5.4 - Genetic Variation PDF

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NAME ______________________________________

Genetic Variation

   Inquiry question 4: H  ow can the genetic similarities and differences within and between species be compared? Students: ■

Conduct practical investigations to predict variations in the genotype of offspring by modelling meiosis, including the crossing over of homologous chromosomes, fertilisation and mutations (ACSBL084)

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Model the formation of new combinations of genotypes produced during meiosis, including but not limited to:

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interpreting examples of autosomal, sex-linkage, co-dominance, incompete dominance and multiple alleles (ACSBL085)

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constructing and interpreting information and data from pedigreess and Punnet squares.

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Collect, record and present data to represent frequencies of characteristics in a population, in order to identify trends, patterns, relationships and limitations in data, for example:

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examining frequency data

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Analysing single nucleotide polymorphism (SNP)

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Student activity:

Complete the following definitions with examples: ● variation  ● gene  ● allele  ● genotype  ● phenotype  ● meiosis  ● homologous chromosome  ● crossing over  ● fertilisation  ● mutation  Recap from previous lessons.

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Conduct practical investigations to predict variations in the genotype of offspring by modelling meiosis, including the crossing over of homologous chromosomes, fertilisation and mutations

Genetic Variation - Meiosis, fertilisation and mutations. In the Year 11 course, you learnt in evolution that individuals have variation (colour, height, etc). In genetics, variation relates to the different forms of a gene (alleles) within a population. For example, we could say coat colour in a population of a species of dog could range from brown, brown/white or brown/black. The genetic inheritance of coat colour in other animals is similarly variable.  There are several ways in which variation arises during sexual reproduction: - During meiosis - During fertilisation Both processes randomly mix TWO sets of parental chromosomes, resulting in individuals with varied genotypes. Mutation also introduces genetic variation. We will briefly mention it here but it will be learnt at length during Module 6. 

How can variation be introduced during meiosis? In IQ2 (Cell Replication), you learnt about the process of meiosis, compared it to mitosis and assessed its effect on the continuity of a species. Here, you will learn how meiosis introduces genetic variation during crossing over, independent assortment and random segregation.  1. Crossing over of homologous

chromosomes ● In prophase I into metaphase of Meiosis I  Crossing over or synapsis is the exchange of alleles between homologous chromosomes, resulting in a mixture of parental characteristics in the offspring. It occurs when the chromatids of homologous chromosomes wrap around each other, break where they meet (termed chiasmata=point at which they meet), exchanging genetic material of paternal and materal chromosomes. This exchange causes a MIX of parental genes and introduces genetic variation. 3

 2. Independent assortment (EDROLO) ● In anaphase I of Meiosis I Each pair of chromosomes separate during anaphase I and one entire chromosome of each pair moves into a daughter cell. This separation of maternal and paternal chromosomes not only halves the chromosome number in gametes but leads to genetic variation, depending on which chromosome of each pair ENDS UP IN WHICH DAUGHTER CELL. This is termed independent assortment. There are 223 possible combinations of chromosomes in the formation of a human gamete, based on independent assortment alone.                        3. During Meiosis II and random segregation (EDROLO) ● In metaphase of meiosis II Mendel's Law of Segregation: The two alleles in a pair segregate (separate from each other) into different gametes during gamete formation. This means that during meiosis, homologous chromosomes are separated. Depending on where a sister chromosome is situated at the metaphase II plate, there is a huge range of variation that could result in the daughter cells. 4

   This clip summarises this as does this clip   Another good site 

What happens if chromosomes don’t separate during cell division? Student activity: Use the link to look at what Nondisjunction of chromosomes means. Students investigate a case study of a nondisjunction mutation - identify the cause (what specific trisomy and the mechanism), risk factors, prevalence, symptoms, treatment and/or management strategies and life expectancy. _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ 5

Student activity: Read and complete the worksheet “Meiosis” provided.  Student activity: Students complete Investigation 5.1 Secondary source and practical investigation to model meiosis, fertilisation and mutations (DONE AS A SPLIT CLASS EXERCISE WITH OTHER MODELS). 

Variation and fertilisation The process of fertilisation, which involves the random meeting of any two gametes, ensures further mixing of genetic material. Self-fertilisation in plants will not result in as much variation as cross-fertilisation. Each human male ejaculate = 300 million sperm cells. All of which have undergone crossing over, independent assortment and random segregation to make them genetically unique. Considering only 1 sperm fertilises an ova, there is a huge amount of variation possible. 

Variation due to mutations A mutation is a permanent change that occurs in our nucleotide sequence of DNA, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light. The mutation can be harmful, beneficial or neutral.  There are TWO types of mutations - somatic mutations (occurring in body cells and not passed on) and genetic mutations (occurring in the sex cells and are inheritable).  Mutations are essential for evolution and create genetic variation (More in Module 6) Examples of mutations

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Model the formation of new combinatios of genotypes produced during meiosis, including but not limited to:

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interpreting examples of autosomal, sex-linkage, co-dominance, incompete dominance and multiple alleles ( ACSBL085)

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constructing and interpreting information and data from pedigreess and Punnet squares.

Student activity:

Definitions:

Term

Defintion with examples

homozygous

heterozygous dominant recessive monohybrid cross pedigree punnett square

 In the last syllabus point, we looked at HOW the variations can be introduced. This syllabus point looks at the genotypes that RESULT from these variations.  We all know genes lie on chromosomes and genes are stretches of DNA that code for proteins. BUT we didn’t always know this.   

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Background (EDROLO) Gregor Johann Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel was an Augustinian monk. He selected a simple biological system, garden peas, and conducted methodical, quantitative analyses using large sample sizes. In 1856, he grew around 29,000 garden pea plants in a monastery’s garden, where he analyzed seven  characteristics of the garden pea plants: flower colour (purple or white), seed texture (wrinkled or round), seed colour (yellow or green), stem length (long or short), pod colour (yellow or green), pod texture (inflated or constricted), and flower position (axial or terminal). Based on the appearance, or phenotypes, of the seven traits, Mendel developed genotypes for those traits. Mendel worked with pure breds (two copies of the same alleles) and hybrids (different alleles), studying the inheritance of one particular trait at a time, through monohybrid  crosses (a  cross between individuals which studies only ONE trait).  

What results did Mendel find in his crosses for flower colour?  In the parental, or P generation, Mendel crossed a pure-breeding violet-flowered plant to a pure-breeding white-flowered plant (as shown on page 9). When he gathered and planted the seeds produced in this cross, Mendel found that 100 percent of the plants in the next generation, or F1 generation, had violet flowers. Conventional wisdom at that time would have predicted that the hybrid flowers should be pale violet—that is, that the parents' traits should blend in the offspring. Instead, Mendel’s results showed that the white flower trait had completely disappeared. He called the trait that was

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visible in the F1generation (violet flowers) the dominant trait, and the trait that was hidden or lost (white flowers) the recessive trait. Mendel did not stop his experimentation there. Instead, he let the F1 plants self-fertilize. Among their offspring, called the F2 generation, he found that 705 plants had violet flowers and 224 had white flowers. This was a ratio of approximately 3 violet flowers to one white flower, or approximately 3:1. For the other six characteristics that Mendel examined, both the F1 and F2 generations behaved in the same way they did for flower colour. One of the two traits would disappear completely from the F1generation, only to reappear in the F2 generation in a ratio of roughly 3:1.    

Mendel found that: ● Inheritance is controlled by what he termed ‘factors’ but we now know are alleles of genes. ● Characteristics are either dominant or recessive. ● Ratios of offspring can be predicted. ● The set of alleles carried by an organism is known as its genotype. Genotype determines phenotype  , an organism's observable features. ● When an organism has two copies of the same allele (say, YY or yy), it is said to be homozygous for that gene. ● If, instead, it has two different copies (like Yy), we can say it is heterozygous.    9

                      Because of Mendel’s work, the fundamental principles of heredity were revealed, which are often referred to as Mendel’s Laws of Inheritance. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or mutated.

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Autosomal genetic crosses Autosomal chromosome = a chromosome that is not a sex chromosome. A Punnett  square can be used to predict genotypes  (allele combinations) and phenotypes  (observable traits) of offspring from genetic crosses. It applies the rules of probability to predict the possible outcomes of a monohybrid cross and their expected frequencies. Look at the example given using a capital T for the dominant tall allele, t for the recessive short allele.                       Education Perfect - Punnet Squares  Student activity: Complete the monohybrid crosses using punnet squares worksheet  . Additonal worksheet ‘Monhybrid Cross’ Biozone International.  Student activity: Complete the ‘Check your understanding’ questions 5.2b on page 170 of you text. Specifically Q1-3

Alternatives to Dominance and Recessiveness Today, we know that not all alleles behave quite as straightforwardly as in Mendel’s experiments. For example, in real life:

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● Allele pairs may have a variety of dominance relationships (that is, one allele of the pair may not completely “hide” the other in the heterozygote). ● There are often many different alleles of a gene in a population. In these cases, an organism's genotype, or set of alleles, still determines its phenotype  , or observable features. However, a variety of alleles may interact with one another in different ways to specify phenotype.

Incomplete dominance: a condition in which the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes For example, in the snapdragon, Antirrhinum  majus, a cross between a homozygous white-flowered plant (CWCW) and a homozygous red-flowered plant (CRCR) will produce offspring with pink flowers (CRCW). 

F1 generation: 100% pink phenotype  enotype 100% CRCW g

F2 generation: 1 : 2 : 1 red: pink : white phenotype  RCW : 1  C  WCW genotype 1 CRCR : 2 C

Co-dominance: a condition in which both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other. The gene controlling human ABO blood groups has three alleles, not just two, and they co-dominate: ● IA and IB are not dominant over one another ● both are dominant over IO

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The table shows the possible genotypes (alleles present) and phenotypes (blood group)

Genotype

Phenotype

IA IA , IAIO

A

IB IB , IBIO

B

IA IB

AB

IO IO

O

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.  INCOMPLETE DOMINANCE 

CO-DOMINANCE

Student activity: Complete the worksheet ‘Codomance’ from Biozone International. Additonal worksheets provided.

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Extension Dihybrid crosses (2 loci) Dihybrid crosses look at 2 traits at once. For example, looking at peas smooth/ wrinkled and green/yellow.                      

Sex linkage  Walter Sutton and Theodor Boveri generally gets credit for the insight that genes were on chromosomes. Sutton, an American, studied chromosomes and meiosis in grasshoppers. Boveri, a German, studied the same things in sea urchins.  Boveri and Sutton's chromosome theory of inheritance states that “genes are found at specific locations on chromosomes, and that the behaviour of chromosomes during meiosis can explain Mendel’s laws of inheritance”.  Thomas Hunt Morgan studied fruit flies, Drosophila melanogaster. He provided the first strong confirmation of the chromosome theory. Morgan discovered a mutation that affected fly eye colour. He observed that the mutation was inherited differently by male and female flies. Based on the inheritance pattern, Morgan concluded that the eye colour gene must be located on the X chromosome. 14

 Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes. When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked.  Because X-linked characteristics are often recessive, and females have two X-chromosomes, so that they can be heterozygous and do not exhibit the trait, X-linked traits are more common in males than in females (in humans). Mendelian ratios do not occur as the two alleles for each characteristic are not necessarily both existent (males only have one X-chromosome).

X linked traits Insects also follow an XY sex-determination pattern and like humans, Drosophila males have an XY chromosome pair and females are XX. Eye colour in Drosophila was one of the first X-linked traits to be identified, and Thomas Hunt Morgan mapped this trait to the X chromosome in 1910.  In fruit flies, the wild-type eye color is red (XW) and is dominant to white eye colour (Xw). Because this eye-colour gene is located on the X chromosome only, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males because each male only has one copy of the gene. Drosophila males lack a second allele copy on the Y chromosome; their genotype can only be XWY or XwY. In contrast, females have two allele copies of this gene and can be XWXW, XWXw, or XwXw.

X linked crosses More info https://courses.lumenlearning.com/boundless-biology/chapter/patterns-of-inheritance/ More detail on this link. 15

 Student activity: C  omplete the worksheet, ‘Inheritance of linked genes’ and Sex-linked genes’ worksheets.  We have been currently working with the fruit fly as they are an excellent model in understanding Mendelian genetics and inheritance of traits. Have a read of the attached information on the benefits of working with fruit flies for genetics and how we, as scientists, go about it. Fruit fly experiments 

Multiple Alleles (1 loci) Mendel's work suggested that just two alleles existed for each gene. Today, we know that's not always, or even usually, the case! Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist in a population level, and different individuals in the population may have different pairs of these alleles.       Image credit: "Characteristics and traits: Figure 5," by OpenStax College, Biology (CC BY 3.0).

As an example, let’s consider a gene that specifies coat colour in rabbits, called the CC gene. The CC  gene comes in four common alleles: CC, Cch, Ch, and c: ● A CC rabbit has black or brown fur.  ● A CchCch rabbit has chinchilla coloration (grayish fur). h h  ● A C C rabbit has Himalayan (color-point) patterning, with a white body and dark ears, face, feet, and tail ● A cc rabbit is albino, with a pure white coat.

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Multiple alleles makes for many possible dominance relationships. In this case, the black C allele is completely dominant to all the others; the chinchilla Cch allele  is incompletely dominant h h to the Himalayan C and albino c alleles; and the Himalayan C allele is completely dominant to the albino c allele. Rabbit breeders figured out these relationships by crossing different rabbits of different genotypes and observing the phenotypes of the heterozygous kits (baby bunnies).

Pedigrees Doctors can use a pedigree analysis chart to show how genetic disorders are inherited in a family. They can use this to work out the probability (chance) that someone in a family will inherit a condition.

Student activity: Read pages 166-169 and create ...