BIO.B.2 Genetics
Topic Summary for Chromosomal Mutation:
There is a molecular reason for genetic disorders. A change in DNA can alter an amino acid sequence, which can change a protein and therefore, the phenotype. Some common inherited disorders result from a change in DNA. They include:
sickle cell disease, in which a defective polypeptide makes hemoglobin in the blood less soluble;
cystic fibrosis, in which a deletion of three bases in a gene causes cell membranes to lose their ability to transport chloride ions;
Huntington’s disease, in which a single codon for a certain amino acid repeats more than 40 times, causing mental deterioration and uncontrolled movements.
Some alleles that cause disease in the homozygote can provide an advantage in the heterozygote. The geographic associations between sickle cell disease and malaria and between cystic fibrosis and typhoid demonstrate how the heterozygous state reduces the risk of infection.
Sometimes, during meiosis, homologous chromosomes fail to separate. This nondisjunction (not coming apart) can create a gamete with an abnormal number of chromosomes, leading to offspring with missing or extra chromosomes. Examples include:
Down syndrome, most often a result of three copies of chromosome 21;
Turner’s syndrome, a female with a single X chromosome;
Klinefelter’s syndrome, a male with an extra X chromosome.
Topic Summary for Chromosomal Mutation:
There is a molecular reason for genetic disorders. A change in DNA can alter an amino acid sequence, which can change a protein and therefore, the phenotype. Some common inherited disorders result from a change in DNA. They include:
sickle cell disease, in which a defective polypeptide makes hemoglobin in the blood less soluble;
cystic fibrosis, in which a deletion of three bases in a gene causes cell membranes to lose their ability to transport chloride ions;
Huntington’s disease, in which a single codon for a certain amino acid repeats more than 40 times, causing mental deterioration and uncontrolled movements.
Some alleles that cause disease in the homozygote can provide an advantage in the heterozygote. The geographic associations between sickle cell disease and malaria and between cystic fibrosis and typhoid demonstrate how the heterozygous state reduces the risk of infection.
Sometimes, during meiosis, homologous chromosomes fail to separate. This nondisjunction (not coming apart) can create a gamete with an abnormal number of chromosomes, leading to offspring with missing or extra chromosomes. Examples include:
Down syndrome, most often a result of three copies of chromosome 21;
Turner’s syndrome, a female with a single X chromosome;
Klinefelter’s syndrome, a male with an extra X chromosome.