What will only produce heterozygous offspring

People who are heterozygous Aa are not healthy carriers. They have the disorder just like homozygous dominant AA individuals. Punnett squares are standard tools used by genetic counselors. Theoretically, the likelihood of inheriting many traits, including useful ones, can be predicted using them. It is also possible to construct squares for more than one trait at a time.

However, some traits are not inherited with the simple mathematical probability suggested here. We will explore some of these exceptions in the next section of the tutorial.

All rights reserved. Previous Topic. Return to Menu. Practice Quiz. Next Topic. Mendel's principles can be used to understand how genes and their alleles are passed down from one generation to the next. When visualized with a Punnett square, these principles can predict the potential combinations of offspring from two parents of known genotype, or infer an unknown parental genotype from tallying the resultant offspring. An important question still remains: Do all organisms pass on their genes in this way?

The answer to this question is no, but many organisms do exhibit simple inheritance patterns similar to those of fruit flies and Mendel's peas. These principles form a model against which different inheritance patterns can be compared, and this model provide researchers with a way to analyze deviations from Mendelian principles. This page appears in the following eBook. Aa Aa Aa. Genes come in different varieties, called alleles.

Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism. Often, it is impossible to determine which two alleles of a gene are present within an organism's chromosomes based solely on the outward appearance of that organism. However, an allele that is hidden, or not expressed by an organism, can still be passed on to that organism's offspring and expressed in a later generation.

Tracing a hidden gene through a family tree. Figure 1: In this family pedigree, black squares indicate the presence of a particular trait in a male, and white squares represent males without the trait.

White circles are females. A trait in one generation can be inherited, but not outwardly apparent before two more generations compare black squares. Figure Detail. The family tree in Figure 1 shows how an allele can disappear or "hide" in one generation and then reemerge in a later generation. In this family tree, the father in the first generation shows a particular trait as indicated by the black square , but none of the children in the second generation show that trait.

Nonetheless, the trait reappears in the third generation black square, lower right. How is this possible? This question is best answered by considering the basic principles of inheritance.

Mendel's principles of inheritance. How do hidden genes pass from one generation to the next? Although an individual gene may code for a specific physical trait, that gene can exist in different forms, or alleles.

One allele for every gene in an organism is inherited from each of that organism's parents. In some cases, both parents provide the same allele of a given gene, and the offspring is referred to as homozygous "homo" meaning "same" for that allele.

In other cases, each parent provides a different allele of a given gene, and the offspring is referred to as heterozygous "hetero" meaning "different" for that allele. Alleles produce phenotypes or physical versions of a trait that are either dominant or recessive. The dominance or recessivity associated with a particular allele is the result of masking, by which a dominant phenotype hides a recessive phenotype. By this logic, in heterozygous offspring only the dominant phenotype will be apparent.

The relationship of alleles to phenotype: an example. Dominance, breeding experiments, and Punnett squares. Figure 4: A brown fly and a black fly are mated. Figure 5: A Punnett square. Figure 6: Each parent contributes one allele to each of its offspring. Thus, in this cross, all offspring will have the Bb genotype.

Figure 7: Genotype is translated into phenotype. In this cross, all offspring will have the brown body color phenotype. The phenomenon of dominant phenotypes arising from the allele interactions exhibited in this cross is known as the principle of uniformity, which states that all of the offspring from a cross where the parents differ by only one trait will appear identical.

How can a breeding experiment be used to discover a genotype? Figure 8. To prepare a Punnett square, all possible combinations of the parental alleles the genotypes of the gametes are listed along the top for one parent and side for the other parent of a grid. The combinations of egg and sperm gametes are then made in the boxes in the table on the basis of which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg.

Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance dominant and recessive is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible in the F 1 offspring.

All offspring are Yy and have yellow seeds. When the F 1 offspring are crossed with each other, each has an equal probability of contributing either a Y or a y to the F 2 offspring.

The result is a 1 in 4 25 percent probability of both parents contributing a Y , resulting in an offspring with a yellow phenotype; a 25 percent probability of parent A contributing a Y and parent B a y , resulting in offspring with a yellow phenotype; a 25 percent probability of parent A contributing a y and parent B a Y , also resulting in a yellow phenotype; and a 25 percent probability of both parents contributing a y , resulting in a green phenotype. When counting all four possible outcomes, there is a 3 in 4 probability of offspring having the yellow phenotype and a 1 in 4 probability of offspring having the green phenotype.

Using large numbers of crosses, Mendel was able to calculate probabilities, found that they fit the model of inheritance, and use these to predict the outcomes of other crosses. Observing that true-breeding pea plants with contrasting traits gave rise to F 1 generations that all expressed the dominant trait and F 2 generations that expressed the dominant and recessive traits in a ratio, Mendel proposed the law of segregation.

This law states that paired unit factors genes must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F 2 generation of a monohybrid cross, the following three possible combinations of genotypes result: homozygous dominant, heterozygous, or homozygous recessive. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes.

Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F 1 offspring will be heterozygotes expressing the dominant trait Figure 8.

Alternatively, if the dominant-expressing organism is a heterozygote, the F 1 offspring will exhibit a ratio of heterozygotes and recessive homozygotes Figure 8. The cross between the true-breeding P plants produces F1 heterozygotes that can be self-fertilized. The self-cross of the F1 generation can be analyzed with a Punnett square to predict the genotypes of the F2 generation.

Given an inheritance pattern of dominant—recessive, the genotypic and phenotypic ratios can then be determined. In pea plants, round peas R are dominant to wrinkled peas r. You do a test cross between a pea plant with wrinkled peas genotype rr and a plant of unknown genotype that has round peas. You end up with three plants, all which have round peas.

From this data, can you tell if the parent plant is homozygous dominant or heterozygous? You cannot be sure if the plant is homozygous or heterozygous as the data set is too small: by random chance, all three plants might have acquired only the dominant gene even if the recessive one is present. Independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics.

Consider the characteristics of seed color and seed texture for two pea plants, one that has wrinkled, green seeds rryy and another that has round, yellow seeds RRYY.

Because each parent is homozygous, the law of segregation indicates that the gametes for the wrinkled—green plant all are ry , and the gametes for the round—yellow plant are all RY. Therefore, the F 1 generation of offspring all are RrYy Figure 8. In pea plants, purple flowers P are dominant to white p , and yellow peas Y are dominant to green y.

What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? Was the final answer of the question wrong? Were the solution steps not detailed enough? Was the language and grammar an issue? Didn't find yours? Ask a new question Get plagiarism-free solution within 48 hours.

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