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SBI 3U Mrs. Holterman/Ms. Powell Name: ___________________ SEX-LINKED GENES The determination of sex is based on the inheritance of a certain combination of chromosomes. In humans, there are 46 chromosomes (23 pairs). Twenty-two of these pairs are known as autosomes. These contain genes for many traits, but are not related to the sex of the individual. The twenty-third pair is known as the sex chromosomes or allosome. Females have a homologous pair of X chromosomes (genotype XX), while males have the XY combination. The X chromosome carries genes for many traits, while the Y chromosome does not carry these genes. Therefore the phenotype of the females will be determined by the combination of two alleles, while the phenotype of the males will be determined by the allele present on their one X chromosome. Characteristics of sex-linked traits: 1. The sex-linked recessive phenotype will occur more frequently in males because they cannot hide it a second, dominant allele. 2. Females have two copies of these genes, so if one copy is the recessive allele, they may still have the dominant phenotype. 3. Heterozygous females are called carriers since they can pass sex-linked traits to their children. 4. A female must be homozygous recessive to display the recessive phenotype (rare). Example: In humans, a disease called Duchenne-type muscular dystrophy is caused by a sex-linked recessive allele, while the normal condition results from the dominant allele. If a female carrier marries a normal male, what are the possible genotypes for their children? Answer: SBI 3U Mrs. Holterman/Ms. Powell Name: ___________________ Let XN = normal, Xn=Duchenne-type muscular dystrophy, Parents: XNXn x Y=male XNY Punnett Square: XN Xn XN XNXN XNXn Y XNY XnY Answer: 1 in 4 children will probably have DMD. None of the girls will have it, but half of the boys probably will.