Transcript Slide 1
Gene • Genetic material • Instructions or code for a particular trait or characteristic http://www.accessexcellence.org/AB/GG/genes.html Allele • One of the possible options for a given characteristic or trait • Pea Plant Examples: – flower color is purple or white – height is tall or short – Flowers are terminal or axial Homologous pair • Two chromosomes that contain genes for the same traits at the same locations • One comes from mom, the other from dad • Humans have 22 pairs of homologous and 1 pair of non-homologous chromosomes Probability •Chance that a given event will take place •Multiplication rule = to calculate the odds of two or more events occurring at the same time, multiply the probabilities for each independent event Dominant • An allele that can mask or hide the expression of another allele for the same trait • Is represented by a capital (big) letter http://www.accessexcellence.org/AB/GG/recessive.html Recessive • An allele that is masked or hidden when present with another allele for the same trait • Is only expressed when two are inherited • Is represented by a lower case (little) letter http://www.accessexcellence.org/AB/GG/recessive.html Homozygous • When the two alleles inherited are the same • Homozygous dominant – Both are dominant alleles – TT = tall tall = a tall plant • Homozygous recessive – Both are recessive alleles – tt = short short = a short plant Heterozygous • When the two alleles inherited are different • Only the dominant allele is expressed or visible – Tt = tall short = a tall plant Phenotype • The actual expression of the genes • What you see Genotype • The actual alleles present • What genes they inherited for that trait Law of Segregation • Each pair of alleles separate during the formation of the gamete (egg or sperm) • Example: If parent is Tt, the offspring will get either the T or the t- not both Tt T t Law of Independent Assortment • Each pair of alleles separate independent from other pairs of alleles during the formation of the gamete • Which chromosome goes into a gamete is random TtPp TP or Tp or tP or tp Gregor Mendel • 19th century Austrian monk • Experimented in breeding pea plants • The father of genetics – Developed terminology used – Stated laws http://www.accessexcellence.org/AB/GG/mendel.html Types of Crosses Individuals • Purebred = have only one allele for a trait TT, tt, PP,pp • Hybrid = have different alleles for a trait Tt or Pp Crosses • Monohybrid = differ for one trait, TT x tt • Dihybrid = differ for two traits, TTpp x ttPP Punnett Squares An easy (nonmathematical) way to calculate the probability of the traits for the offspring of two specific individuals Terminology for Crosses • P1 = parent generation • F1 = first generation of children • F2 = second generation of children F stands for filial, or sons and daughters Monohybrid • A cross between two individuals • Examines only one trait • Parents have contrasting traits P1 = Pp x Pp F1 = 3:1 phenotypic ratio = 1:2:1 genotypic ratio Gametes P ? p Pp ?p p Pp ?p Test Cross • A cross used to determine what genes a parent has • The unknown individual (PP or Pp) is mated with an individual that is homozygous recessive (pp) • Children show the second unknown gene Dihybrid Cross • A cross between two individuals • Examines only two traits at the same time • Parents have contrasting traits P1 = SSYY x ssyy F1 = SsYy x SsYy F2 = 9:3:3:1 phenotypic ratio Incomplete Dominance • Alleles are not dominant or recessive • Both alleles are expressed, or blended • Capital letters are used for both alleles, or the same letter with a prime or subscript is used H and H1 • Examples: R = red H = straight hair r = white H1 = curly hair Rr = pink HH1 = wavy hair Codominance • Both alleles are completely expressed • Neither hides the other • Example: Blood Type – M and N proteins – LM LM produces protein M – LN LN produces protein N – LM LN produces both protein M and N Multiple Alleles • Three or more alleles for a given trait exist • Example: A,B,AB and O blood types result from which 2 of 3 genes you inherit Type A blood results from IAIA or IAi Type B blood results from IBIB or IBi Type AB blood results from IAIB Type O blood results from ii Pleiotropy • One gene influences more than 1 trait • Example in peas: – One gene determines whether seeds are round or wrinkled – Same gene affects starch metabolism and water absorption http://fig.cox.miami.edu/~cmallery/150/gene/14x15sickle.jpg Polygenic Inheritance • Results from the interaction of many genes to determine a single phenotypein other words, many genes work to produce appearance of one trait • Not just 2 or 3 varieties, but a continuous variation or range of phenotypes • The opposite of pleiotropy http://sun.menloschool.org/~dspence/biology/chapter12/images/eye_color.jpg http://fig.cox.miami.edu/~cmallery/150/mendel/polygenic.jpg http://www.specialedprep.net/MSAT%20SCIENCE/Images/PleiotropyPolygenic.jpg Linked Genes • Are on the same chromosome • Are inherited together because they come as a package Sex-linked • Sex chromosomes the last pair, X & Y • Autosomes pairs 1-22 http://www.accessexcellence.org/AB/GG/sex.html Sex-linked Examples of human disorders that are sexlinked are: • Hemophilia • Red-green color blindness • Duchennes’s muscular dystrophy These disorders occur more frequently in men than women, because men only inherit 1 gene for the trait, while women inherit 2 Recessive Autosomal Disorders • Phenylketonuria = unable to break down phenylalanine, results in mental retardation • Sickle-cell anemia = abnormal hemoglobin, unable to transport oxygen • Tay-Sachs disease = unable to break down some lipids, causing nerve damage and ultimately death Dominant Autosomal Disorders Huntington’s disease = mental illness begins in middle age, affecting brain and motor control and leading to total mental and physical incapacity Fraternal Twins • Fraternal twins are the result when two different eggs (ova) are fertilized by two different sperm. • This leads to the development of two separate placentas, each with its own chorion and amnion. • Fraternal twins are more common than identical twins and account for about 2/3 of twin pregnancies. Identical Twins • Identical twins develop when a fertilized egg splits. • Depending on when the split occurs will determine if the twins share a placenta, with either one or two chorions and amnions, or if they each develop their own placentas. • In general, the later the spit occurs, the more likely that the twins will share one placenta. Fraternal vs. Identical Twinning – Even after they are born, it is sometimes difficult to know whether twins are identical or fraternal. It can be easier if they: • share one placenta (identical) • are different sexes (fraternal) • have different blood types (fraternal)