# Genetics problems- punnett squares

• Biology
• colton4286
In summary, a Punnett square is a graphical tool used to predict the possible outcomes of a genetic cross between two individuals. To use a Punnett square, you first need to determine the genotypes of the two parents and then fill in the squares with the corresponding alleles to predict the possible genotypes and phenotypes of the offspring. Its purpose is to help scientists visualize and predict inheritance patterns and the likelihood of certain traits being passed down. There is a difference between a monohybrid and dihybrid Punnett square, with the former predicting outcomes for a single trait and the latter for two different traits. However, Punnett squares are not always accurate as they are based on probability and do not take into account other factors such as genetic mutations
colton4286
I need help with these:

1. Charles was married once before, & he and his first wife had a child who suffers from cystic fibrosis. His current wife Elaine's brother died of cystic fibrosis. What is the probability that Charles & Elaine will have a baby with cystic fibrosis?

Let's say A= normal, a= has cystic fibrosis. If Elaine's brother's genotype is aa, that means both parents carry the recessive allele. From here, I am stuck. How do I know whether the parents are both heterozygous or if one parent is homozygous recessive & the other parent heterozygous. How do I know if Elaine is AA or Aa?

2. T= tall, t= short; Y= yellow, y= green
A green tall pea plant is crossed with a yellow short plant. Their offspring are as follows:

77 green short
81 yellow short
79 yellow tall
84 green tall

What are the genotypes of the parent plants?

Since there are 4 different phenotypes, I'm assuming that the offspring came from the selfing of a plant with genotype YyTt. I found the phenotypic ratios from this data and they all seem fairly close to each other, almost the same (24%, 25%, 25%, 26%) but don't know where to go from here.

colton4286 said:
I need help with these:

1. Charles was married once before, & he and his first wife had a child who suffers from cystic fibrosis. His current wife Elaine's brother died of cystic fibrosis. What is the probability that Charles & Elaine will have a baby with cystic fibrosis?

Let's say A= normal, a= has cystic fibrosis. If Elaine's brother's genotype is aa, that means both parents carry the recessive allele. From here, I am stuck. How do I know whether the parents are both heterozygous or if one parent is homozygous recessive & the other parent heterozygous. How do I know if Elaine is AA or Aa?
You don't know. You have to factor that into your determination of the probability. She could be a carrier or she might be completely unaffected, so you have to work out the probabilities with both choices.

2. T= tall, t= short; Y= yellow, y= green
A green tall pea plant is crossed with a yellow short plant. Their offspring are as follows:

77 green short
81 yellow short
79 yellow tall
84 green tall

What are the genotypes of the parent plants?

Since there are 4 different phenotypes, I'm assuming that the offspring came from the selfing of a plant with genotype YyTt. I found the phenotypic ratios from this data and they all seem fairly close to each other, almost the same (24%, 25%, 25%, 26%) but don't know where to go from here.
Unless there's more to the problem that you haven't shown, there's nowhere to go from here. You've found the answer already.

1. To determine the probability of Charles and Elaine having a baby with cystic fibrosis, we need to first determine their genotypes. Since Elaine's brother had cystic fibrosis, it is likely that both of her parents were carriers of the recessive allele. This means that Elaine has a 50% chance of being a carrier (Aa) and a 50% chance of being normal (AA).

For Charles, since his previous child had cystic fibrosis, we can assume that he is a carrier (Aa) since he did not have the disease himself.

Now, to determine the probability of their child having cystic fibrosis, we can use a Punnett square. The possible genotypes of the child are AA, Aa, and aa.

| | A | a |
| A | AA | Aa |
| a | Aa | aa |

Based on this Punnett square, there is a 25% chance of having a child with cystic fibrosis (aa genotype). However, it is important to note that this is just a probability and does not guarantee that their child will have the disease.

2. To determine the genotypes of the parent plants, we can use the phenotypic ratios to work backwards.

From the data given, we know that the offspring had the following genotypes: YyTt, yyTt, yytt, and YyTT.

To have a green short offspring, the parent plants must have the genotype yytt.

To have a yellow short offspring, the parent plants must have the genotype yyTt.

To have a yellow tall offspring, the parent plants must have the genotype YyTT.

To have a green tall offspring, the parent plants must have the genotype YyTt.

So, the possible genotypes for the parent plants are YyTt and yytt. However, without more information, we cannot determine which of these genotypes belongs to which parent plant.

## What is a Punnett square?

A Punnett square is a graphical tool used to predict the possible outcomes of a genetic cross between two individuals.

## How do you use a Punnett square?

To use a Punnett square, you first need to determine the genotypes of the two parents. Then, you can fill in the squares with the corresponding alleles to predict the possible genotypes and phenotypes of the offspring.

## What is the purpose of a Punnett square?

The purpose of a Punnett square is to help visualize and predict the outcomes of genetic crosses, allowing scientists to better understand inheritance patterns and the likelihood of certain traits being passed down.

## What is the difference between a monohybrid and dihybrid Punnett square?

A monohybrid Punnett square predicts the outcomes of a genetic cross involving a single trait, while a dihybrid Punnett square predicts the outcomes of a cross involving two different traits.

## Are Punnett squares always accurate?

No, Punnett squares are based on probability and can only predict the likelihood of certain outcomes. In reality, there are many factors that can influence the inheritance of traits, such as genetic mutations, environmental factors, and incomplete dominance.

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