Punnett Squares

Jacob Nikodym Author

07/20/2022 Added

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This video demonstrates the process of using Punnett Squares to predict genotypes.

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[00:00:07.710]This video will demonstrate the use
[00:00:09.960]of Punnett squares for predicting genetic inheritance.
[00:00:13.920]Punnett squares are a useful tool in the study of genetics.
[00:00:18.450]Application of this tool allows us to predict
[00:00:21.750]the probability of the genotype, and thereby traits,
[00:00:25.284]of a new sexually-produced offspring
[00:00:28.470]based on the known genotypes of parents.
[00:00:31.890]Punnett squares can likewise be used to deduce
[00:00:34.860]parent genotypes based on the phenotypes
[00:00:38.130]and phenotype ratios of the offspring.
[00:00:41.700]To demonstrate the process of using a Punnett square
[00:00:45.420]in prediction and deductions, we will use an example
[00:00:49.140]of a hypothetical Roundup resistance gene
[00:00:51.787]using a capital 'R' to represent a dominant resistant allele,
[00:00:56.456]and a lowercase 'r' to represent
[00:00:58.965]a recessive susceptible allele.
[00:01:01.920]The capital R allele is dominant
[00:01:04.620]because plants that have one capital R allele per cell,
[00:01:08.475]or the heterozygous genotype capital R, lowercase r,
[00:01:13.075]or two capital R alleles per cell,
[00:01:17.190]the homozygous capital R, capital R genotype,
[00:01:20.940]have the same phenotype response to a Roundup
[00:01:24.240]or glyphosate herbicide application.
[00:01:27.570]The Punnett square accounts for two biological processes
[00:01:32.070]in sexual reproduction.
[00:01:34.260]First, the formation of gametes,
[00:01:36.939]and then the random combination of these gametes
[00:01:41.070]to form the genotypes among the offspring.
[00:01:44.130]Here is how you work with the Punnett square tool.
[00:01:48.240]First, place the pair of genes from one parent
[00:01:51.875]on the top of the square
[00:01:53.715]and the pair from the other parent on the side.
[00:01:57.480]These genes represent the gametes,
[00:02:00.330]with one allele assigned to each row or column
[00:02:04.110]because the pair of genes in the somatic cells
[00:02:07.890]separates when gametes are formed.
[00:02:10.800]These single letters on the top
[00:02:13.154]and the side of the Punnett square represent the genes
[00:02:16.950]inside the gametes, or sex cells.
[00:02:19.800]Next, we will copy each of the alleles
[00:02:22.860]from the top of the square to each of the boxes
[00:02:25.748]in their column, and each of the alleles
[00:02:28.503]from the side of the square
[00:02:30.450]to each of the boxes in their row.
[00:02:33.270]This creates the possible genotypes of the offspring
[00:02:37.260]and the expected ratios at which they will occur
[00:02:40.560]as a result of the gametes randomly uniting
[00:02:43.372]during sexual fertilization of the pollen
[00:02:46.860]onto stigma and style, which lead to the eggs.
[00:02:50.610]In our example, one resulting genotype
[00:02:53.550]is homozygous dominant, or resistant,
[00:02:57.480]two are heterozygous, both also resistant,
[00:03:01.890]and one is homozygous recessive, or susceptible.
[00:03:06.090]This predicts that approximately 75% of the offspring
[00:03:10.890]would be resistant with the homozygous dominant
[00:03:14.670]or heterozygous genotypes.
[00:03:18.330]And approximately 25% of them would be susceptible
[00:03:23.070]with the homozygous recessive genotype.
[00:03:26.250]The Punnett square tool likewise can be applied
[00:03:29.220]to predict parent genotypes if we have information
[00:03:32.640]about their sexually reproduced offspring.
[00:03:35.520]Assume that seeds are collected from a plant
[00:03:38.640]that we know had the Roundup resistant phenotype,
[00:03:42.000]homozygous dominant or heterozygous.
[00:03:45.210]We also know that all the seeds are produced
[00:03:48.480]by a self-pollination in this plant
[00:03:51.030]since we grew it in isolation.
[00:03:53.970]When a sample of these seeds is planted,
[00:03:57.090]and the seedlings treated with Roundup,
[00:03:59.760]both resistant and susceptible offspring are observed.
[00:04:03.960]What is the genotype of the Roundup resistant parent?
[00:04:08.580]If we examine the two Punnett square diagrams shown here,
[00:04:12.390]one for selfing a homozygote and one for a heterozygote,
[00:04:17.430]we can deduce that the parent must have been heterozygous.
[00:04:23.190]We can repeat this Punnett square-based deduction
[00:04:26.880]from a cross-pollinated family.
[00:04:29.400]Assume a female plant that is Roundup resistant
[00:04:32.790]is crossed to a single male,
[00:04:35.010]but we do not have phenotype information on this male.
[00:04:38.850]We observe both Roundup resistant
[00:04:41.250]and susceptible offspring from this cross.
[00:04:44.430]From this observation, we can deduce the female
[00:04:48.510]was heterozygous, and not homozygous dominant.
[00:04:52.980]But the male could be heterozygous or homozygous recessive.
[00:04:59.550]What if we had a large number of offspring in our cross,
[00:05:03.090]and we could determine a fair ratio of the resistant
[00:05:07.020]to the susceptible individuals?
[00:05:09.810]If this ratio is close to one to one,
[00:05:12.930]we would propose that the male
[00:05:14.460]is the homozygous recessive genotype.
[00:05:17.580]If the ratio was closer to three to one,
[00:05:20.310]we would deduce that the male was heterozygous.
[00:05:24.090]Many different cross-breeding scenarios
[00:05:26.670]can be predicted using a Punnett square
[00:05:29.700]by changing the genotypes of the parents.
[00:05:33.150]Ratios of phenotypes observed among offspring
[00:05:36.930]can provide reliable evidence of the genes
[00:05:40.080]the parents possessed and pass on to their offspring.

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Punnett Squares

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