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Punnett Squares were invented by Reginald
Punnett to help predict the genetic outcomes of crosses
between parents carrying specific traits.
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A Punnett Square is made up of four smaller squares.Each square is worth 25%.
T = tall (dominant)
t= short (recessive)
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The different forms of the same trait or gene are called
alleles. For example, the gene that controls height in pea plants has one allele for tall
height and one for short height.
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An offspring inherits two alleles for each trait. An offspring may inherit two
alleles for the same form of a trait or two different
alleles.
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When two alleles are present, the form of the dominant allele appears. A dominant allele is
the one that occurs most often. It is represented with a capital letter. It only takes one
gene for it to show up.
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The allele that is hidden when a dominant allele is
present is called the recessive allele. The form of the trait determined by It
occurs least often. The recessive allele appears only
when two recessive alleles
are inherited.
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Dominant and recessive alleles can be used to
explain Mendel’s observations. An organism
that has two identical (same) alleles for a trait is called a
purebred. Miss Ellie, fromTennessee, wasvoted the ugliestpurebred in the U.S.
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Another name for purebred is homozygous.
The prefix homo means same. A purebred has the
same alleles, either two dominant (TT) or
two recessive (tt).This horse is a purebred Arabian stallion.
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An organism that has two different alleles for a trait is called a hybrid. In a hybrid, the dominant allele appears,
so all the F1 generation plants were tall.
These hybrid Labradoodle puppies had purebred Poodle and Labrador Retriever parents.
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Another name for hybrid is
heterozygous. The prefix hetero means
different. A hybrid has two different alleles
(Tt), but the dominant trait is the one that
shows.
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When plants from the F1
generation produced offspring, some of the
plants received two dominant alleles, some
received one dominant and one recessive allele, and
some received two recessive alleles.
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The plants that received two recessive
alleles were short. All others were tall.
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The phenotype is the physical characteristic that is expressed like blue eyes or brown
eyes. Remember:
phenotype = physical
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T T
t
t
Tt Tt
Tt
Tt
This Punnett Square shows purebred dominant (TT) and purebred recessive
(tt) parents.
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T t
T
t
TT Tt
Tt tt
This Punnett Square shows parents that are both hybrids. They have one dominant
allele (T) and one recessive (t).
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Punnett squares can be used to calculate the probability or
likelihood that a certain combination will occur among the offspring. The higher the probability, the more likely
it is to happen.
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In the above Punnett square, the probability that the flower will
be purple is 3 out of 4 or ¾ which is .75 or 75%.
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Look again at the chart for plant height. What is the probability
that the offspring will be tall?
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Test Practice
1. Genes are carried by structures called _________.
A. chromosomes B. alleles C. hybrids D. dominants
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2. A scientist crossed a tall pea plant with a short pea plant.
Out of the offspring, 42 plants were tall and 14 were short.
What percentage were short?A. 10%B. 13%C. 25%D. 50%
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3. A scientist crossed a purebred long-winged fruit fly with a purebred short-winged
fly. Long wings are dominant over
short wings. What is the probability that the offspring
will have long wings? A) 25% B) 50% C) 75% D) 100%
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4. In humans, the allele for a cleft chin (C) is dominant. A
woman with the genotype cc has children with a man with
the genotype Cc. If the couple has six children, how many do you think would have a cleft
chin?
A) 0 B) 1 C) 3 D) 6