Chapter 54. Genetic Inheritance

Learning Objectives

allele
one version of a gene that can take different forms
chromosome
a tightly wound strand of DNA; normal human cells have 23 pairs of chromosomes
codominant
two alleles that are equally powerful, so both are expressed in the phenotype
DNA (deoxyribonucleic acid)
a special, large molecule that carries a cell’s genetic code
dominant
an allele that is always expressed in the phenotype, whether the child receives one copy or two copies
environment
external influences on development from the physical and social world
gene
segment of a DNA molecule that can synthesize a protein; located on chromosomes
genome
the complete set of genetic instructions for each organism
genotype
the particular alleles that a child receives
heredity
internal genetic contribution to development, carried in the DNA within each body cell
interaction
the combined result of influences from two or more factors; typically, the presence of one factor changes the impact of the other factor
nature-nurture issue
controversy about the relative influence of heredity (genes) and environment (experience in the world)
ovum
the female reproductive cell; sometimes called the egg cell
phenotype
the physical and behavioral traits that are expressed by a particular genotype
recessive
an allele that is expressed in the phenotype only when the child receives two copies
sperm cell
the male reproductive cell
trait
an enduring characteristic of a person, animal, or plant; could refer to physical appearance, behavior, or mental predisposition
Genetic Inheritance
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Learning Objectives:

Understand how the genetic traits of parents are passed to their biological children.

Describe the connection between dominant-recessive traits and genotype-phenotype differences.

Review

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1. Every important human trait is shaped by the interaction of heredity (“nature”) and environment (“nurture”). The hereditary influences are passed from parents to their child through the specific set of genes contained in the father’s sperm cell and the mother’s ovum.

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The figure is an image of a cell. Within the cell is the nucleus, which is a round structure in the middle of the cell. Within the nucleus are chromosomes. Each chromosome is made up of genes. Each gene is comprised of a strand of DNA.

2. Each individual gene is a segment of DNA (deoxyribonucleic acid) that can synthesize a specific protein. The total package of genes a child receives—called the genome—contains a complete set of instructions for assembling the human body, as well as for wiring the brain in ways that will make some behaviors more likely than others. These instructions are carried in the 23 pairs of chromosomes found in every human cell (except the sperm cell and the ovum, which have 23 single chromosomes each).

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3. Many genes come in several different versions called alleles. The particular set of alleles a child receives (one from each parent) is called the genotype. Sometimes one allele is dominant over the other recessive allele, and the child’s phenotype (the expressed traits) will not reflect the presence of the recessive allele.

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4. Let’s consider the genetic disease called sickle-cell anemia, one of the rare traits that is controlled by a single gene pair. If we use an uppercase “S” to refer to the dominant “healthy” allele and a lowercase “s” to refer to the recessive allele, this illustration shows the possible combinations of alleles from the mother and father making up the child’s genotype. Only if the child receives the recessive allele from both parents will the child’s phenotype show the disease.

Practice 1: Principles of Genetic Inheritance

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Practice 1: Principles of Genetic Inheritance

Each of the large letters represents one of the possible alleles at this gene location. Roll over each letter to view the dominant-recessive pattern of that allele.

To review the principles of genetic inheritance, we will consider human blood types within the ABO blood group system: another rare example of an important trait controlled by a single gene pair. This gene comes in three versions, or alleles, named A, B, and O. Some of the alleles display the familiar dominant or recessive outcomes when paired, but one particular pair displays an unusual pattern called codominance.

A
B
O
Dominant-recessive pattern:

A is dominant to O but codominant with B.

B is dominant to O but codominant with A.

O is recessive to both A and B.

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Practice 2: Predicting Phenotypes

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Practice 2: Predicting Phenotypes

Roll over each genotype combination to view the associated phenotype.

Given the dominant-recessive pattern of alleles for blood groups, what phenotypes (blood type) can you predict from the genotypes: the various combinations of alleles?

Letter pairs represent the genotype of the child Father’s contribution
A B O
Mother’s contribution A
AA
AB
AO
B
AB
BB
BO
O
AO
BO
OO
Associated phenotype:

Genotype: AA ➔ Phenotype: Type A blood

Genotype: AB ➔ Phenotype: Type AB blood

Genotype: AO ➔ Phenotype: Type A blood

Genotype: AB ➔ Phenotype: Type AB blood

Genotype: BB ➔ Phenotype: Type B blood

Genotype: BO ➔ Phenotype: Type B blood

Genotype: AO ➔ Phenotype: Type A blood

Genotype: BO ➔ Phenotype: Type B blood

Genotype: OO ➔ Phenotype: Type O blood

Quiz 1

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Quiz 1

Match the terms to their descriptions by dragging each colored circle to the appropriate gray circle. When all the circles have been placed, select the CHECK ANSWER button.

Select the NEXT button and move to Quiz 2.
Perhaps you should go back to review factors involved in genetic inheritance.
allele
chromosome
DNA
gene
genotype
phenotype
a tightly wound strand of DNA
the particular alleles that a child receives
one version of a gene that can take different forms
a special, large molecule that carries a cell’s genetic code
the traits that are expressed by a particular genotype
a segment of a DNA molecule that can synthesize a protein

Quiz 2

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Quiz 2

Given the genotypes and phenotypes of the parents, match each child to the correct genetic outcome by dragging each genotype and phenotype to its proper location next to one of the children. When all the genotype and phenotype labels have been placed, select the CHECK ANSWER button.

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Genotypes:

OO
AO
BO
AB
Blood type A
Blood type B
Blood type AB
Blood type O

Phenotypes:

Conclusion

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