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Mendelian genetics.
*Gregor Mendel is considered the “father of
genetics.”
*In 1843, at the age of 21, he became a monk in
Austria where he was put in charge of tending the
monastery garden.
*In 1851, he was sent to the University of Vienna
where he studied science, math, and statistics.
*Gregor finished college and returned back to
the monastery as a very well-educated monk.
*He began working in the garden again and
noticed several things…..
*Many of the tall plants he grew produced tall
offspring, while some produced short offspring.
*Some of the plants with yellow seeds had
offspring with yellow seeds, while others had
green seeds.
*Mendel believed that plants inherited their
traits from their parents. He decided to see if
he could find a predictable pattern to the way
pea plants inherited traits.
*Mendel developed pure strains of plants by
self-pollinating plants with the trait he wanted.
*The pure strains each had a contrasting trait.
*He called these pure strains the P1 (parental )
generation.
Plants reproduce when pollen from the anther is
transferred to the stigma and the ovules are
fertilized.
*Mendel developed
14 pure plant
strains. Each
characteristic had
two contrasting
traits.
*Mendel cross
pollinated the pure
strains and called
the offspring the
F1 (1st filial)
generation.
*The F1 plants were then self-pollinated, and
Mendel called their offspring the F2 (2nd filial)
generation.
*Think of this as the parents, kids, & grandkids.
*Gregor Mendel made thousands of crosses and
found a predictable pattern.
*In the F1 generation, the recessive trait was
“hidden.”
*In the F2 generation, the recessive trait reappeared in a 3:1 ratio with the dominant trait.
*Mendel concluded that there are three
principles that control heredity.
Mendel’s Principles of Heredity.
Principle of Dominance – when 2 forms of the
same gene are present the dominant allele is
expressed.
Principle of Segregation – in meiosis two
alleles separate so that each gamete (sex cell)
only receives one form of the gene. You get a
trait either from your mother or father, not
both.
Principle of Independent Assortment – each
trait is inherited independently from other
traits. Height and hair color are not
connected.
Genes.
*Mendel’s factors are now called genes.
*A gene is a segment of DNA on a
chromosome that controls a hereditary trait.
*Chromosomes occur in pairs.
*One chromosome comes from each parent.
*The gene segment on each chromosome is
called an allele.
*Dominant alleles are represented by a capital
letters (T, R).
*Recessive alleles are represented by lower
case letters (t,r).
*A homozygous dominant genotype (TT, RR)
always shows the dominant trait.
*A heterozygous dominant genotype (Tt, Rr)
always shows the dominant trait, but is a carrier
of the recessive trait.
*A homozygous recessive genotype (tt, rr)
always shows the recessive trait.
*The genotype is the genetic makeup of the
Individual and is expressed in letters
(TT, Tt, tt).
*The phenotype is what the individual actually
looks like (tall, short).
Genetic crosses.
A monohybrid cross is a cross between
individuals that involves only one pair of
contrasting traits. (TT x Tt)
1. Cross a plant that is homozygous dominant
for the trait of height ( ) with a plant that
is homozygous dominant for that trait ( )
2. Cross a plant that is homozygous
dominant for the trait of height ( ),
with a plant that is heterozygous for
that trait ( ).
3. Cross a plant that is heterozygous
for the trait of height ( ), with a
plant that is heterozygous for that
trait ( ).
Testcross.
*A testcross is the procedure in which an
individual of unknown genotype is crossed with
a homozygous recessive individual.
*A testcross can determine the genotype of any
individual whose phenotype is dominant.
*For example, in rabbits, both of the genotypes
BB and Bb result in a black coat. A black rabbit
is found in your garden and you want to find out
whether it is homozygous dominant (BB), or
heterozygous (Bb).
*You simply breed the mystery rabbit with a
homozygous recessive rabbit.
Determine the genotype of an unknown
black rabbit by performing a test
cross.
Incomplete dominance.
*Incomplete dominance occurs when two or
more alleles influence the phenotype.
*This result in a blending of traits in the
offspring’s phenotype.
*In Japanese four o’clock plants, both the allele
for red flowers (R) and the allele for white
flowers (R’) influence the phenotype, but
neither allele is dominant.
*RR results in red flowers.
*R’ R’ results in white flowers.
*RR’ results in pink flowers which is a blending
of the red and white traits.
CROSS A RED 4 O’CLOCK PLANT WITH A
WHITE 4 O’CLOCK PLANT.
LIST THE OFFSPRING
Dihybrid Crosses.
*A dihybrid cross involves two pairs of
contrasting traits, instead of one trait as in a
monohybrid cross.
*Predicting the results of a dihybrid cross is
more complicated than predicting the results of
a monohybrid cross because there are more
possible combinations of alleles to work out.
*A dihybrid cross such as TtRr X TtRr will yeild
16 offspring.
*A heterozygous X heterozygous dihybrid cross
always results in a 9:3:3:1 phynotypic ratio in
the offspring.
Dihybrid Cross Template
Cross:
Foil:
Genotypes
4/162/162/162/162/16-
1/161/161/161/16-
Ratio
9
3
3
1
Phenotypes
Dihybrid Cross Template
Cross:
Foil:
Genotypes
4/162/162/162/162/16-
1/161/161/161/16-
Ratio
9
3
3
1
Phenotypes
Dihybrid Cross Template
Cross:
Foil:
Genotypes
4/162/162/162/162/16-
1/161/161/161/16-
Ratio
9
3
3
1
Phenotypes