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Chapter 10
Fundamentals of Genetics
Gregor Mendel, the father of
modern genetics
9-1 Mendel’s Legacy
Some stuff on Gregor Mendel:
• his parents were farmers
• he became ordained as a priest
• studied science and mathematics at the
University of Vienna
• His research provided the basis for
heredity- the transmission of
characteristics from parents to
offspring.
A. Mendel’s Pea Experiment
1. He set up truebreeding plants
(bred for many
generations) by
allowing them to
self-fertilize.
• He controlled
pollination, looking
at 1 or 2
characteristics at a
time.
These were the 7 characteristics he looked at:
2. He crossed a true breeding plant with a plant
of the opposite trait (tall x short). He called
this the Parental (P1) generation.
3. In the 1st filial (F1) generation all of the
offspring showed the same trait – all tall.
Next, he let the F1 generation self-pollinate.
Results: ¾ of the F2 generation showed the
dominant trait & ¼ showed the recessive trait.
Mendel’s Results
Analysis
• The F1 generation always displayed one trait
(he later called this the dominant trait)
• The F1 generation must have within it the trait
from the original parents - the white trait
• The F2 generation displayed the hidden trait,
1/4 of the F2 generation had it (he later called
this hidden trait the recessive trait)- 3:1 ratio.
• Each individual has two "factors" that determine
what external appearance the offspring will
have. (We now call these factors genes or
alleles)
Mendel established three principles
(or Laws) from his research:
1. Law of Dominance – some alleles show
and some alleles are “hidden”. The traits that
mask other traits are dominant traits.
EX: Tall plant X Short plant
all offspring are Tall
Tall is the dominant trait!
• Use capital letter to represent
dominant allele
• Use lower case letter to represent
recessive allele
EX: T = tall
t = short
• REMEMBER…
Each organism has a pair of genes for
each trait (diploid)
TT = homozygous dominant (tall)
Tt = heterozygous (tall, but carries an
allele for short)
tt = homozygous recessive (short)
2) Law of Segregation – during meiosis,
one member of a gene pair separates
into different gametes.
Therefore, each gamete only carries one
member of the gene pair.
For example, if a pea plant is Tt for tall,
during meiosis, half the gametes formed
will carry the T, half will carry the t.
3) Law of Independent Assortment –
A random distribution of alleles occurs
during gamete formation. (Genes on
separate chromosomes sort
independently during meiosis)
Using symbols we can depict the cross of tall
and short pea plants in the following manner:
Alleles
• Allele - one alternative form of a given gene
pair.
• Tall and dwarf are the alleles for the height
of a pea plant.
• More than two alleles can exist for any
specific gene, but only two of them will be
found within any individual.
• We use letters to denote alleles, since every
gene has two alleles, all genes can be
represented by a pair of letters.
TT = tall, Tt = tall, tt = dwarf
Other Terms
• 1. Genotype- The genetic makeup of a
trait. We use letters to denote alleles
(BB, Dd, ee, for example).
• 2. Phenotype- Physical appearance (or
what an organism looks like). Examples
would be tall, short, blue, purple, etc.
Other Terms
• 3. Homozygous: when the alleles are the
same, the individual is said to be
homozygous, or true breeding. Letters
designating a homozygous individual could
be capital or lower case, as long as they are
the same. Ex. AA, bb, EE, dd
• 4. Heterozygous: when the alleles are
different, in this case the DOMINANT allele
is expressed. Ex. Pp, Aa
When we cross-breed 2 things, looking at
one factor, we have a:
• 5. Monohybrid cross = a cross
involving one pair of contrasting traits.
Ex. Pp x Pp.
• To solve it, we would use a:
• 6. Punnett Square: used to determine
the PROBABILITY of having a certain
type of offspring given the alleles of the
parents
How to Solve a Punnett Square
1. Determine the genotypes (letters) of
the parents. (Bb x Bb)
2. Set up the punnett square with one
parent on each side.
3. Fill out the punnett square middle
4. Analyze the number of offspring of
each type to calculate the genotypic
and phenotypic ratios.
An example
• In pea plants, round seeds are
dominant to wrinkled. The genotypes
and phenotypes are:
• RR = round
Rr = round
rr = wrinkled
• If a heteroyzous round seed is crossed
with itself (Rr x Rr) a punnett square
can help you figure out the ratios of the
offspring.
Set up your square
Remember, it’s Rr x Rr
Note that the letters get separated on the top
and the side. It DOES NOT MATTER which
parent goes on top or on the side.
Results
3/4 round, 1/4 wrinkled
So,The Phenotypic Ratio is 3:1, Round to Wrinkled
The Genotypic Ratio is 1:2:1, and refers to the
letters. It is 1 RR, 2 Rr, 1 rr.
Another look at how to do it
Predicting Results of
Monohybrid Crosses
• See your textbook for other examples,
and try some on the worksheets that I
will pass out to you for classwork and for
homework.
Other types of crosses
1. Incomplete Dominance- There is no
dominant or recessive, the heterozygous condition
results in a "blending" of the two traits. Example:
Snapdragons can be red, white, or pink (heterozygous)
2. Codominance
• Occurs when both alleles for a gene are
expressed in a heterozygous offspring.
Neither is dominant or recessive.
• Ex- White + Red horse= Roan (white
and red hairs mixed together).
+
CodominanceAlso Observed in Blood Types
• Both A and B are
dominant.
• Type O is
recessive
• Four phenotypes
• Six genotypes
4 Types of Blood
• Type A with A antigens on the red cells and anti B
antibodies in the plasma.
• Type B with B antigens on the red cells and anti A
antibodies in the plasma.
• Type AB with both A and B antigens on the red cells
and no blood type antibodies in the plasma.
• Type O with no antigens on the red cells and both anti
A and anti B antibodies in the plasma
• ** Group O blood cannot be clumped by any human
blood, and therefore people with Group O are called
universal donors.
SEX CHROMOSOMES AND
SEX-LINKED GENES
• A human male has one X chromosome
and one Y chromosome
• A human female has two X
chromosomes
• Whether a sperm cell has an X or Y
chromosome determines the sex of the
offspring
Sex-linked disorders affect
mostly males
• Most sex-linked human
disorders are due to
recessive alleles
– Examples: hemophilia,
red-green color blindness
– These are mostly seen in males
– A male receives a single X-linked allele
from his mother, and will have the
disorder, while a female has to receive the
allele from both parents to be affected
Sex Linked Trait:
Colorblindness
Dihybrid
Crosses:
Crosses that
involve 2
traits.
For these
crosses your
punnett square
needs to be
4x4
In any case where the parents are heterozygous for
both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio as
in the previous example.
• If you cross other combinations, you will need to do a
square. Prepare a Punnett Square in your notes,
using the crossing of RrYy x rryy.
• You use the FOIL method to determine the letters
that go on top and on the side. For example, in
RrYy, the letters would be: RY, Ry, rY, ry.
Results of cross RrYy x rryy:
Now try some others from the
worksheets you will receive.