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PUNNETT SQUARE

VOCABULARY

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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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(?)Some interesting hybrids (?)

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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 genotype is the code for the trait like

BB or bb.Remember:

genotype = genes

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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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In the above example, the alleles for the father are written across

the top of the square (TT).

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The alleles for the mother are down the side (tt).

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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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Fill in your own Punnett square to find out the

percentage of long-winged flies.

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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

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Fill in your own Punnett square to find out how many

children will have a cleft chin.