dihybrid crosses practice problems answer key

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31-01-2025

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Understanding dihybrid crosses is crucial for grasping fundamental genetics concepts. This guide provides answers and detailed explanations for common dihybrid cross practice problems, helping you solidify your understanding of Mendelian inheritance. We'll cover the Punnett Square method and analyze the resulting phenotypic and genotypic ratios.

What is a Dihybrid Cross?

A dihybrid cross involves tracking the inheritance of two different traits simultaneously. Each trait is controlled by a separate gene, with each gene possessing two alleles (variations of the gene). For example, we might consider plant height (tall vs. short) and flower color (red vs. white).

Practice Problem 1: Pea Plant Traits

Let's consider a classic example: pea plants. Assume tall (T) is dominant to short (t), and red flowers (R) are dominant to white flowers (r). A homozygous tall, red-flowered plant (TTRR) is crossed with a homozygous short, white-flowered plant (ttrr).

a) What are the genotypes and phenotypes of the F1 generation?

• Solution: All F1 offspring will be TtRr, exhibiting a tall stature and red flowers. This is because T and R are dominant alleles.

b) What are the expected genotypic and phenotypic ratios in the F2 generation (resulting from a cross between two F1 plants)?

• Solution: To determine this, we'll use a 4x4 Punnett Square:

• Genotypic Ratio: 1 TTRR : 2 TTRr : 1 TTrr : 2 TtRR : 4 TtRr : 2 Ttrr : 1 ttRR : 2 ttRr : 1 ttrr (This simplifies to a 1:2:1:2:4:2:1:2:1 ratio, but the unsimplified version is more useful for understanding individual combinations.)

• Phenotypic Ratio: 9 Tall, Red : 3 Tall, White : 3 Short, Red : 1 Short, White (This is the classic 9:3:3:1 dihybrid ratio).

Practice Problem 2: Fruit Fly Eye Color and Wing Shape

Fruit flies are another common subject in genetics studies. Consider eye color (red, R, dominant; white, r, recessive) and wing shape (normal, N, dominant; vestigial, n, recessive). A homozygous red-eyed, normal-winged fly (RRNN) is crossed with a homozygous white-eyed, vestigial-winged fly (rrnn).

a) Determine the genotypes and phenotypes of the F1 generation.

• Solution: All F1 offspring will be RrNn, displaying red eyes and normal wings.

b) Predict the genotypic and phenotypic ratios in the F2 generation (self-cross of F1 generation).

• Solution: Again, a 4x4 Punnett Square is needed. (Due to space constraints, the full Punnett Square isn't shown here, but you should create it to follow along) The results will yield the same 9:3:3:1 phenotypic ratio as Problem 1, but with different traits:

• Phenotypic Ratio: 9 Red eyes, Normal wings : 3 Red eyes, Vestigial wings : 3 White eyes, Normal wings : 1 White eyes, Vestigial wings

Understanding the Ratios

The 9:3:3:1 phenotypic ratio in dihybrid crosses is a key concept. It arises from the independent assortment of alleles during meiosis. Each trait is inherited independently of the other, leading to this characteristic ratio in the F2 generation. Deviations from this ratio might indicate linkage (genes located close together on the same chromosome), which prevents independent assortment.

Further Practice

To strengthen your understanding, create your own dihybrid cross problems using different traits and alleles. Remember to use Punnett Squares to visualize the possible gamete combinations and resulting offspring genotypes and phenotypes. Understanding these problems is key to mastering Mendelian genetics and progressing to more complex genetic concepts.