CHAPTER 14

Mendel and the Gene Idea

Gregor Mendel

Known as the father of modern genetics.

Austrian monk who studied pea plants in an abbey garden.

Started his experiment in 1857 to study inheritance in pea plants.

Why pea plants?

There are many different varieties of pea plants to work with, in other words there are many traits that can be tested.

Peas have a short generation time, offspring can be collected in a short period of time.

Peas produce a large number of offspring.

Pea plants can be easily self fertilized or cross pollinated

Breeding Experiment

Mendel cross pollinated two true-breeding plants with different traits.

True-breeding: Plants that produce only offspring with the same particular trait as the parent.

Hybridization: Crossing of two different types of true-breeding plants.

Mendel’s Experiment

P Generation: Parental generation

F1 Generation: P generations hybrid offspring.

F2 Generation: Hybrids of F1 generation

Mendel’s Model

Alternative versions of genes account for variations in inherited characters. Alternative versions of genes are called alleles.

For each character an organism inherits two alleles, one from each parent.

If the two alleles are different then one will determine the organisms appearance, called dominant allele. The other allele is masked, and has no noticeable effect, called recessive allele.

Mendel’s Model (cont)

Law of Segregation: Two alleles for a character segregate during gamete formation and end up in different gametes.

An egg or sperm only gets one of the two alleles present in somatic cells.

Punnett Square

Some useful terms

Homozygous: Organism with a pair of identical alleles, can be homozygous dominant or homozygous recessive.

Heterozygous: Organism with a pair of different alleles.

Phenotype: Organism’s appearance, for example a pea plant has Purple flowers

Genotype: Organism’s actual genetic makeup, for example a plant with Purple flowers is heterozygous, Pp.

Practice!

A pea plant that has purple flowers has the genotype Pp is crossed with a pea plant with white flower and genotype pp. Use a punnett square to predict the F1 generations phenotypic and genotypic ratios.

True-breeding tall pea plants are crossed with true-breeding dwarf pea plants. Use a punnett square to figure out the F1 generation, then cross the F1 hybrids to figure out the F2 generation. Write all phenotypic and genotypic ratios. The tall allele (T) is dominant over the dwarf allele (t).

Test cross

You are walking down the street one day when you come across a purple pea plant. Being the AP Bio students that you are, you wonder “what is this plant’s genotype?” How can you figure out the genotype of this “mystery” pea plant?

Perform a test cross to predict the possible outcomes of breeding these peas.

After performing your experiment you find that the plants produced 654 purple plants and 222 white plants. What can you conclude from these results?

Law of Independent Assortment

The crosses we have been doing are known as Monohybrid crosses. This is because they only follow one character.

Mendel’s second law came when he followed two traits at the same time. Seeds of pea plants may be either yellow or green. They may also be round or wrinkled.

Mendel knew that yellow (Y) was dominant over green (y) and that round (R) was dominant over wrinkled (r).

Law of Independent Assortment

Law of Independent Assortment

Crosses that involve two characters are known as dihybrid crosses.

The Law of Independent Assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation.

This only applies to genes that are located on different chromosomes. Genes located near each other on the same chromosome tend to be inherited together.

Practice!

A pea plant homozygous dominant for inflated pods(LL) and for green pod color (GG) is crossed with a pea plant that is homozygous recessive for constricted pods (ii) and yellow pod color (gg). Use a punnett square to figure out the F1 generation and F2 generation as well as the phenotypic ratios.

A pea plant that is heterozygous for yellow seeds and homozygous dominant for round seeds, YyRR, is self pollinated, use a punnett square to figure out the F1 generation’s phenotypic ratios. (Yellow is dominant over green, and round is dominant over wrinkled).

Further Inquiry!

Is it possible to do a cross for three or more characters?

If we were dealing with a pea plant that is self pollinating and heterozygous for Yellow seeds, Tall stems, and purple flowers (YyTtPp), what would the possible gametes be? How big would your punnett square have to be?

Incomplete Dominance

The examples we have been studying are known as Complete dominance, because the dominant trait completely masks the recessive one.

Some alleles show incomplete dominance. Neither allele is completely dominant over the other.

Incomplete Dominance

Practice!

A white snap dragon flower is crossed with a pink snap dragon flower. Use a punnett square to determine the phenotypes of the F1 generation.

Two pink snapdragon flowers are crossed. Using a punnett square determine the F1 generation.

Codominance

In Codominance neither allele is dominant over the other.

An example of this is the MN blood type

Individuals with homozygous for M have only the M molecules, individuals homozygous for N have only N molecules, and heterozygous individuals MN have both molecules.

Why is this different from incomplete dominance?

Multiple Alleles

In the examples thus far only two alleles exist for the trait. In some cases there are more than two alleles.

An example of this is the ABO blood group. Blood type is determined by three alleles I^A, I^B, and i.

I^A and I^B are codominant to each other, but dominant over i. What does this mean?

Practice!

A male with AB blood type and a female with O blood type have a child. Using a punnett square determine their child’s possible blood types.

A male who is heterozygous for the A blood type and a female who is heterozygous for the B blood type have a child. Using a punnett square determine the child’s possible blood types.

Can a male who is homozygous for the B blood type and a female with the O blood type conceive a child with the A blood type? Prove why or why not.