Post on 03-Jan-2016
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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 (non-mathematical) 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
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
Gametes P ?
p Pp ?p
p Pp ?p
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 phenotype- in 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 sex-linked 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)