How do geneticists use principles of probability

By crossing it with a plant with yellow pods gg , if the genotype is GG, all the offspring will have genotype Gg and have green pods. If the genotype is Gg, half the offspring will have genotype Gg, with green pods. The other half will have genotype gg with yellow pods. Monohybrids and Dihybrids Some hybrids have only one trait, involving one pair of alleles, and others involve separate pairs of alleles and two separate traits.

When only one trait is involves, the possibilities are simple, and the Punnett square is a simple 2 X 2 grid. A relatively simple trait in humans is whether earlobes are free dominant or attached recessive. If two individuals are heterozygous for the trait, their offspring might have the following genotypes: EE, Ee, Ee, and ee. If two separate traits are involved, the Punnett square will be larger because of the many different possibilities.

For watermelons, solid color green G is dominant over striped green g , and short length S is dominant over long s. If a green , short watermelon GGSS is crossed with a striped, long watermelon ggss , the grid grows to 4 X 4, with 16 different genotypes in the second generation. Incomplete Dominance Not all alleles are dominant or recessive, however. Sometimes, both alleles are expressed. For example, short tailed cats have one gene for no tail T N and one gene for long tail T L.

The roan reddish-brown color of some cattle also results from incomplete dominance, as well as many other features of plants and animals. Interested in Science tutoring services? Next, you put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:.

Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or the top of the Punnett square. Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares.

This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs. These will be the odds every time a new offspring is conceived by parents with YG genotypes. An offspring's genotype is the result of the combination of genes in the sex cells or gametes sperm and ova that came together in its conception. One sex cell came from each parent. Sex cells normally only have one copy of the gene for each trait e.

Each of the two Punnett square boxes in which the parent genes for a trait are placed across the top or on the left side actually represents one of the two possible genotypes for a parent sex cell. Which of the two parental copies of a gene is inherited depends on which sex cell is inherited--it is a matter of chance.

If you are not yet clear about how to make a Punnett Square and interpret its result, take the time to try to figure it out before going on. Why is it important for you to know about Punnett squares? The answer is that they can be used as predictive tools when considering having children.

Let us assume, for instance, that both you and your mate are carriers for a particularly unpleasant genetically inherited disease such as cystic fibrosis. Of course, you are worried about whether your children will be healthy and normal. Punnett squares can be used to predict and compare the genetic variations that will result from a cross. From Punnett squares, you can predict the probable genotype and phenotype of an organism. Slide 6 A capital letter represents the dominant allele for tall.

A lowercase letter represents the recessive allele for short. Punnett Squares Slide 8 Possible gene combinations for the F 2 offspring appear in the four boxes. Slide 9 Punnett Squares Organisms that have two identical alleles for a particular trait are said to be homozygous. Organisms that have two different alleles for the same trait are heterozygous.

Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait. Slide 10 Punnett Squares All of the tall plants have the same phenotype, or physical characteristics. The tall plants do not have the same genotype, or genetic makeup.