Results and Conclusions of Mendel’s Experiments

Mendel began his experiments by cross pollinating pure breeding plants

• His studies focused on only one characteristic at a time for example, flower color. This type of cross is called a Monohybrid cross

Mendel developed a mathematical vocabulary to identify different generations of pea plants

• P1: Parent generation• F1: First (“filial”meaning offspring) generation

Offspring of P1 cross P1 x P1• F2: Second Generation - Offspring of F1 cross

All F1 plants are purple (Ww )…

Where did the WHITE form go?

3 out of 4 offspring with purple flowers

WW Ww Ww ww

Ww WwWw Ww

What is the resulting ratio of purple to white flowers in the F2 generation?

• Purple 3:1 white

Express this answer as a percent • 75% purple: 25 % white

Now let’s look at another…

Mendel Concluded….

1) Parents transmit information about traits to their offspring.

• Mendel called this information a “factor.” Today we know that “factors” are really genes.

2) Each parent must be contributing a form of a gene to the offspring.

• From our studies in the last unit, we know this is true because chromosomes came in pairs called homologous chromosomes. Each homolog contains the same genes, so a gene pair exists for each gene on the chromosome.

Gene PairGene locus

AlleleAlleleGTACCAGGA ATGCCGGGT

(Yellow) (Green)

2) Each parent must be contributing a form of a gene to the offspring.

• The two alleles in a gene pair can be:

1) identical or 2) different • For example, a gene pair which

codes for seed color in pea plants could contain:

1) yellow, yellow (purebred or2) green, green homozygous)3) yellow, green (hybrid) -

heterozygous

• Gene pairs are described in two ways:1) homozygous: alleles in gene pair are identical Pure breeding plants are homozygous.2) heterozygous: alleles in gene pair are differentHybrid plants are hererozygous.

3. Symbols can be used to represent alleles.

• If you use an uppercase “W” to represent purple as the dominant form of pea color and use a lowercase “w” to represent white as the recessive form of pea color, you can illustrate the allele combinations for the following pea plants. (WW, Ww, ww)

4. Genetic makeup and observed characteristics are not always the same!

GENOTYPE• the exact form of the alleles

in a gene pair• Example: WW, Ww, ww

PHENOTYPE• the observable or expressed

form of a trait resulting from a gene pair

• Example: yellow or green seed color

In pea plants, Green pod color is dominant over yellow.

Description GENOTYPE PHENOTYPE

Homozygous dominant GG Green

Homozygous recessive gg Yellow

Heterozygous Gg Green

Heterozygous recessive?? No such thing!

As a result of his many years of careful study, Mendel’s work was summarized into three

main laws.

Mendel’s Laws

1. The law of Dominance• States that if two alleles in a gene pair are

different, then one allele can control the trait and the other one can be hidden.

• Example: Green pod color is seen in plants with the genotype Gg

Mendel’s Laws

2. The law of Independent Assortment• States that gene pairs segregate into gametes

randomly and independently of each other.• At what stage of meiosis does this occur?

Metaphase I and Anaphase I of meiosis-homologs randomly separate

Mendel’s Laws

3. The law of Segregation• States that each pair of genes segregates, or

separates, during meiosis and each gamete contains one gene from each gene pair.

• What is the chance that a gamete will end up with a maternal homolog at any one chromosome pair?

• ½ or 50%• The chance that all 23 homologs from one parent

will end up in a single gamete is 1 in 8 million (1/2)23

CLOSER

What is a monohybrid cross?

• A monohybrid cross is when a scientist studies the inheritance of one trait at a time.

How is the following statement “Each parent must be contributing a form of a gene to the

offspring” supported by what you know about meiosis and homologous chromosomes?

The chromosomes from each homologous pair come from the parents (one from each). Each homologous pair carries a pair of genes (one gene on each homolog). Therefore one copy of each gene comes from each parent.

What is the difference between homozygous and heterozygous?

• Homozygous means each gene has the same allele.

• Heterozygous means that each gene in the pair has different alleles.

What is the relationship between phenotype and genotype?

• Genotype is what alleles are found in a pair of genes. Phenotype is the outward appearance of those alleles.

What does dominant and recessive mean?

• A dominant allele can control a trait and hide a recessive allele. A recessive allele will only show if the dominant allele is not present.