Question

question 1
the physical appearance of an organism is called its
a gene.
b allele
c genotype.
d phenotype.

question 2
mendel concluded that each pea has two units for each character, and each gamete contains one unit. mendels \unit\ is now referred to as
a a trait.
b an allele.
c the genotype.
d a character.

question 3
the genotype of an organism that expresses a dominant trait can be determined by
a crossing the organism with a homozygous recessive organism.
b crossing the organism with a heterozygous dominant organism.
c observing the phenotype of the progeny from any cross.
d crossing the organism with a homozygous dominant organism.

question 4
in a species of mice, short hair is dominant to long hair. if a short - haired individual is crossed with a long - haired individual and both long - and short - haired offspring result, one can conclude that
a the short - haired individual is heterozygous.
b the long - haired individual is homozygous.
c the long - haired individual is heterozygous.
d the short - haired individual is homozygous.

Explanation:

• Question 1:
• A gene is a segment of DNA that codes for a trait.
• An allele is a variant form of a gene.
• Genotype refers to the genetic makeup of an organism.
• Phenotype is the physical appearance of an organism, which matches the description in the question.
• Question 2:
• A trait is a characteristic of an organism.
• An allele is what Mendel referred to as a "unit" (a variant of a gene).
• Genotype is the genetic composition.
• A character is a general feature.
• Question 3:
• Crossing with a homozygous recessive (test cross) can determine if the dominant - trait organism is homozygous or heterozygous. If all offspring show the dominant trait, the organism is homozygous dominant; if there is a 1:1 ratio of dominant to recessive traits in offspring, the organism is heterozygous dominant.
• Crossing with a heterozygous dominant may not clearly distinguish the genotype of the unknown dominant - trait organism.
• Observing progeny from any cross is too vague.
• Crossing with a homozygous dominant won't help in determining the genotype of the unknown (as all offspring will show the dominant trait regardless of the unknown's genotype if it is also dominant).
• Question 4:
• Let \(S\) be the allele for short hair (dominant) and \(s\) for long hair (recessive). The long - haired individual has a genotype \(ss\) (homozygous recessive). If the short - haired individual was homozygous (\(SS\)), all offspring would have short hair (\(Ss\)). But since there are both long - haired (\(ss\)) and short - haired (\(Ss\)) offspring, the short - haired individual must be heterozygous (\(Ss\)). The long - haired individual is always homozygous (\(ss\)) as the long - hair trait is recessive.

Answer:

• Question 1: D. phenotype
• Question 2: B. an allele
• Question 3: A. crossing the organism with a homozygous recessive organism
• Question 4: A. the short - haired individual is heterozygous