Many human traits are controlled by a single gene with one dominant allele and one recessive allele. As with tall and short pea plants, these human traits have two distinctly different phenotypes, or physical appearances. For example, the allele for a widow's peak, which is a hairline that comes to a point in the middle of the forehead, is dominant over the allele for a straight hairline.
Some human traits are controlled by single genes with two alleles, and others by single genes with multiple alleles. Still other traits are controlled by many genes that act together. Height and skin color are both examples of human traits controlled by many genes. When more than one gene controls a trait, there are many possible combinations of genes and alleles. There is an enormous variety of phenotypes for height, for example, and human skin color ranges from almost white to nearly black, with many shades in between.
Some human traits are controlled by a single gene that has more than two alleles. Such a gene is said to have multiple alleles-three or more forms of a gene that code for a single trait. An example of a human trait that is controlled by a gene with multiple alleles is blood type. There are four main blood types-A, B, AB, and O-controlled by three alleles.
The sex chromosomes are one of 23 pairs of chromosomes in each body cell. The sex chromosomes carry genes that determine whether a person is male or female. They also carry genes that determine other traits. If you are female, you have two X chromosomes. If you are male, you have an X and a Y chromosome. Whether you inherited an X or Y chromosome from your father determines your sex.
Genes on the X and Y chromosomes are often called sex-linked genes. Traits controlled by sex-linked genes are called sex-linked traits. Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele. One example of a sex-linked trait that is controlled by a recessive allele is red-green colorblindness. A carrier is a person who has one recessive allele for a trait and one dominant allele. Although a carrier does not have the trait, the carrier can pass the recessive allele on to his or her offspring. In the case of sex-linked traits, only females can be carriers.
The effects of genes are often altered by the environment-the organism's surroundings. Many of a person's characteristics are determined by an interaction between genes and the environment. Several genes determine human height. However, environment also inﬂuences people's heights. People's diets can affect their height. A poor diet can prevent a person from growing as tall as might be possible.
A ﬁsh is a vertebrate that lives in water and uses ﬁns to move. In addition to living in water and having ﬁns, most ﬁshes are ectotherms, obtain oxygen through gills, and have scales. Scales are thin, overlapping plates that cover the skin. Fishes are the largest group of vertebrates-nearly half of all vertebrate species are ﬁshes.
Fishes get their oxygen from water. Water ﬂows into a ﬁsh's mouth and over its gills. Gills contain many blood vessels. As water ﬂows over the gills, oxygen moves from the water into the ﬁsh's blood, while carbon dioxide, a waste product, moves out of the blood and into the water.
Like all vertebrates, ﬁshes have a closed circulatory system. Fins help ﬁshes swim. Most ﬁshes reproduce by external fertilization. The major groups of ﬁshes are jawless ﬁshes, cartilaginous ﬁshes, and bony ﬁshes.
Jawless ﬁshes are unlike other ﬁshes in that they have no jaws and no scales. They feed by scraping, stabbing, and sucking their food. Their skeletons are made of cartilage, a tissue that is more ﬂexible than bone. Hagﬁsh and lampreys are the only kinds of jawless ﬁshes.
Sharks, rays, and skates are cartilaginous ﬁshes.The cartilaginous ﬁshes have jaws and scales, and skeletons made of cartilage. Cartilaginous ﬁshes are carnivores.
A bony ﬁsh has jaws, scales, a pocket on each side of the head that holds the gills, and a skeleton made of hard bones. A bony ﬁsh also has an organ called a swim bladder, an internal gas-ﬁlled sac that helps the ﬁsh stay stable at different depths in the water. Bony ﬁshes make up about 95 percent of all ﬁsh species.
Salamanders, frogs, and toads are amphibians. An amphibian is an ectothermic vertebrate that spends its early life in water. After beginning their lives in water, most amphibians spend their adulthood on land, returning to water to reproduce.
Amphibian eggs are coated with clear jelly that keeps moisture in and helps protect them from infection. Frog and toad larvae are called tadpoles and look very different from the adults. In contrast, the larvae of salamanders look like the adults. The larvae of most amphibians grow and undergo metamorphosis.
The respiratory and circulatory systems of adult amphibians are adapted for life on land. In addition, adult amphibians have adaptations for obtaining food and moving. Amphibian larvae use gills to obtain oxygen, whereas adults use lungs. Lungs are organs of air-breathing vertebrates in which oxygen gas and carbon dioxide gas are exchanged between the air and blood. Amphibian larvae also have a single-loop circulatory system and two-chambered heart, like that of a ﬁsh. In contrast, the circulatory system of an adult amphibian has two loops and a heart with three chambers. The two upper chambers of the heart are called atria, and the lower chamber is the ventricle. Blood moves to the lungs in one loop, and to the body in the other loop.
Tadpoles are usually herbivores that feed on rotting pond plants. Most adult frogs and toads are carnivores that feed on insects or other small animals. Frogs and toads usually wait until their prey come close; salamanders stalk and ambush their prey. Most adult amphibians have strong skeletons and muscular limbs adapted for moving on land. Frogs and toads have powerful hind-leg muscles for jumping.
Amphibian populations are declining all over the world. One reason is the destruction of their habitats. A habitat is the speciﬁc environment in which an animal lives. Other environmental factors, such as environmental poisons, are probably involved, too.
A reptile is an ectothermic vertebrate that has lungs and scaly skin. Snakes, lizards, turtles, and alligators are all reptiles. The eggs, skin, and kidneys of reptiles are adapted to conserve water.
Reptiles have dry, tough skin covered with scales that protects them and helps keep water in their bodies. Reptiles also have kidneys, which are organs that ﬁlter wastes from the blood, excreting them in a watery ﬂuid called urine.The kidneys of reptiles concentrate the urine so that the reptiles lose very little water.
The eggs of reptiles are fertilized internally. Unlike an amphibian's egg, a reptile's egg has a shell and membranes that protect the developing embryo and keep it from drying out. This type of egg is called an amniotic egg.
Both lizards and snakes are reptiles that have skin covered with overlapping scales.As they grow, they shed their skin and scales, replacing the worn ones with new ones. Lizards have four legs, moveable eyelids, and a pair of lungs. Snakes have no legs, no eyelids, and most have only one lung. Most lizards are carnivores that capture their prey by jumping at it. All snakes are carnivores. Most feed on small mammals, such as mice. However, some snakes eat large prey. A snake's jawbones can spread wide apart, and its skin stretches, too. Lizards walk and run, while most snakes slither.
Alligators, crocodiles, and their relatives are the largest living reptiles. Both alligators and crocodiles are large, carnivorous reptiles that care for their young.
A turtle is a reptile whose body is covered by a protective shell that includes the ribs and the backbone. Some turtles can pull their legs and head into their shells for protection. Turtles have sharp-edged beaks instead of teeth. While some turtles are carnivores, other turtles are herbivores.
Dinosaurs were a major group of reptiles that died out 65 million years ago. Dinosaurs were the earliest vertebrates that had legs positioned directly beneath their bodies. Some biologists think that birds descended from certain small dinosaurs.
A fossil is the hardened remains or other evidence of a living thing that existed a long time ago. A fossil can be an imprint in a rock, such as a footprint or outline of a leaf. Fossils can also be the remains of bones or other parts of living things. Very few organisms become fossils because most living tissues decay rapidly.
Fossils are found most frequently in sedimentary rock. Sedimentary rock is made of hardened layers of sediments-particles of clay, sand, mud, or silt. Sediments can build up when wind blows sand into dunes or when muddy water stands still. Over a very long time, sediments can harden into rock. When a plant or animal dies and is trapped in the sediments, it is sometimes preserved as a fossil.
Paleontologists, the scientists who study extinct organisms, examine fossil structure and make comparisons to present-day organisms. By studying fossils, paleontologists can infer how animals changed over time.
Paleontologists estimate the age of fossils. One way to do that is by studying the sedimentary rocks in which the fossils are found. Paleontologists know that fossils in the higher layers of rock are usually younger than fossils in the lower layers.
Radioactive chemical elements are also used to estimate the age of fossils. Over a long time, radioactive elements decay, or change into other chemical elements. Scientists can measure the amount of the decayed element in the fossil. The greater the amount of decayed form of the element, the older the fossil.
Paleontologists have used fossils to determine a likely pattern of how vertebrates changed over time. Fishes ﬁrst appeared on Earth about 530 million years ago. Amphibians appeared 380 million years ago, and descended from ﬁshes. Then, 320 million years ago, amphibians gave rise to reptiles. Birds and mammals both descended from reptiles. Mammals appeared about 220 million years ago, and birds 150 million years ago.
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