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Traits and
Probability
Genes
• A gene codes for a specific trait (ex: eye color)
• Alleles of a gene are the different forms it can
take (ex: blue, brown, green)
• You inherit two copies of each gene (remember,
those are your homologous chromosomes!)
– One from each parent
• Dominant genes are always expressed
• Recessive genes only show up if no dominant
genes are present
Punnett Squares
• Predicts the ratio of possible genotypes that
can result from a cross.
• Offspring inherit two copies of each gene…
one from each parent.
• When gametes are made during fertilization,
the egg or sperm gets one copy of a gene
• This contributes to genetic diversity in the
offspring!
Monohybrid Cross
• Simple test cross looking at just one trait
• Predicts the ratio of possible genotypes and
phenotypes in offspring
Monohybrid Cross Results
• Homozygous-Homozygous
– All offspring have same
genotype and phenotype
• Heterozygous-Homozygous
– 1:1 ratio of genotypes and
phenotypes
• Heterozygous-Heterozygous
– 1:2:1 genotypic ratio
– 3:1 phenotypic ratio
Monohybrid Practice
1. An albino is a person with no pigmentation in his
skin, eyes or hair. Normal pigmentation is (A) is
dominant over albino (a). A normal pigmented
man (whose mother was albino) marries a
homozygous normal woman. What would the
expected appearances of the children be?
2. If the man in problem #1 married a normal
woman (whose father was albino), what percent
of the children would be expected to be albino?
Monohybrid Practice
1. . In chickens, yellow legs (Y) are dominant over
white legs (y). A yellow legged rooster was
crossed with a white legged hen. Both kinds of
offspring were produced. What are the
genotypes of the parents and the offspring?
2. A yellow legged rooster was crossed with a
white legged hen. All fifteen of the offspring
were yellow legged. What is the genotype of the
rooster?
Practice Test cross
Suppose you work for a company that sells plant seeds.
You are studying a plant species in which the dominant
phenotype is pink flowers (PP or Pp). The recessive
phenotype is white flowers (pp). Customers have been
requesting more plants with pink flowers. To meet this
demand, you need to determine the genotypes of some
of the plants you are currently working with.
• You cross a pink plant (Plant A) with a white plant
(Plant B). The resulting cross yields 6 pink plants and 6
white plants. Use Punnett squares to determine the
genotype of Plant A.
Dihybrid Cross
• Follow the inheritance of two different traits
• Law of Independent Assortment
– Allele pairs separate independently during gamete
formation
– Different traits are inherited separately
Dihybrid Practice
Corn is bred for traits that improve its usefulness for specific purposes.
For example, it may be bred to grow in various climates, to produce
more corn, or to be better tasting. These traits depend on the alleles
inherited by the corn plant. Suppose you are studying the color and
texture of kernels on a cob. Kernels can be either purple (R), which is
the dominant color, or yellow (r). Kernels can also be smooth (T) or
wrinkled (t). You want to to cross two corn plants with the following
genotypes: Rrtt and RrTT.
1. List the possible genotypes and phenotypes that can be produced
by this cross.
2. What is the genotypic ratio resulting from this cross? The
phenotypic ratio?
3. If the genotypes for kernel texture of two plants are tt and tt,
what is the probability of their having offspring with smooth
kernels? Why?
Independent Practice
In summer squash, white fruit color (W) is
dominant over yellow fruit color (w) and diskshaped fruit (D) is dominant over sphere-shaped
fruit (d). If a squash plant true-breeding for white,
disk-shaped fruit (WWDD) is crossed with a plant
true-breeding for yellow, sphere-shaped fruit
(wwdd), what will the phenotypic and genotypic
ratios be for:
a. the F1 generation?
b. the F2 generation?
Independent Practice
1. About 70% of Americans get a bitter taste from the
substance called phenylthiocarbamide (PTC). It is tasteless to
the rest. The "taster" allele is dominant to non-taster. Also,
normal skin pigmentation is dominant to albino. A normally
pigmented woman who is taste-blind for PTC has an albinotaster father. She marries an albino man who is a taster,
though the man's mother is a non-taster. Show the expected
offspring of this couple.
2. In pigeons the checkered pattern is caused by a dominant
allele. A plain (non-checkered) pattern is recessive. Red color
is also caused by a dominant allele and brown color by a
recessive allele.
a.
b.
c.
Show the expected offspring of a cross between a homozygous
checkered red bird and a plain brown one. Carry out this cross
through the F2 generation.
What fraction of the F2 offspring would be expected to show both
recessive traits?
What fraction would be expected to show both dominant traits?

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