<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment . It is named after Reginald C. Punnett, who devised the approach . The diagram is used by biologists to determine the probability of an offspring having a particular genotype . The Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles . These tables can be used to examine the genotypic outcome probabilities of the offspring of a single trait (allele), or when crossing multiple traits from the parents . The Punnett Square is a visual representation of Mendelian inheritance . It is important to understand the terms "heterozygous", "homozygous", "double heterozygote" (or homozygote), "dominant allele" and "recessive allele" when using the Punnett square method . For multiple traits, using the "forked - line method" is typically much easier than the Punnett square . Phenotypes may be predicted with at least better - than - chance accuracy using a Punnett square, but the phenotype that may appear in the presence of a given genotype can in some instances be influenced by many other factors, as when polygenic inheritance and / or epigenetics are at work . It is a square with 2 lines that look like a window with letters . </P> <P> Zygosity refers to the grade of similarity between the alleles that determine one specific trait in an organism . In its simplest form, a pair of alleles can be either homozygous or heterozygous . Homozygosity, with homo relating to same while zygous pertains to a zygote, is seen when a combination of either two dominant or two recessive alleles code for the same trait . For example, using' A' as the representative character for each allele, a homozygous dominant pair's genotype would be depicted as' AA', while homozygous recessive is shown as' aa' . Heterozygosity, with hetero associated with different, can only be' Aa' because' aA' is not biologically correct . The phenotype of a homozygous dominant pair is' A', or dominant, while the opposite is true for homozygous recessive . Heterozygous pairs always have a dominant phenotype . To a lesser degree, hemizygosity and nullizygosity can also be seen in gene pairs . </P> <P> "Mono" means "one"; this cross indicates that the examination of a single trait . This could mean (for example) eye color . Each genetic locus is always represented by two letters . So in the case of eye color, say "B = Brown eyes" and "b = green eyes". In this example, both parents have the genotype Bb . For the example of eye color, this would mean they both have brown eyes . They can produce gametes that contain either the B or the b allele . (It is conventional in genetics to use capital letters to indicate dominant alleles and lower - case letters to indicate recessive alleles .) The probability of an individual offspring's having the genotype BB is 25%, Bb is 50%, and bb is 25% . The ratio of the phenotypes is 3: 1, typical for a monohybrid cross . When assessing phenotype from this, "3" of the offspring have "Brown" eyes and only one offspring has "green" eyes . (3 are "B_" and 1 is "bb") </P>

What does heterozygous mean in a punnett square
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