Mendelian genetics Notes

Mendel's First Law of Genetics (Law of Segregation)

Genetic analysis predates Gregor Mendel, but Mendel's laws form the theoretical basis of our understanding of the genetics of inheritance.

Mendel made two innovations to the science of genetics:

1.   developed pure lines

2.   counted his results and kept statistical notes

Pure Line - a population that breeds true for a particular trait [this was an important innovation because any non-pure (segregating) generation would and did confuse the results of genetic experiments]

Results from Mendel's Experiments

Parental Cross	F1Phenotype	F2 Phenotypic Ratio	F2Ratio
Round x Wrinkled Seed	Round	5474 Round:1850 Wrinkled	2.96:1
Yellow x Green Seeds	Yellow	6022 Yellow:2001 Green	3.01:1
Red x White Flowers	Red	705 Red:224 White	3.15:1
Tall x Dwarf Plants	Tall	l787 Tall:227 Dwarf	2.84:1

Terms and Results Found in the Table

Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait

Mendel's pea plants exhibited the following phenotypes:

• - round or wrinkled seed phenotype
• - yellow or green seed phenotype
• - red or white flower phenotype
• - tall or dwarf plant phenotype

Seed Color: Green and yellow seeds.

Seed Shape: Wrinkled and Round seeds.

What is seen in the F₁ generation? We always see only one of the two parental phenotypes in this generation. But the F₁ possesses the information needed to produce both parental phenotypes in the following generation. The F₂ generation always produced a 3:1 ratio where the dominant trait is present three times as often as the recessive trait. Mendel coined two terms to describe the relationship of the two phenotypes based on the F₁ and F₂ phenotypes.

Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F₁ generation from the cross of two pure lines
Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F₁ generation from the cross of two pure lines and reappears in the F₂ generation

Mendel's Conclusions

1.   The hereditary determinants are of a particulate nature. These determinants are called genes.

2.   Each parent has a gene pair in each cell for each trait studied. The F₁ from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair.

3.   One member of the gene pair segregates into a gamete, thus each gamete only carries one member of the gene pair.

4.   Gametes unite at random and irrespective of the other gene pairs involved.

Mendelian Genetics Definitions

• Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual
• Allelic pair - the combination of two alleles which comprise the gene pair
• Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest
• Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote
• Genotype - the specific allelic combination for a certain gene or set of genes

Using symbols we can depict the cross of tall and short pea plants in the following manner:

The F₂ generation was created by selfing the F₁ plants. This can be depicted graphically in a Punnett square. From these results Mendel coined several other terms and formulated his first law. First the Punnett Square is shown.

	D	d
D	DD (Tall)	Dd (Tall)
d	Dd (Tall)	dd (Short)

The Punnett Square allows us to determine specific genetic ratios.
Genotypic ratio of F₂: 1 DD : 2 Dd : 1 dd (or 3 D_ : 1 dd)
Phenotypic ratio of F₂: 3 tall : 1 dwarf

Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete

Confirmation of Mendel's First Law Hypothesis
With these observations, Mendel could form a hypothesis about segregation. To test this hypothesis, Mendel selfed the F₂ plants. If his law was correct he could predict what the results would be. And indeed, the results occurred has he expected.

From these results we can now confirm the genotype of the F₂ individuals.

Phenotypes	Genotypes	Genetic Description
F2 Tall Plants	1/3 DD 2/3 Dd	Pure line homozygote dominant Heterozygotes
F2 Dwarf Plants	all dd	Pure line homozygote recessive

Thus the F₂ is genotypically 1/4 Dd : 1/2 Dd : 1/4 dd

This data was also available from the Punnett Square using the gametes from the F₁ individual. So although the phenotypic ratio is 3:1 the genotypic ratio is 1:2:1

Mendel performed one other cross to confirm the hypothesis of segregation --- the backcross. Remember, the first cross is between two pure line parents to produce an F₁ heterozygote.

At this point instead of selfing the F₁, Mendel crossed it to a pure line, homozygote dwarf plant.

Backcross: Dd x dd

		Male Gametes
		d
Female Gametes	D	Dd (Tall)
	d	dd (Short)

Backcross One or (BC₁) Phenotypes: 1 Tall : 1 Dwarf

BC₁ Genotypes: 1 Dd : 1 dd

Backcross - the cross of an F₁ hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd; most often, though a backcross is a cross to a fully recessive parent
Testcross - the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous
So far, all the discussion has concentrated on monohybrid crosses.
Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa)
Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair
Remember --- a monohybrid cross is not the cross of two monohybrids.
Monohybrids are good for describing the relationship between alleles. When an allele is homozygous it will show its phenotype. It is the phenotype of the heterozygote which permits us to determine the relationship of the alleles.
Dominance - the ability of one allele to express its phenotype at the expense of an alternate allele; the major form of interaction between alleles; generally the dominant allele will make a gene product that the recessive can not; therefore the dominant allele will express itself whenever it is present

Mendel's Law of Independent Assortment

To this point we have followed the expression of only one gene. Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment. First, a few terms are presented.

Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)

Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)

Again a dihybrid cross is not a cross between two dihybrids. Now, let's look at a dihybrid cross that Mendel performed.

Parental Cross: Yellow, Round Seed x Green, Wrinkled Seed

F₁ Generation: All yellow, round

F₂ Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled

At this point, let's diagram the cross using specific gene symbols.

Choose Symbol	Seed Color: Yellow = G; Green = g
	Seed Shape: Round = W; Wrinkled = w

The dominance relationship between alleles for each trait was already known to Mendel when he made this cross. The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs.

Let's now look at the cross using our gene symbols.

Now set up the Punnett Square for the F₂ cross.

		Female Gametes
		GW	Gw	gW	gw
Male Gametes	GW	GGWW  (Yellow, round)	GGWw  (Yellow, round)	GgWW  (Yellow, round)	GgWw  (Yellow, round)
	Gw	GGWw  (Yellow, round)	GGww  (Yellow, wrinkled)	GgWw  (Yellow, round)	Ggww  (Yellow, wrinkled)
	gW	GgWW  (Yellow, round)	GgWw  (Yellow, round)	ggWW  (Green, round)	ggWw  (Green,
	gw	GgWw  (Yellow, round)	Ggww  (Yellow, wrinkled)	ggWw  (Green, round)	ggww  (Green, wrinkled)

The phenotypes and general genotypes from this cross can be represented in the following manner:

Phenotype	General Genotype
9 Yellow, Round Seed	G_W_
3 Yellow, Wrinkled Seed	G_ww
3 Green, Round Seed	ggW_
1 Green, Wrinkled Seed	ggww

The results of this experiment led Mendel to formulate his second law.
Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair
As with the monohybrid crosses, Mendel confirmed the results of his second law by performing a backcross - F₁dihybrid x recessive parent.
Let's use the example of the yellow, round seeded F₁.

                            Punnett Square for the Backcross

	Female Gametes
		GW	Gw	gW	gw
Male Gametes	gw	GgWw (Yellow, round)	Ggww (Yellow, wrinkled)	ggWw (Green, round)	ggww (Green, wrinkled

		Female Gametes
		GW	Gw	gW	gw
Male Gametes	gw	GgWw (Yellow, round)	Ggww (Yellow, wrinkled)	ggWw (Green, round)	ggww (Green, wrinkled

The phenotypic ratio of the test cross is:

• 1 Yellow, Round Seed
• 1 Yellow, Wrinkled Seed
• 1 Green, Round Seed
• 1 Green, Wrinkled Seed

Backcross with a recessive is called test cross