3.12: Punnett Squares
- Page ID
- 62212
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A Punnett square is a grid or matrix that represents the outcomes of different combinations. They are often presented as proofs of Mendel's Principle of Segregation and Principle of Independent Assortment, but Punnett squares came after Mendel, and I think it's important to understand the steps Mendel went through in his research: empirical observations of pea plant variations, breeding true-breeding plants, crossing specific traits, getting weird results, counting them, working out simple ratios, explaining the ratios as biological Principles as to how the peas (and all life, including humans) reproduce and transmit the information using traits from parent to offspring. Punnett squares are graphic representations of sexual reproduction: all the possible sperm are one axis, all the possible eggs on the other, and in the middle are all the possible combinations of fertilization – the individual zygotes (fertilized egg) who develop into fetuses, babies, and then adults. About a hundred years after Mendel's experiment we got to look in a microscope to confirm Mendel's mathematics and we continue to explore Mendelian traits in humans.
If you take true breeding plants with two different traits, like form of seed and color of seed-coat, cross them together, you first get all of the dominant trait. Then if you cross those new versions again, you get some interesting numbers of outcomes: 9:3:3:1 The numbers reveal that there's no connection between the traits; the traits are independently assorted. We can now explain this with cellular biology because the two traits are on different chromosomes.
Terms to know for Punnett Squares:
Allele: a variant of a gene (Ex: for the gene for eye color, the alleles are blue, green, hazel and brown)
Genotype: the two alleles for a gene, written with a letter that stands for the trait (Ex: TT or Tt)
Phenotype: the physical representation of the trait (Ex: right-handed or Type B blood)
Homozygous dominant: when the genotype has two dominant alleles; it is written with two capital letters (TT)
Homozygous recessive: when the genotype has two recessive alleles; it is written with two lower-case letters (tt)
Heterozygous: when the genotype has one dominant and one recessive (Tt)
Here is an example using Tay-Sachs disease. The HEXA gene on chromosome 15 makes part of an enzyme that is important for maintaining your central nervous system. If you have one or two normal alleles, you're OK, but if both your alleles have a Tay-Sachs mutation, then you'll have different neurological problems usually starting as an infant. If you are a genetic counselor and a couple comes to you planning to have kids, and they are both carriers (heterozygotes), you want to be able to tell them what is the chance their baby will have Tay-Sachs. If we assign symbols to alleles, "t" = a Tay-Sachs mutation, and "T" = normal HEXA allele, then we can diagram the possible outcomes of fertilization.
|
|
T |
t |
|---|---|---|
|
T |
TT |
Tt |
|
t |
Tt |
tt |
Statistically, 25% of their children will be normal (TT), 50% of their children will be carriers (Tt), and 25% of their children will be born with Tay-Sachs (tt). This principle works with most recessive diseases.
