Often Mendelian genetics refers to basic genetics that use Punnett Squares and statistics to predict the phenotypes of offspring. Modern Genetics usually refers to our modern-day understanding of how DNA and genes control the expression of traits. During Mendel's time, biologists did not know about DNA and genes.
Punnett Square Flash Game
DNA From the Beginning
MGHE Drosophila Simulation
Chapter 11.1 - Gregor Mendel
- Developed the
fundamental laws of heredity
- He studied science
and mathematics, using statistics to support his hypotheses
- Blended Concept
of Inheritance (accepted at the time) vs Particulate Theory of Inheritance (Mendel's Theory)
- Mendel chose
garden peas (Pisum sativum)as his subjects as they are easily grown
and their pollination is easily controlled. He controlled pollination by manually
moving pollen between plants
- Developed True-breeding
plants by self-pollination, crossbreeding those to create hybrids
- Mendel examined
varieties of peas for heritable characters and traits for his study. (stem
length, pod shape, seed shape, seed color..etc)
- His publication " Experiments on Plant Hybridization" went largely ignored until long after his death
Law of Segregation (MONOHYBRID CROSS)
- Mendel crossed
true-breeding plants that differed for a given character
- A monohybrid
cross involves one (mono) character and different (hybrid) traits.
- Pollen from
true-breeding pea plants with purple flowers (one
trait) was placed on stigmas of true-breeding plants with white flowers (another
- The F1 seeds
were all purple; the white flower trait failed to appear at all.
- Because the
purple flower trait completely masks the white flower trait when true-breeding
plants are crossed, the purple flower trait is called dominant, and
the white flower trait is called recessive.
- The F1 plants
were allowed to self-pollinate.
- This step was
the monohybrid cross. (or the F1 cross).
- The progeny,
called F2, were examined: roughly 1/4 were white, and 3/4 were purple.
- Mendel proposed
that the units responsible for inheritance were discrete particles - particulate
As Viewed by Modern Genetics
- During production
of gametes, only one of the pair members for a given character passes to the
gamete. (LAW OF SEGREGATION)
- When fertilization
occurs, the zygote gets one from each parent, restoring the pair.
- Mendel's units
of inheritance are now called genes.
- Different forms
of a gene are called alleles.
- Each allele
is given a symbol: In the case of purple flowers, P might represent purple
and p white. By convention, uppercase P represents the dominant; lowercase
p represents the recessive. Never mix your letters! Choose one letter and
use a capital to denote dominate and a lowercase to denote recessive.
individuals would have two copies of the same allele: Purple would be PP.
Ttwo copies of same allele = homozygous. Homo means "the same"
- White true-breeding
would be pp. Two copies of same allele = homozygous.
- Some purple-flowered
plants could be Pp, although they would not be true-breeding. Individuals
that are purple, but had a white parent, are heterozygous: Pp. Hetero
- When an organism
is studied for three different genes and has the alleles AABbCC, it is homozygous
for A and C genes but heterozygous for the B gene.
- The physical
appearance of an organism is its phenotype. Purple-flowered would be
- The actual composition
of the organism's alleles for a gene is its genotype: Pp is a genotype.
- Organisms have
many different genes some have thousands, and complex organisms have 10 times
Task: Show the
P, F1 and F2 generations of a true-breeding round seeded plant crossed with
a true breeding wrinkle-seeded plant.
(Click to see Answer)
The Punnett Square
- a simple box-like
device that helps us to consider all genetic combinations and show the expected
frequencies of genotypes.
- The P and p
symbols represent the single allele each gamete receives.
provides the two alleles for the new individual, one from the male (sperm)
and one from the female (egg).
- The Punnett
square shows that the genotypes and associated ratios for a monohybrid cross
are 1 PP :2 Pp : 1 pp.
- Any progeny
with a P would have the dominant (purple) phenotype, so the phenotypic ratio
is 3 purple to 1 white.
- Now it is known
that a gene is a portion of the chromosomal DNA that resides at a particular
site, called a locus (plural is loci). The gene codes for a particular
function or trait.
- Mendel arrived
at the law of segregation with no knowledge of meiosis or chromosomes. The
mechanism of chromosome separation in meiosis I today explains his law of
One Trait Test
- A test cross
can determine the genotype (heterozygous or homozygous) of an individual with
a dominant trait.
- It involves
crossing the individual to a true-breeding recessive (homozygous recessive).
- If the unknown
is heterozygous, approximately half the progeny will have the dominant trait
and half the recessive trait.
- If the unknown
is homozygous dominant, all the progeny will have the dominant trait
Mendel's Law of
- The second law
describes the outcome of dihybrid (two character) crosses, or hybrid
crosses involving additional characters.
- A dihybrid is
an individual that is a double heterozygote (e.g., with the genotype RrYy
- round seed, yellow seed).
- Mendel's second
law states that the Rr alleles assort into gametes independently of the Yy
- The dihybrid,
RrYy, produces gametes that have one allele of each gene.
- Four different
gametes are possible and will be produced in equal proportions: RY, Ry, rY,
- Random fertilization
of gametes yields the outcome visible in the Punnett square. Note its 4x4
table construction to accommodate 16 possible phenotypes.
- Filling in the
table and adding the like cells reveals a 9:3:3:1 ratio of the four possible
phenotypes: (Round, Yellow - Round, Green - Wrinkled, Yellow - Wrinkled, Green.)
- The Law of
Independent Assortment states that alleles of different genes assort independently
of one another during gamete formation.
- In fact, this
law is not always true - The law of independent assortment is accurate for
genes that are on separate chromosomes, but not necessarily for genes that
are on the same chromosome.
- Genes that are
close to each other on the same chromosome tend to stay together, but crossing
over during meiosis may separate them.
Dihybrid Cross: RrYy x RrYy
TIP: In any
cross that is dihybrid (AaBb x AaBb) you will always get a 9:3:3:1 ratio, if
you memorize this, you can save the trouble of doing a giant square!
Alternative (LAWS OF PROBABILITY)
punnet square is not needed to determine the ratios of genotypes and phenotypes.
Simple statistics and math can save you the trouble of filling out a square.
a monohybrid cross Pp x Pp, each parent produced
P gametes and p
you wanted to determine how many of the offspring are pp:
H is dominate for long hair (h = short) and B is dominate for black eyes
(b = red eyes). If
the parents are...
HhBb x hhBb
many off the offspring will be short haired and red eyed?
Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from
the cross above.
Used to determine
the genotype of an "unknown" by crossing it with an individual that
is homozygous recessive for both traits.
In flies (Long
wings is dominant to short wings, Gray body is dominant to black)
A L __ G ___
is test crossed.
are 1:1:1:1 --> What is the genotype of the unknown parent?
If the offspring are half long winged & gray, and half long winged and black
--> What is the genotype of the unknown parent?