INHERITANCE OF CHROMOSOMES
Non-disjunction during meiosis causes an abnormal chromosome number in gametes produced.
· It is the failure of one or more chromosomes to separate.
· Can occur when homologous chromosomes fail to separate during meiosis I or when sister chromatid fail to separate during meiosis II.
· Results in gametes with either too few or too many chromosomes.
· If abnormal gametes fertilize normal gametes, a monosomy (45 chromosomes), or trisomy (47 chromosomes) results.
A syndrome is the set of symptoms that occur with a medical condition.
A. Syndromes as a result of non-disjunction:
Down Syndrome:
· It is the most common autosomal trisomy, involves chromosome 21.
· Most often, Down syndrome results in three copies of chromosome 21 due to non-disjunction during gametogenesis.
A. In 23% of cases, the sperm had the extra chromosome 21.
B. In 5% of cases, there is translocation with chromosome 21 attached to chromosome 14.
· Chances of a woman having Down syndrome children increase with age.
· Chorionic villi sampling or amniocentesis and karyotyping detects a Down syndrome child.
· Down syndrome childrenhave a tendency for leukemia, cataracts, faster aging, and mental retardation.
Abnormal Sex Chromosomal Inheritance:
1. Turner syndrome (XO) females have only one sex chromosome, an X.
A. Turner females are short, have a broad chest, and webbed neck.
B. Ovaries of Turner females never become functional: therefore, do not undergo puberty.
2. Kleinfelter syndrome males have one Y chromosome and two or more X chromosomes.
A. Affected individuals are sterile males; testes and prostate are underdeveloped, some breast development, sparse facial and body hair, and the ninability to produce sperm.
B. Individuals have large hands and feet and long arms and legs.
3. Triple X syndrome females have three or more X chromosomes.
A. There is no increased femininity; most lack any physical abnormalities.
B. Physical abnormalities could include malformed reproductive and urinary tracts, widely spaced eyes, small head, mild retardation, may be tall, delayed speech and motor skills.
C. There is an increased risk of having triple X daughters of XXY sons.
D. May experience menstrual irregularities, including early onset of menopause.
4. XYY males with Jacob’s syndrome have two Y chromosomes instead of one.
A. Results from non-disjunction during meiosis II.
B. Usually taller than average; suffer from persistent acne; tend to have lower intelligence, infertile.
C. Occurrence is 1 in every 1000 newborn males.
B. Autosomal Genetic Disorders
1. Genetic disorders are medical conditions caused by alleles inherited from parents.
2. A carrier is a heterozygous individual who has no apparent abnormality but can pass on an allele for a recessively inherited genetic disorder.
3. Since these disorders are involving the autosomes they only involve chromosomes 1-22.
A. Autosomal Dominant Disorders
1. Neurofibromatosis
· Affects one in every 3,000 people.
· Individuals have tan skin spots at birth which develop into tumors.
· Most symptoms are mild and patients live a normal life.
· Severe symptoms include; skeletal deformities, blindness and hearing loss due to tumors, learning disabilities and hyperactivity.
2. Huntington’s Disease
· Affects one in 20,000 people.
· Leads to progressive degeneration of brain cells, which cause severe muscle spasm, personality disorders, mental retardation, and death in 10-15 years from onset.
· Most appear normal until they are middle aged and already have children. Can pass the gene to offspring if you do not know you have it.
B. Autosomal Recessive Disorders
1. Cystic Fibrosis
· It is the most lethal genetic disease in Caucasians in the US.
· 1 in 20 Caucasians is a carrier, 1in 2,500 has the disorder.
· Lungs and pancreas are largely affected.
· There is a build up of mucus which becomes thick, dry, and sticky.
Mutations
1. Germ cell mutations
2. Somatic mutations
3. Lethal mutations
4. Chromosome mutations
a) Deletion
b) Inversion
c) Translocation
5. Gene mutations