Ch.14 Mendel and the Gene Idea
These notes are going
to be relatively short
because I believe
practicing with
problems is the best
way to cover
genetics.
Obviously, this is
Gregor Mendel.
Mendel’s discoveries
A Blending- Hereditary
material- Both
parents contribute
genetic material.
B Inheritable factors,
genes are passed
from generation to
generation
Pea experiments
1. Worked with 7
traits
2. Used
purebreeding
plants first then
crossed them.
Mendels Pea Crosses
Law of independent assortment.
a homologous
chromosomes
move
independently.
b Ex.
Chromosome
with A or a will
go separate from
chromosome
with B or b.
Law of Segregation
• Homologous pairs
segregate during
gamete
formation(meiosis
) and pair again
after fertilization
• Ex. If a pea is Pp
then half the
gametes will
receive a P and
half a p.
Know the vocabulary
Homozygous
Heterozygous
Phenotype
Genotype
Testcross- Mating an unkown to a homozygous
recessive
Monohybrid Cross- a cross between parents
that are heterozygous for one trait.
Dihybrid Cross
Incomplete dominance
• Incomplete
dominance leads
to a blending of
traits, red + white
yields pink
Codominance - is not a blending but a
mixing of the traits
Codominance
• Codominance - A
and B are both
dominant to O
but not to each
other, giving the
AB blood type
Epistasis
• One gene has an effect
on another. C leads to
deposition of color
while B or b leads to
color BBcc would be
white even though the
genes code for black
color.
Polygenic inheritance
• Combinations of genes
can yield cumulative
effects. The more
dominant alleles the
more color.
Environmental Effects
• Sometimes the
environment can
change the way a gene
is expressed, like these
hydrangia, and the
color based on pH of
the soil.
Pleiotropy
• One gene can have an
effect on other parts of
the body. The sickle
cell gene which causes
problems with blood
cells has far reaching
affects on the body.
Pedigree analysis- square is male,circle is
female.
Achondroplasia
Amniocentesis
Cystic Fibrosis
Sickle Cell Anemia
Tay- sachs
Huntington’s Corea
Sample problems
Mendel crossed short and tall pea
plants(purebred cross) TT x tt = P gen.
t t
T Tt Tt
All offspring are heterozygous
T Tt Tt
All are tall (Tt)
Mendel crossed round, yellow pea plants
to wrinkled, green pea plants (RRYY x
rryy)
The only possible gametes are RY from
parent 1 and ry from parent 2 so
ry
ry
RY RrYy RrYy All offspring are round
RY RrYy RrYy and yellow (RrYy)
Monohybrid Cross -two heterozygotes
mated
Tt xTt
T
t
T TT Tt
1TT:2Tt:1tt
t Tt
3 tall: 1 short
tt
Dihybrid cross
Heterozygotes for 2 traits are
crossed RrYy x RrYy
You can make the folowing
gametes for each parent
RY,Ry,rY,ry, this means a four by
four box as in the next slide
Dihybrid Cross
Dihybrid crosses can also be done like this;
R
r
Y y
R RR Rr
Y YY Yy
r Rr
y Yy
rr
yy
Now calculate the chances of RRYY (1/4 x 1/4)
RRYy (1/4 x2/4)
RrYY (2/4 x 1/4)
RRyy (1/4 x 1/4)
RrYy ( 2/4 x 2/4)
Rryy (2/4 x 1/4)
rrYY (1/4 x/1/4)
rrYy (1/4 x 2/4)
rryy (1/4 x 1/4)