Genotypic Ratios and Phenotypic Ratios for Punnett Squares

Summary

This video explains how to determine genotype and phenotype ratios using a Punnett square. It covers calculating genotype ratios (homozygous dominant, heterozygous, homozygous recessive) and converting them into percentages. Then, it discusses phenotype ratios, focusing on the dominant and recessive traits (e.g., tall vs. short, red vs. white). A final example introduces codominance, where both traits are equally expressed, and the ratio of genotypes and phenotypes is explained. The video provides clear steps for calculating and understanding genetic outcomes in different inheritance scenarios.

Takeaways

• 🔬 The video focuses on determining genotype and phenotype ratios using a Punnett square.
• 🧬 The genotype ratio includes homozygous dominant, heterozygous, and homozygous recessive types.
• 📊 In the example, the genotype ratio is 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive.
• 📉 Percentages for genotype are calculated based on the Punnett square, with each box representing 25%.
• 🌱 For the example involving tall and short traits, the genotype percentages are 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive.
• 🌿 Phenotype ratio refers to the observable traits, with 75% tall (dominant) and 25% short (recessive) in the first example.
• 🌸 In the second example, involving red and white flowers, heterozygous parents result in a 50% red and 50% white phenotype ratio.
• 🌻 The genotype ratio in the flower example is 50% heterozygous and 50% homozygous recessive.
• 🐓 The final example discusses codominance in chickens, where black, white, and spotted traits are expressed.
• 📑 Codominant genotype ratio in the chicken example is 1 black, 2 spotted, and 1 white, which corresponds to a 25%, 50%, and 25% ratio respectively.

Q & A

What is the difference between genotype and phenotype?

Genotype refers to the genetic makeup or code (e.g., homozygous dominant, heterozygous, or homozygous recessive), while phenotype refers to the physical expression of that genetic code (e.g., tall or short in plants).

How do you calculate the genotype ratio using a Punnett square?

To calculate the genotype ratio, count the occurrences of each genotype: homozygous dominant, heterozygous, and homozygous recessive. Then, express them as a ratio. For example, if there is 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive, the ratio would be 1:2:1.

How is the percentage for each genotype determined?

Each box in a Punnett square represents 25%. So, for each genotype, count how many times it appears, multiply by 25%, and you'll get the percentage. For example, if a genotype appears in 2 boxes, it accounts for 50%.

What is the genotype ratio for a heterozygous parent crossed with a homozygous recessive parent?

The genotype ratio would be 0 homozygous dominant, 2 heterozygous, and 2 homozygous recessive. This can be simplified to a 1:1 ratio of heterozygous to homozygous recessive.

How do you calculate the phenotype ratio?

To calculate the phenotype ratio, determine which traits are expressed. For example, if the dominant trait is tall, count how many offspring have that trait. The ratio is determined by comparing the dominant to the recessive phenotypes.

What is the phenotype ratio for a cross where tall is dominant over short?

If tall is dominant over short, and there are 3 tall and 1 short offspring, the phenotype ratio would be 3:1. This corresponds to 75% tall and 25% short.

How would you express a genotype ratio as a percentage?

To express a genotype ratio as a percentage, divide the occurrences of each genotype by the total number of squares (4 in a Punnett square) and multiply by 100. For example, if 2 boxes show heterozygous, it accounts for 50%.

What is the phenotype ratio in a case of codominance, such as black and white spotted chickens?

In codominance, both traits are expressed equally. For example, if you cross black and white spotted chickens, the phenotype ratio could be 1 black, 2 spotted (black and white), and 1 white, resulting in a 1:2:1 ratio.

What happens in a cross between heterozygous red flowers and homozygous white flowers?

In this cross, red is dominant, so half of the offspring will be heterozygous red (50%) and the other half will be homozygous white (50%), resulting in a phenotype ratio of 1:1 or 50% red and 50% white.

What does a 1:2:1 phenotype ratio indicate in a codominance situation?

A 1:2:1 phenotype ratio in codominance indicates that one trait is expressed fully in 1 offspring, both traits are expressed in 2 offspring (showing the codominant trait), and the other trait is fully expressed in 1 offspring.

Outlines

00:00

🧬 Understanding Genotype and Phenotype Ratios

In this section, the speaker introduces the topic of genotype and phenotype ratios using a Punnett square. The focus is on calculating these ratios and percentages, particularly in an example where tall (dominant) is crossed with short (recessive). For genotype, the ratios are presented as 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive. The percentages are calculated as 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive. For phenotype, tall (dominant) occurs in 75% of cases, while short (recessive) occurs in 25%.

🌸 Genotype and Phenotype Ratios in a Flower Cross

This example examines a flower cross where red (dominant) is crossed with white (recessive). A heterozygous parent is crossed with a homozygous recessive parent. The genotype ratios are explained as 0 homozygous dominant, 2 heterozygous, and 2 homozygous recessive. When simplified, the genotype ratio is 1:1 (50% heterozygous, 50% homozygous recessive). The phenotype ratio shows that red and white each make up 50%, emphasizing the inheritance of dominant and recessive traits in this scenario.

🐔 Co-dominance Example in Chickens

The final example explores a co-dominance scenario involving chickens, where the phenotypes include black, spotted (speckled), and white. The genotypes are represented as BB (black), BW (spotted), and WW (white). The genotype ratio is 1:2:1, meaning 25% black, 50% spotted, and 25% white. Similarly, the phenotype ratio mirrors this distribution, demonstrating how co-dominance results in a blend of traits in the offspring.

Keywords

💡Genotype

Genotype refers to the genetic makeup or code of an organism, specifically the combination of alleles inherited from the parents. In the video, the speaker explains how to calculate the genotype ratio by counting the different combinations of alleles, such as homozygous dominant, heterozygous, and homozygous recessive. For example, in the Punnett square, the genotype ratio includes 1 homozygous dominant (TT), 2 heterozygous (Tt), and 1 homozygous recessive (tt).

💡Phenotype

Phenotype is the observable physical characteristics or traits of an organism, which are influenced by the genotype. The video demonstrates how to calculate the phenotype ratio, where traits like 'tall' or 'short' plants depend on the dominance of certain alleles. For instance, the phenotype ratio is calculated based on the dominant trait (tall) appearing in 3 out of 4 cases, resulting in a 75% tall and 25% short distribution.

💡Homozygous Dominant

Homozygous dominant refers to an organism that carries two dominant alleles for a particular trait. In the video, the speaker identifies this as one of the genotypes in the Punnett square. For example, if 'T' represents the dominant allele for height, a homozygous dominant organism would have the genotype 'TT'. In the example given, there is one occurrence of homozygous dominant in the Punnett square.

💡Heterozygous

Heterozygous refers to an organism that carries one dominant and one recessive allele for a particular trait. In the video, the heterozygous genotype is labeled as 'Tt', and the speaker notes that it contributes to the genotype and phenotype ratios. For instance, heterozygous organisms in the example Punnett square result in a 50% heterozygous ratio and a phenotype that expresses the dominant trait (tall plants).

💡Homozygous Recessive

Homozygous recessive refers to an organism that carries two recessive alleles for a specific trait. In the video, the speaker discusses how to identify and count homozygous recessive genotypes in the Punnett square. For example, if 't' represents the recessive allele for short plants, a homozygous recessive organism would have the genotype 'tt'. In one example, the homozygous recessive makes up 25% of the genotype ratio, leading to the short plant phenotype.

💡Punnett Square

The Punnett square is a diagram used to predict the genotypes of offspring from a particular cross or breeding experiment. The speaker uses a Punnett square to visually represent how alleles from two parents combine and to calculate genotype and phenotype ratios. The example shows different combinations of alleles for traits such as plant height (tall and short) and flower color (red and white).

💡Dominant Trait

A dominant trait is one that is expressed in the phenotype even if only one allele for it is present. In the video, tall plants and red flowers are used as examples of dominant traits. The speaker explains that any organism with at least one dominant allele will express the dominant trait. For example, a 'Tt' or 'TT' genotype will result in a tall plant because the dominant 'T' allele is present.

💡Recessive Trait

A recessive trait is one that is only expressed in the phenotype if the organism has two copies of the recessive allele. The video uses short plants and white flowers as examples of recessive traits. For example, a 'tt' genotype results in a short plant, since no dominant 'T' allele is present to override the recessive trait. The phenotype for recessive traits appears less frequently in offspring ratios when a dominant allele is involved.

💡Codominance

Codominance occurs when both alleles in a genotype are fully expressed, resulting in a phenotype that shows both traits simultaneously. The speaker provides an example using chickens with black and white feathers, where codominance results in spotted chickens. In this scenario, the genotype ratio includes combinations like 'BB' (black), 'BW' (spotted), and 'WW' (white), and the phenotype ratio reflects this distribution.

💡Percentage Calculation

Percentage calculation is the process of converting the ratios of genotypes or phenotypes into percentages to better understand their proportional distribution. The speaker frequently converts the genotype and phenotype ratios into percentages for clarity. For example, in the first Punnett square, the genotype distribution of 1:2:1 is converted into 25%, 50%, and 25% for homozygous dominant, heterozygous, and homozygous recessive, respectively.

Highlights

Introduction to calculating genotype and phenotype ratios using a Punnett square.

Explanation of genotype, which refers to the genetic code, and phenotype, which refers to physical traits.

Example 1: Tall is dominant over short in a plant. The Punnett square is used to determine genotype and phenotype ratios.

Genotype ratio for Example 1: Homozygous dominant (1), heterozygous (2), and homozygous recessive (1).

Explanation of converting genotype ratios to percentages: 25% homozygous dominant, 50% heterozygous, 25% homozygous recessive.

Phenotype ratio for Example 1: 75% tall and 25% short.

Example 2: Red flowers are dominant over white flowers. A heterozygous parent is crossed with a homozygous recessive parent.

Genotype ratio for Example 2: 0 homozygous dominant, 2 heterozygous, and 2 homozygous recessive (1:1 ratio).

Genotype percentages for Example 2: 50% heterozygous and 50% homozygous recessive.

Phenotype ratio for Example 2: 50% red and 50% white.

Example 3: Codominant situation involving chickens with black, spotted, or white feathers.

Genotype ratio for Example 3: BB (1), BW (2), and WW (1), or 1:2:1 ratio.

Genotype percentages for Example 3: 25% BB, 50% BW, and 25% WW.

Phenotype ratio for Example 3: 1 black, 2 spotted, 1 white (1:2:1).

Summary of using Punnett squares to determine genotype and phenotype ratios across different genetic examples.

Transcripts

math and science in this video I'd like

to talk about figuring out the genotype

and phenotype ratio okay so let's go

ahead and get started so we have

completed the punnett square and I've

got it drawn out here and now we need to

calculate how to figure out the ratio

and also maybe the percent and in this

particular example tall is dominant over

short whenever you write the genotype

ratio and remember the genotype is the

genetic code you will write the

homozygous dominant then the

heterozygous then the homozygous

recessive so let's go ahead and write

this out first let's look for homozygous

dominant we have one of those alright of

one and now for heterozygous we have one

two and then for your homozygous

recessive we have one now if we would

like for this to be a percent for your

genotype percent remember each box is

25%

so that would be 25% 50% for

heterozygous and 25% for homozygous

recessive okay so we have 25 percent for

homozygous dominant 50% for heterozygous

25% for homozygous recessive okay

now let's look at the phenotype and the

phenotype will be how many will be tall

and how many will be short so tall will

be anytime it is the dominant trait so

it will be 1 2 3 because anytime you

have a capital letter you get the

dominant trait and then 1 for short

which is the recessive trait and if we

would like you to write that into a

percentage that will be 75% tall and 25%

short ok so there we go let's look at

another example and it has something you

may want to consider okay so we have

let's say red and we'll these will be

flowers red is dominant over

and we have a heterozygous parent

crossed with a homozygous for the

recessive trait so homozygous recessive

there so first let's look for homozygous

dominant for our genotype remember

genotype is your cold and there is zero

for heterozygous we have one two and

four homozygous recessive we have one

two now if you would like you can drop

the zero and that would be 2 to 2 or 1

to 1 now if we want to convert that to a

percentage that would be 1 2 or 50

percent would be heterozygous and 50

percent would be homozygous recessive

okay now let's look at phenotype

remember we're trying to see which one

is red and the percentage or the ratio

of white so the percentage of red to

white and so the red is dominant so that

would be 1/2 and then the white will be

1/2 and if we want to convert that to a

percentage that would be 5050 okay and

finally let's look at one more example

very quickly say you have a codominant

situation and in this particular example

we have a chicken in it can be black

spotted or white spotted is another word

for speculum so here we go I've kind of

set this up to pick up the pace a little

bit

so now instead we will have the ratio of

the genotype B be the genotype B W in

the genotype ww and so that would be 1

be B to be W in 1w or that would be 25

percent 50 percent or 25 percent and the

phenotype ratio kind of the same thing

this will be black this will be

spot it in this will be white so it's a

1 to 2 to 1 so there we go

I hope that helps in setting up a

genotype and phenotype ratio thanks for

watching and moving math uploads a new

math and science video every day please

subscribe and share