia is an X-linked dominant disorder that
causes problems in phosphate transport and results in the softening of
bones. Bone deformity and recurrent fractures are common symptoms of
this disease.

X-linked severe combined immunodeficiency (SCID) is an X-linked
recessive disease. It is often called “bubble boy disease” because one
patient, David Vetter, lived in a plastic, germ-free bubble for 12 years.
The defining characteristic of SCID is a severe defect in T- & B-
lymphocytes, which are an important part of the immune system.

Red-Green Colour Blindness is an X-linked recessive disorder. The
retinal cones which perceive color in light do not develop properly, thus
they are not able to transmit the information to the optic nerve. People
who are affected by this disorder are unable to distinguish between red
and green colours. As a result, patients with Red-Green color blindness
are unable to locate the number on this picture.

Haemophilia is the oldest known X-linked recessive bleeding
disorder. Patients with haemophilia have less than 1% of the
normal amount of clotting factors in their blood. A small cut can
cause these patients to bleed excessively or even bleed to death.

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