• Heredity is passing characteristics from one generation to the next.
  • Genetics is the study of heredity.
  • Gregor Mendel is the “Father of Modern Genetics.” He was an Austrian monk who studied heredity in pea plants. His work was published in 1865.
  • He described “factors”that were passed between generations of plants
  • We now know the factors are genes:chemical factors that determine characteristics.

Mendel’s Peas:

  • Mendel observed true-breeding pea plants produced genetically identical offspring. Ex: Tall plants produced tall offspring, short produced short
  • Mendel studied seven different traits in pea plants.
  • Traits are inherited characteristics that vary from individual to individual
  • Each trait each had two different forms or alleles
  • For Example: Pea plant height can be either tall (T) OR short (t)

7 Pairs of contrasting characters studied by Mendel in pea plant:

HeightTall or Short
Flower LocationTip of stem or along the stem
Pod ShapeInflated or Constricted
Pod ColorGreen or Yellow
Seed ShapeRound or Wrinkled
Seed ColorYellow or Green
Seed Coat ColorGray or White
  • Homozygous means to have 2 identical alleles for a trait.Ex. TT or tt True-breeding pea plants are homozygous.
  • Heterozygous means to have 2 different alleles for a trait.Ex. Tt Hybrid plants are heterozygous.

Generations of Pea Plants:

P = Parent generation= your parents

F1 = First generation offspring. = you

Produces hybrids = crosses between parents with different traits (Tall x short) (TT x tt)

F2 = Second generation offspring = your kids

Formed from hybrid x hybrid. (Tt x Tt)(F1 x F1)

Monohybrid Cross:

  • When fertilization occurs between two true-breeding parents that differ by only the characteristic being studied, the process is called a monohybrid cross, and the resulting offspring are called monohybrids.
  • Mendel performed seven types of monohybrid crosses, each involving contrasting traits for different characteristics.
  • Out of these crosses, all of the F1 offspring had the phenotype of one parent, and the F2 offspring had a 3:1 phenotypic ratio.
  • On the basis of these results, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely.

Principle of Dominance:

Dominant traits are expressed if only one allele is present. (capital letter, first letter of trait ex. Tall= T)

Ex -Tall allele (T) is dominant and short allele is recessive (t)

F1 generation = All plants were tall even though Tt both TT and Tt plants are Tall

Recessive traits are expressed when the dominant allele not present.Two alleles are needed for the recessive trait to be expressed. (lower case letter)

Ex from pea plants-short allele is recessive (t) Only tt plants are short.

The Principle of Dominance explains why genotype differs from phenotype. Genotypes for plant height are TT, Tt, tt. Genotypes can be heterozygous or homozygous. Phenotypes for plant height are tall or short. TT and Tt genotypes both expressed the tall phenotype because the T is dominant to t. Only the tt genotype expressed the short phenotype

Principle of Segregation:

  • The principle of segregation states that an organism possesses two alleles for any particular characteristic. These alleles separate during the formation of gametes. In other words, one allele goes into each gamete.
  • During fertilization, a “t” gamete fertilized a “t” gamete 1/4 of the time, resulting in tt short plants.
  • This accounts for new combinations of alleles that were not present in either parent.

Punnett Square:

A Punnett square is a graphical representation of the possible genotypes of an offspring arising from a particular cross or breeding event. Creating a Punnett square requires knowledge of the genetic composition of the parents. The various possible combinations of their gametes are encapsulated in a tabular format. Therefore, each box in the table represents one fertilization event.

Below Punnett square shows the cross between plants with yellow seeds and green seeds. The cross between the true-breeding P plants produces F1 heterozygotes that can be self-fertilized. The self-cross of the F1 generation can be analyzed with a Punnett square to predict the genotypes of the F2 generation. Given an inheritance pattern of dominant-recessive, the genotypic and phenotypic ratios can then be determined

Dihybrid Cross:

  • A genetic cross between parents that differ in two characteristics, controlled by genes at different loci.
  • Mendel performed a dihybrid cross using pea plants and the characteristics of seed colour and texture: the parental plants had either smooth yellow seeds (SSYY) – the dominant characteristics – or wrinkled green seeds (ssyy) – the recessive characteristics.
  • All the offspring had smooth yellow seeds, being heterozygous (SsYy) for the two alleles. Crossing between these offspring produced an F2 generation of plants with smooth yellow, smooth green, wrinkled yellow, and wrinkled green seeds in the ratio 9:3:3:1.
  • Mendel used these results as the basis for his Law of Independent Assortment

Refer Below Example:

Consider the characteristics of seed color and seed texture for two pea plants: one that has green, wrinkled seeds (yyrr) and another that has yellow, round seeds (YYRR). Because each parent is homozygous, the law of segregation indicates that the gametes for the green/wrinkled plant all are yr, while the gametes for the yellow/round plant are all YR. Therefore, the F1 generation of offspring all are YyRr.

For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed as follows: YR, Yr, yR, and yr. Arranging these gametes along the top and left of a 4 × 4 Punnett square gives us 16 equally likely genotypic combinations. From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green.

Law of Independent Assortment:

  • Mendel’s law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes; every possible combination of alleles for every gene is equally likely to occur.
  • The calculation of any particular genotypic combination of more than one gene is, therefore, the probability of the desired genotype at the first locus multiplied by the probability of the desired genotype at the other loci.
  • The forked line method can be used to calculate the chances of all possible genotypic combinations from a cross, while the probability method can be used to calculate the chance of any one particular genotype that might result from that cross.

Quick Review:

  • Monohybrid Phenotypic ratio = 3 : 1
  • Monohybrid Genotypic ratio = 1 : 2 : 1
  • Dihybrid Phenotypic ratio = 9 : 3: 3 : 1
  • Dihybrid Genotypic ratio = 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1
  • Back cross : F1 hybrid x parent (Dominant /Recessive)
  • Test cross : F1 hybrid x parent (Recessive)
  • Mendel’s 1st Law : Law of dominance
  • Mendel’s 2nd Law : Law of segregation
  • Mendel’s 3rd Law : Law of independent assortment


Leave a Reply

Your email address will not be published. Required fields are marked *