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Identical twins have exactly the same DNA, but they are not exactly alike. Each twin has his or her own personality, talents, likes, and dislikes. There are even diseases that appear in one twin but not the other, including arthritis, diabetes, autism, schizophrenia, cancer, and many others. The differences between identical twins don't come from DNA-they all come from external factors.

Scientists often study twins to understand how genes and the environment work together to affect traits. They compare traits in identical twins, who have identical DNA, and fraternal twins, who share half their DNA, just like any siblings. If a characteristic appears more frequently in identical twin pairs than in fraternal twin pairs, then it has an inherited component.

The differences between identical twins don't come from DNA-they all come from external factors.

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Identical twins have exactly the same DNA, but they are not exactly alike. Each twin has his or her own personality, talents, likes, and dislikes. There are even diseases that appear in one twin but not the other, including arthritis, diabetes, autism, schizophrenia, cancer, and many others.

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The order of building blocks in a strand of DNA makes up a "sequence." We can read a DNA sequence like letters in a book. In fact, we know the sequence of the entire human genome-all 3 billion letters. That's enough information to fill roughly 1,000 200-page books!

Contained within the 3 billion letters of the human genome are about 21,000 genes. Most of our known genes code for proteins, but some code for RNA molecules.

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The order of building blocks in a strand of DNA makes up a "sequence." We can read a DNA sequence like letters in a book. In fact, we know the sequence of the entire human genome-all 3 billion letters. That's enough information to fill roughly 1,000 200-page books!

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A protein’s three-dimensional shape is uniquely suited to its function. The 20 different amino acid building blocks can be arranged in different ways to form a nearly infinite assortment of protein shapes.

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Different organisms have different numbers of chromosomes. Organisms that appear more complex do not always have more chromosomes or genes than less-complex ones.

Humans have 23 pairs of chromosomes. The first 22 pairs are called autosomes. The last pair are called sex chromosomes, and they are different between males and females. Most females have two X chromosomes (XX), and most males have an X and a Y (XY).

Most other species have different numbers of chromosome pairs than we do. Species also vary in the number of chromosome sets they have. In some species of bees and ants, males have just one copy of every chromosome (they develop from unfertilized eggs), while females have two. Some species of plants have four or more copies of each chromosome. Bacteria have just a single chromosome, arranged in a ring.

Species also differ in how chromosomes specify sex. In birds, the sex chromosomes are called Z and W. Females are ZW and males are ZZ. The tiny roundworm C. elegans has just one type of sex chromosome called X. Those with one X chromosome (known as XO) become male, and those with two X chromosomes become hermaphrodites, meaning they make both eggs and sperm. The platypus has ten sex chromosomes. Females have five pairs of X chromosomes, and males have five X chromosomes and five Y chromosomes. Some turtles do not use sex chromosomes at all. The sex of a baby turtle depends on how warm it was during a critical period as it developed inside its egg.

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Humans have 23 pairs of chromosomes. The first 22 pairs are called autosomes. The last pair are called sex chromosomes, and they are different between males and females. Most females have two X chromosomes (XX), and most males have an X and a Y (XY).

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Most commonly, traits are influenced by multiple genes and the environment. So for most traits, patterns of inheritance are complex and unpredictable. For instance, when a sexually reproducing organism inherits a defective or “broken” allele that codes for a non-functional protein, the second copy can often make up for the loss. This type of interplay between alleles, especially when traits are influenced by multiple genes, makes predicting inheritance difficult, if not impossible.

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Mutation Generates New Alleles

The whole human family is one species with the same genes. Mutation creates slightly different versions of the same genes, called alleles. These small differences in DNA sequence make every individual unique. They account for the variation we see in human hair color, skin color, height, shape, behavior, and susceptibility to disease. Individuals in other species vary too, in both physical appearance and behavior.

Genetic variation is useful because it helps populations change over time. Variations that help an organism survive and reproduce are passed on to the next generation. Variations that hinder survival and reproduction are eliminated from the population. This process of natural selection can lead to significant changes in the appearance, behavior, or physiology of individuals in a population, in just a few generations.

Once new alleles arise, meiosis and sexual reproduction combine different alleles in new ways to increase genetic variation.

It's useful to think of mutation as a process that creates genetic variation. We often refer to a mutation as a thing-the genetic variation itself. This approach can be useful when it comes to a gene associated with a disease: the disease allele carries a mutation, a DNA change that compromises the protein's function. However, this approach gives mutation a bad name.

It's important to remember that losing the function of a gene doesn't always affect health. For example, most mammals have hundreds of genes that code for olfactory receptors, proteins that help us smell. Losing one of these genes probably doesn't make all that much difference.

In contrast to variations that cause disease, there are many more examples of variations that are neither good nor bad, but just different-like blood types and eye color. Just like with disease alleles, the process of mutation creates these more neutral variations. But with neutral variations, it can be impossible to tell which allele is the "normal" one that existed first and which is the "mutant"-and the distinction is often meaningless.

Mutation creates variations in protein-coding portions of genes that can affect the protein itself. But even more often, it creates variations in the "switches" that control when and where a protein is active and how much protein is made.

Lactase is an enzyme that helps infants break down lactose, a sugar in milk. Normally the gene that codes for lactase is active in babies and then turned off at about age four. When people who don't make lactase consume milk, they experience gas, nausea, and discomfort. But some people have a variation in a genetic switch that keeps the lactase gene active. This variation is called "lactase persistence," and people who have it can keep milk in their diets even as adults.

Other drivers of mutation: Environmental agents

Radiation, chemicals, byproducts of cellular metabolism, free radicals, ultraviolet rays from the sun-these agents damage thousands of nucleotides in each of our cells every day. They affect the nucleotides themselves: converting one base to another, knocking a base off its backbone, or even causing a break in the DNA strand.

Most of the time, mutation is reversed. DNA repair machines are constantly at work in our cells, fixing mismatched nucleotides and splicing broken DNA strands back together. Yet some DNA changes remain. If a cell accumulates too many changes-if its DNA is so damaged that repair machinery cannot fix it-it either stops dividing or it self-destructs. If any of these processes go wrong, the cell could become cancerous.

When we put on sun screen, we are protecting ourselves against mutation in somatic cells-the cells that make up the body and are not involved in reproduction.&ZeroWidthSpace; Only when DNA changes are carried in egg and sperm cells are they passed to the next generation. Believe it or not, a certain amount of sloppiness is built into the system. Without mutation there would be no variation, and without variation there would be no evolution.

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Mutation Generates New Alleles The whole human family is one species with the same genes. Mutation creates slightly different versions of the same genes, called alleles. These small differences in DNA sequence make every individual unique. They account for the variation we see in human hair color, skin color, height, shape, behavior, and susceptibility to disease.

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Un travail de recherche photographique sur les ressemblances génétiques entre membres de même famille

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29 juillet 2023Les rides d'amertume, également connues sous le nom de plis d'amertume, sont ces lignes verticales qui apparaissent entre...

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29 juillet 2023Les rides d'amertume, également connues sous le nom de plis d'amertume, sont ces lignes verticales qui apparaissent entre...

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Afin de comprendre l'importance de la fécondation sur la diversité génétique de la descendance, on croise des drosophiles hétérozygotes pour deux gènes situés sur des chromosomes différents:

le gène " taille des ailes " situé sur le chromosome 2: compte l'allèle dominant L (ailes longues) et l'allèle récessif vg (ailes vestigiales).

le gène " couleur des yeux " situé sur le chromosome 3: compte l'allèle dominant B (yeux rouge brique) et l'allèle récessif se (yeux sépia).

Complétez l'échiquier du croisement puis indiquez le phénotype des descendants et la fréquence de chaque phénotype.

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Conséquences génétiques de la fécondation

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Afin de comprendre l'importance de la fécondation sur la diversité génétique de la descendance, on croise des drosophiles hétérozygotes pour deux gènes situés sur des chromosomes différents: le gène " taille des ailes " situé sur le chromosome 2: compte l'allèle dominant L (ailes longues) et l'allèle récessif vg (ailes vestigiales).

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29 juillet 2023Les rides d'amertume, également connues sous le nom de plis d'amertume, sont ces lignes verticales qui apparaissent entre...

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FicheEleve-Anatomie de la fleur et du fruit

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Toutes les cellules de notre corps ont le même patrimoine génétique

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On entend souvent dire que toutes les cellules de notre corps ont le même patrimoine génétique. Qu'en est-il réellement ? Al...