Question

section 8.1, continued
introduction to mendelian genetics
practice 2
answer the following questions about mendelian genetics. darken the circle or circles that correspond to
your answer choice(s).

1. which of the following represents a heterozygous genotype?

a aa b aa c aa d ab
2 a homozygous recessive genotype is shown by which of the following?
a tt b tt c tt d ts

1. a recessive trait would appear in the phenotype of which of these?

a bb b bb c bb d bf

1. in pea plants, the allele for purple flowers is dominant to the allele for white flowers. which of the following

correctly describes a pea plant that has white flowers?
a it must be homozygous dominant.
b it must be homozygous recessive.
c it must be heterozygous.
d it could be either heterozygous or homozygous dominant.

1. a homozygous dominant pea plant with purple flowers is crossed with a pea plant with white flowers. which of

the following correctly describes the offspring?
a homozygous with purple flowers c heterozygous with purple flowers
b homozygous with white flowers d heterozygous with white flowers

1. which of the following are true about a recessive gene trait? choose all that apply.

a it is expressed when the genotype is heterozygous.
b it can be masked by a dominant gene trait.
c it is expressed only when a dominant gene isnt present.
d it can be expressed when the genotype is homozygous.
e it cannot be expressed but can only passed onto the next generation.

1. which of the following best describes mendels law of dominance?

a when an organism has two different alleles of the same gene, both genes will be expressed equally in the
organism.
b when an organism has two different alleles of the same gene, one of the genes will be expressed and the
other will be hidden.
c when an organism has two of the same alleles for a gene, that allele is always dominant.
d when an organism has two of the same alleles for a gene, the stronger of the two genes will be expressed in
the organism.

Explanation:

1. A heterozygous genotype has two different alleles. \(Aa\) has one dominant (\(A\)) and one recessive (\(a\)) allele. \(AA\) and \(aa\) are homozygous (two of the same alleles), and \(Ab\) is not a standard genetic notation for a single - gene locus.
2. A homozygous recessive genotype has two recessive alleles. \(tt\) is two recessive alleles. \(TT\) is homozygous dominant, \(Tt\) is heterozygous, and \(TS\) is not a standard genetic notation for a single - gene locus.
3. A recessive trait is expressed when there are two recessive alleles. \(bb\) is two recessive alleles. \(BB\) is homozygous dominant (expresses dominant trait), \(Bb\) is heterozygous (expresses dominant trait), and \(Bf\) is not a standard genetic notation for a single - gene locus.
4. Since purple (\(P\)) is dominant over white (\(p\)), a white - flowered plant must have two recessive alleles (\(pp\)), which is homozygous recessive. A homozygous dominant (\(PP\)) would be purple, and a heterozygous (\(Pp\)) would also be purple.
5. Let \(P\) be the allele for purple (dominant) and \(p\) for white (recessive). The homozygous dominant parent (\(PP\)) can only give \(P\) alleles, and the white - flowered parent (\(pp\)) can only give \(p\) alleles. So the offspring will all be \(Pp\) (heterozygous) and since \(P\) is dominant, they will have purple flowers.

6.

• A recessive gene is masked by a dominant gene in a heterozygous genotype. So it is not expressed when heterozygous (A is false).
• A recessive gene can be masked by a dominant gene (B is true).
• A recessive gene is expressed only when there is no dominant gene (i.e., homozygous recessive) (C is true).
• When the dominant gene isn't present (homozygous recessive), the recessive gene is expressed (D is true).
• A recessive gene can be expressed when homozygous recessive (E is false).

1. Mendel's law of dominance states that when an organism has two different alleles of the same gene, one (the dominant allele) will be expressed and the other (the recessive allele) will be hidden.

Answer:

1. A. \(Aa\)
2. C. \(tt\)
3. B. \(bb\)
4. B. It must be homozygous recessive.
5. C. heterozygous with purple flowers
6. B. It can be masked by a dominant gene trait; C. It is expressed only when a dominant gene isn't present; D. It can be expressed when the dominant gene isn't present.
7. B. When an organism has two different alleles of the same gene, one of the genes will be expressed and the other will be hidden.