Gene Basics  

What Is a Gene?

To understand how genes work, let's review some biology basics. Most living organisms are made up of cells that contain a substance called deoxyribonucleic acid (DNA). DNA is wrapped together to form structures called chromosomes.

Chromosomes and Genes

Most cells in the human body have 23 pairs of chromosomes, making a total of 46. Individual sperm and egg cells, however, have just 23 unpaired chromosomes. You received half of your chromosomes from your mother's egg and the other half from your father's sperm cell. A male child receives an X chromosome from his mother and a Y chromosome from his father; females get an X chromosome from each parent.

So where do genes come in? Genes are sections or segments of DNA that are carried on the chromosomes and determine specific human characteristics, such as height or hair color. Because each parent gives you one chromosome in each pair, you have two of every gene (except for some of the genes on the X and Y chromosomes in boys because boys have only one of each). Some characteristics come from a single gene, whereas others come from gene combinations. Because every person has from 25,000 to 35,000 different genes, there is an almost endless number of possible combinations!

Genes and Heredity

Heredity is the passing of genes from one generation to the next. You inherit your parents' genes. Heredity helps to make you the person you are today: short or tall, with black hair or blond, with green eyes or blue.

Can your genes determine whether you'll be a straight-A student or a great athlete? Heredity plays an important role, but your environment (including things like the foods you eat and the people you interact with) also influences your abilities and interests.

How Do Genes Work?

DNA contains four chemicals (adenine, thymine, cytosine, and guanine - called A, T, C, and G for short) that are strung in patterns on extremely thin, coiled strands in the cell. How thin? Cells are tiny - invisible to the naked eye - and each cell in your body contains about 6 feet of DNA thread, for a total of about 3 billion miles (if all your DNA threads were stretched out straight) of DNA inside you! The DNA patterns are the codes for manufacturing proteins, chemicals that enable the body to work and grow.

Genes hold the instructions for making protein products (like the enzymes to digest food or the pigment that gives your eyes their color). As your cells duplicate, they pass this genetic information to the new cells. Genes can be dominant or recessive. Dominant genes show their effect even if there is only one copy of that gene in the pair. For a person to have a recessive disease or characteristic, the person must have the gene on both chromosomes of the pair.

What Are Genetic Disorders?

Cells can sometimes contain changes or variants in the information in their genes. This is called gene mutation, and it often occurs when cells are aging or have been exposed to certain chemicals or radiation. Fortunately, cells usually recognize these mutations and repair them by themselves. Other times, however, they can cause illnesses, such as some types of cancer. And if the gene mutation exists in egg or sperm cells, children can inherit the mutated gene from their parents.

Researchers have identified more than 4,000 diseases that are caused by genetic variants. But having a genetic mutation that may cause disease doesn't always mean that a person will actually get that disease. Because you inherit a gene from each parent, having one disease gene usually does not cause any problems because the normal gene can allow your body to make the normal protein it needs.

On average, people probably carry from 5 to 10 variant or disease genes in their cells. Problems arise when the disease gene is dominant or when the same recessive disease gene is present on both chromosomes in a pair. Problems can also occur when several variant genes interact with each other - or with the environment - to increase susceptibility to diseases.

If a person carries the dominant gene for a disease, he or she will usually have the disease and each of the person's children will have a 1 in 2 (50%) chance of inheriting the gene and getting the disease. Diseases caused by a dominant gene include achondroplasia), Marfan syndrome (a connective tissue disorder), and Huntington disease (a degenerative disease of the nervous system).

People who have one recessive gene for a disease are called carriers, and they don't usually have the disease because they have a normal gene of that pair that can do the job. When two carriers have a child together, however, the child has a 1 in 4 (25%) chance of getting the disease gene from both parents, which results in the child having the disease. Cystic fibrosis (a lung disease), sickle cell anemia (a blood disease), and Tay-Sachs disease (which causes nervous system problems) are caused by recessive disease genes from both parents coming together in a child.

Some recessive genetic variants are carried only on the X chromosome, which means that usually only guys can develop the disease because they have only one X chromosome. Girls have two X chromosomes, so they would need to inherit two copies of the recessive gene to get the disease. X-linked disorders include the bleeding disorder hemophilia and color blindness.

Sometimes when an egg and sperm unite, the new cell gets too many or too few chromosomes. Most children born with Down syndrome, which is associated with mental retardation, have an extra chromosome number 21.

In some cases, people who are concerned that they might carry certain variant genes can have genetic testing so they can learn their children's risk of having a disease. Pregnant women can also have tests done to see if the fetus they are carrying might have certain genetic illnesses. Genetic testing usually involves taking a sample of someone's blood, skin, or amniotic fluid, and checking it for signs of genetic diseases or disorders.

Changing Genes

Sometimes scientists alter genes on purpose. For many years, researchers have altered the genes in microbes and plants to produce offspring with special characteristics, such as an increased resistance to disease or pests, or the ability to grow in difficult environments. We call this genetic engineering.

Gene Therapy

Gene therapy is a promising new field of medical research. In gene therapy, researchers try to supply copies of healthy genes to cells with variant or missing genes so that the "good" genes will take over. Viruses are often used to carry the healthy genes into the targeted cells because many viruses can insert their own DNA into targeted cells.

But there are problems with gene therapy. Scientists haven't yet identified every gene in the human body or what each one does. Huge scientific efforts like The Human Genome Project and related projects have recently completed a map of the entire human genome (all of the genetic material on a living thing's chromosomes), but it will take many more years to find out what each gene does and how they interact with one another. For most diseases, scientists don't know if and how genes play a role. Plus, there are major difficulties inserting the normal genes into the proper cells without causing problems for the rest of the body.

There are also concerns that people might try changing genes for ethically troubling reasons, such as to make smarter or more athletic children. No one knows what the long-term effects of that kind of change would be.

Still, for many people who have genetic diseases, gene therapy holds the hope that they - or their children - will be able to live better, healthier lives.

Genetic Glossary of Terms

Adenine (A) - a base in DNA and RNA that always bonds with thymine (T) in DNA, and with uracil (U) in RNA.

Alleles - two different forms or versions of a gene that occupy corresponding positions on homologous chromosomes. A person has two alleles of a trait--one from each parent.

Amino Acid - The basic building block of proteins. Each protein consists of a different set of amino acids, put together according to instructions in the corresponding gene. There are 22 amino acids, each encoded by a three-letter "word" of the genetic code.

Anaphase - the third stage of mitosis in which chromatids move to opposite poles of the cell.

Antioxidant - Any compound that prevents or neutralizes the damaging effects of free radicals-reactive oxygen molecules in cells. Some natural antioxidants are produced in the body while others, such as certain vitamins, are found in a variety of foods.

Antisense strand - the portion of DNA double helix that is not transcribed during protein synthesis (as opposed to the sense strand).

Antisense technology - gene silencing or the process of inactivating a gene.

Autosome - any chromosome not involved in sex determination. Twenty two of the human chromosome pairs are autosomes.

Bases - the molecular building blocks of DNA and RNA: adenine (A), cytosine (C), guanine (G), thymine (T), and in RNA only, uracil (U). In DNA, A attaches only to T, and C attaches only to G. In RNA, A attaches only to U, and C attaches only to G.

Base pairs - the pairs of complementary bases that form the rungs of DNA: adenine (A) pairs with thymine (T), cytosine (C) pairs with guanine (G).

Base sequence - the order of bases in DNA.

Biotechnology - the use of biological processes to make products. d

Body Mass Index (BMI) - A measurement using height and weight to express the relative percentages of fat and muscle mass in the human body, the results are used as an index of overweight and obesity.

Carrier - an individual who is heterozygous for a recessive trait.

Cell - the basic unit of life; the smallest basic part of every living thing that can function by itself. It is made of a nucleus surrounded by cytoplasm, organelles (similar to organs in a body) and proteins. Each cell contains the entire genome.

Cell cycle - the time from one cell division to the next. Varies from minutes to days, but the actual division takes less than 10% of the cycle.

Centromere - the region on a chromosome that ensures proper pairing of sister chromatids.

Chromatin - a complex of protein and DNA that make up chromosomes.

Chromosome - a long coiled strand in the nucleus, made up of DNA and protein. There are 46 human chromosomes, each containing DNA for hundreds or thousands of individual genes.

Chromosome jumping - the process of moving rapidly up or down a chromosome in search of a particular gene by jumping over uninformative areas of DNA.

Chromosome map - a diagram showing the locations and relative spacing of genes along a chromosome.

Clone - a genetic duplicate.

Cloning - the process of producing a genetically identical group of cells from a single ancestor.

Codominant - two genes that are neither dominant or recessive; neither masks the other, as in the case of red geraniums crossed with white geraniums yielding pink geraniums.

Codon - a special code to make amino acid; a sequence of three nucleotides.

Complementary DNA (cDNA) - a single strand of DNA made in the lab to complement the bases in a strand of messenger RNA.

Conserved DNA sequences - sequences that have not changed over time.

Controlled breeding - the reproduction of desired characteristics.

Crossing-over/recombination - a process of chromsome exchange during cell division, resulting in new combinations of alleles.

Cytogenics - the study of the genetic implications of chromosome structure and behavior.

Cytoplasm - all the organelles and materials inside a cell between the cell membrane and the nucleus.

Cytosine (C) - a base in DNA that always bonds with guanine (G).

Daughter cell - one of two cells resulting from cell division.

Daughter strand - the DNA, or chromosome, strand newly made.

Deletion - the loss of a segment of genetic material.

Deoxyribonucleic acid - see DNA

Differentiated cells - cells in which some of the DNA shuts off so that the cells can be different; they make liver cell, bone cells, etc.

Dihybrid cross - the graphing of two or more traits in a Punnett Square to predict heredity.

Diploid - a cell or organism that has two complete sets of chromosomes, as opposed to haploid, or those with only one member of each pair of the same chromosomes.

Detoxification - The process by which the body rids itself of unwanted and potentially harmful substances, or toxins. These toxins can come from food, water, air, or from the by-products of normal metabolism. Detoxification generally happens in the liver or kidneys, where toxins are either broken down or attached to a water-soluble, natural chemical to be easily excreted in the urine or sweat.

DNA - The genetic material of living organisms, an abbreviation for deoxyribonucleic acid. The DNA is known as a "double helix" because its molecules have the shape of a twisted rope ladder consisting of two intertwined coils. DNA forms the genetic blueprint; it contains the genes that carry all the information about our appearance, how our bodies function and, sometimes, the diseases we will get. The building blocks of DNA contain four different chemicals-adenine, thymine, cytosine, and guanine, or A, T, C, and G for short-referred to as the "letters" of the genetic code.

DNA (deoxyribonucleic acid) - a large double-stranded, spiraling molecule that contains genetic instructions for growth, development and replication. It is organized into bodies called chromosomes and found in the cell nucleus.

DNA fingerprinting - the use of fragments of DNA to identify the unique genetic makeup of an individual. There are the RFLP method and the PCR method.

DNA replication - the process or copying a DNA molecule just before cell division.

DNA sequencing - the process of determining the order of bases in a segment of DNA.

Dominant gene - a gene which passes on a certain physical characteristic, dominating over a recessive gene present on another chromosome.

Double helix - refers to DNA; the physical structure of DNA resembles the shape of a twisted rope ladder or a spiral staircase.

Egg - the female reproductive cell.

Electrophoresis - a method of separating large molecules such as DNA fragments from a mixture of similar molecules.

Enzyme - A protein that carries out the biochemical reactions essential for the body to metabolize food and produce energy for growth, repair, and movement. Organisms could not function if they had no enzymes.

Eugenics - the use of genetic engineering to produce a superior race.

Eukaryotes - all living things whose cells have nuclei (most living things).

Excision - the removal of a DNA fragment from a chromosome.

Exon - a gene segments that has instructions for making a protein, as opposed to introns, which don’t have these instructions.

Fats - Organic compounds composed of glycerol and fatty acids, which serve as the most concentrated source of energy in foods. Depending on the predominant type of fatty acids they contain, they are divided into saturated and unsaturated molecules.

Founder effect - the change in gene frequency occurring when a population is based on only a few individuals.

Fraternal twins - twins resulting from fertilization of separate eggs; genetically related as siblings.

Free radical - An extremely active portion of an oxygen or other molecule formed in the body as part of normal metabolism. If produced in excess, or not neutralized efficiently, it releases an electric charge that can damage cell structures and proteins.

Gametes - the sex cells in human beings--sperm in men and ova in women. Each gamete cell has 23 chromosomes, half the number found in other body cells.

Gel electrophoresis - see electrophoresis

Gene - the unit of heredity in chromosomes; a segment of double-stranded DNA.

Gene cloning - the process of separating a gene and making identical copies of it.

Gene expression - the way information in a gene is changed to a protein or RNA. The DNA sequence is changed into RNA and usually (not always) then changed into protein.

Gene linkage map - a map of relative positions of genes on a chromosome. Genes inherited together are close to each other on the chromosome, and said to be linked. (see gene mapping)

Gene locus - the position a gene occupies on a chromosome.

Gene mapping - the process of determining the relative positions of genes on a DNA molecule. (see gene linkage map)

Gene pool - the genetic make-up of a specific population.

Gene silencing - the process of inactivating a gene that may cause disease or be defective in another way.

Gene splicing - the process of cutting the DNA of a gene in order to add base pairs.

Gene targeting - the process of changing or turning off a particular gene.

Gene therapy - a method of treating a disease by exchanging the defective gene causing the disease with a healthy one in a cell.

Geneticist - a scientist who studies genes.

Gene transcription - the first step in gene expression where the gene’s DNA sequence is copied into messenger RNA.

Gene translation - the method where transcribed messenger RNA directs the making of proteins from amino acids.

Genetic code - all the genes a living thing has inherited. They determine appearance, function, growth, behavior, etc.

Genetic disease - a disease caused by a genetic mutation.

Genetic engineering - the technology used to genetically manipulate living cells to produce new chemicals or to perform new functions.

Genetic marker - a landmark that identifies a certain spot on a chromosome.

Genetics - the study of genes and heredity, or how characteristics are passed from parents to children.

Genetic testing or screening - a technique used to find out whether a person or organism has a particular gene.

Genome - all the genetic information necessary to build a living organism. It is species-specific.

Genotype - the exact genetic coding and DNA sequence information that an organism has.

Germ cells - the cells that give rise to sperm or egg.

Germplasm - the protoplasm of germ cells containing the chromosomes.

Guanine (G) - a base in DNA and RNA that always bonds with cytosine (C).

Haploid - cells that have a single set of chromosomes, or half the full set of genetic material, as opposed to diploids. Sperm and egg cells are haploid.

Hardy-Weinberg principle - states that the allele frequency for dominant and recessive alleles remains the same over the generations in a particular population as long as certain conditions exist.

Helix - spiral in form.

Heredity - all of the traits passed on to children by parents (plant or animal) or the process of transferring these traits.

Heteromorphic chromosome pairs - chromosomes that visibly differ, like X and Y.

Heterosis/hybrid vigor - the superiority of hybrids.

Heterozygous - refers to having dissimilar alleles of one or more genes, as opposed to homozygous.

Homologous recombination - the process of replacing one DNA segment with a similar sequence.

Homozygous - refers to having an identical pair of alleles, one from each parent, as opposed to heterozygous.

Human Genome Project - a large project to map and sequence all DNA in human chromosomes.

Hybrid - a plant or animal that results from two different breeds or varieties of parents; carries two different genes for a characteristic.

Hybridization - the cross-fertilization or breeding of plants and animals to get the best.

Hybrid vigor/heterosis - the superiority of hybrids.

Identical twins - the offspring resulting from a fertilized egg splitting into two separate embryos.

Interphase - the period of cell growth between divisions.

Introns - DNA sequences without instructions for making protein that come between those sequences with instructions for making protein (exons). Introns are not in messenger RNA, and it is not known what they are for.

Inversion - a mutation occurring where a segment of DNA flips upside down in a sequence, but remains in place.

Karyotype - a photograph of a cell’s chromosomes, arranged in order from largest to smallest.

Linkage - the tendency of genes on the same chromosome to be associated in inheritance.

Linkage group (linked genes) - genes found near each other on a chromosome.

Locus - the position of a particular gene on a chromosome.

Mass selection - large-scale controlled breeding.

Metabolism - The natural process by which all living organisms, including humans, transform food into energy and dispose of their waste products. Metabolism is essential for life.

Maternal inheritance - the inheritance of a trait from the female gamete.

Meiosis - the process of cell division, where the number of chromosomes is reduced by half, then the new cells divide again by mitosis. Results in four daughter cells.

Messenger RNA (mRNA) - a single strand of RNA that directs protein production.

Metaphase - the second stage of mitosis during which the chromatid pairs align at the cell’s equator.

Minerals - inorganic molecules that are essential for normal metabolism, growth and development, and for the regulation of cell function. Minerals are not made by the body and must be obtained through the diet. Insufficient levels of minerals in the diet lead to deficiency.

Mitochondria - the part of a cell that turns food into energy for chemical reactions, movement, and growth.

Mitosis - cell division resulting in daughter (new) cells that are genetically identical to each other and to the parent; the chromosomes are divided before the cell divides.

Molecular genetics - the study of molecules of DNA in order to identify the form and function of the genes in them.

Molecule - The smallest part of any compound or substance that is chemically stable. It consists of two or more atoms joined together by chemical bonding. Nucleus - The central cell structure; it contains the chromosomes.

Monohybrid cross - the graphing of only one trait in a Punnett Square, used for predicting heredity.

MRNA - see messenger RNA.

Multifactorial trait - a trait that is determined by heredity and environment.

Mutagens - substances which increase the likelihood of mutations.

Mutation - a permanent change in a DNA sequence that can be passed on. It can be a change in genetic information, addition of information, or deletion of information.

Natural selection - the process by which only the organisms better adapted to the environment survive and reproduce.

Nuclear transfer technology - the process of taking the nucleus with the DNA from one cell and putting it into another.

Nucleotide - the building block of DNA (rungs on the “ladder” of the DNA double helix) made up of four molecules that occur in pairs--adenine (A) with thymine (T), and cytosine (C) with guanine (G). Thousands of nucleotides are linked to form a DNA or RNA molecule.

Nucleus - the center part of the cell. It contains 6 feet of DNA in 23 pairs of chromosomes and is the largest part of the cell. Living things are divided into eukaryotes (whose cells have nuclei) and prokaryotes (whose cells have no nuclei).

oligonucleotide - a short string of nucleotides; a single-stranded segment of DNA. Used as a probe to find matching sequence of DNA or RNA.

Organelle - an organized structure within a cell, with a specific function.

Outcrossing - an inbreed crossed with a less related.

Ovule - the structure in a seed plant that develops into a seed after fertilization.

Ovum - the human female reproductive cell.

Oxidative Stress - A situation in which the environment within cells becomes highly "oxidized"-that is, comes to contain reactive, unstable molecules, particularly those of oxygen. These reactive molecules can attach themselves to proteins and DNA inside the cell and cause damage. Cells in this highly activated state lose control of their regulatory systems. Oxidative stress has been linked to the development of disease.

Parent cell - the original cell before cell division.

Paternal inheritance - the inheritance of a trait from the male gamete.

PCR (polymerase chain reaction) - a process developed in mid-1980’s to make a large number of copies of a DNA sequence from very little DNA. It is used in forensics when little DNA is available for testing.

Pedigree - a diagram representing family relationships of individuals.

Peptide - two or more amino acids linked together.

Pharmacogenomics - the study of drug-metabolizing genes and the influence of their variants on an individual's ability to use a drug safely and effectively.

Phenotype - all the observable traits a person inherits, as opposed to the genome, or all the traits inherited.

Proteins - Complex, organic compounds that contain carbon, hydrogen, oxygen, and nitrogen. It is the presence of nitrogen that differentiates proteins from carbohydrates and fats. The basic building blocks of proteins are amino acids. Humans need 22 amino acids for the synthesis of their proteins. The human body can make only 13, known as nonessential amino acids because we don't need to get them from the food we eat. There are nine essential amino acids that are not made by the body and can be obtained only from food.

Phytonutrients - bioactive components from plants that have health benefits.

Plasma membrane - the fatty layer surrounding the molecules of a cell.

Plasmids - the rings of DNA in bacteria.

Pollen - the grains that contain the male reproductive cells of a seed plant.

Pollination - the transfer of pollen in a flower from the stamen to the pistil (male to female).

Polymerase chain reaction - see PCR.

Polymorphisms - the differences in DNA sequences that occur naturally. They are used as genetic markers because researchers can use them to tell apart DNA from different sources.

Polyploidy - cells that have extra chromosomes.

Population - all the members of a species that live in a particular location.

Population genetics - the study of how genetic principles apply to entire generations.

Positional cloning - the process of finding the gene for a specific trait by comparing the DNA of people with the trait to the DNA of relatives without the trait.

Principle of segregation - the principle, first noted by Gregor Mendel, that explains why children don’t always directly inherit their parents’ characteristics.

Probe - a radioactive DNA or RNA molecule used in DNA-RNA or DNA-DNA hybridization trials.

Prokaryotes - the living things whose cells do not have nuclei; includes only single-celled bacteria.

Propagation - the multiplication of plants.

Promoter - a segment of DNA that acts as controlling element in expression of a gene. Marks the place of the beginning of gene transcription.

Prophase - the first stage of mitosis or meiosis.

Protein - a molecule made up of amino acids linked together. They are the body’s workhorses whose function can be as enzymes, structural components, or signaling molecules.

Punnett Square - a chart that shows all possible genetic outcomes of a mating.

Recessive gene - a gene which is hidden by a dominant gene; it must be present on both chromosomes in a pair (one from the father, one from the mother) to show outward signs of the characteristic.

Recombinant DNA - the new DNA resulting from combining two or more types of DNA.

Recombinant DNA technology - the technique for cutting apart, splicing together and producing pieces of DNA from different sources.

Recombination/crossing-over - a process of chromosome exchange during cell division, resulting in new combinations of alleles.

Restriction enzyme - an enzyme capable of cutting DNA into fragments.

Retrovirus - a cancer-causing virus that transforms cells by altering the function of host-cell genes or by carrying cancer-causing genes from one host to another.

Ribosome - the small cellular structure where RNA translates the genetic code into proteins.

RFLP (restriction fragment length polymorphism) - considered to be the most accurate DNA test; it takes several weeks to complete, and is becoming more frequently used in forensics cases.

RNA (ribonucleic acid) - single-stranded nucleic acid (similar to double-stranded DNA) containing the bases adenine (A), cytosine (C), guanine (G), and uracil (U). The different forms include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), which are all part of making protein.

sense strand - the portion of the DNA double helix that is transcribed during protein synthesis (as opposed to the antisense strand).

sex chromosome - the X or Y chromosome, which determines sex. Females have two X chromosomes in each cell, and males have an X and a Y.

sex-influenced trait - a trait whose expression depends on the sex of the individual organism.

sex-limited trait - a trait expressed in one sex and not in the other.

sex-linked trait - a trait whose gene is located on or linked to the X chromosome.

single-stranded DNA - the result of genes being changed or copied before a cell division; half of the normal two strands that form the double helix.

SNP - Single nucleotide polymorphism, pronounced "snip." A gene variation that consists of alteration in a single genetic "letter," or base: for example, GGT instead of GCT. Such common, though minute, variations occur in human DNA at a frequency of one in every 1,000 bases.

somatic cell - any cell in the body other than the sex cells.

species - a group of closely related organisms capable of mating and producing fertile offspring.

sperm - the male reproductive cells.

stamen - the male part of a plant, which produces pollen.

Telephase - the fourth stage of mitosis where chromosomes uncoil and new nuclear membranes form.

Thymine (T) - a base in DNA that always bonds with adenine (A).

Toxin - A harmful substance, specifically one produced by an animal, plant, or bacterium. Toxins can enter the body from one of these sources or be generated as by-products of metabolism. Constant exposure to toxins can overwhelm the body's detoxification mechanisms and lead to disease.

Transcription - the process by which DNA passes genetic information to RNA. The first step in producing proteins.

Transfer RNA (tRNA) - a class of RNA that carries amino acids into ribosomes and bonds with mRNA for protein production.

Transformation - a process by which some bacteria can absorb DNA from their surroundings.

Transgenic - organisms with inserted genes.

Translation - process by which RNA makes proteins.

Translocation - a mutation in which a chromosomal piece becomes attached to another chromosome.

Uracil (U) - a base in RNA that bonds only with adenine (A).

Vector - a DNA molecule into which a gene or DNA segment is inserted by recombinant DNA techniques; a cloning vehicle.

Virus - an infectious parasite.

Vitamins - Organic molecules that are essential for normal metabolism, growth and development, and for the regulation of cell function. Some vitamins activate specific enzymes in the body. Insufficient vitamins in the diet lead to deficiency.

Wildtype - the most common phenotype or genotype in a population.

X chromosome - one of the chromosomes that determines sex. Females have two X chromosomes in each somatic cell, and males have one X and one Y chromosome..

Y chromosome - one of the chromosomes that determines sex. Males carry one X chromosome and one Y chromosome in each somatic cell.

Zygote - the new cell formed when the male sperm unites with a female egg.


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