These chromosomes contain between 20, and 25, genes. New genes are being identified all the time. The paired chromosomes are numbered from 1 to 22 according to size. Chromosome number 1 is the biggest.
These non-sex chromosomes are called autosomes. People usually have two copies of each chromosome. One copy is inherited from their mother via the egg and the other from their father via the sperm.
A sperm and an egg each contain one set of 23 chromosomes. When the sperm fertilises the egg, two copies of each chromosome are present and therefore two copies of each gene , and so an embryo forms. The chromosomes that determine the sex of the baby X and Y chromosomes are called sex chromosomes. A person with an XX pairing of sex chromosomes is biologically female, while a person with an XY pairing is biologically male. As well as determining sex, the sex chromosomes carry genes that control other body functions.
There are many genes located on the X chromosome, but only a few on the Y chromosome. Genes that are on the X chromosome are said to be X-linked. Genes that are on the Y chromosome are said to be Y-linked. Parents pass on traits or characteristics, such as eye colour and blood type, to their children through their genes. Some health conditions and diseases can be passed on genetically too.
Sometimes, one characteristic has many different forms. Changes or variations in the gene for that characteristic cause these different forms. These two copies of the gene contained in your chromosomes influence the way your cells work. The two alleles in a gene pair are inherited, one from each parent. Alleles interact with each other in different ways. These are called inheritance patterns. Examples of inheritance patterns include:. An allele of a gene is said to be dominant when it effectively overrules the other recessive allele.
The allele for brown eyes B is dominant over the allele for blue eyes b. So, if you have one allele for brown eyes and one allele for blue eyes Bb , your eyes will be brown. This is also the case if you have two alleles for brown eyes, BB. However, if both alleles are for the recessive trait in this case, blue eyes, bb you will inherit blue eyes. For blood groups, the alleles are A, B and O.
The A allele is dominant over the O allele. Blood group A is said to have a dominant inheritance pattern over blood group O. If the father has two O alleles OO , he has the blood group O. For each child that couple has, each parent will pass on one or the other of those two alleles.
This is shown in figure 1. This means that each one of their children has a 50 per cent chance of having blood group A AO and a 50 per cent chance of having blood group O OO , depending on which alleles they inherit.
The combination of alleles that you have is called your genotype e. The observable trait that you have — in this case blood group A — is your phenotype. If a person has one changed q and one unchanged Q copy of a gene, and they do not have the condition associated with that gene change, they are said to be a carrier of that condition.
It tells every part of the cell what to do. But, how does the nucleus know so much? It contains our chromosomes and genes. As tiny as it is, the nucleus has more information in it than the biggest dictionary you've ever seen.
In humans, a cell nucleus contains 46 individual chromosomes or 23 pairs of chromosomes chromosomes come in pairs, remember? Half of these chromosomes come from one parent and half come from the other parent. Under the microscope, we can see that chromosomes come in different lengths and striping patterns. When they are lined up by size and similar striping pattern, the first twenty two of the pairs these are called autosomes; the final pair of chromosomes are called sex chromosomes, X and Y.
The sex chromosomes determine whether you're a boy or a girl: females have two X chromosomes while males have one X and one Y. But not every living thing has 46 chromosomes inside of its cells. For instance, a fruit fly cell only has four chromosomes! Each gene has a special job to do. The DNA in a gene spells out specific instructions—much like in a cookbook recipe — for making proteins say: PRO-teens in the cell.
Proteins are the building blocks for everything in your body. Bones and teeth, hair and earlobes, muscles and blood, are all made up of proteins. Those proteins help our bodies grow, work properly, and stay healthy.
Scientists today estimate that each gene in the body may make as many as 10 different proteins. That's more than , proteins! Like chromosomes, genes also come in pairs. Each of your parents has two copies of each of their genes, and each parent passes along just one copy to make up the genes you have. Genes that are passed on to you determine many of your traits, such as your hair color and skin color.
Maybe Emma's mother has one gene for brown hair and one for red hair, and she passed the red hair gene on to Emma. If her father has two genes for red hair, that could explain her red hair. Emma ended up with two genes for red hair, one from each of her parents. The extra chromosome was determined to be the result of a defect during meiosis that caused a high frequency of nondisjunction. Nondisjunction is the failure of two sister chromatids to separate during the second meiotic division. Thus, when an egg containing two nondisjoined X chromosomes, each of which carried the mutant white gene, was fertilized by a sperm cell containing the Y chromosome, the product was an XXY female with white eyes.
Rather than disproving the chromosome theory, this "exceptional" female actually provided strong experimental support that genes were in fact located on chromosomes. Morgan's lab also found that the trait for white eyes could appear even if a fly's father didn't have white eyes. This showed that flies could carry the white-eye trait even if they didn't show it themselves.
The trait could vanish and reappear only in certain exceptional moments. This concept forms the basis of our modern understanding of the hereditary substance that exists on chromosomes but is not always apparent in the outward physical traits of an organism.
Whereas Mendel called this substance "elementen" and Darwin called it "gemmules," researchers now use the more familiar term "gene. When considered in view of all this information, the chromosome theory of inheritance was not the work of a single scientist.
Rather, the theory was built on collaboration between multiple researchers working over a period of many decades.
The seeds of this theory were first planted in the s, when Gregor Mendel and Charles Darwin each proposed possible physical elements of heredity. It wasn't until several decades later, following Walther Flemming's Figure 6 discovery of chromosomes and description of their behavior during mitosis, that a probable mechanism for the transmission of traits was uncovered. Subsequently, Theodor Boveri and Walter Sutton's research strengthened the idea of a connection between chromosomes and hereditary elements.
But direct evidence that explicitly demonstrated that traits exist on specific chromosomes wasn't delivered until the Morgan lab's experiments with fruit flies at the beginning of the twentieth century. Thus, after nearly fifty years of speculation, scientists were finally able to confirm what they had long suspected: chromosomes are indeed the physical carriers of hereditary information, and this information exists in the form of genes.
This page appears in the following eBook. Aa Aa Aa. Drosophila chromosome. Scientists first discovered chromosomes in the nineteenth century, when they were gazing at cells through light microscopes. But how did they figure out what chromosomes do? And how did they link chromosomes — and the specific genes within them — to the concept of inheritance? After a long period of observational studies through microscopes, several experiments with fruit flies provided the first evidence.
What is a gene? The first words for genes: Elementen and gemmules. The Original Thinkers. Who was Gregor Mendel? The life and legacy of Charles Darwin.
Figure 1: Gregor Mendel. Figure 2: Charles Darwin. Describing chromosomes. Figure 3: Sample image from Walther Flemming's drawings of chromosome behavior during mitosis. In , German biologist Walther Flemming was the first person to describe what scientists now know as chromosomes. Flemming's elegant drawings showed how chromosomes aligned and were eventually pulled apart during mitosis Figure 3.
Then, in , another German researcher named Theodor Boveri provided the first descriptions of meiosis, also supported by detailed drawings, except these drawings showed how the number of chromosomes in a parent cell was reduced by half in the resulting gametes. Connecting heredity to chromosomes. Figure 4: Walter Sutton.
Confirming the chromosome theory of inheritance. Morgan's lab connects eye color with inheritance of sex chromosomes. Figure 5: Although white-eyed males were bred in several cycles with female flies, only male offspring were passed the unique trait. The exception proves the rule.
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