Chapter 14 Critical Thinking

A true-breeding purple plant is crossed with a true-breeding white plant. What are the parental genotypes?

PP × pp.

What genotype will every F1 offspring have in PP × pp?

Pp.

What phenotype will every F1 offspring have in PP × pp?

Purple.

What cross produces the classic 3:1 phenotypic ratio?

Pp × Pp.

What cross produces the classic 1:2:1 genotypic ratio?

Pp × Pp.

A purple-flowered plant has an unknown genotype. What cross determines whether it is PP or Pp?

A testcross with pp.

What outcome from a testcross proves a purple plant is heterozygous?

Any white offspring.

What outcome from PP × pp identifies the purple allele as dominant?

All F1 offspring are purple.

Why does the reappearance of white flowers in F2 disprove blending inheritance?

The white allele remained intact and was not permanently mixed away.

What are the possible genotypes of a plant with a dominant phenotype?

Homozygous dominant or heterozygous.

What is the probability of a recessive offspring from Pp × Pp?

1/4.

What is the probability of a heterozygous offspring from Pp × Pp?

1/2.

What is the probability of a dominant-phenotype offspring from Pp × Pp?

3/4.

What is the probability of two recessive offspring in a row from Pp × Pp?

1/16.

What is the probability of three dominant-phenotype offspring in a row from Pp × Pp?

27/64.

What rule is used to calculate the probability of three dominant offspring in a row?

The multiplication rule.

What rule is used to calculate the probability of a heterozygote from Rr × Rr?

The addition rule.

Why is the probability of Rr from Rr × Rr equal to 1/2?

Rr and rR are two separate ways to make a heterozygote.

What gametes can an individual with genotype YyRr produce?

YR, Yr, yR, and yr.

What is the probability of yyrr from YyRr × YyRr?

1/16.

What is the probability of Y_R_ from YyRr × YyRr?

9/16.

What is the probability of yyR_ from YyRr × YyRr?

3/16.

What is the probability of Y_rr from YyRr × YyRr?

3/16.

What is the probability of at least one recessive phenotype from YyRr × YyRr?

7/16.

What is the probability of exactly one recessive phenotype from YyRr × YyRr?

6/16.

What phenotypic ratio is expected if two genes assort independently in a dihybrid cross?

9:3:3:1.

Why would a dihybrid cross fail to produce a 9:3:3:1 ratio?

The genes may be linked or another inheritance pattern may be involved.

A heterozygous YyRr individual produces only YR and yr gametes. What is the most likely explanation?

The genes are linked and no crossing over occurred between them.

Which meiotic event gives each gamete one allele for each gene?

Segregation of homologous chromosomes in meiosis I.

Which meiotic event creates different combinations of maternal and paternal chromosomes?

Independent assortment in metaphase I.

What is the probability of girl, boy, girl, boy in that exact order?

1/16.

What is the probability of two girls and two boys in any order?

6/16, or 3/8.

What probability rule explains why two girls and two boys in any order equals 6/16?

The addition rule applied to six equally likely birth orders.

Why is incomplete dominance not true blending inheritance?

Alleles remain distinct and reappear unchanged in later generations.

What phenotype ratio results from CRCW × CRCW snapdragons?

1 red : 2 pink : 1 white.

What genotype ratio results from CRCW × CRCW snapdragons?

1 CRCR : 2 CRCW : 1 CWCW.

How does an incomplete-dominance cross differ from complete dominance in phenotype ratios?

The phenotype ratio matches the 1:2:1 genotype ratio.

How does codominance differ from incomplete dominance?

Codominance expresses both phenotypes, while incomplete dominance produces an intermediate phenotype.

What blood types can result from IAi × IBi?

Type A, type B, type AB, and type O.

What is the probability of type O blood from IAi × IBi?

1/4.

What is the probability of type AB blood from IAi × IBi?

1/4.

What blood types can result from IAIB × ii?

Type A and type B only.

Why can IAIB × ii never produce type O offspring?

The AB parent has no i allele to contribute.

What blood types can result from IAIB × IAIB?

Type A, type AB, and type B.

Why can two type O parents never have a type AB child?

Both parents are ii and can contribute only i alleles.

What parental genotypes are required for a type O child from a type A and type B couple?

IAi × IBi.

Why can a dominant allele be rare in a population?

Dominance does not determine how common an allele is.

What Labrador ratio results from BbEe × BbEe?

9 black : 3 brown : 4 yellow.

Why are there four yellow Labrador offspring instead of three?

Both B_ee and bbee puppies are yellow because ee prevents pigment deposition.

Which genotype class is epistatic in the Labrador coat-color example?

ee.

What is the probability of a yellow puppy from BbEe × BbEe?

4/16, or 1/4.

What is the probability of a brown Labrador from BbEe × BbEe?

3/16.

What is the probability of a black Labrador from BbEe × BbEe?

9/16.

Why are intermediate phenotypes most common in polygenic inheritance?

Many allele combinations produce intermediate numbers of contributing alleles.

What distribution is expected from AaBbCc × AaBbCc for a six-allele polygenic trait?

1:6:15:20:15:6:1.

What phenotype is least common in a six-allele polygenic trait?

Either extreme phenotype.

Why can individuals with the same genotype show different phenotypes?

Environmental conditions can alter gene expression and phenotype.

Why can hydrangeas with similar genotypes have different flower colors?

Soil chemistry changes pigment expression.

Two unaffected parents have an affected child for a rare trait. What inheritance pattern is most likely?

Recessive.

Two affected parents have only unaffected children for a trait. What inheritance pattern is most likely?

Dominant, if both parents are heterozygous.

Why can a recessive trait appear to skip generations?

Carriers can pass on the allele without showing the phenotype.

Why do dominant traits often appear in every generation?

Affected individuals usually have an affected parent.

What genotype must an affected individual have for a recessive disease?

Homozygous recessive.

What genotype does a phenotypically normal carrier have for a recessive disease?

Heterozygous.

What is the probability that two cystic-fibrosis carriers have an affected child?

1/4.

An unaffected person has a sibling with cystic fibrosis. What is that person's probability of being a carrier?

2/3.

Two unaffected people each have a sibling with cystic fibrosis. What is the probability that their child has cystic fibrosis?

1/9.

Why is the probability in the previous question 1/9 rather than 1/4?

Each parent has only a 2/3 chance of being a carrier.

If one parent is confirmed to be a cystic-fibrosis carrier and the other is confirmed not to be a carrier, what is the chance of an affected child?

0.

Why can Huntington's disease remain in families even though it is fatal?

Symptoms often appear after individuals have reproduced.

Why is genetic counseling based heavily on probability?

Genotypes and inheritance outcomes are uncertain but mathematically predictable.

What prenatal test removes amniotic fluid for fetal-cell analysis?

Amniocentesis.

What prenatal test removes chorionic villi from placental tissue?

Chorionic villus sampling.

Why can fetal genetic testing help families without changing the fetus's genotype?

It provides information for medical and reproductive decisions.