Human Genetic Disorders
Learning Objectives
 Explain how small changes in DNA cause genetic
disorders.
 Summarize the problems caused by nondisjunction.
From Molecule to Phenotype
Changes in the DNA sequence of a gene can change proteins:
 Altering amino acid sequences
 Affecting the phenotype
Sickle
cells
Cystic Fibrosis
 Deletion of three bases in the DNA of a single gene
 Body does not produce normal CFTR
Missing
Genetic Advantages
Some alleles that cause disease may also have benefits.
 The sickle cell provides
protection from malaria.
 The CF allele provides
protection from typhoid
fever.
Meiosis Review
When meiosis works
properly, each human
gamete gets exactly
23 chromosomes.
Chromosomal Disorders: Nondisjunction
Nondisjunction: when
homologous chromosomes do
not separate as they should
Gametes with an abnormal
number of chromosomes may
result.
Down Syndrome
The most common form of
trisomy, involving three copies
of chromosome 21, is Down
syndrome.
Other Chromosomal Disorders
Nondisjunction of the X chromosomes can lead to a disorder
known as Turner’s syndrome.
In males, nondisjunction may cause Klinefelter’s syndrome.
Human Genetic Disorders Summarized
 Small changes in a gene’s DNA sequence can change proteins
and may affect phenotype.
 Nondisjunction may cause gametes with an abnormal number
of chromosomes.