SI BIO1101-102 Dr Mark Venable Fern Perkins 30 March 2004
INHERITANCE 2 ANSWERS
1. Crossing over is important because it increases genetic variability. (Why is
genetic diversity important?) Crossing over is also important because it allows
mapping.
2. Crossing over makes mapping possible because it can be used to determine the
location of a gene on a chromosome. This is accomplished by examining the
number of recombinant offspring with regards to a certain trait in order to
determine the distance between two genes.
3. The probability of disrupting linkage genes is proportional to the distance
between them because the further apart two genes are on a chromosome the
greater the chance they will be separated when crossing over occurs (and vice
versa).
4. A centromere is the attachment point for sister chromatids following
chromosomal replication; constricted portion of the chromosome.
5. The location of a gene on a chromosome is described by the number of the
chromosome, the arm of the chromosome (p = short, q = long), and the number of
units from the centromere.
6. If the location of a gene is 4p20 the gene is located 20 units from the centromere
on chromosome 4.
7. Autosomes code for the majority of traits in an organism and are the same in both
genders. Sex chromosomes determine an individual’s gender although, in humans,
some of the X chromosome also codes for other traits.
8. a) For autosomal recessive disorders the genotype must be homozygous
recessive for the person to be affected by the disease. For autosomal dominant
disorders the genotype must be either heterozygous or homozygous dominant for
the individual to be affected by the disease. For a person to be a carrier of an
autosomal recessive disorders they must be heterozygous. There are no carriers
of autosomal dominant disorders.
b) An example of an autosomal recessive disorder is galoctosemia. An example
of an autosomal dominant disorder is progeria.
c) The easiest way to figure these out, I think, is to construct Punnet squares.
autosomal recessive inheritance: autosomal dominant inheritance:
affected carrier affected carrier – none
i: 25% 50% 75%
ii: 0% 50% 100%
iii: 50% 50% 50%
iv: 0% 50% 100%
v: 50% 50% 50%
vi: 0% 0% 100%
vii: 100% 0% 0%
