lecture 14

Chromosomal theory of inheritance. Allelic interactions – Dominance vs recessive, complete dominance, codominance, incomplete dominance, threshold characters

Chromosomal Theory of Inheritance:

• In 1902, Theodor Boveri observed that proper embryonic development of sea urchins does not occur unless chromosomes are present.

• That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis.

• Together, this observation led to the development of the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.

The Chromosomal Theory of Inheritance was consistent with Mendel’s laws and was supported by the following observations:

• During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.

• The sorting of chromosomes from each homologous pair into pre-gametes appears to be random.

• Each parent synthesis gamete that contains only half of their chromosomal complement.

• Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.

• The gametic chromosomes combine during fertilization to produce offspring with the same chromosomes number as their parents.

Evidence for chromosome theory of inheritance:

• Each somatic cell contains 2 copies of each gene. Each somatic cell produced during embryonic and subsequent development receives 2 copies of each gene present in the zygote.

• A gamete contains only one copy of a gene or one allele of a gene. This phenomenon is termed as segregation. It is assumed that 2 alleles of a gene are located in the chromosomes of a homologous pair. Separation of 2 homologous chromosomes during anaphase I of meiosis will account for segregation of 2 alleles.

• Several subsequent studies and Mendel's studies found that 2 or more genes assorted independently to yield typical dihybrid, trihybrid ratios. Members of homologous chromosomes assort independently to that of the other.

• In 1901, McClung discovered the accessory or x chromosome of grass hopper and stated that, this chromosome was involved in sex determination since a specific chromosome is involved in sex determination, the genes governing this trait may be located on x chromosomes.

• In 1910, Morgan described the phenomenon of linkage and crossing over between 2 sex linked genes in drosophila. He proved that the linkage between any 2 genes depends on the distance between them in the chromosome. Morgan explained that the recombination of linked gene is due to exchange of genetic material between homologous chromosomes.

• When a segment of chromosome is inverted, the inverted chromosome shows as inverted gene sequence corresponding to the inverted segment. Hence genes must be located in chromosome.

• In addition to structural changes in chromosomes, numerical chromosome changes also furnish in favour of this theory. In monosomics and trisomics there will be significant deviation from the normal inheritance pattern i.e. from 3:1 ratio in F2 for a single gene.

Bridges proof for Chromosomal theory of inheritance:

   Discovery of non-disjunction of X chromosomes in fruit fly Drosophila

Explanation for the results:

   In some females, the XX chromosomes fail to disjoin at Meiosis and this primary disjunction leads to the production of eggs either with XX chromosomes or with no X chromosome

   Four types of Zygotes were produced when the eggs are fertilized by normal sperms.

Gene interaction:

   When expression of one gene depends on the presence or absence of another gene in an individual it is known as gene interaction. Two types of gene interaction are given below.

Allelic Interaction:

   Allelic gene interaction means expression of character is produced by interaction between alleles of single gene.

Non-Allelic Interactions:

   Non-allelic gene interaction means expression of character is produced by the reaction between two or more genes.

Dominance:

• The suppression of the expression of one trait of a character by another trait of the same character is called dominance. The characters that appear in the F1 generation are called as dominant characters.

• It is able to express itself even in the presence of its recessive allele.

• It does not require another similar allele to produce its effect on the phenotypes, e.g., Tt is tall.

• Dominant allele or factor can form complete polypeptide or enzyme for expressing its effects, e.g., red colour of flower in Pea.

Recessive:

• A recessive gene is a gene that can be masked by a dominant gene. The recessive character appears in the F2 generation.

• Recessive allele or factor is unable to express its effect in the presence of dominant allele.

• It produces its phenotypic effect only in the presence of a similar allele, e.g., tt is dwarf.

• The recessive allele forms an incomplete or defective polypeptide or enzyme so that the expression consists of absent of the effect of dominant allele, e.g., white flower colour in Pea.

Gene action:

• Gene action refers to the manner in which genes control the phenotypic expression of various characters in an organism. Alleles of the gene may interact with one another in a number of ways to produce variability in their phenotypic expression.

• The dominant and recessive relationship is fundamental and is essentially constant with each pair of alleles. Gene action can be of the following types:

• Based on the dominance effect:

  • Complete dominance

  • Incomplete dominance

  • Co-dominance

  • Over dominance

  • Pseudo-dominance

• Based on lethal effects:

  • Dominant lethals

  • Recessive lethals

• Based on epistatic action:

  • Epistatic factors

  • Supplementary factors

  • Duplicate factors

  • Complementary factors

  •  Additive factors

  • Inhibitory factors

• Based on number of genes involved:

  • Monogenic

  • Digenic

  • Oligogenic

  • Polygenic

  • Based on pleiotropism / pleiotropic gene action

1.     Based on dominance effect:

    It was noted by many workers that many characters of F1 hybrid were exactly the same as those of one or the other parent of a hybrid. The phenomenon of F1 hybrid being identical to one of its parents for a character is termed as dominance. The form of a character that is seen in F1 hybrid is called dominant, while that form of the concerned trait which does not appear in F1 is known as recessive. Dominance relationship is of the following types:

   a)     Complete dominance: The phenotype produced by a heterozygote is identical to that produced by the homozygotes for the concerned dominant allele. The dominant allele in such a situation is said to be completely or fully dominant. E.g.: In garden pea, round seed shape is completely dominant over wrinkled.

   b)     Incomplete dominance: In many cases, the intensity of phenotype produced by heterozygote is less than that produced by the homozygote for the concerned dominant allele. Therefore, the phenotype of heterozygote falls between those of the homozygotes for the two concerned alleles. Such a situation is known as Incomplete or partial dominance and the dominant allele is called incompletely dominant or partially dominant.

   Eg : In Mirabilis jalapa (Four ‘O’ clock plant) a partially dominant allele ‘R’ produces red flowers in homozygous state, while its recessive allele ‘r’ produces white flowers in homozygous state. When a red (RR) flower type plant is crossed with white (rr) flower type plant, the hybrid (Rr) has pink flowers. Thus, the intensity of flower colour in F1 is intermediate between the intensities of flower colour produced by two homozygotes. This phenomenon is also called blending inheritance.

   c)     Co-dominance: Both the alleles of a gene express themselves in heterozygotes. As a result, heterozygotes for such genes possess the phenotypes produced by both the concerned alleles. The coat colour of short horned breed of cattle presents an excellent example of co dominance.

   d)     Over dominance:

• In case of some genes, the intensity of character governed by them is greater in heterozygotes than in the two concerned homozygotes. This situation is known as over-dominance. True over dominance is known in case of very few genes.

• Over-dominance is not the property of an allele but is the consequence of heterozygous state of concerned gene. E.g.: white eye gene (W) of Drosophila exhibits over dominance for some of the eye pigments such as sepia pteridine and Himmel blaus.

• These two eye pigments are present in low concentration in the recessive homozygotes (ww), while the dominant homozygotes (WW) have relatively higher concentrations of these pigments. However, the flies heterozygous for this gene (Ww) have an appreciably higher concentration of these two pigments than the two homozygotes.

Transgressive segregation: The appearance of individuals in F2 or subsequent generation which exceed the parental types with reference to one or more characters is known as transgressive segregation. (or) The segregants which fall outside the range of both the parents are called transgressive segregants and the phenomenon is called transgressive segregation.

   e)     Pseudo-dominance: Expression of recessive allele of the gene in the hemizygous state / condition either due to sex linkage (E.g.: colour blindness in human beings) or chromosomal aberrations (deletion in heterozygotes) is known as pseudo-dominance.

1. Based on lethal effect:

• One of the most important assumptions for inheritance of any trait is the equal survival of all gametes and zygotes produced as a result of segregation. The assumptions are true for a vast majority of genes.

• However, some genes affect the survival of those zygotes or individuals in which they are present in the appropriate genotype. Such genes are known as lethal gene. A lethal gene causes death of all the individuals carrying the gene in the appropriate genotype before they reach the adult stage.

• Most of the lethal genes express their lethal effect only when they are in homozygous state while the survival of heterozygotes remains unaffected. The stage of development at which a lethal gene produces a lethal effect varies considerably from one gene to another. Some genes cause the death of embryo very early in developmental stage.

• (Eg: ‘Y’ gene in mice); while others allow survival for a certain period of time and then produce lethal effect (Eg: ‘g’ gene producing albino seedlings in crop plants like rice, barley etc.

a) Dominant Lethal gene action:

    A lethal gene affecting coat colour in mice was discovered by French geneticist Cuenot in 1905. He found that yellow coat colour in mice was produced by a dominant gene ‘Y’ while its recessive allele ‘y’ determines the normal black / grey coat colour. Further, he found that all the mice with yellow coat colour were heterozygous Yy and he was unable to found a mouse homozygous for ‘Y’ allele (YY). The dominant allele ‘Y’ is lethal and hence it causes death of homozygous ‘YY’ embryos at an early stage of development.

   b) Recessive lethals:

• Albino seedling character in plants such as rice and barley is governed by recessive alleles. Whenever these alleles are in the homozygous state the seedlings are near white or almost white and totally devoid of chlorophyll.

• Albino seedlings survive only as long as the food material stored in the seeds is available to them because they are not able to carry out photosynthesis. The heterozygotes, however are normal green and are identical with the dominant homozygotes in their phenotype as well as their survival.

• Segregation of such genes produces 3 green : 1 albino seedling if they are counted within a week from germination. However, if the plants are counted at maturity, there will be only green plants in the progeny.

• The heterozygous individuals carrying the lethal genes without expression in the heterozygous condition but giving rise to lethals in F2 generation is called a carrier. In the above examples Yy, or Gg are carriers

2.Based on epistatic gene action:

• When expression of one gene depends on presence / absence of another gene in an individual, it is known as gene interaction. Interaction of genes at different loci that affect the same character is called epistasis.

• The term epistasis was first used by Bateson in 1909 to describe two different genes which control the same character, out of which one masks / suppresses the expression of another gene.

• Gene that masks the action of another gene is called epistatic gene while the gene whose expression is being masked is called hypostatic gene. Epistatic gene interaction can be of the following types.

3. Based on number of genes involved:

    Polygenic or polymerism: In general, one gene controls or affects a single character. But some characters are known to be controlled by a greater number of genes. Such genes are called poly genes and the phenomenon is called polymerism. e.g.: Yield in plants.

4.Based on pleiotropism / pleiotropic gene action:

• In general, one gene affects a single character. But some of the genes are known to affect or control more than one character. Such genes are called pleiotropic genes and the phenomenon is known as pleiotropism.

• Many folds phenotypic expressions of a single gene is called pleiotropism or pleiotropic gene effects. e.g.: White eye gene effects the shape of sperm storage organs and other structures in Drosophila.

• These genes are found in all crop plants. Good example of pleiotropism has been reported in wheat. A gene governing awn in Ona’s variety of wheat also increases the yield as well as seed weight.