Name
Date
Period / Life Science

Monohybrid Cross Notes

Gregor Mendel

·  Austrian monk who started the study of genetics in his monastery’s garden in the 1860s

·  Studied heredity in garden peas

Why did he study peas?

1.  They had a variety of characteristics that occur in two different forms or ______.

2.  He could easily control which plants pollinated each other.

·  Pea plants can ______, which is when a plant’s pollen, which contains the sperm, is used to fertilize the same plant’s egg. If he wanted pure breeding plants, he could allow the plants to self-pollinate over many generations.

·  Pea plants can also ______, which is when the pollen of one plant is used to fertilize another plant. He could do this by removing stamens or male parts of the flowers. He could brush pollen from one flower to the female parts of another flower.

3. Peas produce a lot of offspring quickly, so he could obtain lots of data quickly.

P generation- ______

F1 generation-______

·  F stands for filial or son.

F2 generation-______

3 Steps of Mendel’s Experiments

1. Produce a true-breeding P generation 2. Producing an F1 generation 3. Producing an F2 Generation

Self-pollination

Long Stemmed Plant Long Stemmed Plant

P Generation

Cross-Pollination Cross-Pollination

Self-pollination

Short Stemmed Plant Short Stemmed Plant

P Generation ¾ Long Stemmed Plants

All Long Stemmed Plants ¼ Short Stemmed Plants

F1 Generation F2 Generation

Mendel’s Law of Segregation- ______

______

·  This is because the two different alleles that an organism has are on different chromosomes in a homologous pair.

Parent Cell

Meiosis I

Meiosis II

Gametes

* When you are only dealing with one gene with two alleles, the individual alleles always make up the two possibilities for gametes.

Practice:

What are the possible genetic contents of the gametes made by a person with the genotype BB? ______

What are the possible genetic contents of the gametes made by a person with the genotype Tt? ______

Making Sense of Mendel’s Findings

Probability-______

·  Scientists use probability to predict the phenotypes and genotypes of the offspring.

Punnett Square- ______

·  Punnett squares show predicted results, not actual results.

Offspring 1 / Offspring 2
Offspring 3 / Offspring 4

Example: Parent 1

Gamete 1 Gamete 2

Gamete 1

Parent 2

Gamete 2

Crosses Involving One Trait (Monohybrid Crosses)

Let’s take a look at Mendel’s crosses that he completed to determine his laws of heredity. We will use L to represent the allele for long stems and l to represent the allele for short stem.

1. Homozygous X Homozygous

When he crossed two pure-breeding plants for the same version of the trait, all of the offspring shared the same phenotype as the parents.

LL X LL

Possible Genotypes

Genotypic Ratio

Possible Phenotypes

Phenotypic Ratio

*Each box is filled in with the letter appearing above it and to the left. These are the genotypes of the offspring.*

2. Homozygous Dominant X Homozygous Recessive

When he crossed pure-breeding plants for two different versions of a trait, all of the offspring showed the dominant version of the trait.

LL X ll

Possible Genotypes

Genotypic Ratio

Possible Phenotypes

Phenotypic Ratio

3. Heterozygous X Heterozygous

When he crossed these dominant offspring, he observed that 75% of the resulting offspring had the dominant trait and 25% had the recessive trait.

Ll X Ll

Possible Genotypes

Genotypic Ratio

Possible Phenotypes

Phenotypic Ratio

A good path to follow when completing a genetics problem is:

1)  Pick a letter to represent the gene, if one isn’t given to you.

2)  Record what characteristics the dominant and recessive alleles represent.

3)  Determine the genotypes of the parents.

4)  Complete a Punnett Square.

5)  Determine your results.

Sample Problem:

1. In peas, wrinkled seeds (W) are dominant to smooth seeds (w). If a plant that is homozygous dominant is bred with a plant that is heterozygous, what phenotypic and genotypic ratios would be expected in the offspring?

2. In peas, purple flowers are dominant to white flower. If a plant that is heterozygous for purple flowers is bred with a plant that has white flowers, what percentage of the offspring will have white flowers?

Test Crosses

We can’t tell by looking at an individual with a dominant phenotype if the individual’s genotype is homozygous dominant (BB) or heterozygous (Bb). What can we do to determine the genotype of an individual with a dominant phenotype?

Test Cross- ______

______

Example: In guinea pigs, the dominant allele B codes for black fur, while the recessive allele b codes for white fur. Jimmy has a black guinea pig and wants to determine its genotype. He decides to do a test cross to determine his guinea pig’s genotype. What phenotypic ratio would he expect in his offspring if his guinea pig was homozygous? If it is heterozygous?

Sample Problems:

1. In a species of lizard, green coloring is dominant to brown coloring.

A.  Sally wants to determine the genotype of her green lizard. What type of lizard should she mate her green lizard with to determine its genotype?

B.  Sally carries out this mating. Three of the baby lizards were green and four were brown. What is the genotype of her lizard? Explain your reasoning.

2. In a species of plant, red flowers are dominant to white flowers. Billy cross-pollinates a red flowered plant with a white flowered plant. All of the offspring plants have red flowers. What is the genotype of the red flowered parent plant? Explain your reasoning.