Cell Biology
Sex chromosomes carry sex-linked genes. Humans have two types of sex chromosomes: the X and Y chromosomes. Most genetic females have two X chromosomes which are homologous, while most genetic males have one X and one Y chromosome, making them hemizygous. The X chromosome is similar in size to the autosomal chromosomes and carries many genes, while the Y chromosome is smaller and carries fewer genes.
Punnett squares can be used to predict the genotypes of offspring for sex-linked genes. Abnormalities of genes located on sex chromosomes can lead to sex-linked disorders, with the majority arising from mutations of genes on the X chromosome, called X-linked disorders. Only XX heterozygotes can be carriers of recessive X-linked disorders, and their symptoms will be masked by a copy of the dominant allele. In contrast, XY individuals are more prone to inheriting recessive X-linked disorders and tend to show more severe symptoms of dose-dependent X-linked disorders since they don't have a backup X chromosome to produce functional gene products.
Lesson Outline
<ul> <li>Introduction to sex chromosomes</li> <ul> <li>Sex chromosomes: X and Y <ul> <li>XX chromosomes in most genetic females</li> <li>XY chromosomes in most genetic males</li> </ul> </li> <li>Differences in structure and function of X and Y chromosomes <ul> <li>X chromosome: similar size to autosomal chromosomes, carries many genes</li> <li>Y chromosome: smaller, carries fewer genes</li> </ul> </li> </ul> <li>Using Punnett squares for sex-linked genes</li> <ul> <li>Explanation of Punnett square setup for XX and XY parents</li> <li>Different notation for sex-linked genes: Xᴬ, Xᵃ, Xᴬ Y, etc.</li> <li>Phenotypes determined by typical patterns of dominance for XX offspring</li> <li>Phenotypes dictated only by inherited X chromosome allele for XY offspring</li> </ul> <li>Sex-linked disorders <ul> <li>Majority arise from mutations on X chromosome (X-linked disorders)</li> <li>Recessive X-linked disorders <ul> <li>Carriers: XX heterozygotes without symptoms</li> <li>Offspring genotypes and phenotypes when a carrier reproduces with an unaffected XY individual</li> <li>XY individuals more prone to inheriting recessive X-linked disorders due to having only one X-chromosome</li> </ul> </li> <li>Dose-dependent X-linked disorders <ul> <li>Severity varies based on number of functional gene copies</li> <li>XX heterozygotes: some symptoms due to half of X chromosomes carrying disordered allele</li> <li>XY individuals: more severe symptoms due to only producing dysfunctional protein</li> </ul> </li> </ul> </li> </ul>
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FAQs
X-linked traits are those controlled by genes located on the X chromosome, one of the two sex chromosomes. Males have one X chromosome (inherited from their mother) and one Y chromosome (inherited from their father), while females have two X chromosomes (one from each parent). Autosomal traits are controlled by genes located on the autosomes, which are the other 22 non-sex chromosomes. Both males and females inherit two copies of each autosomal chromosome, and the inheritance patterns of autosomal traits are not influenced by the sex of the offspring.
Punnett squares are a graphical tool used to predict the genotype and phenotype ratios of offspring from a particular cross. For X-linked traits, Punnett squares display the possible combinations of X and Y chromosomes inherited from the parents, along with any potential alleles, using notation like "XᴬXᵃ". They can help identify the likelihood of an offspring inheriting a certain X-linked trait, taking into consideration the sex of the offspring and the genotypes of the parents. By using Punnett squares, medical students can understand the probabilities of X-linked disorders being passed on and the differences in inheritance patterns between males and females.
X-linked disorders are genetic conditions caused by mutations in genes located on the X chromosome. Some common X-linked disorders include hemophilia (a blood clotting disorder), Duchenne muscular dystrophy (progressive muscle weakness), and color blindness (difficulty distinguishing certain colors). These disorders often affect males more severely than females, as males have only one copy of the X chromosome and therefore do not have another normal copy of the gene to compensate for the mutated one.
In genetic males (XY), X-linked traits are often more likely to be expressed due to the presence of only one X chromosome, which means that any recessive allele on the X chromosome will not be masked by a dominant allele. If a male inherits a recessive X-linked disorder, he will always express the trait as he does not have a second X chromosome to potentially carry a normal allele. Genetic females (XX), on the other hand, have two X chromosomes and may be heterozygous carriers for X-linked traits without showing any symptoms. The presence of a dominant allele on one of the X chromosomes can prevent the expression of the recessive allele, resulting in a milder or non-existent phenotype in females.
Dose-dependent genetic disorders occur when the severity of the phenotype depends on the number of copies of a mutated gene. These disorders are relevant in X-linked inheritance because they often display different manifestations in males and females due to differences in X chromosome dosage. Specifically, males only have one copy of the X chromosome and are more likely to display severe symptoms if they inherit the mutated gene. Females, having two copies of the X chromosome, may show a milder phenotype if one of their X chromosomes carries a normal allele.
