Lecture 20 Class: Chromosomal inheritance
Starter:
A mother and father give birth to a child with 47, XYY. This indicates:
A. Disjunction in the mother
B. Disjunction in the father
C. One X chromosome was inactivated and formed a Barr body
Laws of Segregation and Inheritance
Cystic fibrosis is caused by a recessive gene (c) that produces a faulty Cl- transporter in the
lungs. Joe and Jane are both heterozygous (Cc). What is the probability that a child of theirs will
have cystic fibrosis?
What kind of cross is this?
Guinea pigs have short hair that is dominant to long hair (Ss) and black hair dominant to brown
hair (Bb). If two dihybrids are crossed, what proportion will have long black hair?
Which of Mendel’s laws is demonstrated here?
Segregation
Independent Assortment
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Sex-linked Genes
Red-green color blindness (cb) is a sex-linked recessive trait in humans. Two people with normal
color vision (CB) have a color-blind son. What are the genotypes of the parents?
Pedigrees
Recessive trait:
Dominant trait:
In the sex-linked trait below, circle all ___________________________________.
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Barr bodies
A female inherited a dominant allele on her X chromosome from her father and the
homologous recessive allele from her mother. Which statement is TRUE?
A. Approximately half of her cells show the recessive phenotype
B. Only the chromosome with the recessive allele will become the Barr body
C. Cells without Barr bodies will show an intermediate phenotype
Recombination frequencies:
A BbNn (black, winged) female is crossed with a bbnn (brown, wingless) male. Of the 1000
offspring, 35 are brown winged and 65 are black wingless. The rest look like one of the parents.
What is the recombination frequency?
A series of additional matings determines the distance between wing size and antenna length is
5%. What is the correct order of genes on the chromosome?
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More practice problems:
What does a frequency of recombination of 50% indicate?
A. The two genes are likely on different chromosomes
B. All the offspring look like one of the two parents
C. The genes are on sex chromosomes
D. Independent assortment is hindered
Given the following recombination frequencies, what is the correct order of genes on the
chromosome?
A-C = 10%, B-C = 6%, A-B = 15%
A. ACB
B. ABC
C. CAB
D. BAC
When crossing a homozygous recessive with a heterozygote, what is the chance of getting an
offspring with the homozygous recessive phenotype?
A. 0
B. 0.25
C. 0.5
D. 0.75
E. 1
The fact that all seven of the pea plant traits studied by Mendel obeyed the principle of
independent assortment means that:
A. None of the traits obeyed the law of segregation
B. The genes were located on the same chromosome
C. The genes behaved as if they were on different chromosomes
In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange
color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the
cross of a black female and an orange male?
A. Tortoiseshell females, tortoiseshell males
B. Black females, orange males
C. Tortoiseshell females, black males
D. Black females, black males
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