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 14 th week Check for genetic markers, enzymes, or karyotypes – Chorionic villus sampling 8 th week possible, 10 th 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