human genes & chromosomes. human genetic disorders nondisjunction is a cause of some human...
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Human Genes & Chromosomes
Human Genetic Disorders
Nondisjunction is a cause of some human genetic disorders– In nondisjunction,
• the members of a chromosome pair fail to separate at anaphase,
• producing gametes with an incorrect number of chromosomes.
– Nondisjunction can occur during meiosis I or II.
Meiosis I
Abnormal
Gametes
Homologouschromosomes failto separate.
Meiosis II
Sisterchromatidsfail to separate.
Abnormal Normal
n n n 1n 1 n – 1n 1 n – 1 n – 1
NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II
Nondisjunction
Homologous chromosomes fail to separate
Meiosis I:Nondisjunction
Meiosis II
Nondisjunction
Human Genetic Disorders
Many different human genetic disorders, all with certain causes
Can be a result of autosomal chromosomes or sex chromosomes being affected.
Chromosomal Disorders• Error during meiosis –
homologous chromosomes fail to separate = nondisjunction
• If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosomes may result– Ex. Down Syndrome – 3
copies of chromosomes on pair 21
– Ex. Klinefelter’s – XXY – too many copies of X chromosome in males = infertility
Klinefelter’s
Human Autosomal Genetic Disorders
• Recessive Alleles– Genetic disorders can be carried on recessive
alleles, and need both recessive alleles to have the disease
– Ex. Cystic Fibrosis – recessive – excessive mucus in lungs, pancreas, reduced life span
– Ex. Tay Sachs Disease – recessive – nervous system breakdown and death in first few years of life
Human Autosomal Genetic Disorders
• Dominant Alleles– Effects of dominant allele expressed even when recessive
allele present– Ex. Dwarfism (not all types of dwarfism are a result of this
dominant gene)
Dd x Dd
D d
D
d
D D
D d
D d
d d
D = dwarfism
d = normal height
Human Autosomal Genetic Disorders
– Ex. Huntingon’s Disease – dominant - nervous system breaks down, but not until middle age
Sex-Linked Genes
• Special pattern of inheritance for genes located on X and Y chromosomes
• Many sex-linked genes are found on the X chromosome because many genes are located on it
Colorblindness
• 3 genes associated with color vision are located on the X chromosome
• In males, a defective version of any one of these genes results in colorblindness
• Common in males – 1 in 10 have it• Rare in females – why?
– Males have just one X chromosome and all X-linked alleles are expressed in males, even if they are recessive
– Females need both recessive alleles to have colorblindness = rarer
• Can be passed from daughters to their children
ColorblindnessFather
(normal vision)
ColorblindNormal vision
Mother (carrier)
Daughter(normal vision)
Son(normal vision)
Daughter(carrier)
Son(colorblind)
Male
Female
ColorblindnessFather
(normal vision)
ColorblindNormal vision
Mother (carrier)
Daughter(normal vision)
Son(normal vision)
Daughter(carrier)
Son(colorblind)
Male
Female
Colorblindness
Other Sex-Linked Disorders
• Hemophilia – blood doesn’t clot correctly
• Duchenne Muscular Dystrophy – weakening and loss of skeletal muscle