Topic 3.2: Simple Inheritance Introduction

Question 1

Distinguish between the terms genotype and phenotype

Answer 1

The phenotype of an individual is its appearance, which is determined by the genes it carries for traits carried in its genome. The genes present in the genome comprise its genotype.

Question 2

Explain the purpose of testcross.

Answer 2

A testcross is conducted to determine if an individual that is expressing a dominant allele is heterozygous or homozygous for that gene.

Question 3

Summarize Mendel’s law of segregation

Answer 3

The law of segregation states that each individual has two factors for each trait and these factors separate during gamete formation such that each gamete contains only one factor. At fertilization, the zygote receives two factors for each trait.

Question 4

Summarize Mendel’s law of independent assortment.

Answer 4

The law of independent assortment states that during gamete formation, each pair of factors separates independently of other factors and that all possible combinations of factors can occur.

Question 5

Describe the types of crosses that result in a 3:1 and a 9:3:3:1 ratio.

Answer 5

A phenotypic ratio of 3:1 results from a one-trait cross and a ratio of 9:3:3:1 results from a two-trait cross.

Question 6

Define alleles

Answer 6

Alternative form of a gene; alleles occur at the same locus on homologous chromosomes.

Question 7

Define locus

Answer 7

Physical location of a trait (or gene) on a chromosome.

Question 8

What is the law of segregation and what does it state?

Answer 8

Mendelian principle that explains how, in a diploid organism, alleles separate during the formation of the gametes.

It states:
- Each individual has 2 factors for each trait.
-The factors segregate (separate) during the formation of gametes.
- Each gamete contains only one factor from each pair of factors.
- Fertilization gives each new individual two factors for each trait.

Question 9

Define phenotype.

Answer 9

Visible expression of a genotype—e.g., brown eyes or attached earlobes.

Question 10

Define genotype

Answer 10

Genes of an organism for a particular trait or traits; often designated by letters—for example, BB or Aa.

Question 11

Define dominant allele

Answer 11

Allele that exerts its phenotypic effect in the heterozygote; it masks the expression of the recessive allele.

Question 12

Define recessive allele

Answer 12

Allele that exerts its phenotypic effect only in the homozygote; its expression is masked by a dominant allele.

Question 13

Define homozygous

Answer 13

Possessing two identical alleles for a particular trait.

Question 14

Define heterozygous

Answer 14

Possessing unlike alleles for a particular trait.

Question 15

Define monohybrid cross

Answer 15

Cross between parents that differ in only one trait.

Question 16

Define Punnett square

Answer 16

Visual representation developed by Reginald Punnett that is used to calculate the expected results of simple genetic crosses.

Question 17

Define testcross

Answer 17

Cross, between an individual with a dominant phenotype and an individual with a recessive phenotype, to determine whether the dominant individual is homozygous or heterozygous.

Question 18

What is the law of independent assortment and what does it state?

Answer 18

Mendelian principle that explains how combinations of traits appear in gametes; see also independent assortment.
- Each pair of factors separates independently (without regard to how the others separate.)
- All possible combinations of factors can occur in the gametes.

Question 19

Define dihybrid cross

Answer 19

Cross between parents that differ in two traits.

Question 20

The law of segregation states all of the following except
a) factors separate during formation of the gametes
b) each individual has two factors for each trait
c) gametes contain a single factor for each trait
d) factors assort independently of each other by meiosis.

Answer 20

d) factors assort independently of each other by meiosis.

Question 21

If two parents with short fingers (dominant) have a child with long fingers, what is the chance their next child will have long fingers?
a) There is no chance.
b) 1/16
c) 1/2
d) 3/16
e) 1/4

Answer 21

e) 1/4

Question 22

Using the products rule, determine the probability that an Aa individual will be produced from an Aa x Aa cross.
a) 25%
b) 50%
c) 75%
d) 100%
e) 0%

Answer 22

b) 50%

Question 23

According to the law of independent assortment,
a) all possible combinations of factors can occur in the gametes
b) only the parental combinations of gametes can occur in the gametes
c) only the nonparental combinations of gametes can occur in the gametes

Answer 23

a) all possible combinations of factors can occur in the gametes

Question 24

The genotype of an individual with the dominant phenotype can be determined best by reproduction with
a) a homozygous recessive individual
b) an individual with the dominant phenotype
c) a heterozygote
d) none of the above

Answer 24

a) a homozygous recessive individual

Question 25

In a dihybrid cross of AaBb x AaBb, what is the probability that offspring will have the dominant phenotype for trait A but the recessive phenotype of trait B?
a) 1/16
b)3/16
c) 7/16
d) 916
e) none of the above

Answer 25

a) 1/16