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Interactives -- DNA

Genetics : Punnett Square

An English geneticist, Reginald Punnett, devised an easy way to calculate the probability that a certain trait will be inherited. It involves what is now called a Punnett square. In order to use a Punnett square, it helps to know some common terminology:

Allele: The alternative forms of a gene, like the "tall" and "short" versions of the gene for height in garden peas.

Dominant: An allele that produces the visible or measurable trait in an organism and is expressed over recessive genes. Dominant alleles are represented by a capital letter ("T").

Genotype: The specific combination of alleles possessed by an individual. Example: "homozygous dominant," which means possessing two copies of the dominant allele.

Homozygous: Possessing two copies of the same allele, both dominant or both recessive. Example: "TT" or "tt."

Heterozygous: Possessing two different alleles. Example: "Tt."

Phenotype: The detectable or measurable characteristic of an organism. Example: tall. The phenotype can, but doesn't always, indicate the genotype.

Recessive: An allele that is expressed only when the dominant allele is not present. Recessive alleles are represented by a lowercase letter ("t").

Trait: A feature or characteristic of an organism that can be tested for or observed.

Problem: Cystic Fibrosis

Fill out this Punnett square with the correct letters if both parents are carriers of the cystic fibrosis gene.


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