These problems are based on variations from the classical
Mendelian patterns we studied earlier
- INCOMPLETE DOMINANCE, CODOMINANCE, or BLENDING all occur
when both alleles of a gene pair are expressed, like the
snapdragons on the Tobin and Dushkin textbook p.
195 . Many human traits work this way. You solve
the problems just like the classical
Mendelian problems, except that you must remember that
heterozygotes have an intermediate phenotype so that there are
three types of genotypes and three types of phenotypes.
- POLYGENIC INHERITANCE is when the phenotype is affected
by more than one pair of genes, like eye
color as shown on the Tobin and Dushkin textbook p. 350
and height, as shown
in the Tobin and Dushkin textbook p. 351. Many more human
traits are polygenic, like skin color, shoe size, and the genetic
contribution to intelligence. When only two pairs of genes
are involved, you can solve the problems with Punnett
squares. With three or four or more pairs of genes, the
phenotype distribution begins to resemble a bell-shaped
curve; then it's easier to estimate or "eye-ball"
predictions. This technique will be demonstrated in class.
- Sex linkage is
explained on another page. The sex-linked sample problems
are there, too
- LINKAGE in general applies to somatic chromosomes in addition to
the sex chromosome. This term refers to the fact that with
at least 30,000 genes and only 23 pairs of human chromosomes, many
genes are inherited together, not independently (described on
pp.203-205 of Tobin & Dushkin textbook). We will
not work problems with linkage, but you for the upcoming test, you
need to know what linkage means and why it's important.
Linkage has two important applications:
- Sometimes in trying to diagnose a genetic disease, we don't
know yet how to find its gene on a chromosome, but we can
identify its probable presence by a known marker gene
which is nearby on the same chromosome.
- When genes are linked, they sometimes separate during
crossing-over in Meiosis Prophase I. The closer the two
genes are, the less likely they are to be separated. By
calculating how often several genes do recombine, we can
estimate their relative locations on the chromosome. We
call this analysis mapping.
- MULTIPLE ALLELES, like blood types, will be covered later and
will not be on this test; but for the curious, you
can look.
SAMPLE PROBLEMS
- A red snapdragon crossed with a white snapdragon flower produces
seeds which grow into many snapdragons plants, all with pink
flowers. If the pink snapdragons are interbred, will their
offspring be pink, too?
- In the previous problem, which flowers are the F-1
generation?
- Two palomino horses (with a golden color) are stabled together;
after about a decade they have produced four palomino colts, but
two extremely light-colored (Cremello) colts and two darker sorrel
colts. What's with palominos?
REAL PROBLEMS for another take-home quiz
III.
answers
1. only 1/2 will be pink; 1/4 will be
red and 1/4 will be white.
2. the pink ones with the red parent and white parent.
3. they're hybrids with incomplete dominance.
(like PsPc if sorrels
are PsPs
and cremellos are PcPc
help me |
