Honors BIOLOGY

Honors BIOLOGY
Ch.12 Genetics
Problem Set III
NON-Mendelian Genetics
name:
block:
score:
/30
SEX-LINKED TRAITS:
A sex-linked characteristic is a characteristic that is
carried on a sex chromosome. Therefore it determines
sex as well as the characteristic. Most sex-linked traits
are carried on the X chromosome because it carries many
more chromosomes than the Y chromosome. Because
males get only one X chromosome (always from mom), if
that gene is faulty then there is no allele on the Y to
override it and the male is affected. Because females get
two X’s, they have two chances to be normal. Sex-linked
genotypes always use the XX or XY sex genotypes
“carrying” the linked trait as superscript letters.
NON-DISJUNCTION:
Non-disjunction occurs during Meiosis II when chromatids
fail to separate (disjoin), resulting in both sister chromatids
in one gamete and none in the other. When the sister
chromatids are joined by the homologous chromatid at
fertilization, the result is called a trisomy.
Use tally marks ( I ) to represent autosomal chromosomes
to make a Punnett Square illustrating nondisjunction.
1 pts.
!
In a certain animal, the gene for black coat color (B) is
dominant to the gene for orange coat color (b). The
characteristic is sex-linked. Determine the probable
genotypic and phenotypic ratios among the male and
female offspring produced by a heterozygous female and
B b
B a black male. (X X x X Y )
!
!
List the genotypes in:
!
!
!
1 pt. the sperm:
1 pt. and egg:
Find the ratios for the offspring:
1 pt. male genotypic ratio:
1 pt. male phenotypic ratio:
1 pt. female genotypic ratio:
1 pt. female phenotypic ratio:
What genotypes and phenotypes must each parent have
to produce an orange female? Hint: Write out the
genotype for the orange female, and what you know about
the parents’ genotypes i.e. work it out backwards.
Klinefelter’s Syndrome is a human trisomy resulting from
nondisjunction of the X chromosome from the mother of a
boy. This results in the male have XXY for sex
chromosomes. (see p.213 Snow Leopard) Tortoise Shell
cats carry the orange and black alleles (codominant) for
coat color on their X chromosomes. Male Tortoise Shell
cats are very rare (and valuable).
How is a male Tortoise Shell cat similar to a human male
afflicted with Klinefelter’s Syndrome?
1 pt. Explanation:
1 pt. male genotype(s):
1 pt. male phenotype(s):
1 pt. Punnett Square:
1 pt. female genotype(s):
1 pt. female phenotype(s):
Mrs. Loyd !
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LINKED GENES:
Genes are said to be linked when they are on the same
chromosome. For now, assume that they do NOT cross
over and become unlinked. Hint: when drawing meiosis,
KEEP LINKED GENES LINKED!
1 pt. Explain why blue eyes and blond hair are commonly
inherited together.
In a certain organism, one chromatid contains the genes
A and b. Its homologous chromatid contains a and B.
What combinations of genes might be found in the
gametes if crossing-over does occur? (Refer back to the
second problem under Gene Linkage for the number of
gametes produced without crossing-over. How do they
compare?)
1 pt. What combinations of genes might be found in the
gametes if crossing-over does occur?
In a certain organism, one chromatid contains the genes
A and b. Its homologous chromatid contains a and B.
2 pts. What combinations of genes would be found in
gametes of this organism if no crossing-over occurs?
Find the genotypic ratios produced when two individuals
with these gametes are crossed. A Punnett Square would
be helpful. Refer to your “Check list” for a hint on how to
draw linked genes for Punnett Squares.
1 pt. Genotypic ratio:
CROSSING-OVER:
When chromosomes overlap, they sometimes exchange
fragments, resulting in an exchange of genes. This is
called crossing-over and results in new combinations of
genes on chromosomes in gametes.
1 pt. In which phase of meiosis does this occur?
In a certain plant, green seeds (G) are dominant to yellow
(g) and round seeds (R) are dominant to wrinkled seeds
(r). Also assume that the alleles are on the same pair of
homologous chromosomes (linked!).
Find the F1 when a plant homozygous for green, round
seeds is crossed with a plant with yellow, wrinkled seeds.
1 pt. What is the term given to a pair of homologues (with
their chromatids) arranged side-by-side?
1 pt. G:
1 pt. Explain why crossing-over has survival value for a
population.
1 pt. P:
Find the genotypic and phenotypic ratios for the F2.
1 pt. G:
1 pt. P:
1 pt. How does this compare to a two trait cross in which
the genes assort independently?
Mrs. Loyd !
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GENE MAPPING:
The farther apart two genes are on a chromosome, the more likely a break will occur between them due to crossing-over. In
fact, if two genes are more than a half-chromosome length away from each other, they will cross over so frequently (50%) that
they appear to be on different chromosomes. That is, they appear to assort independently. Relative distance between two
genes on a chromosome can be measured by observing how frequently the chromosome crosses over between them. The
closer they are to each other, the less frequently they cross over.
1.0% recombination frequency = 1 map unit or 1 centimorgan (cM).
You will use the recombination frequencies given between several genes to determine their relative distance from each other.
You will have to use some gene distances as a “bridge” to others. Approach it as a puzzle where all the pieces have to fit.
Gene maps are rather sloppy because we need to use observed frequencies which may not be exactly what we would expect
if we could see the genes on the chromosomes. But the relative order of the genes should be correct.
Notice in the drawing of a chromosome map below (the line being the chromosome itself), that the yellow gene is arbitrarily
chosen as the left end of the map with a position of 0.0 cM, and the position of the other genes are assigned by summing the
recombination frequencies between nearest genes. Thus yellow and rudimentary are separated by .58 cM on the map. Find
a distance .58 cM from yellow and label it with an “r” and the distance following the example of y and w.
0.0 .01
.1
.2
.3
.4
.5
.6
.7
3 pts
y
w
A graduate student worked long hours to collect the following crossing-over (recombinations) frequencies. Use the values to
determine the location of the genes on the chromosome and then a relative order of the genes: y, w, __, __, __.
RECOMBINATION
GENES
FREQUENCY (%)
Yellow (y)
White (w)
.01 (done for you)
Yellow (y)
Vermilion (v)
.32
Yellow (y)
Miniature (m)
.36
Vermilion (v)
Miniature (m)
.03
White (w)
Vermilion (v)
.30
White (w)
Miniature (m)
.33
White (w)
Rudimentary (r)
.57
Vermilion (v)
Rudimentary (r)
.27
1 pt What is the relative order of the genes starting with Yellow (y,
OPTIONAL PROBLEMS
Carnivals used to have “freakshows” which would exploit
people with conditions or deformities that weren’t
understood. The “Bearded Woman” was one of these
sideshows. Now we understand that this condition is
caused by a sex-limited trait.
Explain why sex-limited traits appear using the phrases:
male hormones (androgens), gene for beard,
abnormal expression. Is it normal for women to
have a gene coding for beard type?
w, __, __, __
)?
Baldness is sex-influenced trait. The phenotypic results
show that it is dominant in males and recessive females.
PHENOTYPE
GENOTYPE
MALE
FEMALE
BB
Bald
Very thin hair
Bb
Bald
Normal
bb
Normal
normal
Refer to the chart above to find the genotypic and
phenotypic results of a cross between a male with normal
hair growth and a heterozygous female. Describe the
results for both boys and girls.
Genotypic ratio:
Male phenotypic ratio:
Female phenotypic ratio:
Mrs. Loyd !
[email protected]
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