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Inheritance Probability Screens

Most of us look in the mirror and wonder how we got our mother's nose, blonde hair, and blue eyes. Bigger questions arise when our siblings have the exact opposite facial features. The answer lies in the field of genetics. Genetics can help us determine the odds of inheriting certain traits. Geneticists, or people who study, explain, and predict the probability of inheritance in family lineages, may also expand their research into understanding the genetic makeup of plants and animals. This can help plant and animal breeders in developing selective traits.

Before a geneticist can determine the odds of inheriting a specific trait, he or she must apply a technique invented by Reginald Punnett, an early Twentieth century geneticist. He developed a technique commonly known as the Punnett square, a visual graph that illustrates all of the possible genotype combinations that can occur in newborns based on the traits of their parents. A genotype is the genetic makeup of a cell, organisms, or person, usually defined by a specific characteristic. The Punnett square reveals the odds of the offspring receiving specific genotypes. Anybody can use the Punnett square to determine these odds.

How to Use a Punnett Square

  1. Draw a graph of perpendicular lines. It should look similar to a tick-tack-toe game. Be sure that the graph has a total of nine boxes.
  2. Place one parent genotype across the top of the graph and then the other parent down the left-hand column. For example, if the parent rose plant genotypes are YY and GG, then the setup should appear with the YY across the top and the GG along the left-hand column.
  3. Only place one letter in each box for the parent's genotype. Do not worry about which parent is charted on the top or along the left-hand column of the Punnett square.
  4. Copy each of the row and column-head letters across or down the available squares. This reveals all potential genotype combinations among the offspring, regardless of how many times reproduction occurs.
  5. Examine the graph carefully to understand its different combinations. In this example, the offspring has a one hundred percent chance of becoming heterozygous (YG). A heterozygous genotype refers to two different alleles of a gene that develops into a particular trait. Since the rose plant has a yellow (Y) dominant allele, then one hundred percent of the offspring will have a yellow phenotype. A phenotype is the observable or detectable expression of a genotype.
  6. Redo the Punnett Square by alternating the parent's genotype letters "YG" across the top and left-hand column. In this example, both parents have heterozygous "YG" genotypes; therefore, the offspring has varying probability ratios. For instance, there will be a twenty five percent chance that the offspring will develop a "YY" genotype, fifty percent for a "YG" genotype, and a twenty five percent chance for developing a "GG" genotype. This probability is based on the fact that each of the four offspring boxes in the existing Punnet square is twenty five percent, or one out of four. This means that the phenotypes will have a seventy five percent chance of being "Y," and only twenty five percent will be "G." These odds will occur every time a new offspring is conceived by parents with "YG" genotypes.

An offspring's genotype is determined by the combination of the parent's genes contained within the sex cells or gametes. A gamete is the infusion of both sex cells from the male and female, also known as the sperm and ova. Sex cells typically only have one copy of the gene for each trait, or the "Y" and "G" forms as expressed in the above example. Each of the Punnet squares that illustrate either of the parentís genes actually represents one of the two genotype possibilities of a parent sex cell. The inheritance of either parental gene largely depends on chance. Placing each of the two gene copies in its own box gives it a fifty percent chance of it being inherited.

The Importance of Using Punnett Squares to Determine the Probability of Inherited Traits

The Punnett square can provide the answer for expectant mothers who are anxious to know the genetic traits of their children. It can determine if the newborn will have brown hair and eyes, or long fingers and toes. This can also be used to determine the likelihood of the child having an inherited disorder such as albinism. In this example, the "A" represents the dominant allele, or a normal genetic trait that masks a recessive trait, and will likely appear in an individual with a heterozygous or homozygous dominant genotype (Aa). The "a" represents the recessive allele, or an abnormal genetic trait masked by a dominant allele, and will likely appear in a homozygous recessive genotype (aa). If both parents have the disease, then they possess heterozygous genotypes. The Punnett square illustrates that during each birth, the offspring will have a twenty five percent chance of having an unaffected homozygous (AA) child, a fifty percent change of having an unaffected heterozygous (Aa) child, or a twenty five percent chance of having a homozygous recessive (aa) child who will likely suffer greatly from this condition. In other words, if a carrier parent (Aa) with a recessive gene reproduces with some who has albinism (aa), then the child has a greater likelihood of developing the condition. In general, half of the children will have heterozygous (Aa) genotypes, which makes them carriers of the condition. The other half will inherit the two recessive alleles (aa), leading to the development of albinism.

Some of us may have a greater number of recessive alleles. A percentage of these alleles may cause severe defects in the child if they are inherited by both parents. Some of these alleles may even cause life-threatening conditions. Common recessive disorders include Tay-Sachs, beta-thalassemia, and cystic fibrosis. However, some disorders are passed down through the dominant alleles of the parents genotypes, such as Huntington disease, dwarfism, and poly-dactyly. In this case, parents who are heterozygous are not healthy carriers for their newborns, because they have the disorder similar to homozygous dominant (AA) people. For this reason, most parents will seek the guidance of genetic counselors to understand the probability and consequences of having a child who may develop the disorder.

Follow these links to learn more about the Punnett Square:




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