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)