Genetics Exam 1

• incomplete dominance is when the heterozygous phenotype is an intermediate of the dominant and recessive traits
• codominance is when the heterozygote shows traits from both parents (spotted and dotted)
• pleiotropy is when one gene can contribute to several characteristics (example: Sickle cell disease)
• lethality occurs in some pleiotrophic genes where heterozygous and one homozygous survive, but the other homozygous results in inviable offspring (ratio becomes 2:1)
• recessive epistasis is when the epistatic gene must be homozygous recessive; results in a 9:3:4 phenotypic ratio in a dihybrid cross (genotype ee masks effects of all B genotype)
• reciprocal recessive epistasis is when homozygous recessive allele of each gene masks the dominant alleles of other genes; results in a 9:7 phenotypic ratio in dihybrid cross (dominant alleles for both genes are needed for dominant phenotype)
• type I dominant epistasis is when the dominant allele of one gene masks BOTH alleles of another gene; results in a 12:3:1 phenotypic ratio in dihybrid cross (dominant B allele masks AA, Aa, and aa)
• type II dominant epistasis is when the dominant allele of one gene hides the effects of the dominant allele of the other gene ONLY; results in a 13:3 ratio in dihybrid cross (B allele hides A allele, so A_bb has different phenotype)
• redundant genes are when only one dominant allele needs to be present for phenotype to be expressed; results in 15:1 phenotypic ratio in dihybrid cross
• heterogenous traits have the same phenotype but are caused by mutations (ex: human deafness)
• conditional lethal are mutants that are lethal under some conditions; permissive conditions leads to wild-type function but restrictive conditions leads to defective function
• phenocopy is a phenotype arising from an environmental agent that mimics the effect of a mutant gene; not heritable and can be deleterious or beneficial
• homologs are chromosomes that contain the same set of genes, but can have different alleles for some genes
• leptotene is the first stage of Prophase I where chromosomes begin to condense and centromeres begin to move towards opposite poles
• zygotene is the second stage of Prophase I where homologous chromosomes enter synapsis and synaptonemal complex forms
• pachytene is the third stage of Prophase I where synapsis is complete, chromosomes are in tetrad, and crossing over can occur between non-sister chromatids of homologous pair
• diplotene is the fourth stage of Prophase I where synaptonemal complex dissolves, a tetrad of four chromatids is visible, crossover points appear as chiasmata holding non sister chromatids together, and meiotic arrest can occur
• diakinesis is the fifth stage of Prophase I where chromatids thicken and shorten, nuclear membrane breaks down, and spindle begins to form
• The probability of two independent events occurring together is the product of their individual probabilities
• the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities