Module 5: Heredity

Good marks aren't hereditary, they're earned! Read this Beginner's Guide to Biology to get a cellular understanding of heredity.

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Studying heredity is vital to understanding how living organisms reproduce and pass their characteristics on to their offspring. It also helps us understand what makes us unique from one another in general.

 

In this Beginner’s Guide, we will cover Module 5: Heredity and its topics:

 

Topic 1: Reproduction and Cell Replication

Reproduction is the making of a new offspring via sexual or asexual means. Sexual reproduction occurs mainly in animals and is the process of fertilisation of an egg by a sperm to form an offspring. The fertilisation can happen internally (such as in humans) or externally (such as in fish). Asexual reproduction occurs predominantly in plants and bacteria and requires only one individual for the making of an identical copy of itself.

The summary of these process are as follows:

AnimalPlant
Sexual ReproductionAsexual ReproductionAsexual ReproductionSexual Reproduction
Internal Fertilisation
Involves the fertilisation of
the egg INSIDE the female body.
It is more advantageous for
the protection of offspring.
1. Budding
2. Binary Fission (in Bacteria)
3. Fragmentation
4. Parthenogenesis
1. Rhizomes
2. Runners
3. Tubers
4. Cuttings
5. Budding
There are three steps involved:
1. Pollination
2. Fertilisation
3. Seed Dispersal
External Fertilisation
Involves the fertilisation of
the egg OUTSIDE the female body.
It requires less energy and
nurturing of the offspring.

 

In mammals specifically, a number of hormones are involved in pregnancy and birth of the offspring:

  • Human chorionic gonadotropin (hCG) maintains the corpus luteum (so it can produce progesterone) and stops ovulation. It also increases blood supply to the pelvic area.
  • Progesterone prepares the uterus and prevents lactation and uterine contractions before birth.
  • Oestrogen/Estrogen assists in organ development of the foetus e.g. kidneys, liver and lungs. It also promotes growth of breast tissue.
  • Prolactin stimulates lactation (milk secretion).
  • Oxytocin is a hormone released during childbirth causing contractions of uterine muscles. The pressure of the baby’s head against the opening uterus causes more oxytocin to be released and additional uterine contractions. This positive feedback loop continues until the baby is born.
image showing the cycle oxytocin production described above beginners-guide-to-year-12-biology-heredity-secretion-of-oxytocin

By OpenStax – https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30131134

 

Cell Replication is the other important aspect for survival and continuity of living organisms. This is the process of cell division in order to make new cells and occurs through mitosis and meiosis.

Mitosis is the process of making new body (somatic) cells for the purpose of growth and repair. In humans, a parent cell divides into two genetically IDENTICAL Diploid daughter cells that each contain 46 chromosomes.

Meiosis produces sex cells (gametes) such as sperm and egg for the purpose of sexual reproduction. In humans, the parent cell divides into four genetically UNIQUE haploid daughter cells each containing 23 chromosomes. Each daughter cell is unique from the parent cell due to the various process that occur during meiosis. These processes include crossing-over, random segregation and independent assortment. These processes are vital in producing genetic variability in offspring to ensure their continuity (we will cover the processes in Topic 3). The stages of mitosis and meiosis are summarised in the diagram below:

Image illustrating the processes of mitosis and meiois discussed above

Mitosis by Mysid [Modified] – Vectorized in CorelDraw by Mysid from http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1414818
Meiosis by Rdbickel [Modified] – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49599354

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Topic 2: DNA and Polypeptide Synthesis

In order to understand DNA and polypeptide synthesis, it is important for you to understand the structure of DNA.

DNA (Deoxyribonucleic Acid) is a large molecule containing all the genetic material necessary to pass characteristics onto the offspring. It contains segments called genes that are like recipes for making polypeptides (that will become proteins). These proteins are required for cellular functions inside the organism. This DNA can be packed into larger structures called chromosomes (the structures passed into daughter cells mentioned earlier).

DNA is found in both eukaryotic and prokaryotic cells, without DNA life would not exist!

DNA is made up of three repeating subunits that combine to form a double helix structure. These subunits are sugar (deoxyribose), a phosphate group and nitrogenous bases (adenine which pairs with thymine and guanine which pairs with cytosine). The sugar and phosphate linkage is what makes the “backbone” of the double helix, whereas the pairing of the nitrogenous bases in the middle of the ladder is what forms the “rungs” of the ladder holding the double helix in place.

This is shown below:

Diagram showing the composition of DNA

Polypeptide synthesis in eukaryotes and prokaryotes is the process by which a gene is expressed to make polypeptide chains that are folded into a protein. The process involves:

 

Transcription: DNA to mRNA

Transcription is the copying of a gene onto a portable single strand transcript called mRNA. This takes place in the cell nucleus.

  • The section of DNA containing the gene of interest unwinds to expose its base sequences to the RNA polymerase.
  • Free floating mRNA nucleotides (NTPs) undergo base pairing with the exposed DNA bases to make the mRNA transcript.
  • In order to stabilise this mRNA and prevent its breakdown, a cap called 5 Prime is added to one end and a poly A tail is added to the other end (Prokaryotes lack this step).
image describing the process of transcription described above in the beginners-guide-to-year-12-biology-heredity

Translation: mRNA to Polypeptide

Translation occurs in the cytoplasm of the cell. It is the process where the mRNA transcript is deciphered to produce a polypeptide chain. The process is split into three stages:

  • Initiation: This is where a ribosome attaches to the start of the mRNA transcript. The ribosome molecule will read the mRNA three bases at a time (codons) starting with the start codon.
  • Elongation: The ribosome continues reading the mRNA codons and recruits transfer RNA (tRNA) with complementary anticodons that carry amino acids. The ribosome joins the amino acids in a chain to create a polypeptide.
  • Termination: Once a STOP codon is reached the growth of this polypeptide chain stops and the chain detaches from the ribosome unit.
image illustrating the process of translation described above in beginners-guide-to-year-12-biology-heredity

The polypeptide is still non-functional at this final step and needs be folded into a 3D structure to start functioning as a protein! You will learn more about protein structure and types in this course.

 

 

Topic 3: Genetic Variation and Inheritance Pattern in a Population

DNA maintains genetic continuity but also introduces genetic variation. The main processes that introduce variation include:

  1. Gamete formation (meiosis)
  2. Sexual reproduction
  3. Mutation

1. Gamete formation (meiosis)

Gamete formation (meiosis)produces gametes that are genetically unique from one another resulting in variation in the offspring. This is caused by – crossing over, random segregation and independent assortment.

  • Crossing over refers to the exchange of a section of genetic material between maternal and paternal chromosomes to form new combination of characteristics in the offspring.
  • Random segregation is the process of randomly distributing chromosomes into different gametes
image illustrating assortment
  • Independent assortment refers to the fact that genes that are not close to each other on a chromosome are inherited independently. This means that inheritance of one trait does not depend on another (e.g. hair colour does not depend on height).
image illustrating assortment

2. Sexual Reproduction

The process of fertilisation is random and since each gamete is genetically unique, the union of sperm and egg results in an even greater genetic variability in the offspring.

 

3. Mutation

A permanent change to the DNA can occur during meiosis and create a new characteristic, affecting the phenotype of the offspring.

Inheritance patterns depict how traits are passed onto the offspring through genes. Mendelian genetics illustrates the inheritance of characteristics controlled by a simple dominant/recessive relationship. In this case, there are two variations of a gene called alleles, allele expression is dependent on whether it is dominant or recessive affecting the phenotype of the offspring accordingly.

This topic depicts allele combinations and their respective genotype and phenotype ratios using Punnett squares. You will go through examples of Punnett squares in class so you can have a better understanding of constructing them.

image showing punnett squares

Punnett Squares are drawn according to the following: Dominant allele is represented by an UPPER case letter (e.g. TT ) Recessive allele is represented by a LOWER case letter (e.g tt) Genotype represents allele combination ratios in gametes Phenotype represents physical trait ratios in gametes

 

Other inheritance types occurring across a population can follow more complex patterns including:

  1. Sex-linkage
  2. Co-dominance
  3. Incomplete dominance
  4. Multiple alleles
  • Sex linkage is when a gene controlling a characteristic/disease is located on a sex chromosome (usually the X chromosome). It affects more males than females as males only have one X chromosome (unlike females that have two) and thus no backup X-chromosome to mask the trait if they inherit the faulty X-chromosome. An example of this is colour-blindness.
  • Co-dominance refers to both alleles of a gene being expressed simultaneously. An example of this is human blood types. This is where both A and B alleles can be expressed at the same time to produce an offspring with type AB blood.
  • Incomplete Dominance is when both alleles mix together rather than being fully expressed simultaneously. An example includes the crossing of a red flower with a white flower to produce a pink flower.
  • Multiple alleles refer to the possibility of having more than two alleles across a whole population (NOT present in an individual as we only carry two alleles for each gene). An example includes the leaf pattern of white clover plants controlled by up to 7 alleles giving rise to 22 different patterns on the leaf as a result.

We can also recognise inheritance patterns through use of pedigrees which are family tree diagrams that show genetic relationships between individuals across generations. Pedigrees represent traits that are autosomal or sex-linked. This module teaches you how to construct pedigrees so you will need to familiarise yourself with the standard symbols used as shown below.

image showing standard symbols for notation

There are four main types of inheritance patterns detected by pedigrees:

1. Autosomal recessive:

This is where the trait SKIPS a generation (such as in generation II below). Another clue to look for is that unaffected parents can have affected children.

 

2. Autosomal dominant:

This is where the trait affects EVERY generation. Besides that, affected parents can have unaffected children.

 

 

3. Sex-linked recessive:

This type of trait affects more males than females as shown below.

beginners-guide-to-year-12-biology-heredity-sex-linked-recessive.


4. Sex-linked dominant:

This trait involves an affected father passing the trait to ALL his daughters only. This type of trait is very rare, however.

Technologies can be used to examine population inheritance patterns and identify individuals through regions in their DNA that make each individual unique. One type of mutation that makes us unique is called Single Nucleotide Polymorphism (SNP).

During DNA replication, SNP involves a natural mutation of a single base in the polypeptide sequences that ultimately affects the phenotype.

In this module, you will learn about the steps involved in various technologies that use DNA to identify individuals and to reflect population inheritance patterns.

An example of such technology includes DNA sequencing and profiling that can be used in areas like crime scene investigations and paternity tests. For example, The Golden State Killer was apprehended thanks to DNA sequencing and profiling.

You will go through examples to try to determine the suspect of a crime!

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