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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 phenotype. 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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