Ch.12 Genetics
Problem Set III
NON-Mendelian Genetics
SEX-LINKED TRAITS: name: block: score: /30
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.
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 a black male. (X B X b x X B 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.
1 pt. male genotype(s):
1 pt. male phenotype(s):
1 pt. female genotype(s):
1 pt. female phenotype(s):
Mrs. Loyd  cschmittloyd@waukeeschools.org
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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.
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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. Punnett Square:
LINKED GENES:
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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.
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:
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 F
1
when a plant homozygous for green, round seeds is crossed with a plant with yellow, wrinkled seeds.
1 pt. G:
1 pt. P:
Find the genotypic and phenotypic ratios for the F
2
.
1 pt. G:
1 pt. P:
1 pt. How does this compare to a two trait cross in which the genes assort independently?
GENE MAPPING:
Mrs. Loyd  cschmittloyd@waukeeschools.org
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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?
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?
1 pt. What is the term given to a pair of homologues (with their chromatids) arranged side-by-side?
1 pt. Explain why crossing-over has survival value for a population.
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3 pts
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
y w
.1 .2 .3 .4 .5 .6 .7
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
, __, __, __.
Yellow (y)
Yellow (y)
Yellow (y)
Vermilion (v)
White (w)
White (w)
GENES
White (w)
Vermilion (v)
Miniature (m)
Miniature (m)
Vermilion (v)
Miniature (m)
White (w)
Vermilion (v)
Rudimentary (r)
Rudimentary (r)
1 pt What is the relative order of the genes starting with Yellow ( y
, w
, __, __, __ )?
RECOMBINATION
FREQUENCY (%)
.01 (done for you)
.32
.36
.03
.30
.33
.57
.27
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?
Baldness is sex-influenced trait. The phenotypic results show that it is dominant in males and recessive females.
GENOTYPE
BB
Bb
PHENOTYPE
MALE FEMALE
Bald
Bald
Very thin hair
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
 cloyd@waukee.k12.ia.us
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