Question

In pea plants, tall ( $T$ ) is dominant over short ( $t$ ) and green $(G)$ is dominant over yellow $(g)$. If a pea plant that is heterozygous for both traits is crossed with a plant that is recessive for both traits, approximately what percentage of the progeny plants will be tall and yellow? (A) $0\mathrm{\%}$ (B) $25\mathrm{\%}$ (C) $66\mathrm{\%}$ (D) $75\mathrm{\%}$

Short answer

(B) $25\mathrm{\%}$

Step-by-step solution

Set up the cross

First, set up the cross with the given parent genotypes: heterozygous for both traits (TtGg) and recessive for both traits (ttgg). We write the cross as (TtGg) x (ttgg).

Determine possible gametes

Determine the possible gametes for each parent plant by separating the alleles in each genotype. The heterozygous plant (TtGg) can produce the following gametes: (TG, Tg, tG, tg), while the recessive plant (ttgg) can only produce one type of gamete (tg).

Set up the Punnett square

Create a Punnett square with the heterozygous plant's gametes (TG, Tg, tG, tg) and the recessive plant's gametes (tg) to find the possible genotypes of the offspring: ``` | TG | Tg | tG | tg | -------------------------- tg | TTGg | TtGg | ttGg | ttgg | ```

Determine the genotypes of tall and yellow progeny

Now, we will look for the genotypes that result in tall and yellow offspring. For the progeny plants to be tall, they must have at least one dominant T allele. For the plants to be yellow, they need to have two recessive g alleles. In our Punnett square, the genotypes TtGg and ttGg meet these criteria.

Calculate the percentage of tall and yellow progeny

There are a total of 4 possible offspring genotypes in the Punnett square, and 1 of them (25%) is tall and yellow (TtGg). So, the answer is: (B) $25\mathrm{\%}$

Key concepts

Genetics

Genetics is the branch of biology concerning the study of genes, heredity, and genetic variation in living organisms. It is essentially a set of instructions for building and maintaining an organism, encoded in the organism's DNA.

At the most basic level, genetics explains how traits are passed down from parents to their offspring. These traits come in the form of genes, which are segments of DNA carrying genetic information. Each organism receives two copies of each gene, one from each parent, which then interact to determine the characteristics of the offspring.

Understanding genetics is fundamental to deciphering how traits are inherited and can help predict the likelihood of an organism having a certain phenotype - the set of observable characteristics.

Dominant and Recessive Traits

Dominant and recessive traits are at the core of how genetic inheritance is expressed in an organism. A dominant allele is one whose traits always show up in the organism even if the gene's counterpart (allele) is different.

On the other hand, a recessive allele is an allele that is masked by the presence of a dominant allele in a heterozygous genotype, and it only shows its effects when a pair of recessive alleles is present. This means that in order for a recessive trait to be expressed, an organism must inherit two copies of the recessive allele, one from each parent.

In a typical Mendelian cross, such as the one involving pea plants, dominant traits are represented by uppercase letters (like T for tall plants), while recessive traits are designated with lowercase letters (like t for short plants).

Mendelian Inheritance

Mendelian inheritance refers to the patterns of inheritance that were first described by Gregor Mendel in 1865, based on his experiments with pea plants. Mendel's principles of inheritance include the law of segregation and the law of independent assortment.

The law of segregation states that during the production of gametes, the two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. The law of independent assortment indicates that the alleles of different genes get sorted into gametes independently of one another.

These foundational laws of genetics make it possible to predict the outcome of genetic crosses using tools like the Punnett square and understand the distribution of traits across generations.

Heterozygous Genotype

A heterozygous genotype refers to the makeup of a gene that contains two different alleles - one inherited from each parent. In our example, the allele 'T' is dominant for tallness, and 't' is recessive for shortness. Similarly, 'G' is dominant for the green color, and 'g' is recessive for yellow color.

When an organism, like our heterozygous pea plant, has a genotype of TtGg, it means it has both dominant and recessive alleles for both traits. This genetic diversity contributes to the variations observed in offspring. In genetics, heterozygosity is considered important because it enables the potential for a greater diversity of traits and can influence an organism's adaptation and survival.