Chromosomal Basis of Inheritence

Imagine crossing a pea heterozygous at the loci for flower color

(white versus purple) and seed color (yellow versus green) with

a second pea homozygous for flower color (white) and seed

color (yellow). What types of gametes will the first pea

produce?

A. two gamete types: white/white and purple/purple

B. two gamete types: white/yellow and purple/green

C. four gamete types: white/yellow, white/green, purple/yellow, and

purple/green

D. four gamete types: white/purple, yellow/green, white/white, and

purple/purple

E. one gamete type: white/purple/yellow/green

C. four gamete types: white/yellow, white/green, purple/yellow, and

purple/green

Pea plants were particularly well suited for use in

Mendel's breeding experiments for all of the following

reasons except that

A. peas show easily observed variations in a number of characters, such

as pea shape and flower color.

B. it is possible to control matings between different pea plants.

C. it is possible to obtain large numbers of progeny from any given cross.

D. peas have an unusually long generation time.

E. many of the observable characters that vary in pea plants are controlled by single genes.

D. peas have an unusually long generation time.

A cross between homozygous purple-flowered and

homozygous white-flowered pea plants results in

offspring with purple flowers. This demonstrates

A. the blending model of genetics.

B. true breeding.

C. dominance.

D. a dihybrid cross.

E. the mistakes made by Mendel.

C. dominance.

Imagine a genetic counselor working with a couple who have just had a

child who is suffering from Tay-Sachs disease. Neither parent has TaySachs,

nor does anyone in their families. Which of the following

statements should this counselor make to this couple?

A. "Because no one in either of your families has Tay-Sachs, you are not likely to have another

baby with Tay-Sachs. You can safely have another child."

B. "Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you

have has a 50% chance of having the disease."

C. "Because you have had one child with Tay-Sachs, you must each carry the allele. Any child

you have has a 25% chance of having the disease."

D. "Because you have had one child with Tay-Sachs, you must both carry the allele. However, since

the chance of having an affected child is 25%, you may safely have thee more children without

worrying about having another child with Tay-Sachs."

E. "You must both be tested to see who is a carrier of the Tay-Sachs allele."

C. "Because you have had one child with Tay-Sachs, you must each carry the allele. Any child you have has a 25% chance of having the disease."

Albinism in humans occurs when both alleles at a locus produce

defective enzymes in the biochemical pathway leading to melanin. Given

that heterozygotes are normally pigmented, which of the following

statements is/are correct?

A. One normal allele produces as much melanin as two normal alleles.

B. Each defective allele produces a little bit of melanin.

C. Two normal alleles are needed for normal melanin production.

D. The two alleles are codominant.

E. The amount of sunlight will not affect skin color of heterozygotes.

A. One normal allele produces as much melanin as two normal alleles

Imagine that the last step in a biochemical pathway to the red skin pigment of an

apple is catalyzed by enzyme X, which changes compound C to compound D. If an

effective enzyme is present, compound D is formed and the apple skin is red.

However, if the enzyme is not effective, only compound C is present and the skin is

yellow. Thinking about enzyme action, what can you accurately say about a

heterozygote with one allele for an effective enzyme X and one allele for an

ineffective enzyme X?

• The phenotype will probably be yellow

but cannot be red.

• The phenotype will probably be red

but cannot be yellow.

• The phenotype will be a yellowish red.

• The phenotype will be either yellow or red.

• The phenotype will be either yellowish

red or red

The phenotype will be either yellowish

red or red.

In humans, alleles for dark hair are genetically

dominant, while alleles for light hair are recessive.

Which of the following statements is/are most likely to

be correct?

A. Dark hair alleles are more common than light hair alleles in all areas of Europe.

B. Dark hair alleles are more common than light hair alleles in southern Europe

but not in northern Europe.

C. Dark hair alleles are equally common in all parts of Europe.

D. Dark hair is dominant to light hair in southern Europe but recessive to light hair

in northern Europe.

E. Dark hair is dominant to light hair in northern Europe but recessive to light hair

in southern Europe.

B. Dark hair alleles are more common than light hair alleles in southern Europe

but not in northern Europe.

Imagine a locus with four different alleles for fur color in an animal. The alleles

are named Da

, Db

, Dc

, and Dd

. If you crossed two heterozygotes, DaDb and DcDd

,

what genotype proportions would you expect in the offspring?

• 25% DaDc, 25% DaDd, 25% DbDc, 25% DbDd

• 50% DaDb, 50% DcDd

• 25% DaDa, 25% DbDb, 25% DcDc, 25% DdDdDcDd

• 50% DaDc, 50% DbDd

• 25% DaDb, 25% DcDd, 25% DcDc, 25% DdDd

• 25% DaDc, 25% DaDd, 25% DbDc, 25% DbD

Envision a family in which the grandfather, age 47, has just

been diagnosed with Huntington's disease, which is caused by

a dominant allele (and the father is a heterozygote). His

daughter, age 25, now has a 2-year-old son. No one else in the

family has the disease. What is the probability that the

daughter will contract the disease?

A. 0%

B. 25%

C. 50%

D. 75%

E. 100%

C. 50%

Review the family described in the previous

question. What is the probability that the baby will

contract the disease?

A. 0%

B. 25%

C. 50%

D. 75%

E. 100%

B. 25%

When a disease is said to have a multifactorial

basis, it means that

• both genetic and environmental factors contribute to the disease.

• it is caused by a gene with a large number of alleles.

• it affects a large number of people.

• it has many different symptoms.

• it tends to skip a generation.

both genetic and environmental factors contribute to the disease.

• Mendel's "hereditary factors" were...

genes:

• Today we know that genes are located on chromosomes

chromosome theory of inheritance

-Mendelian genes have specific loci (positions) on chromosomes

-Chromosomes undergo segregation and independent assortment

• The behavior of chromosomes during meiosis can account for Mendel's laws of segregation and independent assortment

Thomas Hunt Morgan

began with model

organism, Drosophila melanogaster, in 1907

• Morgan noted wild-type, or normal, phenotypes

• Traits alternative to the wild type are called mutant phenotypes

Thomas Hunt Morgan proposed:

• gene for white eye color in fruit flies resides on the X chromosome and that

the Y chromosome does not carry an allele for this gene.

• This is the hypothesis of X-linked inheritance (or X-linkage). Females would then have two copies of the gene and males would have only one

In humans and other mammals, there are two varieties of sex

chromosomes:

a larger X chromosome and a smaller Y chromosome

• Only the ends of the Y chromosome have regions that are

homologous with corresponding regions of the X chromosome

• The SRY gene on the Y chromosome is required for the developments

of testes

sexlinked

gene

A gene that is located on either sex chromosome

Y-linked genes

Genes on the Y chromosome, Few Y linked genes; mainly encodes genes related to sex determination

X-linked genes

Genes on the X chromosome

• have genes for many characters unrelated to sex

• X-linked genes follow specific patterns of inheritance

• For a recessive X-linked trait to be expressed

• A female needs two copies of the allele (homozygous)

• A male needs only one copy of the allele (hemizygous)

• X-linked recessive disorders are much more common in males than in

females

• If a trait appears equally often in males and females, it is likely to be

autosomal.

If males are much more likely to have the trait, it is usually

X-linked

• Ex. Hemophilia- X-linked trait resulting from a recessive allele

Barr body

• In mammalian females, one of the two X chromosomes in each cell is

randomly inactivated during embryonic development

• The inactive X condenses into a Barr body

mosaic

• If a female is heterozygous for a particular gene located on the X chromosome,

she will be a mosaic for that character

linked genes

Each chromosome has hundreds or thousands of genes (except the Y

chromosome)

• Genes located on the same chromosome that tend to be inherited together

are called linked genes

Recombination

occurs when the combination of alleles on the chromosomes

of progeny is different from the combinations of alleles present in the

parental generation

parental types

Offspring with a phenotype matching one of the parental phenotypes

recombinant types, or recombinants

Offspring with nonparental phenotypes (new combinations of traits)

Mendel observed that combinations of traits in some offspring differ from either parent

these were recombinants

genetic map

Alfred Sturtevant, one of Morgan's students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome

Alfred Sturtevant proposed that

the farther apart two genes are, the higher the probability

that a crossover will occur between them and therefore the higher the

recombination frequency

• Distances between genes can be expressed as map units; one map unit represents a

1% recombination frequency

Genes that are far apart on the same chromosome can have a recombination frequency near

50%

• Such genes are physically linked, but genetically unlinked, and behave

as if found on different chromosomes

Cytogenetic maps

indicate the positions of genes with respect to chromosomal features

Using methods like chromosomal banding, geneticists can develop

cytogenetic maps of chromosomes

Large-scale chromosomal alterations in humans and other mammals often lead to

spontaneous abortions (miscarriages) or cause a variety of

developmental disorders

• Plants tolerate such genetic changes better than animals do

Breakage of a chromosome can lead to four types of changes in chromosome

structure

• Deletion

• Duplication

• Inversion

• Translocation

translocation

moves a segment from one chromosome to another

inversion

reverses orientation of a segment within a chromosome

duplication

repeats a segment

deletion

removes a chromosomal segment