Genetics
Chapter 29
Essential Must Know Terminology
• Chromosome: structure of DNA seen in cell division
– Homologous chromosomes
• Autosomes: 22 homologs w/o effecting sex
• Sex chromosomes: 1 homolog determines sex
– Karyotype: display of homologs
• Gene: trait determining sequence of DNA
– Alleles: different or alternate versions
• Dominant: masks presence of other alleles (A)
• Recessive: easily masked by other alleles (a)
– Genotype: allelic combination
• Homozygous dominant: two dominant alleles (AA)
• Heterozygous: one dominant and one recessive allele (Aa)
• Homozygous recessive: two recessive alleles (aa)
– Phenotype: allelic combination physically seen/shown
Sources of Genetic Variation
• Segregation
– Random alignment in metaphase
for separation in anaphase
• Independent assortment
– Homologs segregate
independently of other homologs
• Crossing over
– Synapse of tetrad and exchange
at chiasma (when?  review)
• Random fertilization
– No control over which sperm or
egg
Solving Genetic Problems
• Punnett Squares helpful to ‘predict’ probability
– Each event independent
– Larger # of offspring = better prediction
– Key points:
• All individuals have 2 alleles for EVERY gene
– 1 from mom and 1 from dad
• Meiosis produces haploid gametes from diploid cells
– Aa mother = A or a eggs
• Practicing
– Place gametes (haploid) of one parent along top,
other along the left side
– Combine all possible female gametes with all
possible male gametes = fertilization
– Boxes with 2 alleles = possible offspring (diploids)
Patterns of Inheritance
• Dominant-recessive
– Presence of dominant allele always
masks recessive
– Tongue rolling, freckles, widow’s peak,
unattached earlobes
– Disorders discussed later
• Incomplete dominance
– No alleles masked in heterozygote =
intermediate phenotype
– Flower color
– Sickle cell
• ss vs Ss vs SS
Patterns of Inheritance (cont.)
• Multiple alleles/ Codominance
– Traits w/ 2+ alleles
– Varying relationships
– Blood type
• Sex-linked
– Genes on sex chromosomes
– X-linked when on X only (most)
• Males only need 1 = hemizyogus
• Females need 2; 1 = carrier
– Colorblindness and hemophilia
(discussed later)
Patterns of Inheritance (cont.)
• Polygenic
– 2+ genes in multiple locations working together
– Broad spectrum of phenotypes
– Skin color (3 genes), height, intelligence
• AABBCC = darkest, aabbcc = lightest, others = in between
• Environmental relationships
– Maternal drug use/environment during embryonic
development
– Nutrition and diet
– Hormonal alterations
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
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
Sex-linked Disorders
• Need to represent sex chromosome and version
– XN = trait not present, Xn = trait present
• Red-green colorblindness
• Hemophilia
– Allele for clotting factor VIII mutated
Genetic Screening
• Pedigrees trace traits through
generations
– Family tree using phenotypic information
• 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
• Similar to above