Which one of Mendel’s laws relates to the
inheritance of alleles for a single character?
Which law relates to the inheritance of alleles for two characters in a
Mendel derived the from following a single character, which states that the two alleles for a heritable character segregate during gamete formation and end up
in different gametes. Using a dihybrid
cross Mendel developed the , which states that each
pair of alleles segregates independently of other pairs of alleles during
What is the physical basis of Mendel’s laws?
The physical basis for the law of
segregation is the separation of homologs in . The physical basis for the law of independent
assortment is the alternative arrangements of homologous chromosome pairs in .
Propose a possible reason that the first
naturally occurring mutant fruit fly Morgan saw involved a gene on a sex
To show the mutant phenotype, a
male needs to possess mutant allele.
If this gene had been on a pair of , two mutant alleles would
have had to be present for an individual to show the mutant phenotype, a much
less probable situation.
A white-eyed female Drosophila is mated with a red-eyed (wild-type) male, the
reciprocal cross of that is shown. What
phenotypes and genotypes do you predict for the offspring?
Because the gene for this
eye-color character is located on the X chromosome, all female offspring will
be red-eyed ; all male offspring
will inherit a Y chromosome from the father and be white-eyed .
Neither Tom nor Rhoda has Duchenne muscular
dystrophy, but their firstborn son does have it. What is the probability that a second child
of this couple will have the disease?
What is the probability if the second child is a boy? A girl?
There is a ¼ chance that the firstborn son does have it, a chance that the child will
inherit a Y chromosome from the father and be male multiplied by a chance that he will
inherit the X carrying the disease allele from his mother. If the child is a boy, there is a chance he
will have the disease; a female would have zero chance (but chance of being a
During early embryonic development of female
carriers for color blindness, the normal allele is inactivated by chance in
about half the cells. Why, then, aren’t
50% of female carriers color-blind?
The cells in the eye responsible
for color vision must come from multiple cells in the early embryo. The descendants of of those cells
express the allele for normal color vision and the allele for color
blindness. Having the number of
mature eye cells expressing the normal allele must be sufficient for normal color vision.
When two genes are located on the same
chromosome, what is the physical basis for the production of recombinant
offspring in a testcross between a dihybrid parent and a double-mutant
Crossing over during in
the parent produces some gametes with recombinant genotypes for
the two genes. Offspring with a
recombinant phenotype arise from fertilization of the recombinant gametes by
homozygous recessive gametes from the double-mutant parent.
Genes A, B, and C are located on the same
chromosome. Testcrosses show that the
recombination frequency between A and B is 28% and between A and C is 12%. Can you determine the linear order of these
. To determine which possibility is correct,
you need to know the recombination frequency between and .
More common than completely polyploidy animals
are mosaic polyploids, animals that are diploid except for patches of
polyploidy cells. How might a mosaic
tetraploid, an animal with some cells containing four sets of chromosomes
At some point during development,
one of the embryo’s cells may have to carry out mitosis after
duplicating its chromosomes. Subsequent
normal cell cycles would produce genetic copies of this tetraploid cell.
About 5% of individuals with Down syndrome have
a chromosomal translocation in which a third cop of chromosome 21 is attached
to chromosome 14. If this translocation
occurred in a parent’s gonad, how could it lead to Down syndrome in a child?
In meiosis, a combined 14-21
chromosome will behave as one chromosome.
If a gamete receives the combined 14-21 chromosome and a normal copy of
chromosome 21, trisomy 21 will result when this gamete combines with a normal
gamete during .
The ABO blood type locus has been mapped on
chromosome 9. A father who has type AB
blood and a mother who has type O blood have a child with trisomy 9 and type A
blood. Using this information, can you
tell in which parent the nondisjunction occurred?
. The child can be either IAIA
or IAii. A sperm of genotype
IAIA could result from nondisjunction in the father
during , while an egg with the genotype ii could result from
nondisjunction in the mother during either or .
Gene dosage, the number of active copies of a
gene, is important to proper development.
Identify and describe two processes that establish the proper dosage of
Inactivation of an chromosome in
females and . Because
of X inactivation, the effective dose of genes on the X chromosome is in males and females. As a result of
genomic imprinting, allele of certain genes is phenotypically
Reciprocal crosses between two primrose
varieties, A and B, produced the following results: A female X B male =>
offspring with all green (nonvariegated) leaves. B female X A male => offspring with
spotted (variegated) leaves. Explain.
The genes for leaf coloration are
located in within the cytoplasm.
Normally, only the parent transmits plastids genes to
offspring. Since variegated offspring
are produced only when the female parent is of the variety, we can conclude
that variety B contains both the wild-type and mutant alleles of pigment genes,
producing variegated leaves.
Mitochondrial genes are critical to the energy
metabolism of cells, but mitochondrial disorders caused by mutations in these
genes are generally not lethal. Why not?
The situation is similar to that
for chloroplasts. Each cell contains
numerous mitochondria, and in affected individuals, most cells contain a
variable mixture of normal and mutant mitochondria. The mitochondria carry out enough
cellular respiration for survival.