Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics Flashcards
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
dominant or recessive
the situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable
complete dominance
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:
incomplete dominance
This third, intermediate phenotype results from flowers of the heterozygotes having less red pigment than the red
homozygotes.
figure 14.10 incomplete dominance in snapdragon color, animation incomplete dominance in “MendAliens”
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
one red to two pink to one white.
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,
inheritance is particulate.
Another variation on dominance relationships between alleles is called __________________ ; the two alleles each affect the phenotype in separate, distinguishable ways.
codominance
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
M and N molecules.
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
N molecules
But _______ M and N molecules are present on the red blood cells of individuals heterozygous for the M and N alleles (MN)
both
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.
dominant
Alleles are simply variations in a gene’s
nucleotide sequence
a human genetic disease caused by recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain.
Tay-Sachs disease
Tay-Sachs disease can lead to
seizures, blindness, and degeneration of motor and mental performance
Only children who inherit _____________ of the Tay-Sachs allele (homozygotes) have the disease
two copies
Thus, at the organismal level, the Tay-Sachs allele qualifies as
recessive
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
codominant.
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
polydactyly
The low frequency of polydactyly indicates that the recessive allele, which results in five digits per appendage, is far more prevalent than the
dominant allele in the population.
Only two alleles exist for the pea characters that Mendel studied, but most genes exist in more than
two allelic forms.
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:
Iּּᴬ , Iּּᴮ and i
A person’s blood group may be one of four types:
A, B, AB, or O.
These letters refer to two _________________—A and B—that may be found attached to specific cell-surface molecules on red blood cells.
carbohydrates
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
neither (type O)
figure 14.11
the ability of a single gene to have multiple effects
pleiotropy
In humans, for example, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as
cystic fibrosis and sickle-cell disease
Dominance relationships, multiple alleles, and pleiotropy all have to do with the effects of the alleles of a
single gene.
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.
epistasis, polygenic inheritance
the phenotypic expression of a gene at one locus alters that of a gene at a second locus.
epistasis
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
chocolate Labs.
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
yellow, regardless of the genotype at the black/brown locus (yellow Labs).
In this case, the gene for pigment deposition (E/e) is said to be epistatic to the gene that codes for
black or brown pigment (B/b) .
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
law of independent assortment
figure 14.12 an example of epistasis
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
quantitative characters
Quantitative variation usually indicates __________________________________, an additive effect of two or more genes on a single phenotypic character.
polygenic inheritance
In 2014, a genomic study of over 250,000 people found almost 700 genetic variations associated with over
180 genes that affect height
Skin pigmentation in humans is also controlled by many separately
inherited genes
figure 14.13 a simplified model for polygenic inheritance of skin color, Biointeractive video, scientific skills exercise
Another departure from simple Mendelian genetics arises when the phenotype for a character depends on
environment as well as genotype
Other characters, such as a person’s blood count of red and white cells, vary quite a bit, depending on such factors as the
altitude, the customary level of physical activity, and the presence of infectious agents.
figure 14.14 the effect of environment on phenotype
many factors, both genetic and environmental, collectively influence phenotype.
multifactorial
bbc video Genetics vs Environment
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
physical appearance, internal anatomy, physiology, and behavior.
similarly, the term genotype can refer to an organism’s entire genetic makeup, not just its alleles for a single
genetic locus
