Genetic Inheritance: Beyond Mendel
AP Biology
Mendel’s work laid the foundation for an understanding of inheritance. However, it’s clear his
work explaining traits with two alleles, one dominant, one recessive, represent just one possible
type of inheritance.
Some traits are determined by multiple genes; this is polygenic inheritance. In these cases, there
are more genotypes, which results in a greater range of phenotypes. Some genes have more than
two alleles in the population, or multiple alleles; again, more phenotypes result. In some traits
alleles blend, or are expressed equally, in the heterozygote; this describes incomplete
dominance and codominance, respectively. Other traits result from genes on the X
chromosome, of which men have just one; these are sex-linked (X-linked) traits.
Yet other traits are linked to one another, as the genes for each are close together on the same
chromosome. These linked genes are less likely to be parted via crossing over and so are less
likely to independently assort during meiosis. The relative distance between two genes can be
calculated from offspring whose phenotypic ratio does not show the expected outcome from a
standard Punnett. The greater this distance, the further apart on the chromosome the genes are,
and the grater the likelihood the genes independently assort.
A. Melanin Pigmentation
Work in pairs. Each student tosses three coins – for example a penny (Pp), nickel (Nn) and
dime (Dd) – representing three different genes that together determine the degree of skin
pigmentation. Let heads represent the pigment allele, and tails the recessive.
1. Determine the number of pigment alleles required to make each of the phenotypes.
Record in Table A. Which skin phenotype(s) are most likely? Why? Discuss with your
lab partner(s) and write a conditional, justified hypothesis before continuing.
2. At the same time as your partner, toss your three coins. Count the number of pigment
alleles in the genotype and record this trial with a tally in the appropriate column. Repeat
for a minimum of 50 trials
3. Graph your results as a histogram. Analyze statistically: find the mean, median, mode,
standard deviation, & standard error; and via chi2 test (‘expected’ Punnett uploaded)
C. Linked Genes : Morgan’s Flies
Linkage is determined by crossing a hybrid organism with one that is recessive for both traits.
Recombination information regarding three genes on the same arm of the same chromosome is
summarized in Table 2. Determine the relative location of each of genes from the centromere.
1.
Determine the expected frequencies of each phenotype from a cross of a heterozygote
and a homozygous recessive and a) the genes sort independently and b) the genes are
adjacent on a chromosome (linked)
Table 2
Black body & normal wing
Purple eye & normal wing
X
x
Gray body & vestigial wing
Red eye & vestigial wing
Black body & purple eye
x
Gray body & red eye
BbVv Bbvv
bbVv
bbvv
RrVv
Rrvv
rrVv
rrvv
BbRr
Bbrr
bbRr
bbrr
965
185
944
1339
151
154
1195
1159
67
78
1017
206
2.
3.
dominant alleles are in bold type.
Do Morgan’s data match either expected frequencies? Recombination of the parent pairs
occurred but infrequently, indicating linked genes that occasionally are broken up by
crossing over. The closer the genes are to one another on the chromosome, the more the
traits show up together. The further apart, the more likely genetic recombination occurs.
Determine the percent recombination – phenotypes that do not match either parent –
between each pair of traits. To find this value, use the following formula:
recombined phenotypes
---------------------------- x 100 = percent recombination
total offspring
4.
Every 1% of recombination equals 1 Linkage Map Unit (LMU) between two traits. Use
the three recombination percents to reflect the distances between the three genes on a
chromosome. Sequence them on a line to reflect their positions relative to one another.
C. Discussion:
1. Explain the coin analogy: why does each partner have a coin set? What do the heads and
tails of each coin represent? How would you describe the genotype & phenotype of these
parents?
2. What pattern does the graphed data A produce? Why does this pattern appear? Infer from
your results about human hair color frequency.
3. What if parents are homozygous for pigmentation for two of the three genes – how would
this affect the frequency distribution? Can med-pigmented parents have a dark child?
Explain.
4. What do linkage maps tell us? What do they not tell us?
Table A
Number of
melanin
alleles
pigmentation
phenotype
darkes
t
dark
brown
medium/
dark
brown
medium
brown
medium/
light
brown
light
brown
lightest
Darkest
skin
dark
brown
medium/
dark
brown
medium
brown
medium/
light
brown
light
brown
lightest
skin
Coin tally
(# heads)
Coin
Genotype(s)
Total
phenotype
Number of
melanin
alleles
Coin toss
results
(tally)
Genotypes
(coin
combos)
Total