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Genetics: Punnett Square Notes
First…some vocab!
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Let’s Review… What is heredity? What is a gene? What is an allele?
The passing of traits to offspring What is a gene? A specific section on a strand of DNA that codes for a certain trait What is an allele? Different versions of a gene
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Vocab: Trait- physical characteristic that we see
Dominant- expressed allele (overpowers and covers up recessive allele, if present) Recessive- unexpressed allele (covered up by dominant gene)
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Phenotype- the physical appearance of a trait in an organism
Genotype- the genes of an organism; for one specific trait we use two letters to represent the genotype. Uppercase letter= dominant allele Lowercase letter= recessive allele. Phenotype- the physical appearance of a trait in an organism
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Homozygous- two same alleles (2 dominant or 2 recessive) [purebred]
Heterozygous- two different alleles- 1 dominant and 1 recessive [hybrid]
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Tt QQ Ff rr Yy hh mm Ss GG PP Ll Bb dd
Underline the following that are homozygous and circle the ones that are heterozygous: Tt QQ Ff rr Yy hh mm Ss GG PP Ll Bb dd
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B= brown eyes b= blue eyes
What would the genotype be for an individual who is heterozygous? ______ What would be the phenotype be for an individual who is heterozygous? _________ If a person has blue eyes, what must their genotype be? _______ Bb Brown eyes bb
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Punnett Square What is it?
A diagram used to predict the probability of traits being passed from parents to offspring. What do I need to know before doing a Punnett Square? Both parents’ genotypes for that trait (Tt x tt) Does the Punnett Square guarantee anything?? No! It just predicts the probability of traits being passed on to offspring.
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Important Note! Probability is NOT influenced by previous results
Each offspring has the exact same chance of getting a trait as previous offspring Ex: every time a baby is born, there is a 50% chance of being a boy or a girl.
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Step-by-Step Punnett square review
1st. Draw your square Parental Cross: Tt x tt
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2nd. Then place the parent’s genotypes on the outside
2nd. Then place the parent’s genotypes on the outside. Each allele goes next to its own box!! Parental Cross: Tt x tt
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T t t t 3rd. Complete the square. (“Drag & Drop”)
Parental Cross: Tt x tt 3rd. Complete the square. (“Drag & Drop”) T t t t
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Genotypes (letters): Phenotypes (physical appearance):
4th. Summarize your results, using percentages.
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Sample Problem #1 1. An elephant with large ears (EE) is crossed with an elephant with small ears (ee). What are the expected offspring?
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Sample Problem #2 2. A heterozygous white chicken, which is the dominant allele, is crossed with a red chicken (recessive). What is the probability of having a red chicken?
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Now it is your turn! Refer to vocab list as you work
You must show a completed Punnett Square for each problem. Answer Key will be posted on screen in a few minutes
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Answer Key- check yourself!
1. BB x bb G: 100% Bb P: 100% Black bulls 2. Pp x pp G: 50% Pp, 50% pp P: 50% horned, 50% polled 3. Ss x Ss G: 25%SS, 50%Ss, 25%ss P: 75% Large, 25% Small First Generation- SS x ss G: 100% Ss P: 100% Long hair Second Generation- Ss x Ss G: 25%SS, 50%Ss, 25%ss P: 75% Long, 25% short “Genetic Problem Solving” wkst is due TOMORROW.
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