Genetics
The scientific study of heredity.
Homework Handout
Some vocab for ya!!!!!
• 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 self-
pollinate.
The work of Mendel
• Mendel had true-
breeding 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 Y Y
and yy
y Yy Yy
y Yy Yy
Homework
• Handout (Due Monday)
Crossing true-breeding parent generation
•Trait
P Generation
•Trait
–Green pea –Yellow pea
–recessive –dominant
•Genes Cross Y Y •Genes
YY and
(alleles) yy (alleles)
–yy y Yy Yy –YY
•Gametes y Yy Yy •Gametes
formed formed
F1 Generation
–y and y –Y and Y
Crossing the F1 generation
•Trait F1 Gen. •Trait
–Yellow pea –Yellow pea
•Genes Cross Y y •Genes
Yy and
(alleles) Yy (alleles)
–Yy Y YY Yy –Yy
•Gametes y Yy yy •Gametes
formed formed
F2 Generation
–Y and y –Y and y
Probability and Genetics
•Probability -
likelihood that a
particular event
will occur
Cross Y y
YY
and
yy
Y YY Yy
y Yy yy
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.
Some more vocab for ya!!!
• 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 T t
Tt and
Tt
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 heterzygous 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
Parent:
the parents?
• Second, how RrYy
many different
gametes can be RY Ry rY ry
formed?
• Third, what are
the different
gametes?
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.
Homework
• Studyguide 11-3
• Read Textbook 11-5
• Quiz Tomorrow
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.
Quiz tomorrow Chapter 11
Cross two heterozygous green pod and tall
plant
Give me the ratio of the phenotype
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
• Codominance -
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.