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