Exam Critical Concepts Genetics Chapters

 True-breeding: produce offspring of the same variety when they self-pollinate  Hybridization: Mating two contrasting true-breeding varieties  F 1 generation: hybrid offspring of the P generation  F 2 generation: if F 1 individuals self-pollinate  Homozygous: Two identical alleles for a trait  Heterozygous: Two different alleles for a gene Unlike homozygotes, heterozygotes are not true-breeding  Phenotype: physical appearance  Genotype: genetic makeup

 Multiplication rule: that the probability that two or more independent events will occur together is the product of their individual probabilities  Rule of addition: the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities  Probability rule: apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters  A dihybrid or other cross is equivalent to two or more independent monohybrid crosses occurring simultaneously

 In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together  Complete dominance: phenotypes of the heterozygote and dominant homozygote are identical  Incomplete dominance: phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties ◦ Result is a blend ◦ Red crossed with white yield pink

 Codominance: two dominant alleles affect the phenotype in separate, distinguishable ways ◦ Both traits “show” in offspring ◦ Red crossed with white yield both red and white flowers  Most genes exist in populations in more than two allelic forms ◦ four phenotypes of the ABO blood group in humans:  I A I A & I A i= type A  I B I B & I B i= type B  I A I B = type AB  ii= type O

 Epistasis: gene at one locus alters the phenotypic expression of a gene at a second locus ◦ In mice and many other mammals, coat color depends on two genes  Polygenic inheritance: an additive effect of two or more genes on a single phenotype ◦ Skin color in humans is an example  Pedigree: family tree that describes the interrelationships of parents and children across generations  Carriers: heterozygous individuals who carry the recessive allele but are normal

 Pleiotropy: Most genes have multiple phenotypic effects ◦ Are responsible for the multiple symptoms of certain hereditary diseases:  cystic fibrosis  sickle-cell disease  Huntington's disease: human disorder caused by dominant allele ◦ A degenerative disease of the nervous system

 Punnett Squares: can you work them?  Independent assortment: gametes formed from adult

 Sex-linked gene: located on either sex chromosome is called sex-linked ◦ usually refers to a gene on the larger X chromosome ◦ For a recessive sex-linked trait to be expressed  A female needs two copies of the allele  A male needs only one copy of the allele ◦ Sex-linked recessive disorders are much more common in males than in females

 Common human sex-linked disorders: 1.Color blindness 2.Duchenne muscular dystrophy 3.Hemophilia  Barr body: a condensed inactive X ◦ Mosaic: a female is heterozygous for a particular gene located on the X chromosome  Linked genes: genes located on the same chromosome that tend to be inherited together  Genetic recombination: genes that do not assort independently, and are on the same chromosome

 Aneuploidy: the fertilization of gametes in which nondisjunction occurred ◦ Offspring with this condition have an abnormal number of a particular chromosome  Monosomic zygote: has only one copy of a particular chromosome  Trisomic zygote: has three copies of a particular chromosome

 Polyploidy: a condition in which an organism has more than two complete sets of chromosomes ◦ Triploidy (3n) is three sets of chromosomes ◦ Tetraploidy (4n) is four sets of chromosomes  Common in plants, but not animals  Breakage of a chromosome can lead to four types of changes in chromosome structure: 1.Deletion: removes a chromosomal segment 2.Duplication: repeats a segment 3.Inversion: reverses a segment in a chromosome 4.Translocation: moves a segment from one chromosome to another

 Down syndrome: an aneuploid condition that results from three copies of chromosome 21  Klinefelter syndrome: result of an extra chromosome in a male ◦ producing XXY individuals  Turner syndrome, Monosomy X, X0 females ◦ are sterile ◦ only known viable monosomy in humans

 Genomic imprinting: the phenotype depends on which parent passed along the alleles for those traits ◦ silencing of certain genes that are “stamped” with an imprint during gamete production ◦ the result of the methylation (addition of –CH 3 ) of DNA ◦ thought to affect only a small fraction of mammalian genes ◦ Most imprinted genes are critical for embryonic development