Mendelian Genetics (2024)

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

Genetic analysis predates Gregor Mendel, but Mendel's lawsform the theoretical basis of our understanding of the genetics ofinheritance.

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 truefor a particular trait [this was an important innovation because any non-pure(segregating) generation would and did confuse the results of geneticexperiments]

Results from Mendel's Experiments

Terms and Results Found in the Table

Phenotype - literally means "the form thatis 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 oneof the two parental phenotypes in this generation. But the F₁ possesses theinformation needed to produce both parental phenotypes in the followinggeneration. The F₂ generation always produced a 3:1 ratio where the dominanttrait is present three times as often as the recessive trait. Mendel coinedtwo terms to describe the relationship of the two phenotypes based on theF₁ and F₂ phenotypes.

Dominant - the allele that expresses itselfat the expense of an alternate allele; the phenotype that is expressed inthe F₁ generation from the cross of two pure lines

Recessive - an allele whose expression issuppressed in the presence of a dominant allele; the phenotype that disappearsin the F₁ generation from the cross of two pure lines and reappears in theF₂ 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
• hom*ozygote - an individual which contains only one allele at the allelic pair; for example DD is hom*ozygous dominant and dd is hom*ozygous recessive; pure lines are hom*ozygous 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 peaplants in the following manner:

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

The Punnett Square allows us to determine specific geneticratios.

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 theother member to form the genetic constitution of the gamete

Confirmation of Mendel's First LawHypothesis

With these observations, Mendel could form a hypothesis aboutsegregation. To test this hypothesis, Mendel selfed the F₂ plants. If hislaw was correct he could predict what the results would be. And indeed, theresults occurred has he expected.

From these results we can now confirm the genotype of theF₂ individuals.

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

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

Mendel performed one other cross to confirm the hypothesisof segregation --- the backcross. Remember, the first cross is between twopure line parents to produce an F₁ heterozygote.

At this point instead of selfing the F₁, Mendel crossedit to a pure line, hom*ozygote dwarf plant.

Backcross: Dd x dd

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

BC₁ Genotypes: 1 Dd : 1 dd

Backcross - the cross of an F₁ hybrid toone of the hom*ozygous parents; for pea plant height the cross would be Ddx DD or Dd x dd; most often, though a backcross is a cross to a fully recessiveparent

Testcross - the cross of any individualto a hom*ozygous recessive parent; used to determine if the individual ishom*ozygous dominant or heterozygous

So far, all the discussion has concentrated on monohybridcrosses.

Monohybrid cross - a cross between parentsthat differ at a single gene pair (usually AA x aa)

Monohybrid - the offspring of two parentsthat are hom*ozygous 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 betweenalleles. When an allele is hom*ozygous it will show its phenotype. It is thephenotype of the heterozygote which permits us to determine the relationshipof the alleles.

Dominance - the ability of one allele toexpress its phenotype at the expense of an alternate allele; the major formof interaction between alleles; generally the dominant allele will make agene product that the recessive can not; therefore the dominant allele willexpress itself whenever it is present

Copyright © 2000. Phillip McClean