Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics

Question 1

the inheritance of characters determined by a single gene deviates from simple Mendelian patterns when alleles are not completely ____________________, when a particular gene has more than two alleles, or when a single gene produces multiple phenotypes

Answer 1

dominant or recessive

Question 2

the situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable

Answer 2

complete dominance

Question 3

For some genes, however, neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties. This phenomenon, called ____________________ is seen when red snapdragons are crossed with white snapdragons:

Answer 3

incomplete dominance

Question 4

This third, intermediate phenotype results from flowers of the heterozygotes having less red pigment than the red

Answer 4

homozygotes.

Question 5

figure 14.10 incomplete dominance in snapdragon color, animation incomplete dominance in “MendAliens”

Question 6

At first glance, incomplete dominance of either allele seems to provide evidence for the blending hypothesis of inheritance, which would predict that the red or white trait could never reappear among offspring of the pink hybrids. In fact, interbreeding F1 hybrids produces F2 offspring with a phenotypic ratio of

Answer 6

one red to two pink to one white.

Question 7

The segregation of the red-flower and white-flower alleles in the gametes produced by the pink-flowered plants confirms that the alleles for flower color are heritable factors that maintain their identity in the hybrids; that is,

Answer 7

inheritance is particulate.

Question 8

Another variation on dominance relationships between alleles is called __________________ ; the two alleles each affect the phenotype in separate, distinguishable ways.

Answer 8

codominance

Question 9

For example, the human MN blood group is determined by codominant alleles for two specific molecules located on the surface of red blood cells, the

Answer 9

M and N molecules.

Question 10

Individuals homozygous for the M allele (MM) have red blood cells with only M molecules; individuals homozygous for the N allele (NN) have red blood cells with only

Answer 10

N molecules

Question 11

But _______ M and N molecules are present on the red blood cells of individuals heterozygous for the M and N alleles (MN)

Answer 11

both

Question 12

It is important to understand that an allele is called _______________ because it is seen in the phenotype, not because it somehow subdues a recessive allele.

Answer 12

dominant

Question 13

Alleles are simply variations in a gene’s

Answer 13

nucleotide sequence

Question 14

a human genetic disease caused by recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain.

Answer 14

Tay-Sachs disease

Question 15

Tay-Sachs disease can lead to

Answer 15

seizures, blindness, and degeneration of motor and mental performance

Question 16

Only children who inherit _____________ of the Tay-Sachs allele (homozygotes) have the disease

Answer 16

two copies

Question 17

Thus, at the organismal level, the Tay-Sachs allele qualifies as

Answer 17

recessive

Question 18

Extending our analysis to yet another level, we find that heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. Thus, at the molecular level, the normal allele and the Tay-Sachs allele are

Answer 18

codominant.

Question 19

an example of a rare dominant allele, about one baby out of 400 in the United States is born with extra fingers or toes, a condition known as

Answer 19

polydactyly

Question 20

The low frequency of polydactyly indicates that the recessive allele, which results in five digits per appendage, is far more prevalent than the

Answer 20

dominant allele in the population.

Question 21

Only two alleles exist for the pea characters that Mendel studied, but most genes exist in more than

Answer 21

two allelic forms.

Question 22

The ABO blood groups in humans, for instance, are determined by that person’s two alleles of the blood group gene; there are three possible alleles:

Answer 22

Iּּᴬ , Iּּᴮ and i

Question 23

A person’s blood group may be one of four types:

Answer 23

A, B, AB, or O.

Question 24

These letters refer to two _________________—A and B—that may be found attached to specific cell-surface molecules on red blood cells.

Answer 24

carbohydrates

Question 25

An individual’s blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or ___________________, as shown in Figure 14.11

Answer 25

neither (type O)

Question 26

figure 14.11

Question 27

the ability of a single gene to have multiple effects

Answer 27

pleiotropy

Question 28

In humans, for example, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as

Answer 28

cystic fibrosis and sickle-cell disease

Question 29

Dominance relationships, multiple alleles, and pleiotropy all have to do with the effects of the alleles of a

Answer 29

single gene.

Question 30

We now consider two situations in which two or more genes are involved in determining a particular phenotype. In the first case, called ______________, one gene affects the phenotype of another because the two gene products interact in the second case, called ____________________, multiple genes independently affect a single trait.

Answer 30

epistasis, polygenic inheritance

Question 31

the phenotypic expression of a gene at one locus alters that of a gene at a second locus.

Answer 31

epistasis

Question 32

In Labrador retrievers (commonly called “Labs”), black coat color is dominant to brown. Let’s designate B and b as the two alleles for this character. For a Lab to have brown fur, its genotype must be bb; these dogs are called

Answer 32

chocolate Labs.

Question 33

A second gene determines whether or not pigment will be deposited in the hair. The dominant allele, symbolized by E, results in the deposition of either black or brown pigment, depending on the genotype at the first locus. But if the Lab is homozygous recessive for the second locus (ee), then the coat is

Answer 33

yellow, regardless of the genotype at the black/brown locus (yellow Labs).

Question 34

In this case, the gene for pigment deposition (E/e) is said to be epistatic to the gene that codes for

Answer 34

black or brown pigment (B/b) .

Question 35

What happens if we mate black Labs that are heterozygous for both genes (BbEe)? Although the two genes affect the same phenotypic character (coat color), they follow the

Answer 35

law of independent assortment

Question 36

figure 14.12 an example of epistasis

Question 37

Mendel studied characters that could be classified on an either-or basis, such as purple versus white flower color. But many characters, such as human skin color and height, are not one of two discrete characters, but instead vary in the population in gradations along a continuum. These are called

Answer 37

quantitative characters

Question 38

Quantitative variation usually indicates __________________________________, an additive effect of two or more genes on a single phenotypic character.

Answer 38

polygenic inheritance

Question 39

In 2014, a genomic study of over 250,000 people found almost 700 genetic variations associated with over

Answer 39

180 genes that affect height

Question 40

Skin pigmentation in humans is also controlled by many separately

Answer 40

inherited genes

Question 41

figure 14.13 a simplified model for polygenic inheritance of skin color, Biointeractive video, scientific skills exercise

Question 42

Another departure from simple Mendelian genetics arises when the phenotype for a character depends on

Answer 42

environment as well as genotype

Question 43

Other characters, such as a person’s blood count of red and white cells, vary quite a bit, depending on such factors as the

Answer 43

altitude, the customary level of physical activity, and the presence of infectious agents.

Question 44

figure 14.14 the effect of environment on phenotype

Question 45

many factors, both genetic and environmental, collectively influence phenotype.

Answer 45

multifactorial

Question 46

bbc video Genetics vs Environment

Question 47

The term phenotype can refer not only to specific characters, such as flower color and blood group, but also to an organism in its entirety—all aspects of its

Answer 47

physical appearance, internal anatomy, physiology, and behavior.

Question 48

similarly, the term genotype can refer to an organism’s entire genetic makeup, not just its alleles for a single

Answer 48

genetic locus