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                                                         Topic 16: Inheritance
16.1 Passage of information from parents to offspring

 

​Students should be able to:

1) explain the meanings of the terms haploid (n) and diploid (2n)
2) explain what is meant by homologous pairs of chromosomes
3) explain the need for a reduction division during meiosis in the production of gametes
4) describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of meiosis, but not the sub-divisions of prophase I, are expected: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II and telophase II)
5) interpret photomicrographs and diagrams of cells in different stages of meiosis and identify the main stages of meiosis
6) explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes
7) explain that the random fusion of gametes at fertilisation produces genetically different individuals

 

1. Explain the meanings of the terms haploid (n) and diploid (2n)

In biology, the terms haploid and diploid refer to the number of sets of chromosomes present in a cell. Chromosomes are long molecules of DNA that carry genetic information.

  • A diploid cell, represented as 2n, contains two complete sets of chromosomes—one set inherited from the mother and one set from the father. This means that for every chromosome in the mother’s set, there is a corresponding homologous chromosome in the father’s set. Most of the cells in an animal or plant’s body are diploid, such as skin cells or muscle cells. For example, human diploid cells contain 46 chromosomes arranged in 23 homologous pairs.

  • A haploid cell, represented as n, contains only one set of chromosomes. Haploid cells have half the number of chromosomes as diploid cells. In animals and plants, gametes (sperm and egg cells) are haploid. This reduction in chromosome number is crucial so that when fertilization occurs—the fusion of sperm and egg—the resulting zygote has the correct diploid number of chromosomes.

Thus, haploid cells ensure that the chromosome number remains stable across generations, preventing the doubling of chromosomes each time reproduction happens.
 

 

2. Explain what is meant by homologous pairs of chromosomes

Homologous chromosomes are pairs of chromosomes that have the same size, shape, and gene sequence, with one chromosome of each pair inherited from each parent. Each member of a homologous pair carries genes for the same traits arranged in the same order, although the versions of those genes, called alleles, may differ between the two chromosomes.

For example, in humans, chromosome 1 inherited from the mother will pair with chromosome 1 inherited from the father. These homologous chromosomes pair up during meiosis to allow for accurate segregation and recombination. The concept of homologous pairs is central to sexual reproduction because it ensures offspring receive a balanced mix of genetic material from both parents.
 

 

3. Explain the need for a reduction division during meiosis in the production of gametes

Meiosis is a special type of cell division that produces haploid gametes from diploid cells. The reason for this reduction division is to halve the chromosome number, preventing the doubling of chromosomes in every generation.

If gametes were diploid and fertilization combined two diploid cells, the resulting offspring would have twice as many chromosomes as their parents, leading to genomic instability and serious biological issues. Instead, meiosis reduces the chromosome number by half through two rounds of cell division: meiosis I and meiosis II. This reduction division ensures that when gametes fuse during fertilization, the diploid chromosome number is restored, maintaining species stability over generations.
 

 

4. Describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane, and the spindle (names of the main stages only)

Meiosis occurs in several distinct stages, each with characteristic changes in chromosomes and cellular structures:

  • Prophase I: Homologous chromosomes condense, pair up closely in a process called synapsis, and crossing over occurs (exchange of genetic material). The nuclear envelope begins to break down, and spindle fibers start to form from the centrioles (in animals) or microtubule organizing centers.

  • Metaphase I: Homologous chromosome pairs align along the metaphase plate (equator of the cell), attached to spindle fibers. The nuclear envelope has completely disappeared by this point.

  • Anaphase I: Homologous chromosomes are pulled apart by spindle fibers to opposite poles of the cell. Sister chromatids remain attached.

  • Telophase I: Chromosomes reach the poles, the nuclear envelope reforms around the two new nuclei, and the cell may undergo cytokinesis, dividing into two haploid daughter cells.

  • Prophase II: In each haploid daughter cell, chromosomes condense again, the nuclear envelope breaks down, and spindle fibers form.

  • Metaphase II: Chromosomes align individually along the metaphase plate.

  • Anaphase II: Sister chromatids finally separate and move to opposite poles.

  • Telophase II: Nuclear envelopes reform, chromosomes decondense, and cytokinesis divides cells, resulting in four genetically unique haploid gametes.

Throughout meiosis, the spindle apparatus is crucial for moving chromosomes, and the nuclear envelope dissolves and reforms to allow chromosome movement. The cell surface membrane participates in cytokinesis, splitting the cytoplasm to form separate cells.
 

 

5. Interpret photomicrographs and diagrams of cells in different stages of meiosis and identify the main stages of meiosis

To correctly identify meiosis stages from images, focus on key features:

  • Prophase I: Paired homologous chromosomes visible; crossing over may be inferred by chromosome thickening and chiasmata.

  • Metaphase I: Homologous pairs lined up at the center.

  • Anaphase I: Homologous chromosomes separating toward poles.

  • Telophase I: Two nuclei forming, chromosomes still doubled.

  • Prophase II: Chromosomes re-condensing in haploid cells.

  • Metaphase II: Individual chromosomes aligned in the center.

  • Anaphase II: Sister chromatids separating.

  • Telophase II: Four distinct nuclei forming, chromosomes decondensing.

Understanding these features helps distinguish meiosis from mitosis and identify errors or abnormalities in meiosis.
 

 

6. Explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes

Two important mechanisms contribute to genetic variation during meiosis:

  • Crossing over occurs during prophase I, where homologous chromosomes (with each chromosome being made up of 2 sister chromatids joined by the centromere) exchange corresponding segments of DNA at points called chiasmata. This recombination results in chromosomes containing new combinations of alleles different from either parent, increasing diversity in gametes.

  • Random orientation (independent assortment) happens during metaphase I when homologous chromosome pairs align randomly at the cell equator. This means that the maternal and paternal chromosomes are assorted independently into daughter cells. Since each chromosome pair segregates independently, the number of possible chromosome combinations is extensive (2^n, where n is the haploid number).

Together, these processes ensure each gamete contains a unique set of genetic information, providing the raw material for evolution and variation in populations.
 

 

7. Explain that the random fusion of gametes at fertilisation produces genetically different individuals

After meiosis produces genetically diverse gametes, the next step is fertilization, the fusion of a sperm and egg cell. This process is random—any sperm can fertilize any egg—and it combines two unique sets of chromosomes.

Because each gamete is genetically distinct due to crossing over and independent assortment, their fusion produces a zygote with a unique genetic makeup. This randomness in fertilization adds another layer of genetic variation among offspring, ensuring that siblings have different combinations of traits, which is vital for adaptation and survival in changing environments.


 

16.2 The roles of genes in determining the phenotype

 

​Students should be able to:

1) explain the terms gene, locus, allele, dominant, recessive, codominant, linkage, test cross, F1, F2, phenotype, genotype, homozygous and heterozygous
2) interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid crosses and dihybrid crosses that involve dominance, codominance, multiple alleles and sex linkage
3) interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of dihybrid crosses that involve autosomal linkage and epistasis 
4) interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of test crosses
5) use the chi-squared test to test the significance of differences between observed and expected results
6) explain the relationship between genes, proteins and phenotype with respect to the:
• TYR gene, tyrosinase and albinism
• HBB gene, haemoglobin and sickle cell anaemia
• F8 gene, factor VIII and haemophilia
• HTT gene, huntingtin and Huntington’s disease
7) explain the role of gibberellin in stem elongation including the role of the dominant allele, Le, that codes for a functional enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme

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16.3 Gene control

 

​Students should be able to:

1) describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes
2) explain genetic control of protein production in a prokaryote using the lac operon 
3) state that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription
4) explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription

 

1. Describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes

Genes in organisms can be broadly classified into two categories based on their roles: structural genes and regulatory genes. Structural genes contain the instructions for synthesizing proteins that perform specific functions within the cell or organism. For example, a structural gene might code for an enzyme involved in metabolism or a protein that forms part of the cell’s structure.

On the other hand, regulatory genes produce proteins, often called regulators, which control the expression of other genes. These regulatory proteins include repressors and activators that bind to DNA sequences near structural genes and either prevent or promote the transcription of these genes. Regulatory genes ensure that proteins are produced only when needed, allowing cells to respond to environmental changes and conserve resources.

Regarding enzymes, there are two main types based on how their synthesis is controlled: repressible enzymes and inducible enzymes.

  • Repressible enzymes are typically involved in biosynthetic pathways, where the end product of the pathway inhibits enzyme production when abundant. This feedback mechanism is energy-efficient because the cell stops making enzymes when the product is plentiful, conserving energy and raw materials. In this case, the default state of enzyme synthesis is “on” but can be turned “off” (repressed).

  • Inducible enzymes are usually involved in catabolic pathways, breaking down substances that are not constantly present in the cell’s environment. These enzymes are produced only when their substrate is available. For example, the enzymes that break down lactose in bacteria are produced only when lactose is present. In this case, enzyme production is normally “off” but can be turned “on” (induced) when needed.

This regulation ensures that enzymes are synthesized only when beneficial for the cell, illustrating the importance of gene control in cellular efficiency.
 

 

2. Explain genetic control of protein production in a prokaryote using the lac operon 

In prokaryotes such as Escherichia coli (E. coli), gene expression is often controlled using operons—clusters of genes with related functions transcribed together. The lac operon is a classic example of how bacteria regulate genes encoding enzymes for lactose metabolism.

The lac operon consists of structural genes that code for enzymes necessary to break down lactose, such as β-galactosidase, alongside a promoter region, an operator region, and a regulatory gene that codes for the lac repressor protein.

  • When lactose is absent, the lac repressor binds tightly to the operator region, physically blocking RNA polymerase from transcribing the structural genes. This prevents the production of lactose-metabolizing enzymes, which conserves cellular energy since lactose is not available for metabolism.

  • When lactose is present, a form of lactose called allolactose binds to the repressor protein, causing it to change shape and release from the operator. RNA polymerase can then bind to the promoter and transcribe the structural genes, leading to the production of enzymes that break down lactose into glucose and galactose.

This system exemplifies an inducible operon, where gene expression is switched on only in the presence of the substrate, allowing the bacteria to adapt efficiently to its environment.
 

 

3. State that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription

In eukaryotic cells, gene expression is controlled by proteins known as transcription factors. These proteins bind to specific DNA sequences near genes, such as promoters or enhancers, and influence the activity of RNA polymerase, the enzyme responsible for transcribing DNA into messenger RNA (mRNA).

Some transcription factors act as activators that increase the rate of transcription by helping RNA polymerase bind more efficiently or by modifying the chromatin structure to make the gene more accessible. Others act as repressors, decreasing transcription by blocking RNA polymerase binding or recruiting proteins that condense chromatin and make the gene less accessible.

The precise combination and concentration of transcription factors in a cell determine which genes are expressed, at what levels, and in which cells or developmental stages. This sophisticated control enables eukaryotes to have complex patterns of gene expression necessary for growth, development, and response to stimuli.
 

 

4. Explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription

In plants, gibberellins regulate growth by controlling gene expression at the transcriptional level. DELLA proteins are key repressors that inhibit transcription factors involved in growth-promoting gene expression.

Under normal conditions without gibberellin, DELLA proteins bind to and inhibit transcription factors that would otherwise activate genes involved in processes like stem elongation. This repression prevents excessive or untimely growth.

When gibberellin levels increase, the hormone binds to its receptor, triggering a signaling cascade that leads to the targeted degradation of DELLA proteins via the proteasome pathway. With DELLA repressors removed, the previously inhibited transcription factors become active and promote the transcription of genes responsible for cell division and elongation.

This mechanism highlights how a plant hormone can indirectly activate gene expression by removing protein repressors, allowing growth to proceed. It also demonstrates a sophisticated layer of gene regulation in response to hormonal signals.

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