Ch.15 Chromosome Inheritance

Question 1

explain how the observations of cytologists and geneticists provided the basis for chromosome theory of inheritance

Answer 1

the behavior of homologous chromosomes during meiosis accounts for segregation of alleles at each genetic locus to different gametes, while the behaviour of nonhomologous chromosomes account for independent assortment of alleles of two or more genes located on different chromosome

Question 2

Explain why Drosophila melanogaster is a good experimental organism for genetic studies.

Answer 2

Drosophila genes could be mapped easily to investigate genetic transmission

Question 3

Describe how sex is genetically determined in humans and explain the significance of the SRY

Answer 3

Sex is determined by the Y chromosome. The Y chromosome contains male genes. SRY gene forms the gonads into testis, and if its missing they turn into ovaries

Question 4

Explain why sex-linked diseases are more common in human males

Answer 4

Males, who have only a single copy of the X chromosome, are more likely to be affected by a sex-linked disorder than females, who have two copies

Question 5

Describe the inheritance patterns and symptoms of color blindness, Duchenne muscular dystrophy, and hemophilia

Answer 5

Color blindness; sex-linked recessive
Some types: red, green, and red-green
Difficulty distinguishing reds and/or greens
Duchenne muscular dystrophy; sex-linked recessive
Progressive weakening of muscles
Rarely live past early 20s
Lack the protein dystrophin, a link between the cytoskeleton and cell membrane
Hemophilia; sex-linked recessive
Lack one of the blood proteins required for clotting
Controlled by injections of recombinant clotting factor

Question 6

Describe the process of X inactivation in female mammals. Explain how this phenomenon produces the tortoiseshell coloration in cats

Answer 6

X inactivation
Single active copy of the X chromosome is required
In females: one remains condensed, the Barr body
Inactivation involves DNA methylation
Inactivation is random
Tortoiseshell example
X-linked heterozygous genes randomly turned off
Black allele on one X; orange allele on the other
Half of the cells have inactivated black, the other orange

Question 7

Define and compare linked genes and sex-linked genes. Explain why the inheritance of linked genes is different from independent assortment.

Answer 7

Comparison
Linked genes are located on the same chromosome
Sex-linked are on the X or Y chromosome
Genes on the same chromosome tend to separate together, not randomly

Question 8

Describe the independent assortment of chromosomes during Meiosis I. Explain how independent assortment of chromosomes produces genetic recombination of unlinked genes

Answer 8

homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other

Question 9

Distinguish between parental and recombinant phenotypes

Answer 9

Parental phenotypes are offspring that have the same phenotype as one or both of the parents. Recombinant phenotypes are offspring whose phenotypes differ from both parent

Question 10

Explain why linked genes do not assort independently. Explain how crossing over can unlink genes.

Answer 10

Because linked genes are within the same chromosome, for independent assortment to occur, the genes must be on different chromosomes.

when crossing over occurs, a portion of one chromosome is replaced by a region of a homologous chromosome. This could cause the alleles that were presently on a linked gene to be separated

Question 11

Explain how Sturtevant created linkage maps. Define a map unit

Answer 11

he computed the distance in an arbitrary unit he called the “map unit,” which represented a recombination frequency of 0.01, or 1%

Question 12

explain why Mendel did not find linkage between seed color and flower color, despite the fact that these genes are on the same chromosome.

Answer 12

although they were on the same chromosome, the distance between the genes was too great so that the linkage is no observed in genetic crosses. They also behaved as if they were located on different chromosomes during Mendel’s experiment

Question 13

Explain how genetic maps are constructed for genes located far apart on a chromosome.

Answer 13

Gaps are filled in with newly discovered genes
Overlapping mappable segments are connected

Question 14

explain the impact of multiple crossovers between loci.

Answer 14

Multiple crossovers cause underestimation of distance

Question 15

explain what additional information cytological maps provide over linkage maps.

Answer 15

Crossover frequencies are not uniform along the chromosome
Cytological maps locate genes with respect to physical features

Question 16

Explain how nondisjunction can lead to aneuploidy.

Answer 16

Nondisjunction
Failure of homologous chromosomes to separate properly singly or complete spindle or cytokinetic failure
Each chromosome has many genes, pleiotropic effects
Altered ratios of genes disrupt function
Types
Aneuploidy: abnormal chromosome number
Trisomy: one extra chromosome; three of one type
Monosomy: one missing chromosome; only one of one type Triploidy: three sets of chromosomes; 3N
Polyploidy: many sets of chromosomes; common in plants

Question 17

Explain how nondisjunction can lead to aneuploidy.

Answer 17

Chromosomal mutations all result from breakage and rejoining
Many genes may be involved; ratio and pleiotropic effects
Points of breakage and rejoining may be damaged; mutation
Types
Deletions: loss of a section of a chromosome
Duplications: doubling of a section
Inversions: flipping over a section
Translocations: a piece of a chromosome becomes attached to a non-homologous chromosome

Question 18

Define trisomy, triploidy, and polyploidy. Explain how these major chromosomal changes occur and describe possible consequences

Answer 18

TRISOMY—A chromosome is present in triplicate in the zygote (2n+1 chromosomes)
i. Mitosis will transmit the anomaly to all embryonic cells, producing a set of traits caused by the abnormal dose of genes associated with the extra or missing chromosome (Down syndrome)
ii. Occurs from nondisjunction

TRIPLOIDY—Organism that has 3 complete chromosome sets in all somatic cells.
i. Occurs from the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes.

POLYPLOIDY—Organism that has 3 or more complete chromosome sets in all somatic cells
i. Typically not as extreme as aneuploidy; one extra (or missing) chromosome affects the organism more than an entire set.

Question 19

Describe the type of chromosomal alterations implicated in the following human disorders: Down syndrome, Klinefelter’s syndrome, extra Y, triple-X syndrome, Turner syndrome, cri du chat syndrome, and chronic myelogenous leukemia.

Answer 19

Down syndrome: trisomy 21
Klinefelter’s syndrome: XXY
Extra Y: XYY
Triple-X syndrome: XXX; two Barr bodies
Turner syndrome: XO
Cri du chat syndrome: deletion from chromosome 5
Chronic myelogenous leukemia: 22 è 9 translocation

Question 20

Define genomic imprinting. Describe the evidence that suggests that the Igf2 gene is maternally imprinted

Answer 20

Genomic imprinting is when variation in phenotype is dependent on whether an allele is inherited from the male or female parent. THe evidence to show that it is paternally imprinted is the methylation (addition of methyl groups to cytosine nucleotides of one of the alleles) of certain cytosines on the paternal chromosome

Question 21

Explain why extranuclear genes are not inherited in a Mendelian fashion

Answer 21

Extranuclear genes (or cytoplasmic genes) are genes found in organelles in the cytoplasm, mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules. Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg