Inheritance of Traits Lesson

Why do siblings resemble each other? Has anyone ever mentioned that you look like one of your parents or grandparents? Well, that is because children can inherit traits, genetically determined characteristics or conditions, from their parents.

Heredity is the passing of traits genetically from one generation to the next generation. Traits are located on genes. The human body contains around 24,000 genes that are located on our chromosomes. Take a look at the following picture showing all 23 chromosomes present in humans and traits that are carried on each chromosome. Remember that you have two of each chromosome (for a total of 46) - one you received from your mother, and one from your father.

Human Genome Poster_2009 from Gene Gateway Image

to see information displayed on poster, visit the following url:

http://www.ornl.gov/hgmis/posters/chromosome

On a smaller scale, recall that DNA is made of up of nucleotides. A gene is made up of a certain order of nucleotides.

Therefore, different genes have a different order of bases. The order of nucleotides in a gene for hair color will determine whether the hair will be black, brown, auburn, or blonde. Alleles are the different forms of a gene. You inherit one allele from each parent. You will get an allele for hair color from your mother and an allele for hair color from your father. Of the two alleles, only one will be dominant. The trait that is expressed physically and/or visibly in an organism is controlled by the dominant allele. The allele that is not expressed, hidden, or masked is called the recessive allele. The dominant allele determines the phenotype of an organism. A phenotype is an organism's physical appearance or visible traits. Size, color, and height are some examples of phenotypes.

Can you find the genes for brown hair color, dyslexia, and diabetes on the chromosome map below?

Some human traits are controlled by multiple alleles instead of two alleles. Blood types are controlled by a gene that has more than two alleles. There are four blood types (A, B, AB, and O) that are controlled by a single gene with three alleles. The alleles for blood type are A, B, and O. Each person carries only two of the three alleles for blood type. Chromosomes exist as pairs within your body. Therefore each chromosome has a gene with one of the three alleles for blood type. Alleles A and B are codominant. When they are inherited together, neither allele is not expressed or masked. Both alleles are expressed and the person has type AB blood. The alleles A and B are dominant over the allele for type O blood. A person who inherits type O blood has two recessive alleles. If a person inherits the alleles A and O, their genotype will be AO and he or she will have type A blood. The genotype is an organism's genetic makeup or allele combinations. The possible phenotypes and genotypes for blood type are listed below.

Human traits such as hair, eye, and skin color are controlled by more than one gene. The variation of skin pigmentation in individuals from the same family is due to the combination of alleles from about five different genes. At least two genes contribute to the various hues (green, brown, hazel, blue, and gray) for eye color.

A few traits are inherited based on an individual's gender. These genes are located on our sex chromosomes. One of the 23 pairs of chromosomes in the human body is the sex chromosomes. They carry genes that will determine if a person will be a male or female as well as genes for other traits. Red-green color blindness, hemophilia, and baldness are genetic disorders linked to the X chromosome. Are all sex linked traits associated with genes on the X chromosome?

To learn more about genes and the abilities they unlock for us, watch the video Genes, Genetics, and DNA. While watching listen and look for answers to the following questions

What is it that makes each of us different?
How are traits passed down?
Why do single-celled organisms reproduce exact copies of themselves?
Why do multi-celled organisms seldom reproduce exact copies of themselves?

The video is approximately 25 minutes long. Please pause the video when necessary to make notes.

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