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Genetics
Vocabulary
 Trait - specific characteristic that varies from one
individual to another
 Gene - sequence of DNA that codes for a protein and
thus determines a trait
 Allele - one of a number of different forms of a gene
 Hybrid - offspring of crosses between parents with
different traits
 The principle of dominance states that some alleles
are dominant and others are recessive.
Gregor Mendel –
Father of Modern Genetics
 True-breeding term used to
describe
organisms that
produce offspring
identical to
themselves if
allowed to selfpollinate.
Mendel’s Work
 Mendel had truebreeding pea plants.
 He asked the question:
What would happen if
he breed pea plants
with different traits?
Dominant and Recessive Traits &
Gregor Mendel’s Peas
 P – Parent generation
 F1 – first generation of offspring (f – filial from
latin filius “son”)
Punnett Square
 diagram showing the gene combinations that
might result from a genetic cross
Cross
YY and
yy
Y
Y
y
Yy
Yy
y
Yy
Yy
Crossing true-breeding parent
generation
P Generation
 Trait
 Green pea
 recessive
 Genes (alleles)
 yy
 Gametes
formed
 y and y
 Trait
 Yellow pea
 dominant
Cross
YY and
yy
Y
Y
y
Yy
Yy
y
Yy
Yy
F1 Generation
 Genes (alleles)
 YY
 Gametes formed
 Y and Y
Crossing the F1 generation
F1 Gen.
 Trait
 Yellow pea
 Genes (alleles)
 Yy
 Gametes formed
 Y and y
 Trait
 Yellow pea
Cross Yy
and Yy
Y
Y
YY Yy
y
Yy yy
y
F2 Generation
 Genes (alleles)
 Yy
 Gametes formed
 Y and y
Probability and Genetics
 Probability likelihood that a
particular event
will occur
Cross
YY
and yy
Y
Y
y
YY Yy
Yy yy
y
Crossing the F1 generation
 Segregation - separation of alleles during
gamete formation
 When each F1 plant flowers, the two alleles
are segregated from each other so that each
gamete carries only a single copy of each
gene. Therefore, each F1 plant produces two
types of gametes—those with the allele for
tallness and those with the allele for shortness.
More vocabulary…
 Homozygous - term used to refer to an
organism that has two identical alleles for a
particular trait
 Heterozygous - term used to refer to an
organism that has two different alleles for the
same trait
 Phenotype - physical characteristics of an
organism
 Genotype - genetic makeup of an organism
Genotypes and
Phenotypes
 Phenotypes and
Genotypes
Although these
plants have different
genotypes (TT and
Tt), they have the
same phenotype
(tall).
What are the ratios for
genotype and phenotype?
Cross Tt and Tt
T
t
T
TT
Tt
t
Tt
tt
 Genotype ratio - 1:2:1
 1 HoZ tall, 2 HeZ, 1 HoZ short
 Phenotype ratio – 3:1
 3 tall pea plants, one short pea plant
Using ratios from Punnett Squares
 Ratio of tall to short pea plants is 3:1
 If we breed heterozygous tall pea plants
with each other and in one generation we
made 1000plants, How many tall pea
plants and short pea plants should we
have?
Law of Independent Assortment
 The principle of independent assortment states
that genes for different traits can segregate
independently during the formation of gametes.
 In other words, genes (alleles) of different traits
separate independent of one another. Genes
(alleles) of one trait do not affect genes of
another trait.
 This allows us to cross genes for different traits
at the same time.
Cross of heterozygous
yellow and round peas.
 First, what is the
genotype of the
parents?
 Second, how
many different
gametes can be
formed?
 Third, what are
the different
gametes?
Parent:
RrYy
RY
Ry
rY
ry
Cross of heterozygous
yellow and round peas.
 How many
different
phenotypes do we
have?
4
• What is the ratio
for the
phenotypes?
9:3:3:1
Summary of Mendel’s
Principles
 The inheritance of biological characteristics is
determined by individual units known as genes. In
organisms that reproduce sexually, genes are passed
from parents to their offspring.
 In cases in which two or more forms of the gene for a
single trait exist, some forms of the gene may be
dominant and others may be recessive.
 In most sexually reproducing organisms, each adult
has two copies of each gene—one from each parent.
These genes are segregated from each other when
gametes are formed.
 The alleles for different genes usually segregate
independently of one another.
 1. List the four basic principles of genetics that Mendel discovered in his
experiments. Briefly describe each of these principles.
 2. What is probability? How does probability relate to genetics?
 3. In pea plants, the allele for yellow seeds is dominant to the allele for
green seeds. Predict the genotypic ratio of offspring produced by crossing
two parents heterozygous for this trait. Draw a Punnett square to illustrate
your prediction.
 4. Designing Experiments In sheep, the allele for white wool (A) is
dominant over the allele for black wool (a). How would you determine the
genotype of a white ram, or male sheep?
 5. Inferring Suppose Mendel crossed two pea plants and
got both tall and short offspring. What could have been
the genotypes of the two original plants? What genotype
could not have been present?
 6. Applying Concepts In guinea pigs, the allele for a
rough coat (R) is dominant over the allele for a smooth
coat (r). A heterozygous guinea pig (Rr) and a
homozygous recessive guinea pig (rr) have a total of nine
offspring. The Punnett square for this cross shows a 50
percent chance that any particular offspring will have
smooth coats. Explain how all nine offspring can have
smooth coats.
Beyond dominant and
recessive alleles.
 Most genes do not follow the simple
patterns of dominant and recessive
alleles.
 Some alleles are neither dominant nor
recessive, and many traits are controlled
by multiple alleles or multiple genes.
Beyond dominant and
recessive alleles
 incomplete
dominance situation in which
one allele is not
completely
dominant over
another
 There is no white
no red: new
phenotype pink
Beyond dominant and
recessive alleles
 Co dominance situation in which
both alleles of a
gene contribute
to the phenotype
of the organism
Beyond dominant and
recessive alleles
 multiple alleles - three or more alleles of the same gene
Beyond dominant and
recessive alleles
 polygenic trait trait controlled by
two or more genes
Linkage and Gene Maps
 It’s easy to see how genes located on different
chromosomes assort independently, but what
about genes located on the same
chromosome?
 Wouldn’t they generally be inherited together?
 Thomas Hunt Morgan’s studies back in 1910
helped us to answer this question.
Linkage and Gene Maps
 Just because two genes
are located on the same
chromosome does not
mean that they are linked
together forever.
 Crossing-over
 Also, the further apart
they are the more likely
they are to separate. The
closer they are the less
likely they are to separate.