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Chapter 2: Problem 5

How can you determine whether an organism is heterozygous or homozygous for a dominant trait?

Short Answer

Expert verified
In genetics, an organism expressing a dominant trait can be determined to be homozygous or heterozygous by cross breeding it with an organism that is homozygous recessive. If any offspring expresses the recessive trait, the tested organism is heterozygous; if none do, it is likely homozygous dominant.

Step by step solution

01

Understanding Genetics Terms

A homozygous organism has two copies of the same allele for a particular trait while a heterozygous organism has two different alleles for that trait. A trait is said to be dominant if the trait is expressed when at least one dominant allele is present.
02

Identifying the trait expression

The dominant trait will be expressed in both homozygous and heterozygous organisms. This is because homozygous organisms carry two dominant alleles and heterozygous organisms carry one dominant allele and one recessive allele. In both cases, the dominant trait will show up as it takes precedence over the recessive trait (which requires two copies to express itself).
03

Test Crossing

Determining if an organism exhibiting a dominant trait is homozygous or heterozygous for that trait involves breeding it with an organism that is homozygous recessive. This process is called a test cross. If any offspring express the recessive trait, then the original organism must be heterozygous. If all offspring express the dominant trait, the organism is likely homozygous. It's important to note that due to randomness in genetics, more offspring increase the accuracy of this test.

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Most popular questions from this chapter

Ectrodactyly, also known as "lobster claw syndrome," is a recessive disorder in humans. If a phenotypically unaffected couple produces an affected offspring, what are the following probabilities? A. Both parents are heterozygotes. B. An offspring is a heterozygote. C. The next three offspring will be phenotypically unaffected. D. Any two out of the next three offspring will be phenotypically unaffected.

Why did Mendel's work refute the idea of blending inheritance?

On rare occasions, an organism may have three copies of a chromosome and therefore has three copies of the genes on that chromosome (instead of the usual number of two copies). For such a rare organism, the alleles for each gene usually segregate so that a gamete will contain one or two copies of the gene. Let's suppose that a rare pea plant has three copies of the chromosome that carries the height gene. Its genotype is TTt. The plant is also heterozygous for the seed color gene, \(Y y\), which is found on a different chromosome. With regard to both genes, how many types of gametes can this plant make, and in what proportions? (Assume that it is equally likely that a gamete will contain one or two copies of the height gene.)

A cross was made between a white male dog and two different black females. The first female gave birth to eight black pups, and the second female gave birth to four white and three black pups. What are the likely genotypes of the male parent and the two female parents? Explain whether you are uncertain about any of the genotypes.

With regard to genotypes, what is a true-breeding organism?
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