TOPIC 11 DAY 4 DIHYBRID CROSSES

What is the difference between a monohybrid cross and a dihybrid cross?

• A monohybrid cross involves a single parent, whereas a dihybrid cross involves two parents.

• A monohybrid cross produces a single offspring, whereas a dihybrid cross produces two offspring.

• A dihybrid cross follows the inheritance of two traits and a monohybrid only one.

• A monohybrid cross results in a 9:3:3:1 ratio whereas a dihybrid cross gives a 3:1 ratio.

Answer explanation

A dihybrid cross examines the inheritance of two traits simultaneously, while a monohybrid cross focuses on one trait. This distinction is crucial in understanding genetic variation and inheritance patterns.

Mendel's law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes: every possible combination of alleles for every gene is equally likely to occur.
Which gametes can a plant with genotype SsYy produce?

• Ss and Yy

• SY and sy

• SY, Sy, sY, and sy

• Ss, Yy, SY and sy

Answer explanation

The genotype SsYy can produce gametes by combining alleles from each gene. The possible combinations are SY, Sy, sY, and sy, reflecting independent assortment of alleles.

A tall pea plant with red flowers has the genotype AaDd. This plant is crossed with another tall pea plant with white flowers, having the genotype Aadd.
What is the probability of getting dwarf plants with red flowers in the F1 progeny?

• 1/8

• 2/8

• 3/8

• 4/8

Answer explanation

To get dwarf plants (aa) with red flowers (A_), we need AaDd x Aadd. The probability of aa is 1/4 and A_ is 1/2. Thus, 1/4 * 1/2 = 1/8. Therefore, the probability of getting dwarf plants with red flowers is 1/8.

Six puppies from a single litter have the following genotypes for a certain trait: AA, Aa, AA, aa, aa, Aa. Which of the following describes how these alleles were passed on from parent to offspring?

• 1. Each parent dog randomly passed on a dominant allele (A) or a recessive allele (a) to each offspring.

• 1. The male dog passed on a dominant allele (A) to each female offspring and a recessive allele (a) to each male offspring.

• 1. The female dog chose to pass on either homozygous dominant alleles (AA) or homozygous recessive alleles (aa) to each offspring.

• 1. One parent dog passed on a dominant allele (A) to each offspring, and the other parent dog passed on a recessive allele (a) to each offspring.

Answer explanation

The correct choice states that each parent randomly passed on either a dominant (A) or recessive (a) allele. This is supported by the variety of genotypes in the offspring, indicating random allele distribution.

The Punnett square shows the cross between two parents heterozygous for two traits. In horses, the allele for black color (B) is dominant over the allele for white color (b). Long hair (H) is dominant over short hair (h).

What is the genotype and phenotype of the offspring in block E?

• bbhh, black color with long hair

• bbHh, black color with short hair

• bbhh, white color with short hair

• Bbhh, white color with long hair

Answer explanation

In block E, the genotype is bbhh, which means both traits are recessive. This results in a phenotype of white color (bb) and short hair (hh), confirming the correct answer is 'bbhh, white color with short hair'.

The Punnett square shows the cross between two parents heterozygous for two traits. In watermelons, the allele for solid green color (G) is dominant over the allele for striped pattern (g). Short shape (L) is dominant over the long shape (l).

What is the phenotype of the offspring shaded in the table?

• Color: Solid green

  Shape: Short

• Color: Solid green

  Shape: Long

• Color: Striped

  Shape: Short

• Color: Striped

  Shape: Long

Answer explanation

The shaded offspring are heterozygous for both traits (GgLl). The dominant alleles (G for solid green and L for short shape) determine the phenotype, resulting in solid green color and short shape.