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SOLUTIONS
Chapter 15 – Chromosomes and Heredity
Section A: Linked and Sex-linked Genes
1. Who were the scientists in 1902 that discovered that the behavior of chromosomes during
meiosis was related to the behavior of “factors” or alleles in Mendel’s experiments?
Walter Sutton and Theodor Boveri
2. Linked Genes are those genes that are located on the same chromosome.
A
a. Would you expect the results of breeding experiments to follow those expected
from Mendel’s laws? Explain why or why not.
Mendel ran simple breeding experiments where alleles for different
characteristics were located on separate chromosomes (e.g seed colour and seed
texture are on different chromosomes). Therefore, the alleles assorted
independently of one another. If alleles are located on the same chromosome
(and are close together), then they will be inherited together, as opposed to
independently. Therefore, the results of breeding experiments involving linked
genes will not coincide with the results of Mendel’s experiments.
b
C
b. Based upon what you know about genetics so far, what event or process during
meiosis might occasionally “break” the physical connection between genes on
the same chromosome?
Crossing over, which occurs during Prophase I in Meiosis occasionally breaks the
physical connection between genes on the same chromosome. (Crossing over segments from the paternal non-sister chromatid trade places with segments from the
maternal non-sister chromatid.)
c. In reality, the genes for seed colour and seed shape in peas are said to be linked (i.e.
on the same chromosome). Why do the alleles for seed colour and seed shape always
segregate and assort independently?
The alleles for seed colour always segregate and assort independently of the allele for
seed shape because the distance between these genes on the chromosome is so great,
that crossing over almost always occurs. In general, the probability of crossing over is
proportional to the distance between genes on the same chromosome. This means that
the greater the distance between genes, the higher the probability of crossing over and
the lesser the distance between genes, the lower the probability of crossing over.
3. Define: sex-linked inheritance. Why are very few Y-linked traits known?
Sex-linked inheritance is the transmission of genes that are located on one of the sex
chromosomes. Few Y-linked traits are known because the Y chromosome is a lot smaller
than the X, thus containing less genes.
4. Explain two reasons why fruit flies are excellent experimental organisms for Morgan?
1. Fruit flies are prolific breeders – a single mating can produce hundreds of
offspring and a new generation can be bred every two weeks
2. Fruit fly genome – only has four pairs of chromosomes that are easily
distinguishable under a microscope
5. In Drosophila flies, describe the genotype and phenotypes of the mutant and wild-type flies for
eye colour only.
Trait(s)
G/T
Phenotype
Genotype
Phenotype
Genotype
Eye Colour
Body Colour and
Wing Type
Wild-Type (Dominant)
Red Eyes
+
w w+ or w+ w
Normal colour and wings
bl+ bl+ vg+ vg+
Mutant (Recessive)
White Eyes
ww
Black, vestigial wings
bl bl vg vg
6. Explain the full experiment and results that Morgan used which proved that some genes are
sex-linked. Use Punnett Squares to demonstrate the crosses. Use appropriate genotype
notation.
F2 Generation
F1 Generation
Xw
Y
Xw+
Xw+ Xw
Xw+ Y
Xw+
Xw+ Xw
Xw+ Y
Xw+
Y
Xw+
Xw+ Xw+
Xw+ Y
Xw
Xw+ Xw
Xw Y
All female have red eyes
Eye colour gene is located on X,
no locus for eye colour on Y
In Morgan’s first cross (when he mated red eyed female with a white eyed male), all offspring
has red eyes (both males and females). This led him to conclude that red eyes was dominant
over white eyes. Then, he took male and female offspring from this cross (F1) and mated them.
The result was the typical 3:1 ratio of dominant (red) to recessive (white) phenotypes. He
noticed that no female had white eyes, and only half of the males had white eyes. If eye colour
was not sex-linked, then you would expect there to be 50% female white eyes and 50 % male.
Therefore, Morgan concluded that the locus for eye colour was located on the X chromosome
only. It is possible for females to have white eyes, but their father must have white eyes and
their mother must either be a carrier or have white eyes as well. The white eyed trait is passed
down through transmission of X chromosomes, not Y.
7. If females have two X chromosomes and males only have one, why is it that both males and
females produce the same amounts of proteins coded by genes on X chromosomes?
Males and females produce the same amount of proteins coded for by genes on X
chromosomes because one of the X chromosomes in females is inactivated and becomes a
BARR BODY (which one that becomes inactivated is random). The X chromosome will become
reactivated when a cell turns into an egg cell (meiosis).
Section B – Patterns of Inheritance and Nondisjunction
1. Based upon the readings, hypothesize the genotypes of individuals with genetic diseases for
each type of inheritance pattern. Assume allele A is dominant over allele a.
a. Autosomal Dominant
i. disorder: AA, Aa
ii. normal: aa
c. X-Linked Dominant
i. disorder: MALE: XAY
FEMALE: XAXA, XAXa
ii. normal: MALE: XaY
FEMALE: XaXa
b. Autosomal Recessive
i. disorder: aa
ii. normal: Aa, AA
iii. carrier: Aa
d. X-Linked Recessive
i. disorder: MALE: XaY
FEMALE: XaXa
ii. normal: MALE: XAY
FEMALE: XAXA, XAXa
iii. carrier: FEMALE: XAXa
3. Some genetic diseases are caused by nondisjunction.
a. Define nondisjunction. When can non-disjunction occur during meiosis?
Nondisjunction occurs when members of a pair of homologous chromosomes do
not move apart properly during meiosis I or sister chromatids fail to separate
during meiosis II. Specifically, this can occur during anaphase I or anaphase II.
b. Hypothesize the meaning of: trisomy and monosomy.
Normal individuals, following interphase, have 2 of each chromosome
Trisomy – the condition of having an extra chromosome (3)
Monosomy – the condition of having one less chromosome (1)
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+
n+
n1
n+1
n –1
n–1
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
n
n
(b Nondisjunction of sister
chromatids in meiosis II
c. The following are karyotypes of different individuals. For each answer the following
questions:
i. What problem is shown?
ii. What is the resulting disorder?
iii. Would this be labeled as monosomy, trisomy or neither?
iv. What is the gender of the individual?
Condition
Syndrome
Trisomy 21
Down
Trisomy 18
Edward
Trisomy 13
Patau
XXY
Klinefelter
XYY
Jacobs
XXX
XO (1 X
chromosome
only)
Triple X
Turner
Characteristics
Autosome
Intellectual disabilities, abnormal patterns of palm creases,
almond-shaped eyes, flattened face, short stature
Intellectual and physical disabilities, facial abnormalities,
extreme muscle tone, early death
Intellectual and physical disabilities, wide variety of defects in
organs, large triangular nose, early death
Sex Chromosome
Sexual immaturity (inability to produce sperm), breast
swelling
Typically no unusual symptoms; some individuals may be
taller than average
Tall and thin, menstrual irregularities
Short stature, webbed neck, sexually underdeveloped
A.
i. This individual has two X chromosomes and 1 Y chromosome
ii. This individual has Klinefelter Syndrome
iii. This is an example of a trisomy of the sex chromosomes
iv. This individual is male due to the presence of the Y chromosome
B.
i. This individual has two Y chromosomes and 1 X chromosome
ii. This individual has Jacob’s Syndrome
iii. This is an example of a trisomy of the sex chromosomes
iv. This individual is male due to the presence of the two Y chromosomes
C.
i. This individual has an extra chromosome 18.
ii. This individual has Edward’s Syndrome
iii. This is an example of a trisomy of chromosome 18 (Trisomy 18)
iv. This individual is male