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)