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 – X N = trait not present, X n = trait present Red-green colorblindness Hemophilia – Allele for clotting factor VIII mutated

Pedigrees trace traits through generations – Family tree using phenotypic information 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 Similar to above Genetic Screening