Genetics

1. Genetics Chapter 29

2. 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

3. 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

4. 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 • Aamother = 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)

5. 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 • ssvsSsvs SS

6. 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)

7. 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

8. 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

9. 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

10. 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

11. 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