AP Biology
Genetics Problems 1
(Chapter 15)
1. Human females are XX and males are XY.
a. Does a male inherit the X from his mother or father?
b. With respect to X-linked genes, how many different types of gametes can a male reproduce?
c. If the female is homozygous for an X-linked gene, how many types of gametes can she produce with
respect to that gene?
d. If a female is heterozygous for an X-linked gene, how many types of gametes can she produce with
respect to that gene?
2. Suppose one allele of a Y-linked gene results in nonhairy ears in males. Another allele results in rather long hairs,
a condition called hairy pinnae.
a. Why would you not expect females to have hairy pinnae?
b. Any son of a hairy-eared male will also be hairy -eared, but no daughter will be. Explain this.
3. Suppose you carry two linked genes with alleles Aa and Bb respectively, as shown in the diagram below.
If the crossover frequency between these two genes is zero, what genotypes would be expected among the
gametes you produce, and with what frequencies?
4. In D. melanogaster, a gene governs wing length. A dominant allele at this gene locus results in the formation of
long wings. A recessive allele results in the formation of vestigial (short) wings. You can cross a homozygous
dominant, long winged fly with a homozygous recessive vestigial-winged fly. You ask a technician to expose the
fertilized eggs to a level of X-rays known to induce mutation and deletions. Later, when irradiated fertilized eggs
develop into adults, most of the flies are heterozygous and have long wings. A few have vestigial wings. What might
explain these results?
5. Say you have alleles for left handedness and straight hair on one chromosome and alleles for right handedness
and curly hair on the homologous chromosomes. If the two loci for the alleles are vary close together along the
chromosomes, how likely is it that crossing over will occur between them? If they are distant from each other , is a
crossover more or less likely to occur?
6. Individuals affected by Down syndrome typically have an extra chromosome 21. In other words, their body cells
contain a total of 47 chromosomes.
a. At which stages of meiosis I and II could a mistake occur that could result in the altered chromosomes
number?
b. In a few cases , 46 chromosomes are present. Included in this total are two normal-appearing
chromosomes 21 and longer -than normal chromosome 14. Explain how these few individuals can have a
normal chromosome number.
7. For the mugwump, a tree dwelling mammal, the male is XX and the female is XY. However, sex-linked genes are
found to have the same effect as in humans. For instance , a recessive X-linked allele c produces red-green color
blindness. If a normal female mugwump mates with a phenotypicaly normal male mugwump whose mother was
color blind, what is the probability that a male offspring from the at mating will be color blind? A female offspring?
8. One type of childhood muscular dystrophy is a recessive, X-linked trait. A slowly progressing loss of muscle
function leads to death, usually by age twenty or so. Unlike color blindness, this disorder is restricted to males.
Suggest why?
Biology The Unity and Diversity of Life Cecie Starr and Ralph Taggart; Wadsworth Publish Company, 4th edition
1998
AP Biology
1a. X from mother
allele, the
Genetics Problems 1
ANSWERS TO GENETICS PROBLEMS
1b. Y without gene and X, with linked gene
(Chapter 15)
1c. female XX 1d. one X with one
other with the other allele
2a. Female can’t have hairy pinnae since on Y
2b. gene for H. pinnae father to son only.
3. 0% crossover; 50% AB & 50% ab
4. X-ray may delete dominant allele or irradiation may have mutated dominant allele.
5. Chance of crossing over is greater if distance between genes is further apart.
6a. Either anaphase I or anaphase II
6b. Downs’ syndrome due to 14/21 translocation since individual has
pair of chromosomes 21 too.
7. C(Y) female x Cc Male: results>Daughers will be 50% C(Y) and c(Y) {note sex linkage is reverse of humans;
males are “carriers” and females are commonly affected.
8. Males with muscular dystrophy rarely father children due to an early death prior to childbearing.
From Teacher’s Manual Biology; The Study of Life chapter 25
Probability in Genetics, pp. 25-16:
1. 1/4
2. 1/2
3. 1/2
4. 1/4
5. 1/4
6. 1/4
From Teacher’s Manual Biology; The Study of Life chapter 26
Sex-linked Traits pp. 26-12
1. XHXh X XH Y 2 normal females: 1 normal male; 1 “bleeder” male
2. XHXh X Xh Y normal female; bleeder female; normal male; bleeder male
3. mother XHXh father Xh Y
4a. 1/2
4b.XHXh X XH Y
5. O
6. XCXc X Xc Y 1/2, 1/2
7. XcXc X XC Y 1/2
8. XRXr X Xr Y 1/2
7. 1/4