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Genetics
Chapter 29
Essential Must Know Terminology
• Chromosome• Homologous (homologs) chromosomes
– Autosomes– Sex chromosomes– Karyotype
• Gene• Alleles
– Dominant (A)– Recessive (a)
• Genotype– Homozygous dominant (AA)– Heterozygous (Aa)– Homozygous recessive (aa)
• Phenotype
Sources of Genetic Variation
• Segregation – 1 set of genes to each gamete
• Independent assortment– 1 allele per trait to a gamete
• Crossing over– Equal gene exchange at
chiasmata• Random fertilization– Only takes 1 of each gamete
Punnett Squares
• Predict probability– Parent alleles on sides– Cross over & down
• Accuracy increased w/ occurrences– Independent events– Multiply w/ multiple events
Dominant – Recessive Inheritance
• Either or situation• Common examples– Dominant: tongue rolling, freckles, and widow’s
peak– Recessive: hitchhiker’s thumb, attached earlobes,
and long palmar muscle• Disorders– Dominant less common, but exceptions exist– Recessive more frequent b/c carriers
Incomplete Inheritance
• Heterozygote w/ own phenotype
• Sickle-cell anemia– SS normal– ss w/ condition– Ss situation specific
• Common w/ African descent
Codominance
• 2+ alleles varies genotypic/phenotypic relationship
• ABO blood type
Sex-linked Inheritance
• Genes on sex chromosomes• X-linked when on X only (most)– Males only need 1– Females need 2; 1 = carrier
• Colorblindness and hemophilia
Polygenic Inheritance
• 2+ genes w/ continuum of phenotypes
• Skin color (3 genes)– AABBCC = darkest– aabbcc = lightest, others = in
between
Environmental Relationships
• Maternal drug use/environment during embryonic development
• Nutrition and diet
• Hormonal alterations
• Pedigrees trace traits through generations
• Fetal testing w/ known risk– Amniocentesis• Most common; after 14th week• Check for genetic markers, enzymes, or
karyotypes
– Chorionic villus sampling• 8th week possible, 10th recommended• Karyotypes
Genetic Screening
Autosomal Disorders
• Dominant– Rare because always expressed embryo/fetal death– Huntington’s disease: impairs motor functioning
• Onset after reproductive age, increase probability of passing
• Recessive– More frequent, because of masking – Albinism: lack of pigmentation– Cystic fibrosis: thick mucus build up in lungs and GI
tract
Abnormal Chromosome Number Disorders
• Normally due to nondisjunction• Turner syndrome: female w/ 45 chromosomes (X0)– Underdeveloped reproductive organs = sterile, short,
broad neck, lack 2° sex characteristics• Klinefelter syndrome: male w/ 47 chromosomes
(XXY)– Underdeveloped reproductive organs = sterile, lack facial
hair, some breast enlargement, long appendages• Down syndrome: 47 chromosomes (trisomy 21)– Slanted eyes, flattened face, short stature, gradient of
mental retardartion