D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 23
HUMAN GENETICS, MUTANT ALLELES & CHROMOSOMAL DISORDERS
1.
Human Genetics:
• pedigree analysis: tracking a trait through a family tree (ppts. 1 & 2)
- substitute for performing crosses to study inheritance; e.g., widow’s
peak vs. normal hairline & attached earlobes vs. free earlobes (fig. 14 – 15 & ppts. 3
& 4)
• mutant allelic disorders:
- often the allele is rare in the population due to concurrent sterility or
premature lethal effects (makes matings involving heterozygotes unlikely)
- often preserved in population due to pleiotropic effects (harmful alleles
also causing some beneficial phenotype, e.g. see sickle cell anemia below
- recessive conditions – usually result from matings between carriers
(heterozygotes)
- Tay-Sachs (1/3600 in E. European Jews) – failure of lipid metabolism in
brain cells of children causes multiple neural disorders & early death
- cystic fibrosis (1/2500) – faulty membrane transport protein, resulting
in thick mucus afflicts many organs: lungs – breathing difficulties, intestine – poor
nutrient absorption, usually results in premature death
- sickle cell anemia (1/400 African Americans) – abnormal hemoglobin
causes ‘sickling’ of red blood cells in hypoxic conditions; clogged blood vessels and
excessive destruction of red blood cells results (anemia) (fig. 5 - 22 & ppt. 5);
recessive allele also provides protection against malaria
Bio 102, spr,13
lec. 23 - p. 2
- albinism - lack of normal pigmentation resulting in extreme sensitivity
to sunlight (fig. 14 – 16); consanguineous matings (matings between close relatives)
increase the chance of inheriting a recessive condition (increased risk of two
heterozygotes mating)
- dominant conditions – syndactyly, Huntingdon’s disease (lethal),
achondroplasia (fig. 14 – 17)
• chromosomal disorders: deletions, duplications, inversions, translocations (figs.
15 – 15, 15 – 17 & ppt. 6) result from errors in crossing over
- aneuploidies, e.g. trisomy 21: Down’s syndrome (fig. 15 – 16 & ppt. 7),
result from nondisjunction in anaphase I or in anaphase II (fig. 15 - 13 & ppt. 8)
• genetic counseling and testing: laws of probability and Mendelian
inheritance patterns govern predictions (e.g. if two people who marry, each had a
sibling afflicted with a recessive condition, their chance of having an afflicted
offspring would be 1/9 (2/3 x 2/3 x 1/4)
- fetal testing (fig. 14 – 18 & ppt. 9):
- amniocentesis – sampling of amniotic fluid in utero, provides
material for chemical testing and for cell culturing
- chorionic villus sampling – entails biopsy of fetal placenta (for
karyotyping – analysis of chromosomal constitution) (fig. 13 – 3 & ppts. 10 & 11)