Chapter 9

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Mendel conducted his most memorable experiments on


Varieties of plants in which self-fertilization produces offspring that are identical to the parents are referred to as


CHARACTER is an inherited feature that varies from individual to individual.


TRAIT is one particular variation of a character


Genetic cross involving parents that differ in a single character is called a MONOHYBRID CROSS.


Most human genes come in alternate versions called ALLELES.


If an organism has 2 non-identical version of a gene, the one that is expressed in the organism is called DOMINANT allele.


If an organism has 2 non-identical versions of a gene, the one that is not expressed in the organism is called the RECESSIVE allele.


The physical traits of an organism is called its PHENOTYPE.


The GENOTYPE is the genetic makeup of an organism.


In a test cross, F2 refers to _____.

the second filial generation, or the "grandchildren" of the original mating pair

Imagine a particular trait that is determined by a single gene. If this gene is present in two forms, how can you tell which allele is dominant and which is recessive?
Imagine a particular trait that is determined by a single gene. If this gene is present in two forms, how can you tell which allele is dominant and which is recessive?

Do a test cross between two true-breeding individuals and observe the trait expressed by the F1 plants.

Mendel was a meticulous experimentalist. One set of crosses he performed to test his idea that a pair of hereditary determinants segregated into gametes was to allow self-fertilization of F2 individuals to produce F3 offspring. What proportion of the purple-flowered F2 individuals did Mendel predict to be true-breeding?


An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype _____.


Research since Mendel's time has established that the law of the segregation of genes during gamete formation

applies to all sexually reproducing organisms.

All the offspring of a cross between a black-eyed mendelien and an orange-eyed mendelien have black eyes. This means that the allele for black eyes is ________ the allele for orange eyes.

dominant to

All the offspring of a cross between a black-eyed mendelien and an orange-eyed mendelien have black eyes. What is the expected phenotypic ratio of a cross between two orange-eyed mendeliens?

0 black-eyed:1 orange-eyed

Assume that in cattle a spotted coat is dominant to an even coat, short horns are dominant to long horns, and the traits for coat type and horn length assort independently. In a cross between cattle that are each heterozygous for both traits, what proportion of their offspring are expected to have long horns?


The phenotypic ratio resulting from a dihybrid cross showing independent assortment is expected to be


If A is dominant to a and B is dominant to b, what is the expected phenotypic ratio of the cross: AaBb × AaBb?


Mendel's law of independent assortment states that

each pair of alleles segregates independently of the other pairs of alleles during gamete formation

Imagine that we mate two black Labrador dogs with normal vision and find that three of the puppies are like the parents, but one puppy is chocolate with normal vision and another is black with PRA (progressive retinal atrophy, a serious disease of vision). We can conclude that

the alleles for color and vision segregate independently during gamete formation

Assuming that the probability of having a female child is 50% and the probability of having a male child is also 50%, what is the probability that a couple's first-born child will be female and that their second-born child will be male?


Why are lethal dominant alleles so much more rare than lethal recessive alleles?

Lethal dominant alleles are harmful whether they are carried in homozygous or heterozygous form, so there is always strong selection against these alleles.

A carrier of a genetic disorder who does not show symptoms is most likely to be ________ to transmit it to offspring.

heterozygous for the trait and able

Most people afflicted with recessive disorders are born to parents who were

not affected at all by the disease.

Akin to urban legends, there are curious genetics legends - things like eye color being determined by one gene, with a brown eye allele being completely dominant to blue. The problem comes when simple myth meets the complex reality of how eye color and many other traits are transmitted. Why is the inheritance of so many traits difficult to explain using only Mendel's view of genetics?`

Mendel was correct for the traits he investigated, but his principles must be extended (not discarded) to explain many more complex patterns of inheritance.

Hypercholesterolemia is a disorder in which blood cholesterol levels are elevated. The H allele is incompletely dominant to the h allele, with hh homozygotes having extremely high levels of blood cholesterol. A husband and wife are both Hh heterozygotes. What is the chance that their first child will have normal levels of blood cholesterol?


Snapdragons show incomplete dominance in their flowers. A pink snapdragon is crossed with a red snapdragon. What color(s) are the offspring?

50% red, 50% pink

All the offspring of a cross between a red-flowered plant and a white-flowered plant have pink flowers. This means that the allele for red flowers is ________ to the allele for white flowers.

incompletely dominant

The expression of both alleles for a trait in a heterozygous individual illustrates


A person with AB blood illustrates the principle of


Genes located close together on the same chromosomes are referred to as ________ genes and generally ________.

linked . . . do not sort independently during meiosis

You conduct a dihybrid cross. A ________ ratio would make you suspect that the genes are linked.


Crossing over ________ genes into assortments of ________ not found in the parents.

recombines linked . . . alleles

What is the normal complement of sex chromosomes in a human male?

one X chromosome and one Y chromosome

Assume tall (T) is dominant to dwarf (t). If a homozygous dominant true-breeding individual is crossed with a homozygous true-breeding dwarf, the offspring will _____.

all be tall

If a homozygous dominant (AA) is crossed with a homozygous recessive (aa) for a given character, the offspring will be _____.

all of the dominant phenotype

What is an allele?

an alternative version of a gene

Homologous pairs of chromosomes frequently _____.

contain different alleles

An AABb individual is mated with another AABb individual. The possible number of genetically different kinds of offspring is _____.


Pea plants are tall if they have the genotype TT or Tt, and they are short if they have the genotype tt. A tall plant is crossed with a short plant. Half the offspring are tall, and half are short. This allows us to conclude that the tall plant _____.

was heterozygous

Two identical twins are raised in different environments. They possess _____ genotypes and _____ phenotypes.

identical ... variable

If hair color, eye color, and the presence or absence of freckles were consistently inherited together, the best explanation would be that _____.

the genes for these characters are linked on the same chromosome

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