Biology & Life Sciences Chapter 12 Because The Ween Observed And Expected Values

CHAPTER 12 THE CHROMOSOMAL BASIS OF INHERITANCE

Scientific Skills Exercise

Teaching objective: 2 value for a given data set and determine

whether or not to reject a hypothesis about the data.

Teaching tips: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.

Students often stumble on the step of calculating expected numbers of offspring for each

population of offspring.

recombinant (non-parental) phenotypes: One way is for genes on two different chromosomes to

move independently of each other and the other is the result of crossing over between two linked

generation parents, while a recombinant phenotype is a phenotype that is different from both.

Another way of 2 2 may be used to measure the probability that

this data set could have been obtained by random sampling from a population for which the

hypothesis is correct. If the probability is less than 5%, it is very unlikely that the observed data

could have resulted randomly if the hypothesis were true, so the hypothesis must be rejected.

Some important considerations, mentioned briefly in the exercise, are sample size and the

strength of the linkage (how close the two genes are, if they are linked). With small sample sizes,

only very strong linkages (genes that are very close) will be detectable, while with larger sample

sizes we are capable of detecting even weak linkages. For this reason, we must be cautious with

2 probabilities. We rarely have sufficient evidence to

prove our hypothesis, but rather we simply fail to disprove it. That is, our failure to detect any

Answers:

1.

Offspring from

testcross of AaBb

(F1) x aabb

Purple stem/

short petals

(A-B-)

Green stem/

short petals

(aaB-)

Purple stem/

long petals (A-

bb)

Green stem/

long petals

(aabb)

2.

Testcross offspring Expected

(e)

Observed

(o)

Deviation

(o – e)

(o – e)2

(o – e)2/e

Suggested Answers for End-of-Chapter Essay Questions

See the general information on grading short-answer essays and the suggested rubric at the

beginning of this document.

10. Scientific Inquiry

(a) For each pair of genes, you had to generate an F1 A and B genes as

an example. You obtained homozygous parental flies, either the first with dominant alleles of the

(b)

11. Focus on Evolution

It has been suggested that the regions of internal homology within the Y chromosome may pair

with each other during meiosis I, exchanging genetic information. This exchange could allow the

12. Focus on Information

Sample key points:

A chromosome consists of one DNA molecule, carrying the heritable information, along with

associated proteins.

Genes are stretches of DNA along the chromosome.

Sample top-scoring answer:

Chromosomes contain the DNA that carries the heritable information. One chromosome is made

up of one DNA molecule and associated proteins. Genes, in different forms called alleles, are

present as stretches of DNA at specific locations along the chromosomal DNA molecule. The

13. Synthesize Your Knowledge

A male butterfly zygote would be XX. During the first cell division, if nondisjunction occurred

for one of the X chromosomes, the two daughter cells would have been XXX and XO. The XXX