4.3&10.2 Theoretical Genetics
IB Biology HL I
Mrs. Peters
Spring 2014
Genetic History Genetic History:
Gregor Mendel: Austrian monk who worked with garden peas in the 1860’s, developed fundamental principles of inheritance
Mendel’s Laws: Law of segregation: two
alleles for a specific character are packaged into separate gametes
Law of Independent Assortment: pairs of alleles for different characters segregate independently of each other in the formation of gametes (they line up at the metaphase plate in metaphase I randomly)
Important Terms
Genotype: all alleles of an organism
Phenotype: the characteristics of an organism
Allele: one specific form of a gene
Homozygous: having two identical alleles of a gene
Heterozygous: having two different alleles of a gene
Important Terms
Dominant Allele: an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state
Recessive Allele: an allele that only has an effect on the phenotype when present in the homozygous state
Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote (formerly incomplete dominance)
Practice
Determine if these are genotypes or phenotypes: Red hair brown eyes hitchiking thumb TT tt Tt
Determine if these are homozygous or heterozygous: Tt tt TT
Determine if these are dominant or recessive traits: Tt tt TT
Practice Answers
Determine if these are genotypes or phenotypes: Red hair brown eyes hitchiking
thumb TT tt Tt
Determine if these are homozygous or heterozygous: Tt tt TT
Determine if these are dominant or recessive traits: Tt tt TT Tt will show the dominant trait, but carries the recessive
trait also.
Genetic Crosses Monohybrid cross: a cross
involving the inheritance of a single character (only one trait) Each parent provides two
alleles which are separated during meiosis
A cross looks at the possible combinations of alleles are for the next generation
Punnett square: a diagramming device used to predict simple genetic crosses
Genetic Crosses
Monohybrid Cross Trait: Seed ShapeAlleles: R: round; r: wrinkled
Parents: RR x RrOffspring??? Let’s find out!
1: draw a chart with 4 sections2: separate each parent’s alleles
RR= R; RRr = R; r
3: place alleles at top and left side of chart4: Fill in each inner box, with two letters, the
one from the top and the one from the left5. Determine the possible offspring ratios:
Genotype: 2RR:2RrPhenotype: all round
RR Rr
RR Rr
R r
R
R
Genetic Crosses
Dihybrid Crosses: A cross involving the inheritance of two characters (two traits) This is looking at two
traits that are generally not located on the same chromosome.
Looking at the possible combinations of two traits or 4 alleles in the next generation.
Genetic Crosses
Dihybrid CrossTraits: Seed Shape & Seed ColorAlleles: R: round; r: wrinkled; Y: yellow; y:
green
Parents: RrYy x RrYyOffspring??? Let’s find out!
1: draw a chart with 4 columns and 4 rows2: separate each parent’s alleles
RrYy = RY; Ry; rY; ryRrYy = RY; Ry; rY; ry
3: place alleles at top and left side of chart4: Fill in each inner box, with two of each
letter, the one from the top and the one from the left (alpha/cap order)
5. Determine the possible offspring ratios: Genotype: 1RRYY,Phenotype:
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
RY Ry rY ry
RY
Ry
rY
ry
Practice Time!
Genetic Crosses
Codominance: (the old incomplete dominance) Two alleles If homozygous
produce a certain color, if heterozygous produce a combined color
Ex: Alleles for Red (RR) and White (WW) flowers, when heterozygous (RW) are Pink
Genetic Crosses
Multiple Alleles: genes that exist with more than 2 alleles Blood Type Alleles:
A, B, OiAi iBi
iAi iBi
iA iB
i
i
Genetic Crosses
Sex Linked Traits: genes located on X or Y chromosomes color blindness and
hemophilia in humans are sex linked traits
Seeing color is dominant and not seeing certain colors is recessive
Hemophilia is a recessive trait
Both are carried on the X chromosome
XBXb XbY
XBXb XbY
XB Y
Xb
Xb
B is not color blind, b is color blind
Genetic Crosses
Polygenic Inheritance: when two or more genes are part of the phenotype Skin color and height
in humans At least three genes
contribute to determining skin color
Genetic Crosses
Test Cross: testing a suspected heterozygote by crossing it with a known homozygote recessive
Important Terms
Locus: the particular position on a homologous chromosome of a gene
Pedigree: a family tree with the history of a particular trait shown across generations
Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele
Pedigrees
Members without trait are usually not colored in
Members with a trait are colored in
Carriers can either be left without color, have a lighter color, or have a split box half colored in
Always look at all the information provided before making statements about pedigrees
Pedigree for Hemophilia
Genetic Testing Testing for disorders
Amniocentesis A needle is inserted into the
uterus and extracts about 10ml of amniotic fluid, cells in the fluid are cultured and analyzed using karyotyping (takes several weeks to get results)
Chorionic villus sampling (CVS)
A narrow tube is inserted through the cervix of the uterus and a tiny tissue sample from the placenta is suctioned out, karyotyping can happen immediately (takes a few hours to get results, more advantageous procedure)