Classical Mendelian Genetics Vocabulary

Classical Mendelian Genetics

9.1 Introduction

• Genetics is the study of variation and heredity.
• It examines the origin, maintenance, and inheritance of variation across generations.
• Gregor Mendel is considered the father of genetics.
• Between 1856 and 1863, Mendel cultivated and tested approximately 28,000 pea plants.
• Mendel's mathematical skills helped him understand F2 ratios as a result of random combinations of cellular factors.
• He linked the distribution of factor pairs to the expansion of the binomial expression (A+a)(A+a) or (A+a)^2.
• Mendel published his study results in 1866, proposing two laws of inheritance in sexually reproducing organisms.
• He proposed that a pair of factors control each character transmitted from parents to offspring, laying the groundwork for the particulate theory of inheritance.
• His results were largely ignored until the early 1900s.
• Genetics has applications in medicine, the pharmaceutical industry, forensics, and conservation.

9.2 Objectives

• Define genetics and explain its significance.
• Explain the effect of the environment on gene expression.
• Explain and apply Mendel’s First Law of Segregation and of the monohybrid cross.
• Explain and apply knowledge of the dihybrid cross and Mendel’s Second Law of Independent Assortment.
• Use the Punnett square or probabilities to find the phenotypic ratios in the F2 of a dihybrid cross involving two characters determined by two independent genes.
• Explain the principle behind the Chi-squared test (\chi^2 Test) and apply it to solve genetic problems.

9.3 Common genetic terms

9.3.1 Genotype:

• The genetic constitution (make-up) of an individual.
• Indicates the types of alleles present.
• Examples:
  • TT: Homozygous tall pea plants
  • Tt: Heterozygous tall
  • tt: Homozygous short

9.3.2 Phenotype:

• The physical appearance or function of an organism, resulting from its genotype and environment.
• Results from the expression of genetic information through the protein (polypeptide) product.
• Examples:
  • Tall and short are the two phenotypes for height in pea plants.

9.3.3 Dominant allele:

• An allele that masks (hides) the expression of another allele of the same gene.
• Produces a dominant phenotypic character.
• Example:
  • The allele (T) for tall height in pea plants is dominant to the allele (t) for short height (T > t).
  • TT and Tt plants both have the tall phenotype.

9.3.4 Codominant alleles:

• Contrasting alleles that are both expressed in a heterozygote (F_1).
• The heterozygote exhibits characteristics of both parents.
• Example:
  • Coat color in cattle: R (red) and W (white).
  • RR (red) x WW (white) produces RW (roan) offspring, which have both red and white hairs.

9.3.5 Incompletely dominant alleles:

• A heterozygote exhibits an intermediate phenotype between the two homozygous parental phenotypes.
• Example:
  • Flower color in the four o’clock plant (Mirabilis jalapa): RR (red), RW (pink), and WW (white) in the F_2 generation.

9.3.6 Recessive alleles:

• An allele whose effect is masked (hidden) by the presence of a dominant allele of the same gene.
• A recessive character is only expressed by recessive alleles in their homozygous state.
• Example:
  • The allele (t) for short height in pea plants is recessive to the allele (T) for tall height (T > t).
  • tt plants are short; TT and Tt plants are tall.

9.4 Environmental effects on gene expression

• The expression of certain genes is influenced by the environment.
• The environment can enhance or inhibit gene expression.
• Example:
  • Height in humans is influenced by diet and general health.
• Studies of identical twins show that although they carry the same genes, their phenotypes can differ if raised in different environments.

9.5 Mendel’s experiments

• Mendel observed that certain characters of garden pea plants had two alternative forms.
• Some plants bred true for one character, while others bred true for the alternative character.
• Mendel used seven characters of garden peas in his study (Table 9.1).
• The characters chosen by Mendel were constant and easily recognizable.
• The characters have contrasting forms (e.g., smooth vs. wrinkled seeds).
• The hybrids are fertile, and both cross- and self-fertilization are possible.
• Self-pollination is naturally favored because the reproductive parts are covered and only open after pollination (cleistogamy).
• Cross-pollination is possible by emasculation involving opening the young bud of the female parent, removing the keel and stamen with forceps, and dusting the stigma with pollen from a specific male parent plant.
• Pea seeds are cheap, readily available, take up little space, have a short generation time, and produce many offspring.
• In studying stem height inheritance, Mendel crossed tall plants with short plants and obtained hybrid plants that were all tall.
• Self-pollinating the hybrid tall plants produced a mixture of tall and short plants.
• Similar results were obtained with other characters.
• In other experiments, he studied the simultaneous inheritance of two characters (e.g., seed shape and seed color) and observed that the two forms of each character are inherited independently of each other.
• From this work, Mendel was able to understand the nature of inheritance and came up with his two principles or laws of inheritance:
  • The Law of Segregation.
  • The Law of Independent Assortment.

9.6. Mendel’s Laws of Heredity

9.6.1 Mendel’s First Law - The Law of Segregation

• States that during gamete formation, contrasting forms of a gene separate in equal numbers.
• Best illustrated by a monohybrid cross.

9.6.1.1 Monohybrid cross

• A cross between homozygous parents that differ in only one pair of alleles of one gene controlling one character.
• Example:
  • Stem height in garden peas is controlled by one gene, where a pure-breeding tall plant has the genotype TT and a pure-breeding short plant has the genotype tt.
• Mendel carried out separate monohybrid crosses for each of the seven characters and obtained similar results.

9.6.1.2 F_1 hybrids

• The progeny or offspring of a monohybrid cross is called the first filial generation or F_1 hybrid.
• Reciprocal crosses are usually made in a monohybrid cross.
• Example:
  • Pollen from tall plants fertilizes ovules of short plants in one cross, and pollen from short plants fertilizes ovules of tall plants in the other cross, see Figure 9.1.
• Mendel observed that when seeds from the F_1 plants were grown, they were identical: they all resembled the tall plants with no short or intermediate forms.
• Mendel reasoned that both tall and short forms were present in the hybrid, but only the tall form was expressed.
• He called the tall form dominant and the short form recessive.
• Tall was dominant in both cases of the reciprocal cross.

9.6.1.3 The F_2 generation

• The second filial or F2 generation is raised by allowing the F1 hybrids to self-pollinate.
• When Mendel examined the F_2 plants, the recessive (short) character reappeared along with the dominant (tall) character.
• The numbers of plants were in the ratio of 3 tall : 1 short.
• This phenotypic ratio represents a 1:2:1 genotypic ratio, which is the binomial expression: (T + t) (T + t) = 1TT + 2Tt + 1tt.
• This ratio gave Mendel the idea that the character passed on during reproduction existed in two alternative forms (T and t) that combine randomly in pairs.

9.6.1.4 The Punnett square

• R.C. Punnett devised the Punnett Square method, which shows how the alternative forms of a character combine to produce the F_2 progeny.
• The F_2 progeny is depicted by the 2x2 Punnett square (Table 9.2) of a monohybrid cross.
• Phenotypic ratio = 3T_ : 1tt and Genotypic ratio = 1TT: 2Tt: 1tt

9.6.1.5 Testcross

• A breeding test in which a dominant phenotype (F_1 hybrid) is crossed with the recessive homozygote to verify whether it is a heterozygote or homozygous dominant genotype.
• The results confirm that there are two kinds of alleles in the hybrid.
• A testcross in which an individual is crossed with one of its own parents is referred to as a backcross.
• When 1/2 of the progeny are tall and 1/2 are short, the dominant genotype is confirmed to be heterozygous (F_1 hybrid).
• When all the progeny are tall, the dominant genotype is confirmed to be homozygous.

9.6.2 Mendel’s 2nd Law – The Law of Independent Assortment

• States that when two or more unlinked pairs of genes are brought together in a cross, their alleles assort (segregate) independently of each other as a result of meiosis.
• Best illustrated by a dihybrid cross.

9.6.2.1 Dihybrid cross

• In the F_2 generation of a dihybrid cross, the genotypes do not show any pattern, while the phenotypes show a regular pattern

• There are four possible phenotypes in the F_2:

  • (i) Round yellow (Parental),
  • (ii) Round green (Recombinant),
  • (iii) Wrinkled yellow (Recombinant),
  • (iv) Wrinkled green (Parental).

• The phenotypic ratio is:

  • 9/16 round yellow: 3/16 round green: 3/16 wrinkled yellow: 1/16 wrinkled green = 9:3:3:1
  • 9/16 RY : 3/16 Ryy : 3/16 rrY : 1/16 rryy

9.6.2.2 Dihybrid inheritance test cross

• Cross between a double dominant phenotype and a double recessive phenotype to confirm whether the double dominant phenotype is homozygous or heterozygous.
• A reciprocal cross is recommended.

9.7 The Chi-squared test

• The Chi-squared test (\chi^2 Test) is a statistical test used to confirm whether the differences between observed and expected values are significant.
• It tests whether deviations in observed ratios from expected Mendelian ratios are due to chance or real differences.
• The number of observations should be based on 30 or more individuals for reliable results.
• Mendel worked with large numbers of plants because statistical methods were not available during his time.
• To decide whether results are within the expected ratio, the calculated \chi^2 value is compared with the tabulated \chi^2 value.
• If the calculated value is greater than the tabulated (critical value), the departure from the expected ratio is significant.
• \chi^2 = \sum \frac{(O-E)^2}{E}, where:
  • \sum = sum of
  • O = observed values
  • E = expected values
• The tabulated \chi^2 value is obtained from the \chi^2 statistical table at 5% probability and at the appropriate degrees of freedom (df).
• df = number of classes – 1.

9.7.1 Worked example

• Cross between two red-flowered F_1 plants: Rr \times Rr.
  • R is the dominant allele for red flowers.
  • r is the recessive allele for white flowers.
• Results from 200 F_2 plants: 148 red-flowered plants and 52 white-flowered plants.
• Use the chi-squared test to determine whether the results are in the expected Mendelian ratio of a F_2 progeny of a monohybrid cross.
• Solution:
  • Under perfect conditions, expect a 3 red: 1 white phenotypic ratio in the F_2 progeny.
  • For a sample of 200, expect 150 red and 50 white-flowered plants.
    \chi^2 Test Calculation:

The calculated \chi^2 value is 0.11.

• df = 2 – 1 = 1
• From the \chi^2 table, the value for 1 degree of freedom at 5% probability = 3.841
• Conclusion:
  • The calculated \chi^2 value of 0.11 is less than the table \chi^2 value of 3.841.
  • The calculated \chi^2 value is not significant, and the observed values agree with the expected values.
  • The results are according to the expected Mendelian phenotypic ratio of 3:1.

9.8 Revision exercise

List of questions for review.

9.9 Summary

• Classical Mendelian genetics is based on the Laws of Heredity proposed by Gregor Mendel.
• It explains the genetic basis of cell growth and cell division and lays the foundation for statistical analysis of genetic data.
• The fundamental genetic terms are also discussed.