Genetics Notes
Miller Levine Biology Book – (Chapter 11 Sections 1-3)
Additional Notes
Key Terms:
Gregor Mendel
F1 Generation
Allele
Homozygous
Punnett Square
Codominance
Trait
True breeding
P Generation
F2 Generation
Dominant
Recessive
Law of Dominance Law of Segregation Gene
Heterozygous
Phenotype
Genotype
Complete Dominance Incomplete Dominance
Introduction:
Genetics is the study of how parents pass traits onto their offspring. The
modern understanding of inheritance began with the work of an Austrian monk
named Gregor Mendel who studied the inheritance of traits in peas.
People have probably always had some kind of awareness that parents
pass traits to their children. After all, family resembles family. Farmers have
known for years that traits are inherited in animals and even crop plants and
have constructed breeding programs to capture these selected traits. However,
for most of human history, people did not understand why traits were passed
from parents to offspring, or why some traits seemed to appear more often than
others.
Mendel’s work laid the foundation for the current understanding of
inheritance as is represented as three main ideas called Mendel’s Laws:
I. The Law of Dominance: basically says that one form of a trait can hide
another form of a trait. In other words, organisms have two genetic
messages fro every trait, but only one message (or version) may be
visible in the organism. For example, if a brown-eyed person has
children with a blue eyed person, their children may have brown eyes
even though they inherited messages for blue eyes from one of their
parents. The effect of the messages for brown eyes hides that message
for blue eyes are even there. Mendel said that the form of the trait that
is visible is dominant to the hidden form of the trait, which is recessive.
II. The Law of Segregation basically says that when adults make gametes
(egg and sperm), they give only one of their two messages for each trait
to each gamete. In other words, the two messages for each trait in
adults separate from each other, or segregate, when gametes are made.
Gametes get together with other gametes to make a new organism, so it
makes sense for gametes to get only one copy of each genetic message.
The other copy of each genetic message will come from the other
gamete.
III. The Law of Independent Assortment basically says that each pair
genetic messages sort itself independently from other pairs of genetic
messages. In other words, when pairs of genetic messages are getting
split up and sent to gametes, each pair is doing this process on its own
– independent of other traits.
The Basics: (learning the terms)
An inheritable feature that varies among individuals, such as flower color,
is called a character. Each variant for a character, such as purple or white color
for flowers, is called a trait. A trait is coded for by a series of nucleotides from a
gene (a section of DNA). A different version of a gene is called an Allele. If an
individual has two identical genes (alleles) the gene is said to be Homozygous. If
an individual has different alleles the gene is said to be Heterozygous. The
organism can look (an organism’s phenotype) different than its alleles (its
genotype).
A plant is said to be true-breeding if it is able to self pollinate. That means that
the flower has both male (pollen) and female (ovary) parts. When two different
flowers swap gametes the process is called hybridization.
Mendel’s Experiments: Round One….
It is easiest to understand heredity when you follow just one trait at a time.
For example, Mendel did a cross between a pure-breeding plant that produces
round peas and a pure breeding plant that produces wrinkled peas (parental
generation (P1)). The offspring (first filial generation (F1)) of this mating all
produced round peas. However, when the round peas offspring (F1) were mated
among themselves, both round and wrinkled pea plants were produced (F2).
Because the wrinkled trait appeared again in the offspring of the first generation
(F2), Mendel knew that the wrinkled pea trait must be hidden by the first
generation (F1) plants even though he couldn’t see it.
Mendel’s experiment makes sense if you think about it in the following ways:
The parental plants (P1) are pure-breeding, so they have only one
kind of allele for the trait. Round pea plants can give messages only for
round peas to their offspring, and wrinkled pea plants can give messages for only
wrinkled peas to their offspring.
Each pea plant has two alleles for each trait. Plants can have two copies
of the round-shaped allele, two copies of the wrinkled-shape allele (homozygous),
or one copy of each round and wrinkled (heterozygous).
Each pea plant gives only one allele to its offspring. When pea plants
make gametes, they separate the two alleles for each trait and give one allele for
each trait to each gamete.
The first generation plants get an allele for round peas from one
parent and an allele from wrinkled peas from the other parent. Thus,
the first generation plants have two different messages for pea shape. Because
their two alleles for this trait are different, they’re heterozygous for this trait.
The allele for round peas is dominant to the allele for wrinkled
peas. Although the first generation plants had two different messages, you see
only the effect of the round pea message (its is a dominant gene). The round pea
message is hiding the wrinkled pea message.
When the first generation plants reproduce, they can give either
their allele for round peas or their allele for wrinkled peas to their
offspring. Some plants in the second generation (F2) had wrinkled peas even
though all the first generation had round peas.
The Laws of probability govern Mendelian inheritance:
Mendel’s laws of segregation and independent assortment reflect the same
rules of probability that apply to tossing coins, rolling dice and drawing cards
from a deck.
 The probability scale ranges from 0 to 1, an event that is certain to
occur has a probability of 1, while an event that is certain not to occur
has a probability of 0.
o With a coin that has heads of both sides the probability f tossing
heads is 1 and the probability of tossing tails is 0.
o With a normal coin, the chance of tossing heads is ½, and the
chance of tossing tails is ½.
o For every toss, the probability of heads is ½. The outcome of
any particular toss is unaffected by what has happened on
previous trials. Each toss is independent of the next toss.
o Rules of Multiplication and Addition applied to Punnett
Squares:
 The multiplication rule states that to determine this
probability, we multiply the probability of one event (in
the case of heads ½) by the probability of the other event
(heads again ½). So the probability of getting two heads
in row is (1/2) X (1/2) = ¼.
o To figure out the probability that an F2 plant from a
monohybrid cross will be heterozygous rather than homozygous,
we need to invoke a second rule. F1 gametes can combine to
produce Rr offspring in two mutually exclusive ways:
 For any particular heterozygous F2 plant, the dominant
allele can come from the egg or the sperm, but not from
both.
 According to the addition rule, the probability that any
one of two or more mutually exclusive events will occur is
calculated by adding their individual probabilities.
 The probability for one possible way of obtaining an F2
heterozygote, the dominant allele from the egg and the
recessive allele from the sperm is ¼. The probability for
the other possible way- the recessive allele from the egg
and the dominant allele from the sperm is also ¼. Using
the rule of addition, then, we can calculate the probability
of an F2 heterozygote as ¼ + ¼ = ½.