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Genetics
The Study of Heredity
Cell Division
Cells Divide for two reasons:
1. Growth
2. Reproduction
The Cell
Has a nucleus
Nucleus contains:
1. Chromosomes: carries and protects
the DNA.
2. A single strand of DNA: blueprint of
life (carries the code)
Chromosomes
with DNA
Chromosomes
Human cells have 23 pair (46 total)
Camels 35 pair
Porcupines 17 pair
Dogs 39 pair
Leopard frog 13 pair
Fruit fly 4 pair
Potatoes 24 pair
Human Karyotype
Mitosis
Cell division for growth
One cell copies its DNA (chromosomes)
Grows and expands
The two strands of DNA (inside
chromosomes) to opposite sides of cell.
Cell membrane splits dividing
cytoplasm.
1 cell to 2 cells
Mitosis
Meiosis
Cell division for reproduction
Forms sperm and egg
Chromosome numbers half. (Haploid)
Mitosis occurs
Then the two cells split again splitting
chromosome pairs.
1 cell to 2 cells to 4 repro. cells.
Meiosis
History of Genetics
Gregor Mendel
Priest in monastery in Central Europe
Studied Pea Plants
Why?
1. Have many traits that occur in only two
forms (alleles): Stems are either tall or
short (not medium).
2. Produce many offspring quickly. (Gave
Mendel LOTS of data.)
Mendel’s Experiments
Studied 7 traits: height, seed shape,
seed color, seed coat color, pod shape,
pod color, and flower position.
Mendel always started with PUREBRED
plants.
Purebred: always produces offspring
with the SAME trait as the parent.
Mendel’s Pea Plants
Experiments Continued
Mendel crossed (bred) plants with
opposite alleles of a trait
Tall plant
x
short plant
Parents are called P generation.
Offspring are called f1
The offspring WERE ALL TALL!
What happed to the short?
Mendel crossed the f1 generation (selfpollinated)
New offspring: f2
75% Tall, 25% short
How did the shortness return?
Alleles
Dominant Allele: ALWAYS shows up
when it is present.
Recessive Allele: Trait is not seen if a
dominant allele is present, but is seen
w/out D.A.
Using Symbols in Genetics
We use letters to describe alleles
(traits)
Dominant Allele: capital letter (T=tall)
Recessive Allele: lower case (t=short)
Letters of a trait must MATCH
example (B=blue, b=red)
Shortness?
Mendel’s cross:

TT (purebred Tall) x tt (purebred Short)
f1 gets ONE allele from each parent.

ONE T(tall) and ONE t (short)
Genotype (the genes): Tt
Phenotype (how it looks): Tall
FOR ALL OFFSPRING OF THIS CROSS!
Why did the shortness come back?
This cross was Tt (tall) x Tt (Tall)
Of 4 offspring;



1 would be TT (tall)
2 would be Tt (tall)
1 would be tt (short)
Are you confused yet?
Punnett Squares
Help us see the outcomes of genetic crosses.
Tt(tall) x Tt(tall)
Parent
Alleles
here.
T
T
t
t
Tall
Tall
TT
Tt
Tall
short
tt
Tt
Combine
to get
offspring
alleles!
Genotype vs. Phenotype
Genotype: the genes (the letters: TT,
Tt, tt)
Phenotype: what it looks like (tall or
short)
Genotypes:
Phenotypes:
1TT
3 Tall
2Tt
1 Short
1tt
Ratio of phenotypes- 3:1

Tall: Short
Punnett Problems-make the Punnett squares!
1. Round peas (R) is dominant and Wrinkled peas (r)
is recessive. If we crossed a Round (Rr) plant with a
Wrinkled (rr) plant and there were 4 offspring how
many would be wrinkled? Round?
2. A green pod (G) is dominant to a yellow
pod (g). If we crossed a green pod (Gg) with
a green pod (GG) and there were 4 offspring
how many would be green? How many
yellow?
3. A yellow seed (Y) is dominant to a green
seed (y). If we cross a yellow seed (Yy) with
a yellow seed (Yy) and there are 8 offspring
how many will be yellow? How many will be
green?