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.