AP Bio - Chapter 12

Question 1

What are the three main characteristics of the “Chromosome Theory of Inheritance”?

Answer 1

1) genes have specific loci along chromosomes
2) chromosomes undergo segregation
3) chromosomes undergo independent assortment

Question 2

What is meant by wild type?

Answer 2

the character most commonly observed in natural populations.

ex. red eyes

In contrast, a mutant phenotype would be white eyes.

Question 3

What is a sex-linked gene?

Answer 3

Located on either X or Y chromosome.

If X- linked, on X.

If Y-linked, on Y

Question 4

What is meant by the sentence:

All single-copy Y-linked genes are hemizygous except in cases of aneuploidy.

Answer 4

Hemizygous means that genes are present on only one chromosome. Aneuploidy means that there are multiple copies of the gene due to non-disjunction (eg XYY or XXYY).

Question 5

How many genes are on the X and Y chromosomes? Give an example of a gene on the Y chromosome. What is its function?

Answer 5

There are 100-200 genes on the Y chromosome

There are about 1200 genes on the X chromosome

The gene responsible for sex determination is located on the Y chromosome. It is abbreviated SRY which stands for Sex determining Region Y.

Did you know: Many X chromosome genes, about one-fifth, appear to play a role in human cognition and brain development.

Question 6

Why are X linked diseases more common in males?

Answer 6

Males inherit a Y from their father and an X from their mother. If mom passes a recessive allele for hemophilia, for example, there is no corresponding allele to mask the recessive allele.

Females need to inherit 2 recessive alleles for hemophilia to be affected by the disease.

Question 7

If a carrier female has children with a colorblind male, what is the chance that their sons have the disorder?

What is the chance that they will have a color-blind son?

Answer 7

50%

25% (this is an AND statement): .5 * .5 = 25%

Question 8

What is a Barr body?

What causes inactivation?

Answer 8

The inactive X in each cell of a female.

Females have 2 X. If both were active, there would be twice as many of the proteins encoded by the one X in males.

In early development, almost all of one X chromosome in each cell in females becomes inactivated by making the x-chromosome inaccessible (histones, rna from the xist gene)

This is too cool to leave out:

The XIST gene does not encode a protein but rather produces a 17 kilobase (kb) functional RNA molecule. Hence, it is a noncoding RNA (Costa, 2008). More on this in the next chapter.

XIST RNA is only expressed in cells containing at least two Xs and is not normally expressed in male cells (Figure 2). Higher XIST expression can be seen in cells with more X chromosomes, as a counting mechanism dictates that only one X per cell can remain active. In such cells, XIST is expressed from all supernumerary Xs.

XIST RNA remains exclusively in the nucleus and is able to “coat” the chromosome from which it was produced (Figure 3).

Paradoxically, XIST RNA is expressed from an otherwise inactive X chromosome.

Also see: https://www.youtube.com/watch?v=mHak9EZjySs

Question 9

Is it possible to have a male tortoiseshell cat? Why or why not?

Answer 9

No. not usually.

Males only have one X. There are no Barr Bodies. If males inherit a black allele on the X, then they will be solid black.

However, cats that are XXY (“Klinefelters” in cats) will have orange and black spots… and be male.

Question 10

What are linked genes?

Answer 10

genes located near each other on a chromosome

Question 11

How are parental offspring different from recombinant offspring?

Answer 11

Parental types = inherit a phenotype that matches either of the parents

Recombinants: new combinations of traits.

Question 12

What are the genotypes of F1 that can be formed if there is non-disjunction of X and Y in meiosis 1 and normal separation of the other parent?

Answer 12

XXY

X

Question 13

What is monosomy and what is it due to?

Answer 13

a missing chromosome as a result of non-disjunction during meiosis

Question 14

How many chromosomes does a human with trisomy have

Answer 14

47

Question 15

suppose there is a chromosomal deletion in a chromosome with the following sequence. AGGCTA. Write the new sequence.

Answer 15

Anything other than AGGCTA with ≥ 1 missing nucleotide.

Question 16

suppose there is a chromosomal duplication in a chromosome with the following sequence. AGGCTA. Write the new sequence.

Answer 16

extra copy of a piece of a chromosome

Question 17

suppose there is a chromosomal inversion in a chromosome with the following sequence. AGGCTA. Write the new sequence.

Answer 17

any sequence where the nucleotides have reversed their order

Question 18

Take a look at the chromosome. Which type of alteration is shown? Deletion, translocation, inversion, or duplication?

Answer 18

Translocation

Question 19

What does this Karyotype show?

Answer 19

47 XXX

Question 20

Women born with an extra X are generally healthy and are indistinguishable from XX women.

What is the likely explanation?

Answer 20

If XXX, two X are inactivated leaving one X active like XX women. Two Barr bodies can be seen in XXX cells.

Question 21

In terms of the amino acid sequence of the protein generated, what is more detrimental, a deletion of 1 nitrogenous base or a deletion of 3 nitrogenous bases. Why?

Hint: nitrogenous bases are read in groups of three

Hint #2: 3 nitrogenous bases define which amino acid is chosen.

Answer 21

a deletion of 1 base because now all amino acids after the deletion are different.