Bio Mod 5 Week 4 Biology PDF

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HSCBIO HSC Module 5 heredity notes, furth inquiry question...

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Overview of Inquiry Question #1 Learning Objective #1 – Predicting variations in offspring’s genotype by modelling meiosis – crossing over, fertilisation and mutation Learning Objective #2 – Model the formation of new combinations of alleles during meiosis and sexual reproduction – autosomal, sex linkage, co-dominance, incomplete dominance and multiple alleles inheritance Learning Objective #3 – Constructing and interpreting Punnett Squares Learning Objective #4 – Constructing and interpreting Pedigrees Learning Objective #5 – Examining and monitoring the frequency of characteristics in a population in order to identify trends, patterns, relationships and limitations in data 

Allele frequency, genotype frequency

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Trait frequency such as phenotype

Learning Objective #6 – Exploring single nucleotide polymorphism NEW HSC Biology Syllabus Video – Genetic Variation Week 4 Homework Questions Week 4 Curveball Questions Week 4 Extension Questions Solutions to Week 4 Questions

Learning Objective #1 - Predicting variations in offspring's genotype by modelling meiosis - crossing over, fertilisation and mutation Genetic Variation created by crossing over Crossing over is process involving the exchange of corresponding gene segments of nonsister chromatids between homologous chromosome pairs (double-stranded chromosome pairs for the most organisms).

This effectively creates creates new allele combinations, known as recombination. It is important to stress that stating ‘new allele combinations being created as a result of crossing over’ is critical in HSC Biology as it is what appears on the marking criteria. Below is a diagram illustrating the crossing over process between one pair of homologous chromosomes.

Notice that the crossing over occurs between non-sister chromatids, one from each doublestranded chromosome of the homologous pair. Each parent of the organism contribute one double-stranded chromosome. 

For example, perhaps the blue one can be from the father (paternal chromosome) and the pink one can be from the mother (maternal chromosome).

Chiasma is the point where crossing over occur. Plural is chiasmata. There can be multiple chiasmata, usually the longer chromosome, the more points of crossing over. The chiasma the visible component of the homologous chromosomes during crossing over in Prophase I in meiosis I. Recall from Week 2’s notes that crossing over does not occur in mitosis because the homologous chromosomes are NOT aligned side-by-side along the equator of the cell to allow overlapping segments of non-sister chromatids along the equator in the nuclear membrane.

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Fun Fact: Crossing over can in fact occur between sister chromatids in one doublestranded chromosome (with requiring another double-stranded chromosome). However, it is ONLY crossing over between non-sister chromatids in a homologous pair where where new allele combinations are created. You may be wondering why?

Well, if you recall that sister chromatids are exact copies of each other, this means that the alleles on sister chromatids (coding for the any of the genes) are identical! Take a look at the diagram above. Each red dashed line highlights a locus on each chromatid. Prior to crossing over, each locus (position) on each sister chromatid has the same allele. For example, take the blue double-stranded chromosome, both of the sister chromatids have the ‘B’ allele that codes for black hair colour. Now, look at the other double-stranded chromosome (pink) in the homologous pair. At the locus of both sister chromatids that carries the gene that codes for eye colour, they both have the ‘b’ allele that codes for blue eye colour. NOTE: In this example, ‘B’ is an allele means that it codes for black eye colour and ‘b’ is an allele codes for blue eye colour. The letters (or alleles to be exact) can code for different colours or genes depending on the question given to you on the day. For instance, in the exam, the question will tell you that what gene each letter codes for AND what the capital and lowercase of each letter would mean in terms of alleles. 

By convention, capital letter represents a dominant allele and lowercase represents a recessive allele. We will talk about dominant and recessive alleles soon in this week’s notes.

Just to enhance your prior knowledge from Week 2: As you can see from the diagram shown above, the blue and red double-stranded chromosomes are called homologous pairs because, at the each locus (each locus shown by a red dashed line) of the chromosomes in the homologous pairs there are alleles that code for the same gene. Fun Fact: The longer the distance genes are separated along each of the chromosome (chromatid to be precise), the greater the chance of crossing over. Generally, this will depend on the length of the chromosome. The greater the length, the more chance of crossing over and more chiasmata will exist during crossing over. In the previous diagram, I have drew light green arrows on the homologous chromosomes after crossing over to indicate the alleles that belong to each of the four chromatids. If I were to separate the four chromatids in the previous diagram on its own, the resulting allele combinations for the four chromatids would be as follows:

As you can see, there are two NEW allele combinations that has been created as a result of crossing over. These new allele combinations are bHC and BhC which did not exist prior to crossing over. Extra crossing over scenario that is BEYOND the HSC Biology Course (You WILL NOT be asked about this in HSC Biology Exams)

Note that this diagram illustrating crossing over process, before and after, is slightly different to the previous diagrams. Can you spot the difference? The difference is that the chiasma, i.e. the point of crossing over, is NOT located between the genes that codes for eye colour and height unlike in the previous diagram. As you can see in the products after crossing over, there is still new allele combinations formed. However, there is one consequence. This type of crossing over usually is hard to detect (often undetected by normal means) compared to the previous crossing over scenarios. This means that the average number of crossing over detected would be lower than the true amount of crossing over that actually happened. Scientists often perform crossing over experiments in attempt to determine the locus of chromosome (and thus DNA) that holds alleles that codes for a particular gene. So, having undetected crossing over event that in fact actually occurred would yield inaccurate results. This in fact relates to single nucleotide polymorphism which we will be covering as the last learning objective in this week’s notes. Significance of crossing over

The process of crossing over creates new allele combinations adds diversity of the gene pool of the population as the four gametes formed in meiosis will not be identical but rather can have different an allele for each gene. Also, upon fertilisation, two random gametes with some variation in their alleles for certain genes will combine and produce an offspring with an unique allele combination to their parents. This therefore increases genetic variation in a population and can introduce new phenotypes that potentially* could be expressed. Genetic variation in a population provides a pathway for evolution to occur. Without genetic variation, there will be no mechanisms for evolution. Genetic variation can be introduced into a population’s gene pool via:  

Sexual reproduction (crossing over, independent assortment, random segregation and fertilisation) or Mutation

*The word ‘potentially’ is used because whether or not the allele will be expressed will depend on whether or not it is recessive and dominant which also depends on the complementary allele for the same gene. Dominant and Recessive Alleles Alleles are alternative (or different) versions of a gene that differ by their DNA sequence but codes for a protein that responsible for a same trait (e.g. hairy or hairless). An allele can be classified as dominant or recessive. Dominant alleles are always expressed over recessive alleles if they are both present in the genotype for a gene of the organism. 

By convention, dominant alleles are denoted with capital letters.

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Recessive alleles are denoted with lower-case letters.

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If the the alleles are coding for the same trait, the same letter is used. If not, different letters are used.

For example, in the diagrams shown previously, for the gene that codes for hair colour, there are two types of alleles, B and b. The ‘B’ allele would be the dominant allele for hair colour and b would be the recessive allele for hair colour. In HSC Biology, the question will tell you what the hair colour for dominant allele as well as for the recessive allele for hair colour. Some question requires you to determine what trait each allele expresses and whether or not they are dominant or recessive, in that case, the question will give you sufficient information to allow you to work it out. In that case, you will need to use Punnett Squares which we will learn this week. There will be these questions in this week’s homework set to allow you to practice. Give it a go. Solutions will be uploaded soon. Anyhow, below are some rules of thumb about dominant and recessive allele combinations:

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If the organism (parent and/or offspring) has two dominant allele for a particular gene, the dominant allele will be expressed.

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If the organism (parent and/or offspring) has two alleles for a particular gene, one dominant and the other is recessive, the dominant allele will be expressed.

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If the organism (parent and/or offspring) has two recessive alleles for a particular gene, the recessive allele will be expressed. Fertilisation

As illustrated in previous diagram, ‘Four different combinations of alleles after crossing over’, each chromatid begin segregating in Anaphase II and be completely segregated into a different gamete after Telophase II. 

This occurs for all 23 pairs of homologous chromosome (one pair being sex chromosomes, XX or XY).

In the previous diagrams , we only drew two chromosomes (a homologous pair) that are crossing over just to satisfy the purpose of a simplistic illustration of the crossing over of chromosomes in a homologous pair. As you may know now, when the gametes fuse together (egg and sperm except for fauna), a zygote is formed which has a diploid number of chromosomes. That is, the zygote has 46 chromosomes (23 homologous pairs). So, in meiosis, the process of crossing over effectively creates the genetic variation in the gametes due to their difference in alleles for various genes (as we have mentioned earlier). Also, the random process of fertilisation between gametes (egg and sperm) also give rise to the genetic variation in the zygote whereby the two gamete have different alleles such that the zygote will have different allele combinations than its parents. 

Due to meiosis, each gamete will have one allele that codes for a particular gene. Upon fertilisation where the female and male gametes fuse together to form a zygote, the zygote will then two alleles that code for each gene (e.g. gene for eye colour).

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Therefore, the process of fertilisation creates new genotypes for the offspring that are, for the most part, different from each individual parent.

Note that if both parents the homozygous dominant for the same gene. Homozygous dominant just means that the individual has two same alleles (homozygous) and the alleles are dominant (capital letter). So, if both parents have homozygous dominant alleles for a trait, then the offspring will have genotype that is exactly the same as the parent (homozygous dominant too) though ONLY for that particular gene. It is unlikely that both parents will have EXACTLY the same genotype (allele combinations) for EVERY GENE. In that case, they be twins. LOL. 

We will further explore and talk about homozygous dominant later when we look at Punnett Squares later in this week’s notes.

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Also note, in reality, most traits (e.g. height) are governed by more than one gene.

Recall that independent assortment and random segregation both facilitate in increasing the genetic variation in offsprings of the new generations. Although they do not create new allele combinations, they do introduce variations in the way which alleles that specifies different genes on non-homologous chromosomes are assorted independently relative to each other (independent assortment) and eventually different chromatids that carries different alleles for certain genes can be segregated into different gametes (random segregation). 

This means that these two independent assortment and random segregation introduce (genetic) variation into the alleles that each gamete inherit after Cytokinesis II in Telophase II.

The specifics in how independent assortment and random segregation increases the genetic variation in the offspring was discussed in Week 2’s note. If required, please revisit that section to refresh memory. Mutation Recall that crossing over introduces new combinations of alleles that already exist in the chromatids by exchanging corresponding gene segments between non-sister chromatids. Mutation, however, alters the allele’s identity because the DNA sequence is altered. This new DNA sequence can lead to an alternative expression of the gene (e.g. blue eye colour instead of black eye colour), hence new allele (different DNA sequence for same gene) is created. In most cases, mutation only involves the modification of a DNA nucleotide, changing its nitrogenous base (point mutation). In some more extreme cases, it can modify the DNA sequence of a portion of chromosome that involves one or more genes (chromosomal mutation). We will explore point mutation and chromosomal mutation in Module 6 – Inquiry Question 1. Anyhow, after learning about protein-synthesis in last week’s notes, where the creation of mRNA uses the DNA sequence of a gene as a template, it is clear that altering the nucleotide sequence of a gene will result in the specification of a different protein OR may modify the structure and function of the protein such that it will no longer functions as efficiently or completely not functional at all. Suppose, you have an allele that codes for blue eye colour. This allele can be exchanged with another non-sister chromatid during crossing over. However, mutation completely alters the identity of this allele originally coded for blue eye colour. This means that it may code for something slightly or completely different instead. Perhaps maybe, green eye colour. Since alleles are altered, this would mean that through mechanisms of sexual reproduction and fertilisation, mutated allele will be inherited by gamete and possible give rise to offspring when fertilised. This would mean that a new allele will be introduced into the

population. This would mean that there would be an increase in genetic variation in the population as allele frequency will increase. Note that mutation on genes in autosomes and sex chromosomes will be passed onto offspring if ONLY the mutation occurs in the parents’ germ cell. The gametes derived from the parent germ cell then becomes an offspring when gamete is fertilised where the offspring can inherits the mutation. Most mutations passed onto offspring may not expressed as they could be recessive so offspring needs to inherit one mutated allele from each parent in order to express the mutated characteristic. Whether or not the zygote will express the new allele due to mutation will depend on the zygote’s genotype (dominant or recessive for that gene) which we will explore very shortly when we deal with Punnett Squares. If the mutated allele is expressed, it is important to consider whether or not the expressed trait that it codes for will become an improvement or hinderance to the organism’s daily activities. 

Mutations are permanent changes to an organism’s DNA sequence.

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Mutation and its implications will be discussed in greater detail in Module 6.

Mutation is considered the principal and original source in creating genetic variation in a population. This is because they provide the creation of new alleles by altering DNA sequence. The parent can be copy such mutated DNA via meiosis I which gametes and, when fertilised, the offspring can inherit such mutated DNA. 

Not all of the four gametes must inherit such mutated DNA though.

Anyways, similar to crossing over, independent assortment, random segregation, we should now know that mutation also creates genetic variation in a population which provides a pathway for evolution to occur. Without genetic variation, there will be no mechanisms for evolution. This ties nicely to support your responses when asking questions regarding ‘ensuring’ the continuity of species as per inquiry question one. As genetic variability in a population increases, the species’ population ability to adapt to its environment over time also increases. It is through such adaptability of species in a population overtime whereby we witness evolution due to shifts in dominant or new characteristics in the species’ population.

Learning Objective #2 - Model the formation of new combinations of genotype produced during meiosis, including and interpreting examples of: - Autosomal Inheritance - Sex-Linkage Inheritance

- Co-dominance Inheritance - Incomplete Dominance Inheritance - Multiple alleles Inheritance An autosome is a chromosome in an organism that is not a sex chromosome. In humans, we have 22 pairs of autosomes and one pair of sex chromosomes. Genes are present in both autosomes and sex chromosomes. The process of transferring genes (DNA) present in the parents’ autosomes to offspring is called autosomal inheritance. The process of transferring genes (DNA) present in the parents’ sex chromosomes to offspring is called sex-linked inheritance or X-linked inheritance. The genes that is present in autosomes and sex chromosomes, when passed to offsprings, exhibit different inheritance patterns or combinations of genotypes in terms of phenotype. 

More specifically, the likelihood of an offspring exhibiting a recessive trait that is passed on autosomal inheritance is equal for both males and females. However, the likelihood of offspring exhibiting a recessive trait that is passed on via sex chromosomes inheritance is greater for males than in females.

Let’s have a look at why this is the case by exploring both modes of inheritance. Autosomal Inheritance Recall that in humans, there are two alleles for a given gene where the alleles help determine the trait of organism (both the parents and offspring). For illustration purposes, let’s use the gene that codes for eye colour. The gene that codes for eye colour will be present in both the female and male. 

Just to get a clearer picture and as a recap, do recall that a gene is basically a segment of a DNA or a sequence of DNA nucleotides. Chromosomes are made up of DNA (wrapped around proteins called histones) for eukaryotes. Approximately 40% of a chromosome is made up of DNA and the other 60% is made up of proteins.

Now, let’s return to our example. So, each parent will have two alleles for its eye colour gene. Suppose that:  

The mother has an allele that codes for blue eye colour and an allele that codes for black eye colour. The father have both alleles that code for black eye colour.

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Let’s also suppose that the allele that codes for black eye colour is denoted by ‘B’ and the blue eye colour is denoted by ‘b’.

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As mentioned earlier, the dominant gene...