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HSC Biology Module 5: Heredity Genetic Variation
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Sources of Variation
Genetic variation is important because when conditions change (food becomes scarce, environment changes), some individuals in a population will be more likely to have variations that will allow them to survive. Those who reproduce pass their genes to the next generation. Variation helps species survive, and it's the reason species change over time.
In asexual reproduction, variation comes mainly from mutation. Mutation is a natural process that introduces permanent changes in a DNA sequence. However, microbes also acquire genetic variation through transformation, transduction, and conjugation (gene transfer). These mechanisms often come into play when conditions are harsh.
In sexual reproduction, variation comes from both mutation and recombination. Mutation creates the different versions (or alleles) of the same gene. Parental alleles are then shuffled—or recombined—during meiosis. Because of recombination, sexual reproduction produces more variation than asexual does.
mutation alleles transformation transduction conjugation
Genetic Variation__________________________________________________________________________
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Sex linkage
Organisms that reproduce sexually do so via the production of sex cells, also called gametes. In humans, male gametes are spermatozoa (sperm cells) and female gametes are ova or eggs. Male sperm cells may carry one of two types of sex chromosomes. They either carry an X chromosome or a Y chromosome. However, a female egg cell may carry only an X sex chromosome. When sex cells fuse in a process called fertilization, the resulting cell (zygote) receives one sex chromosome from each parent cell. The sperm cell determines the sex of an individual. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be (XX) or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be (XY) or male.Genes that are found on sex chromosomes are called sex-linked genes. These genes can be on either the X chromosome or the Y chromosome. If a gene is located on the Y chromosome, it is a Y-linked gene. These genes are only inherited by males because, in most instances, males have a genotype of (XY). Females do not have the Y sex chromosome. Genes that are found on the X chromosome are called X-linked genes. These genes can be inherited by both males and females. Genes for a trait may have two forms or alleles. In complete dominance inheritance, one allele is usually dominant and the other is recessive. Dominant traits mask recessive traits in that the recessive trait is not expressed in the phenotype.gamete chromosome zygote allele dominant recessive
Sex Linkage
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Johann Gregor Mendel (1822-1884)Father of Genetics
Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits. He recognized the mathematical patterns of inheritance from one generation to the next. Mendel's Laws of Heredity are usually stated as:
1) The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization.
2) The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.
3) The Law of Dominance: An organism with alternate forms of a gene will express the form that is dominant.
The genetic experiments Mendel did with pea plants took him eight years (1856-1863) and he published his results in 1865. During this time, Mendel grew over 10,000 pea plants, keeping track of progeny number and type.
Mendelian Inheritance
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The Punnett SquareThe Punnett square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment.
The diagram is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a tabular summary of possible combinations of maternal alleles with paternal alleles. These tables can be used to examine the genotypical outcome probabilities of the offspring of a single trait (allele), or when crossing multiple traits from the parents.
The Punnett square is a visual representation of Mendelian inheritance. It is important to understand the terms "heterozygous", "homozygous", "double heterozygote" (or homozygote), "dominant allele" and "recessive allele" when using the Punnett square method. Phenotypes may be predicted with at least better-than-chance accuracy using a Punnett square.
heterozygous homozygous
The Punnett Square
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Single nucleotide polymorphism
Single nucleotide polymorphisms, frequently called SNPs (pronounced “snips”), are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA building block, called a nucleotide. For example, a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA.
SNPs occur normally throughout a person’s DNA. They occur almost once in every 1,000 nucleotides on average, which means there are roughly 4 to 5 million SNPs in a person's genome. These variations may be unique or occur in many individuals; scientists have found more than 100 million SNPs in populations around the world. Most commonly, these variations are found in the DNA between genes. They can act as biological markers, helping scientists locate genes that are associated with disease. When SNPs occur within a gene or in a regulatory region near a gene, they may play a more direct role in disease by affecting the gene’s function.
Most SNPs have no effect on health or development. Some of these genetic differences, however, have proven to be very important in the study of human health.
Single nucleotide polymorphism
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DNA FingerprintingJust like the traditional fingerprinting techniques made famous by detective fiction, DNA fingerprinting of individuals takes place by sampling their DNA and comparing it with a sample
found at a crime scene. DNA sequencing, by contrast, determines the sequence of a stretch of DNA. Although DNA sequencing and DNA fingerprinting involve some of the same techniques, the ultimate aim of each is different and they have different applications.
DNA
Your DNA is a chain of chemical units called base pairs, each of which is typically represented by a letter: either A, G, C or T. The sequence of those "letters" determines the function of a piece of DNA, just as the sequence of ones and zeroes in binary computer code determines what tasks the computer will perform. In DNA sequencing, scientists take a piece of DNA and determine the sequence of letters it contains in an effort to use it or find out more about its function. Your complete DNA sequence is called your genome. Each individual's genome is unique, just like a fingerprint.
Fingerprinting
Unlike sequencing, fingerprinting does not attempt to determine sequence. The goal of fingerprinting is to determine whether a sample of DNA-containing material like blood came from a given individual. Certain regions of the genome are pretty similar from one individual to another but certain other regions are highly variable. The most important variable regions for DNA fingerprinting are called microsatellites. These microsatellites contain a short sequence that is repeated many times. The number of repeats varies greatly from one individual to another. By comparing the number of repeats in certain specific microsatellite regions, forensics experts can determine with high probability whether the DNA from two different samples is a match.
DNA Fingerprinting
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GenomicsUntil recently, the term genome was used to describe the complete set of chromosomes that made up a given species. Today, scientists use the term genome to refer to the complete set of DNA
sequences derived from each chromosome of a given species. Genomics is a relatively new and ever-expanding field dedicated to the study of defining genomes in this more specific way.The direct analysis of the genome of an organism, or the genomes of a group of organisms, is now possible through advances in the efficiency of DNA sequencing and large-scale genetic screening. These new high-throughput methods allow researchers to collect vast amounts of information about genetic variation in very short periods of time.
Genomics has also shown that the size of a genome (i.e. the number of nucleotide pairs it contains) is not necessarily proportional to the number of genes contained within it. Some organisms, like the fruit fly, fit a considerable number of genes into a relatively small genome, whereas humans and mice possess many extra "unused" nucleotide pairs that do not encode genes
Genome comparison chartGenome size is the total number of base pairs in an organism. While the number of genes in an organism's DNA (red bars) varies from species to species (numbers at right), it is not always proportional to genome size (blue bars, in millions of base pairs). Note how many genes a fruit fly can squeeze out of its relatively small genome.
What is genomics?
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https://www.thoughtco.com/sex-linked-traits-373451
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/sex-linkage
http://www.old-ib.bioninja.com.au/standard-level/topic-4-genetics/43-theoretical-genetics.html
https://en.wikipedia.org/wiki/Sex_linkage
https://learn.genetics.utah.edu/content/basics/inheritance/
https://www.broadinstitute.org/medical-and-population-genetics/human-genetic-variation