Unit 3: Gene interaction and cytogenetics - Subjective Questions

Dominance is the relationship between two alleles of a gene in which one allele determines the phenotype of a heterozygous individual, while the expression of the other allele is masked.

In complete dominance, the heterozygote has the same phenotype as the homozygous dominant individual.

Example: Plant height in pea

• = allele for tallness
• = allele for dwarfness
• and plants are tall, whereas plants are dwarf.

Cross:

All offspring are and tall. On selfing the generation:

The generation shows:

• Genotypic ratio:
• Phenotypic ratio: tall dwarf

Thus, the dominant allele completely masks the recessive allele in the heterozygous condition.

Incomplete dominance is a dominance relationship in which neither allele is completely dominant. Consequently, the heterozygote exhibits an intermediate phenotype.

Example: Flower colour in snapdragon or Mirabilis jalapa

• = red flowers
• = white flowers
• = pink flowers

Parental cross:

All plants are and produce pink flowers.

Selfing of :

The offspring are:

• = red
• = pink
• = white

Therefore:

• Genotypic ratio:
• Phenotypic ratio: red pink white

Unlike complete dominance, the genotypic and phenotypic ratios are identical in incomplete dominance.

Codominance occurs when both alleles in a heterozygote are fully and simultaneously expressed. Multiple allelism refers to the presence of more than two alternative alleles of a gene in a population, although a diploid individual carries only two alleles.

The human ABO blood group is controlled by three alleles:

• produces antigen A.
• produces antigen B.
• produces no A or B antigen.

The alleles and are codominant to each other, while both are dominant over .

Genotype–phenotype relationships:

• or = blood group A
• or = blood group B
• = blood group AB
• = blood group O

In an individual with genotype , both A and B antigens are expressed, demonstrating codominance. The occurrence of , , and in the population demonstrates multiple allelism.

The three major dominance relationships can be compared as follows:

Key point:

• Complete dominance masks one allele.
• Incomplete dominance produces an intermediate phenotype.
• Codominance allows simultaneous expression of both alleles.

Epistasis is an interaction between genes at different loci in which one gene masks, modifies, or suppresses the phenotypic expression of another gene.

The gene that masks or modifies the other gene is called the epistatic gene, while the gene whose expression is affected is called the hypostatic gene.

Difference between dominance and epistasis:

Thus, dominance concerns the relationship between alleles at one locus, whereas epistasis concerns functional interactions between different loci.

Recessive epistasis occurs when the homozygous recessive genotype at one locus masks the expression of another gene.

Consider coat colour controlled by two genes:

• permits pigment production, while prevents pigment production.
• In pigment-producing individuals, produces black colour and produces brown colour.

Crossing two dihybrid individuals gives:

The genotypes are classified as follows:

• = black, with a frequency of
• = brown, with a frequency of
• = albino, with a frequency of
• = albino, with a frequency of

Since prevents pigment formation, the last two groups combine:

Therefore, the modified phenotypic ratio is . The genotype is recessively epistatic to the locus.

Dominant epistasis occurs when a dominant allele at one locus masks the expression of alleles at another locus.

In summer squash fruit colour:

• The dominant allele prevents pigment formation and produces white fruit.
• In the absence of , the allele produces yellow fruit.
• The genotype produces green fruit.

For the cross:

The classes are:

• = white,
• = white,
• = yellow,
• = green,

Combining the two white classes gives:

The dominant allele is epistatic because one copy is sufficient to suppress the expression of the colour locus.

Dominant inhibitory epistasis occurs when a dominant allele at one locus suppresses the expression of a gene at another locus. It is also called dominant suppression.

Suppose:

• permits colour development.
• is a dominant inhibitor that prevents colour development.
• Colour appears only in individuals with genotype .

In the cross:

The offspring are grouped as follows:

• = colour inhibited,
• = colour inhibited,
• = coloured,
• = colourless,

The colourless groups combine to produce:

This interaction may be observed in traits such as plumage colour in certain poultry varieties. The dominant inhibitor suppresses colour regardless of the allele present at the colour locus.

Complementary gene interaction, also called duplicate recessive epistasis, occurs when dominant alleles at two different loci are both required for the expression of a phenotype.

In sweet pea, purple pigment is produced through a biochemical pathway requiring the products of two genes, and :

Only plants with genotype produce purple flowers. A recessive homozygous condition at either locus blocks the pathway.

For the cross:

The groups are:

• = purple,
• = white,
• = white,
• = white,

Combining the white classes gives:

The genes are called complementary because at least one dominant allele at each locus is necessary for pigment production.

Duplicate dominant epistasis occurs when a dominant allele at either of two loci is sufficient to produce the same phenotype. Only the double recessive genotype produces the alternative phenotype.

Suppose genes and independently produce a particular character:

• = dominant phenotype
• = dominant phenotype
• = dominant phenotype
• = recessive phenotype

In the cross:

The phenotypic classes are:

• =
• =
• =
• =

The first three classes show the same phenotype and combine as:

Therefore, the ratio is:

This ratio is found when two genes have duplicate or redundant functions, as in certain seed-capsule shape traits of shepherd's purse.

The ratio results from an interaction in which the presence of dominant alleles at both loci produces one phenotype, a dominant allele at either locus alone produces a second phenotype, and the double recessive condition produces a third phenotype.

In summer squash:

• = disc-shaped fruit
• or = spherical fruit
• = long fruit

For the cross:

The classes are:

• = disc-shaped
• = spherical
• = spherical
• = long

The two spherical classes combine:

Thus, the phenotypic ratio becomes:

This demonstrates that genes can act jointly to produce a phenotype different from that produced by either dominant gene alone.

Cytoplasmic inheritance is the transmission of hereditary characters through genetic material located outside the nucleus, mainly in mitochondria and chloroplasts. It is also called extranuclear inheritance or organelle inheritance.

Characteristic features:

• Genes responsible for the trait occur in cytoplasmic organelles rather than nuclear chromosomes.
• In many organisms, inheritance is uniparental, usually maternal, because the egg contributes most of the zygote's cytoplasm.
• Reciprocal crosses often give different results.
• Cytoplasmic traits may not follow Mendelian segregation ratios.
• Organelles and their genomes are distributed randomly during cell division, causing replicative segregation.
• Cells may be homoplasmic, containing one organelle genome type, or heteroplasmic, containing normal and mutant organelle genomes.
• Phenotypic expression may depend on the proportion of mutant organelles and a threshold effect.

Examples include leaf variegation in Mirabilis jalapa, mitochondrial disorders in humans, and petite mutants in yeast.

Leaf variegation in Mirabilis jalapa, the four-o'clock plant, is a classic example of maternal chloroplast inheritance.

The plant may possess three types of branches:

• Green branches: contain normal chloroplasts.
• White branches: contain defective chloroplasts lacking functional chlorophyll.
• Variegated branches: contain a mixture of normal and defective chloroplasts.

Crossing results:

• Flowers from green branches produce mainly green offspring, irrespective of the pollen source.
• Flowers from white branches produce white offspring, which usually die because they cannot photosynthesize.
• Flowers from variegated branches may produce green, white, or variegated offspring.

Pollen contributes little or no cytoplasm to the zygote, while the egg supplies nearly all the chloroplasts. Therefore, the phenotype of the progeny depends primarily on the maternal branch.

The variable progeny from variegated branches results from the random distribution of normal and defective chloroplasts during egg formation and cell division. Reciprocal crosses produce different outcomes, confirming cytoplasmic inheritance.

Mitochondrial inheritance is the transmission of traits controlled by genes present in mitochondrial DNA, or mtDNA. Human mitochondrial DNA is generally inherited from the mother because the egg contributes almost all the mitochondria to the zygote, while paternal mitochondria are usually eliminated.

Pedigree characteristics:

• An affected mother may transmit the mutation to both sons and daughters.
• An affected father generally does not transmit the mutation to his children.
• Both sexes may be affected.
• Only females normally continue transmission to subsequent generations.

Factors affecting expression:

• Heteroplasmy: A cell contains both normal and mutant mitochondrial DNA.
• Replicative segregation: Mitochondria are distributed randomly into daughter cells.
• Threshold effect: Disease appears when the proportion of mutant mtDNA exceeds a critical level.
• Tissue specificity: High-energy tissues such as the brain, muscles, heart, and retina are often most affected.

Examples include Leber hereditary optic neuropathy and mitochondrial encephalomyopathy. The severity may vary among siblings because they can inherit different proportions of mutant mitochondria.

Nuclear and cytoplasmic inheritance differ in the location and transmission of hereditary material.

Thus, nuclear inheritance is associated with chromosomal genes and regular segregation, whereas cytoplasmic inheritance depends on extranuclear genetic material and organelle distribution.

Although both phenomena may cause offspring phenotype to depend on the mother, they have different genetic bases.

In maternal effect, the offspring's own genotype may determine the phenotype of the next generation. In cytoplasmic inheritance, the transmitted extranuclear DNA itself is responsible for the inherited character.

Cytology is the branch of biology concerned with the study of cells. It examines cell structure, organelles, cell division, chromosome morphology, and cellular functions.

Genetics is the branch of biology that deals with heredity and variation. It studies genes, alleles, inheritance patterns, mutation, recombination, gene expression, and genetic variation.

Cytogenetics is the field that combines cytology and genetics to study chromosomes and their relationship to heredity, variation, development, and disease.

Main focus of each field:

• Cytology: Structural and functional organization of cells.
• Genetics: Transmission and expression of hereditary information.
• Cytogenetics: Chromosome number, structure, behaviour, abnormalities, and their genetic consequences.

For example, observing chromosomes during meiosis is cytological, studying the inheritance of a trait is genetic, and linking a chromosomal deletion with an inherited disorder is cytogenetic.

Cytogenetics studies chromosomes at the cellular and genetic levels. Its scope includes chromosome structure, number, behaviour, inheritance, evolution, and abnormalities.

Major areas of study:

• Identification of normal chromosome complements.
• Detection of numerical abnormalities such as monosomy and trisomy.
• Detection of structural changes such as deletion, duplication, inversion, and translocation.
• Study of mitosis, meiosis, recombination, and chromosome segregation.
• Diagnosis of genetic disorders, cancers, infertility, and prenatal abnormalities.
• Comparison of chromosomes among species for evolutionary studies.

Major techniques:

• Karyotyping: Arrangement of metaphase chromosomes according to size, centromere position, and banding pattern.
• Chromosome banding: Techniques such as G-banding reveal characteristic patterns on chromosomes.
• Fluorescence in situ hybridization: Fluorescent DNA probes identify specific chromosome regions or abnormalities.
• Comparative genomic hybridization: Detects gains or losses of chromosome material.
• Microscopy and chromosome staining: Permit direct examination of chromosome morphology and behaviour.

These methods connect visible chromosome changes with genetic traits and diseases.

Cytology, genetics, and cytogenetics are closely related fields that provide complementary information about heredity.

• Cytology provides knowledge of cell structure, chromosomes, mitosis, meiosis, and organelles.
• Genetics explains how hereditary characters are transmitted and how genes produce variation.
• Cytogenetics combines these approaches by associating genetic phenomena with visible chromosome structure and behaviour.

Examples of their interrelationship:

1. Mendel's law of segregation is explained cytologically by the separation of homologous chromosomes during meiosis.
2. Independent assortment is related to the random orientation of different homologous chromosome pairs at metaphase I.
3. Genetic recombination is associated with crossing over between homologous chromosomes during prophase I.
4. Chromosome mutations observed under a microscope can be connected with abnormal phenotypes or diseases.
5. Cytological study of mitochondria and chloroplasts supports the genetic understanding of cytoplasmic inheritance.

Therefore, cytology supplies the cellular basis, genetics supplies the principles of heredity, and cytogenetics integrates both to explain inheritance at the chromosome level.

Cytogenetic analysis detects changes in chromosome number or structure and relates them to abnormal phenotypes, inheritance, and disease.

Numerical abnormalities:

• They commonly arise through nondisjunction, or failure of chromosomes to separate normally.
• Trisomy 21 causes Down syndrome and produces the karyotype in a female or in a male.
• Monosomy X causes Turner syndrome, represented as .
• An additional X chromosome in a male causes Klinefelter syndrome, represented as .

Structural abnormalities:

• Deletion: Loss of a chromosome segment; deletion of part of chromosome 5p causes cri-du-chat syndrome.
• Duplication: Repetition of a chromosome segment.
• Inversion: Reversal of a segment within a chromosome.
• Translocation: Transfer or exchange of chromosome segments; the Philadelphia chromosome in chronic myeloid leukaemia results from a translocation between chromosomes 9 and 22.

Karyotyping, chromosome banding, and fluorescence in situ hybridization can identify these changes. Cytogenetics therefore links cellular chromosome observations with genetic diagnosis, counselling, prognosis, and treatment planning.