Chapter 15 Critical Thinking

How does chromosome behavior physically explain Mendel's law of segregation?

Homologous chromosomes separate in meiosis I, separating the two alleles.

How does chromosome behavior physically explain independent assortment?

Each homologous pair aligns independently during metaphase I.

Why does fertilization maintain chromosome number across generations?

It combines two haploid gametes to restore diploidy.

Why can homologous chromosomes carry different alleles while still carrying the same genes?

They have the same loci but may contain different allele versions.

Why did Morgan's white-eye discovery support the chromosome theory?

The trait followed X-chromosome inheritance rather than ordinary autosomal inheritance.

A red-eyed homozygous female fruit fly is crossed with a white-eyed male. What is the female genotype?

Xw+Xw+.

A red-eyed homozygous female fruit fly is crossed with a white-eyed male. What is the male genotype?

XwY.

What are the F1 daughters from Xw+Xw+ × XwY?

Xw+Xw, red-eyed carriers.

What are the F1 sons from Xw+Xw+ × XwY?

Xw+Y, red-eyed.

What phenotype appears in all F1 offspring of Morgan's original cross?

Red eyes.

What cross produces Morgan's F2 generation?

Xw+Xw × Xw+Y.

What is the overall F2 eye-color ratio in Morgan's cross?

3 red-eyed to 1 white-eyed.

Which F2 sex showed white eyes in Morgan's original experiment?

Males only.

Why were white-eyed F2 females absent in Morgan's original cross?

Their father contributed a normal X allele.

What unusual F2 result suggested white eye was X-linked rather than autosomal?

All white-eyed offspring were male.

What would ordinary autosomal recessive inheritance predict for white eyes?

Equal likelihood in males and females.

A carrier female has genotype XNXn and mates with a normal male XNY. What fraction of all children are affected?

1/4.

In XNXn × XNY, what fraction of sons are affected?

1/2.

In XNXn × XNY, what fraction of daughters are affected?

0.

In XNXn × XNY, what fraction of daughters are carriers?

1/2.

In XNXn × XNY, what fraction of all children are carrier daughters?

1/4.

An affected male XnY mates with a homozygous normal female XNXN. What are the daughters?

All are normal carriers, XNXn.

An affected male XnY mates with a homozygous normal female XNXN. What are the sons?

All are normal, XNY.

Why can an affected father not pass an X-linked recessive allele directly to his sons?

He gives sons his Y chromosome, not his X chromosome.

An affected female XnXn mates with a normal male XNY. What fraction of sons are affected?

All sons.

An affected female XnXn mates with a normal male XNY. What fraction of daughters are affected?

None; all are carriers.

Two unaffected parents have a son with Duchenne muscular dystrophy. What is the mother's likely genotype?

XNXn, a carrier.

Two unaffected parents have a son with Duchenne muscular dystrophy. What is the father's genotype?

XNY.

For carrier mother XNXn and normal father XNY, what is the probability the next child has Duchenne muscular dystrophy?

1/4.

For carrier mother XNXn and normal father XNY, what is the probability the next child has Duchenne muscular dystrophy if the child is known to be male?

1/2.

For carrier mother XNXn and normal father XNY, what is the probability the next child has Duchenne muscular dystrophy if the child is known to be female?

0.

Why can a heterozygous female show patches of two X-linked phenotypes?

Random X inactivation creates cell populations with different active X chromosomes.

Why are calico coat patches evidence of X inactivation?

Different skin cells express different active X-linked coat-color alleles.

Why is a male with one X-linked recessive allele affected while a heterozygous female may not be?

The male has no second X allele to mask it.

A testcross produces offspring in a 1:1:1:1 ratio. What does this suggest about the genes?

They are unlinked or assort independently.

A testcross produces many parental types and fewer recombinant types. What does this suggest?

The genes are linked.

Why do linked genes produce more parental than recombinant offspring?

Crossing over occurs only sometimes between linked genes.

What are the parental gametes for an individual with genes arranged AB/ab?

AB and ab.

What are the recombinant gametes for an individual with genes arranged AB/ab?

Ab and aB.

When does crossing over generate recombinant chromatids?

Prophase I of meiosis.

Why does greater distance between two linked genes increase recombination frequency?

Crossovers are more likely to occur between genes that are farther apart.

Why can two genes on the same chromosome appear to assort independently?

They may be far enough apart for recombination frequency to approach 50%.

What does a 50% recombination frequency indicate?

Genes are unlinked or so far apart that they behave as unlinked.

What does a 17% recombination frequency indicate?

The genes are about 17 map units apart.

How is recombination frequency calculated?

Recombinant offspring divided by total offspring times 100.

A testcross has 206 gray-vestigial and 185 black-normal recombinants out of 2,300 offspring. What is the recombination frequency?

17%.

If the recombination frequency is 17%, what is the map distance?

17 map units, or 17 centimorgans.

Why are map units not exact physical distances along DNA?

Recombination frequency varies across chromosome regions.

A linked-gene testcross produces 232 Mn/mN, 240 mN/mN, 15 MN/mn, and 13 mn/mn offspring. Which classes are recombinants?

MN/mn and mn/mn.

What is the map distance for the previous cross?

5.6 map units.

Genes A and B are 35 map units apart, B and C are 10 apart, C and D are 15 apart, C and A are 25 apart, and D and B are 25 apart. What is the gene order?

A-D-C-B.

Why are genes with the highest recombination frequency often placed at opposite ends of a linkage map?

Greater recombination usually indicates greater distance.

What happens if homologous chromosomes fail to separate during meiosis I?

Nondisjunction occurs.

What gametes result from meiosis I nondisjunction?

Two n + 1 gametes and two n − 1 gametes.

What happens if sister chromatids fail to separate during meiosis II?

Nondisjunction occurs in one of the two cells.

What gametes result from meiosis II nondisjunction?

Two normal gametes, one n + 1 gamete, and one n − 1 gamete.

A gamete with n + 1 chromosomes fuses with a normal gamete. What condition results?

Trisomy.

A gamete with n − 1 chromosomes fuses with a normal gamete. What condition results?

Monosomy.

Why is aneuploidy usually more harmful than polyploidy in animals?

It disrupts dosage for particular chromosomes rather than adding balanced full sets.

Why are polyploid plants often more viable than polyploid animals?

Plants tolerate extra complete chromosome sets better.

What chromosome condition causes Down syndrome?

Trisomy 21.

What chromosome condition causes Klinefelter syndrome?

XXY.

What chromosome condition causes Turner syndrome?

X0.

What chromosome event can lead to Down syndrome, Klinefelter syndrome, or Turner syndrome?

Nondisjunction.

A chromosome loses a segment containing several genes. What structural mutation occurred?

Deletion.

A chromosome repeats a segment containing several genes. What structural mutation occurred?

Duplication.

A chromosome segment flips its orientation without leaving the chromosome. What mutation occurred?

Inversion.

A chromosome segment moves to a different chromosome. What mutation occurred?

Translocation.

Why can chromosome deletions be harmful?

Important genes may be lost.

Why can chromosome duplications be harmful?

Gene dosage becomes abnormal.

Why can translocations contribute to cancer?

They can disrupt or misregulate genes involved in cell division.

What chromosome change is associated with cri du chat syndrome?

A deletion on chromosome 5.

What chromosome change is associated with chronic myelogenous leukemia?

A translocation.

A trait is passed by affected mothers to children of both sexes, but affected fathers usually do not pass it on. What inheritance pattern is most likely?

Mitochondrial inheritance.

Why is mitochondrial inheritance usually maternal?

The egg provides the zygote's cytoplasm and mitochondria.

Which organelles besides mitochondria contain their own DNA?

Chloroplasts and other plant plastids.

How does genomic imprinting differ from mitochondrial inheritance?

Imprinting involves parent-specific silencing of nuclear genes, while mitochondrial inheritance involves cytoplasmic organelle genes.

A phenotype depends on whether an allele came from the mother or father even though the DNA sequence is the same. What mechanism is likely involved?

Genomic imprinting.

Why does genomic imprinting not follow standard Mendelian expectations?

One parental allele may be silenced based on parent of origin.

Why can a person inherit one chromosome from each parent but still inherit mitochondrial genes from only one parent?

Nuclear chromosomes are biparental, but mitochondria usually come from the egg.