monohybrid cross

Summary

This video explains key genetic concepts, including monohybrid crosses and test crosses. It covers the inheritance of traits, such as black and white coat colors in animals, and how to determine genotypes through Punnett squares. The video also illustrates how to predict the phenotype ratio in offspring from different parental crosses, emphasizing the importance of using homozygous recessive parents in test crosses. A brief humor break with the Energizer Bunny joke adds a lighthearted touch to the topic. Overall, the video offers a clear breakdown of fundamental genetic principles with practical examples.

Takeaways

• 😀 A homozygous dominant black (Big B) crossed with a homozygous recessive white (small b) results in 100% black offspring, all heterozygous (Big B small b).
• 😀 A cross between two heterozygous individuals (Big B small b × Big B small b) produces a genotype ratio of 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive.
• 😀 In the example of black and white animals, if both parents are heterozygous, the offspring have a 75% chance of being black and a 25% chance of being white.
• 😀 The phenotype ratio for the offspring of two heterozygous parents is 75% black and 25% white.
• 😀 When working backward, if all offspring are tall, the other parent must carry at least one recessive allele for height, making the other parent homozygous dominant.
• 😀 A test cross is used to determine the genotype of an unknown parent by crossing it with a homozygous recessive individual.
• 😀 In a test cross, the unknown parent's genotype is revealed by observing the offspring, as the homozygous recessive parent can only contribute a recessive allele.
• 😀 In a test cross with known homozygous dominant parent, if all offspring are tall, the other parent is likely heterozygous for height (Big T small t).
• 😀 The test cross method cannot use a dominant parent to determine the unknown genotype—only a recessive parent should be used for this purpose.
• 😀 The concept of working backward in genetics involves using known offspring phenotypes to infer the genotypes of the parents through test crosses.
• 😀 The joke about the Energizer Bunny being arrested for 'battery' adds humor to the script but doesn't contribute to the genetic discussion.

Q & A

What is the result of crossing a homozygous dominant black (Big B) with a homozygous recessive white (small b) in terms of phenotype?

The result will be 100% black offspring, as Big B (dominant) masks the effect of small b (recessive), leading to all heterozygous Big B small b individuals, which are black.

What is the genotype ratio when two heterozygous black individuals (Big B small b) are crossed?

The genotype ratio will be 1 homozygous dominant (Big B Big B), 2 heterozygous (Big B small b), and 1 homozygous recessive (small b small b), leading to a 1:2:1 genotype ratio.

What is the expected phenotype ratio when two heterozygous black individuals (Big B small b) are crossed?

The expected phenotype ratio is 75% black (Big B small b or Big B Big B) and 25% white (small b small b).

In a test cross, if one parent is homozygous dominant (Big T Big T) and the other is homozygous recessive (small t small t), what will the phenotype of the offspring be?

The offspring will all be tall because they will all inherit one Big T allele from the homozygous dominant parent and one small t allele from the homozygous recessive parent, resulting in all heterozygous Big T small t offspring, which are tall.

What is the purpose of a test cross?

The purpose of a test cross is to determine the genotype of an unknown parent by crossing it with a homozygous recessive individual. This helps reveal whether the unknown parent is homozygous dominant or heterozygous.

What should be used in a test cross to determine an unknown parent’s genotype?

In a test cross, a homozygous recessive individual should be used to determine the genotype of the unknown parent, as it allows for clear differentiation in offspring phenotypes.

What would the offspring be if the known parent in a test cross is homozygous dominant (Big T Big T) and the unknown parent is homozygous recessive (small t small t)?

The offspring would all be heterozygous (Big T small t), showing the tall phenotype, because they would inherit one Big T allele from the homozygous dominant parent and one small t allele from the homozygous recessive parent.

Can a dominant parent be used in a test cross to determine the genotype of the other parent?

No, a dominant parent cannot be used in a test cross because it would not provide clear information about whether the unknown parent is homozygous dominant or heterozygous. Only a homozygous recessive parent can be used for this purpose.

What is the main takeaway when performing a test cross with an unknown genotype?

The main takeaway is that the use of a homozygous recessive parent allows for the identification of the unknown parent’s genotype based on the offspring’s phenotypes.

How does a test cross help in genetic analysis?

A test cross helps by allowing researchers to determine whether an individual showing a dominant phenotype is homozygous dominant or heterozygous, by observing the phenotypes of its offspring when crossed with a homozygous recessive individual.