Anita G. Santora
Just as chromosomes are paired, the genes on those chromosomes are also paired; so that we have two genes for every genetically influenced trait or function. One of those genes came from within the sperm contributed by the father and one from within the egg contributed by the mother.
A dominant trait will be expressed (show, or take control) even if only one copy of the gene is present.
Dominant Inheritance
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If one affected parent has a single faulty gene, D, which dominates its counterpart d, with each pregnancy there is a fifty percent chance of the child inheriting the dominant gene D.
A recessive trait is only expressed when two copies of the gene for that trait are present. But a gene for the recessive trait remains part of the chromosomal structure that may be passed on to offspring even if it is not observed in the parents. Both parents may exhibit the same normal trait, however, if either or both are not pure for that trait (e.g. the unexpressed recessive gene is also a part of their genetic makeup) either, or both may pass the unexpressed gene and not the normal gene to their offspring.
Recessive Inheritance
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In a recessively inherited genetic disorder, both parents carry a defective gene, “g”, but are protected by the presence of a normal gene, G. The condition caused by g is not observed in either parent. However, each child has a twenty-five percent risk of inheriting a g gene from each parent and exhibiting the trait, a twenty-five percent chance of inheriting two normal genes; and a fifty percent risk of inheriting a Gg combination and being a carrier.
The genes located on the X chromosome (the sex chromosome passed by mothers to both male and female offspring, and passed by fathers to their female offspring) are called X-linked genes. The genes located on any of the other twenty-two pairs of chromosomes are referred to as autosomal. In an X-linked recessive condition, if a female has a defective gene on one of her two sex chromosomes, she is protected against the defect because her normal sex chromosome, X, compensates for the defect on the other X*. A male with a defective gene on the X(*) chromosome inherited from his mother would not be protected because he only has one X chromosome.
X-linked Recessive Inheritance
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If the mother has a defective gene on one of her two X chromosomes and the father has a normal X chromosome and a normal Y chromosome, each male child has a fifty percent risk of inheriting and being affected by the faulty gene on the X(*) chromosome, and a fifty percent chance of inheriting the normal X chromosome. Each female child has a fifty percent risk of inheriting the faulty X(*) chromosome and becoming a carrier like her mother, and a fifty percent chance of inheriting two normal X chromosomes.
Genes located on the Y chromosome (the sex chromosome passed by the father to his male offspring) are called Y-linked genes.
Mutations are genetic characteristics that were not part of the original genetic makeup of either parent. They may occur because of the rearranging of the chemical code into new sentences that occurs in the production of sex cells. However, there are many, many other causes that can contribute to mutation.
Pedigree charts are used for tracing known patterns of inheritance in a family. The diagram that follows is a pedigree chart showing three generations of one family
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Mary and Paul are symbolized by a square for a male and a circle for a female. Their marriage is shown with a connecting line. On the next line, their children are indicated in order of birth. Tom, a son, is the oldest; followed by fraternal twins, Ann and Bob. Twins are marked with symbols attached to lines beginning at the same point. The diagonal line through Bob’s symbol indicates he is not living. Mary and Paul’s youngest child is Sue. Tom is married to Joan and they have three children; identical twins Bill and Ed are noted by attaching their symbols to lines beginning at the same point as for Ann and Bob; and, because they are identical, joining the symbols with a horizontal line. Their younger brother is Joe. Sue married Tim and they have three daughters, Kim, Jen and Liz.
Jen has a disease that did not previously appear in the family. A filled in symbol in this drawing shows that a person is affected by the trait being traced. A half-filled symbol shows a carrier. Through testing, it was determined that Jen’s parents are both carriers of the disease even though neither actually had the disease. Jen’s mother, Sue, had inherited the gene from her mother, Mary. When she married Tim, who also unknowingly carried a gene for the disease, it meant that it was now possible for some of their children to receive a double dose of the gene and have the disease.
Sample lesson plan Pedigree Charts
Objective To introduce pedigree charts.
Materials needed
One copy of the previous page explaining pedigree charts for each student.
Crayons
Pencils or pens
Rulers
Paper
Method Pass introduction sheets and give children time to silently read them. Working at the chalkboard, recreate the chart one line at a time as the children take turns reading aloud. After you have worked through the sheet with the class, ask questions such as: Who is Joe’s father? Who is Joe’s aunt? Who is Liz’s cousin? Name all the grandchildren of Paul and Mary. Name Sue’s sister and brother.
When children are comfortable determining relationships, pass the crayons, pencils or pens, rulers, and paper and allow time for children to draw a pedigree chart descriptive of a family through one set of grandparents.
Collect and display pedigree charts.
Extension Childrens’ familiarity with pedigree charts will enable you to use information sheets on inherited diseases available from the March of Dimes Birth Defects Foundation. (See resources.)