Study Guide for Module #8

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Exploring Creation with Biology by: Wile and Durnell, 2nd Edition, Module #8 Study Guide.

True Breeding

If an organism has a certain characteristic that is always passed on to its offspring, we say that this organism bred true with respect to that characteristic.


One of a pair of genes that occupies the same position on homologous chromosomes.


Two-letter set that represents the alleles an organism possesses for a certain trait.


The observable expression of an organism's genes.

Homozygous Genotype

A genotype in which both alleles are identical.

Heterozygous Genotype

A genotype with two different alleles.

Dominant Allele

An allele that will determine phenotype if just one is present in the genotype.

Recessive Allele

An allele that will not determine phenotype unless the genotype is homozygous in that allele.

Mendel's Principle of Genetics

1. The traits of an organism are determined by its genes.
2. Each organism has two alleles that make up the genotype for a given trait.
3. In sexual reproduction, each parent contributes ONLY ONE of its alleles to the offspring.
4. In each genotype, there is a dominant allele. If it exists in an organism, the phenotype is determined by that allele.


A diagram that follows a particular phenotype through several generations.

Monohybrid Cross

A cross between two individuals, concentrating on only one definable trait.

Dihybrid Cross

A cross between two individuals, concentrating on two definable traits.


Chromosomes that do not determine the sex an individual.

Sex Chromosomes

Chromosomes that determine the sex of an individual.


A protein that, when introduced into the blood, triggers the production of an antibody.

Autosomal Inheritance

Inheritance of a genetic trait not on a sex chromosome.

Genetic Disease Carrier

A person who is heterozygous in a recessive genetic disorder.

Sex-Linked Inheritance

Inheritance of a genetic trait located on the sex chromosomes.


A radical chemical change in one or more alleles.

Change in Chromosome Structure

A situation in which a chromosome loses or gains genes during meiosis.

Change in Chromosome Number

A situation in which abnormal cellular events in meiosis lead to either none of a particular chromosome in the gamete or more than one chromosome in the gamete.

Three pea plants have the following alleles for yellow ("Y") and green ("y") peas. What is the genotype and phenotype of each? Note whether they are homozygous or heterozygous.
a. YY b. Yy c. yy

A. Homozygous genotype is YY, it's phenotype is yellow peas.
B. Heterozygous genotype is Yy, it's phenotype is yellow peas.
C. Homozygous genotype is yy, it's phenotype is green peas.

A pea plant which is homozygous in the dominant, axial flower allele ("A") is crossed with a pea plant that is heterozygous in that allele. What are the possible genotypes and phenotypes, along with their percentage chances, for the offspring?

AA, AA, Aa, Aa. Both have a 50% chance of that genotype and a 100% chance of the flower phenotype.

A woman is heterozygous in the ability to roll her tongue when extended. If she marries a man who cannot roll his tongue, what percentage of their children will be able to roll their tongues? Remember the allele for being able to roll your tongue is dominant.

Rr, Rr, rr, rr. 50% of their children will be able to roll their tongues.

Recall that in guinea pig coat color, black (filled circles and squares) is dominant and white (hollow circles and squares) is recessive. What is the genotype of the male parent in the cross bellow?

D D O O(white)

The genotype is Bb.

The following pedigree is for the presence or absence of wings on certain insect. The hollow circles and squares represent insects without wings, while the filled circles and squares represent insects with wings. Which is the dominant allele? What are the genotypes for 1-4?

D---_---O D---_---O
-------------- -------------
D (D) O D (O) (O) D O

No wings ("N") is the dominant allele. 1 2 3 and 4 all have the Nn genotype.

Give the possible phenotypes and the percentage chance for each in the dihybrid cross between a pea plant that is homozygous in producing smooth, yellow peas, and a pea plant that produces wrinkled, green peas. The smooth and yellow alleles are dominant.

100% chance of the offspring have the "SsYy" genotype and the smooth, yellow phenotype.

Give the possible phenotypes and the percentage chance for each in the dihybrid cross between pea plant that is heterozygous in producing smooth, yellow peas and another with the same genotype.

Smooth yellow peas (genotypes SSYY, SsYy, SSYy, SsYY) 9 out of 16 or 56.25% chance.
Smooth green peas (genotypes SSyy, Ssyy) 3 out of 16 chance or 18.75% chance.
Wrinkled yellow peas (genotypes ssYY, ssYy) 3 out of 16 chance or 18.75% chance.
Wrinkled green peas (genotype ssyy) 1 out of 16 chance or 6.25% chance.

In fruit flies, the color of the eye is a genetic trait that is sex-linked. What is the percentage of males that will have white eyes when a heterozygous, re-eyed female is crossed with a white-eyed male? What is the percentage of females that will have white eyes from the same cross?

50% of the females will be white eyed and 50% of the males will be white eyed.

In the case of the fruit fly color again, what must be the genotype of a male fly, when crossed with a heterozygous female, there is no possibility of having a female with white eyes?


If a gamete has two alleles for the same genetic trait, what type of genetic disorder will result in a zygote formed with this gamete?

Change in chromosome number.

A person carries a genetic disorder but does not have the disorder. How is that possible?

The genetic disorder must be recessive.

Do sex-linked genetic disorders affect men and women the same? If not, which sex is affected more and why?

Sex-linked disorders affect men more frequently than women. This is because men have only one allele in sex-linked traits.

Two individuals have the exact same genotype for a certain trait, but they are not identical when it comes to that trait. How is this possible?

While the genetics are the same, the environmental and spiritual factors were probably different.

A woman with type O blood marries a man with type AB blood. What blood types are possible for their children? What is the percentage chance of each?

A (50%) and B (50%)

If a person has type B- blood, what are the possible genotypes for that person? Include the possible genotypes related to the type of blood as well as the Rh-factor.

The genotype must be either BB or BO.

What term do we use to refer to genetic traits that are governed by more than one set of genes?

Polygenetic Inheritance.

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