Difference between revisions of "Make a Punnett Square"
m (importing article from wikihow) |
m (Update ref tag) |
||
| Line 11: | Line 11: | ||
#*If you don't know which gene is dominant, use different letters for the two alleles. | #*If you don't know which gene is dominant, use different letters for the two alleles. | ||
#Check the parents' genotypes. Next, we need to know the genotype each parent has for that trait. Each parent has two alleles (sometimes the same one) for the trait, just like every sexual organism, so their genotype will be two letters long. Sometimes, you'll already know exactly what this genotype is. Other times, you'll have to work it out from other information: | #Check the parents' genotypes. Next, we need to know the genotype each parent has for that trait. Each parent has two alleles (sometimes the same one) for the trait, just like every sexual organism, so their genotype will be two letters long. Sometimes, you'll already know exactly what this genotype is. Other times, you'll have to work it out from other information: | ||
| − | #*"Heterozygous" means it has two different alleles (Ff).<ref>http://www.stat.washington.edu/thompson/Genetics/1.3_genotypes.html</ref> | + | #*"Heterozygous" means it has two different alleles (Ff).<ref name="rf1">http://www.stat.washington.edu/thompson/Genetics/1.3_genotypes.html</ref> |
#*"Homozygous dominant" means it has two copies of the dominant allele (FF). | #*"Homozygous dominant" means it has two copies of the dominant allele (FF). | ||
#*"Homozygous recessive" means it has two copies of the recessive allele (ff). Any parent that shows the recessive trait (has yellow fur) belongs to this category. | #*"Homozygous recessive" means it has two copies of the recessive allele (ff). Any parent that shows the recessive trait (has yellow fur) belongs to this category. | ||
| − | #Label the rows with one parent's genotype. Pick one parent – traditionally the female (mother), but either will work.<ref>https://www.youtube.com/watch?v=prkHKjfUmMs</ref> Label the first row of the grid with one of that parent's allele. Label the second row of the grid with the second allele. | + | #Label the rows with one parent's genotype. Pick one parent – traditionally the female (mother), but either will work.<ref name="rf2">https://www.youtube.com/watch?v=prkHKjfUmMs</ref> Label the first row of the grid with one of that parent's allele. Label the second row of the grid with the second allele. |
#*For example, the female bear is heterozygous for fur color (Ff). Write an F to the left of the first row, and an f to the left of the second row. | #*For example, the female bear is heterozygous for fur color (Ff). Write an F to the left of the first row, and an f to the left of the second row. | ||
#Label the columns with the other parent's genotype. Write the second parent's genotype for the same trait as labels for the columns. This is typically the male's, or father's. | #Label the columns with the other parent's genotype. Write the second parent's genotype for the same trait as labels for the columns. This is typically the male's, or father's. | ||
| Line 23: | Line 23: | ||
#*The bottom left box inherits an f from both parents, to make ff. | #*The bottom left box inherits an f from both parents, to make ff. | ||
#*The bottom right box inherits an f from both parents, to make ff. | #*The bottom right box inherits an f from both parents, to make ff. | ||
| − | #Interpret the Punnett square. The Punnett square shows us the likelihood of creating offspring with certain alleles. There are four different ways the parents' alleles can combine, and all four are equally likely. This means that the combination in each box has a 25% chance to occur. If more than one box has the same result, add up these 25% chances together to get the total chance.<ref>http://knowgenetics.org/dominant-inheritance/</ref> | + | #Interpret the Punnett square. The Punnett square shows us the likelihood of creating offspring with certain alleles. There are four different ways the parents' alleles can combine, and all four are equally likely. This means that the combination in each box has a 25% chance to occur. If more than one box has the same result, add up these 25% chances together to get the total chance.<ref name="rf3">http://knowgenetics.org/dominant-inheritance/</ref> |
#*In our example, we have two boxes with Ff (heterozygous). 25% + 25% = 50%, so each offspring has a 50% chance of inheriting the Ff allele combination. | #*In our example, we have two boxes with Ff (heterozygous). 25% + 25% = 50%, so each offspring has a 50% chance of inheriting the Ff allele combination. | ||
#*The other two boxes are each ff (homozygous recessive). Each child has a 50% chance of inheriting ff genes. | #*The other two boxes are each ff (homozygous recessive). Each child has a 50% chance of inheriting ff genes. | ||
#Describe the phenotype. Often, you're more interested in the children's actual traits, not just what their genes are. This is easy to find in the most basic situation, which is what Punnett squares are usually used for. Add up the chance of each square with one or more dominant alleles to get the chance that the offspring expresses the dominant trait. Add up the chance of each square with two recessive alleles to get the possibility that the offspring expresses the recessive trait. | #Describe the phenotype. Often, you're more interested in the children's actual traits, not just what their genes are. This is easy to find in the most basic situation, which is what Punnett squares are usually used for. Add up the chance of each square with one or more dominant alleles to get the chance that the offspring expresses the dominant trait. Add up the chance of each square with two recessive alleles to get the possibility that the offspring expresses the recessive trait. | ||
#*In this example, there are two squares with at least one F, so each offspring has a 50% chance to have black fur. There are two squares with ff, so each offspring has a 50% chance to have yellow fur. | #*In this example, there are two squares with at least one F, so each offspring has a 50% chance to have black fur. There are two squares with ff, so each offspring has a 50% chance to have yellow fur. | ||
| − | #*Read the problem carefully for more information about the phenotype. Many genes are more complex than this example. For example, a flower species might be red when it has the RR alleles, white when it has rr, or pink when it has Rr. In cases like this, the dominant allele is then referred to as an '''incomplete dominant allele.'''<ref>http://anthro.palomar.edu/mendel/mendel_3.htm</ref> | + | #*Read the problem carefully for more information about the phenotype. Many genes are more complex than this example. For example, a flower species might be red when it has the RR alleles, white when it has rr, or pink when it has Rr. In cases like this, the dominant allele is then referred to as an '''incomplete dominant allele.'''<ref name="rf4">http://anthro.palomar.edu/mendel/mendel_3.htm</ref> |
<span id="background"></span> | <span id="background"></span> | ||
===Background Information=== | ===Background Information=== | ||
#Review genes, alleles, and traits. A gene is a piece of "genetic code" that determines a trait in a living organism – for example, eye color. But eye color can be blue, or brown, or various other colors. These variations of the same gene are called '''alleles'''. | #Review genes, alleles, and traits. A gene is a piece of "genetic code" that determines a trait in a living organism – for example, eye color. But eye color can be blue, or brown, or various other colors. These variations of the same gene are called '''alleles'''. | ||
| − | #Understand genotype and phenotype. All your genes together make your '''genotype''': the entire length of DNA that describes how to build you. Your actual body and behavior are your '''phenotype''': how you ended up, partly because of genes but also because of diet, possible injury, and other life experiences.<ref>http://evolution.berkeley.edu/evosite/evo101/IIIA1Genotypevsphenotype.shtml</ref> | + | #Understand genotype and phenotype. All your genes together make your '''genotype''': the entire length of DNA that describes how to build you. Your actual body and behavior are your '''phenotype''': how you ended up, partly because of genes but also because of diet, possible injury, and other life experiences.<ref name="rf5">http://evolution.berkeley.edu/evosite/evo101/IIIA1Genotypevsphenotype.shtml</ref> |
#Learn about gene inheritance. In sexually reproducing organisms, including humans, each parent passes on one gene for each trait. The child keeps the genes from both parents. For each trait, the child might have two copies of the same allele, or two different alleles. | #Learn about gene inheritance. In sexually reproducing organisms, including humans, each parent passes on one gene for each trait. The child keeps the genes from both parents. For each trait, the child might have two copies of the same allele, or two different alleles. | ||
| − | #*An organism with two copies of the same allele is '''homozygous''' for that gene.<ref | + | #*An organism with two copies of the same allele is '''homozygous''' for that gene.<ref name="rf1" /> |
#*An organism with two different alleles is '''heterozygous''' for that gene. | #*An organism with two different alleles is '''heterozygous''' for that gene. | ||
#Understand dominant and recessive genes. The simplest genes have two alleles: one dominant and one recessive. The dominant variation shows up even if a recessive allele is also present. A biologist would say that the dominant allele is "expressed in the phenotype." | #Understand dominant and recessive genes. The simplest genes have two alleles: one dominant and one recessive. The dominant variation shows up even if a recessive allele is also present. A biologist would say that the dominant allele is "expressed in the phenotype." | ||
| − | #*An organism with one dominant allele and one recessive allele is '''heterozygous dominant'''. These organism are also called '''carriers''' of the recessive allele, since they have the allele but don't show the trait.<ref | + | #*An organism with one dominant allele and one recessive allele is '''heterozygous dominant'''. These organism are also called '''carriers''' of the recessive allele, since they have the allele but don't show the trait.<ref name="rf1" /> |
#*An organism with two dominant alleles is '''homozygous dominant'''. | #*An organism with two dominant alleles is '''homozygous dominant'''. | ||
#*An organism with two recessive alleles is '''homozygous recessive'''. | #*An organism with two recessive alleles is '''homozygous recessive'''. | ||
| Line 51: | Line 51: | ||
== Tips == | == Tips == | ||
*You can use any letter you want – it doesn't have to be F and f. | *You can use any letter you want – it doesn't have to be F and f. | ||
| − | *There's no special part of the genetic code that makes one allele dominant. We just see which trait is visible with only one copy of it, then call the allele that caused that trait "dominant."<ref>http://learn.genetics.utah.edu/content/inheritance/patterns/</ref> | + | *There's no special part of the genetic code that makes one allele dominant. We just see which trait is visible with only one copy of it, then call the allele that caused that trait "dominant."<ref name="rf6">http://learn.genetics.utah.edu/content/inheritance/patterns/</ref> |
| − | *You can study the inheritance of two genes at once by using a 4 x 4 grid, and a four-allele code for each parent.<ref>http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html</ref> You can scale this up to any number of genes (or genes with more than two alleles), but the square quickly becomes huge.<ref>http://scienceprimer.com/punnett-square-calculator</ref> | + | *You can study the inheritance of two genes at once by using a 4 x 4 grid, and a four-allele code for each parent.<ref name="rf7">http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html</ref> You can scale this up to any number of genes (or genes with more than two alleles), but the square quickly becomes huge.<ref name="rf8">http://scienceprimer.com/punnett-square-calculator</ref> |
== Related Articles == | == Related Articles == | ||
