BIO 321 : Genetics
MENDELIAN GENETICS
Mendel's Monohybrid Cross
Mendel worked with peas:
•
Many varieties available from seed merchants. These varieties varied in easily
scored traits such as seed color and shape, pod color and shape, flower color and
location on the stem. These varieties were pure-bred in the sense that the traits
Mendel was interested in were stably inherited and invariant in each variety if
inbred such that purple flowering plants always gave rise to purple flowering
plants and white flowering plants always gave rise to white flowering progeny.
•
Short generation times-a number of generations could be raised in a single
growing season
•
Large numbers of progeny can be obtained in crosses which allowed Mendel to
carry out statistical analyses of his results.
•
Peas are self-fertilizing. The flower is closed at the time the pollen and ova
mature. This is important because you can be confident that foreign pollen
wasn't blow in or carried into the pea patch and mess up the results.
•
It is possible to cross fertilize pea plants by cutting open the flower, cutting off
the anthers carrying the immature pollen, and apply foreign pollen with a paint
brush. Thus Mendel had good control of the crosses.
• The reason why Mendel was able to infer the rules of
inheritance was because be approached the problem
by:
• 1. Studying simple variant traits in pure-bred lines.
• 2. Applied a statistical analyses of his results.
• Example: Mendel crossed pure-bred green seeded
plants with pure-bred yellow seeded plants.
• All the progeny contained yellow seeds.
• When these yellow hybrid seeds were grown into
plants and allowed to self-fertilize, Mendel observed
6,022 yellow seeds and 2,001 green seeds on these
plants.
•
When Mendel planted these seeds and allowed the resulting plants to selffertilized, he found that the following pattern:
•
Of 519 of the yellow seeds, 166 gave plants with only yellow seeds and these
always bred true.
•
The remaining 353 yellow seeds gave plants with both yellow and green seeds in
a 3:1 ratio
•
All the green seeds gave plants that bore only green seeds.
Yellow X Green
All Yellow Seeds
↓
6,022 Yellow
Parental
F1
Self cross
+ 2,001 Green
F2
166 All Yellow All Green
+
353 3/4 Yellow
1/4 Green
F3
A 3:1 ratio of Y:G
Self cross
Of the yellow, 1:2
pure-bred: hybrid
•
•
•
•
•
Mendel inferred from these results
There is a single character encoding seed color, for which there are two forms: yellow and green.
Each seed (or plant) has two copies of this character.
A hybrid seed that has one yellow and one green form is yellow because yellow is dominant to green.
From now on, I will refer to the trait by a single letter, in this case Y because yellow is dominant. I will
represent the yellow form of the trait in upper case and the green form in lower case. A genetically YY
or Yy plant is yellow, yy is green.
A hybrid plant produces equal numbers of gametes (ova and pollen) carrying each of the forms of this
character.
By these rules we can explain the results of the cross as follows (assigning Y for the yellow form and y for
the green form):
Yellow (YY) X Green (yy)
↓
All Y
↓
All Yellow Seeds (Yy)
↓
pollen
1/2Y
½y
Parental
↓
All y
Parental gametes
F1
ova
½Y
½y
↓
Self cross
6,022 Yellow
+ 2,001 Green
F2
(1/4 YY, ½ Yy, 1/4 yy)
↓
1/3 YY
2/3 Yy
↓
↓
166 All Yellow
353 3/4 Yellow
F1 gametes
A 3:1 ratio of Y:G
Self cross
1/4 Green
Some terms defined:
•
Gene- the character or trait, seed color here
•
Allele- the form of the trait, yellow or green here
•
Dominant allele- the form of the trait that is expressed in the hybrid, yellow is
the dominant trait
•
Recessive- the unexpressed trait in the hybrid, green is recessive to yellow
•
Genotype- the genetic composition of an organism, a hybrid is Yy, a green seed is
yy, a yellow seed can be YY or yy
•
Phenotype- the appearance of an organism, yellow or green here
•
Homozygote- an organisms that has two copies of the same allele, a green seed is
homozygous or a genotype can be homozygous yellow (YY)
•
Heterozygote- an organism with two different alleles for a gene, a Yy seed is
heterozygous here
SS or Ss-smooth phenotype, ss wrinkled phenotype
Pure-bred Smooth, Yellow x Pure-bred Wrinkled, Green
(SSYY)
(ssyy)
↓
All Smooth, Yellow
F1
(SsYy)
↓
self cross
↓
315 Smooth, Yellow
101 Wrinkled, Yellow
108 Smooth, Green
32 Wrinkled, Green
16 Fig 2-12b
9
3
3
1
How do these arise? Mendel inferred by
independent assortment of the alleles into the
gametes.
We expect in a hybrid cross 3/4 dominant and 1/4
recessive. (3/4 x 3/4= 9/16)
• The sum of all the fertilization events that can occur gives
(DRAW)
•
1/16 SSYY, 2/16 SsYY, 2/16 SSYy, 4/16 SsYy - 9/16 SY
•
•
1/16 ssYY, 2/16 ssYy - 3/16 sY
1/16 SSyy, 2/16 Ssyy - 3/16 Sy
•
1/16 ssyy - 1/16 sy Fig 2-12b
• We could arrive at the same answer by assuming if 3/4 of
the progeny are smooth and 3/4 are yellow, and if the traits
assort independently, than 3/4 x 3/4 = 9/16 smooth and
yellow. Etc.
• We can explain these results based upon what we know
about meiosis if we assume that these two genes are on
different chromosomes
• Mendel also carried out back-crosses between the
heterozygote SsYy and each of the pure-bred parents:
•
YySs x YYSS All progeny have dominant
phenotype
• YySs x yyss: 55 YS, 44 Ys, 51 yS, 53 ys
of all four possible phenotypes
equal numbers
• Mendel sent 12 copies of his paper out to famous
scientists of the time. One went to Charles Darwin,
Mendel's results provided a mechanism for the
inheritance of rare traits required for Darwin's theory
of evolution. Darwin never read the paper…
• Mendel's principles can predict the outcome of any cross, or in
many cases infer the genotypes of individuals from a cross.
AaBbCcDdEe X AaBbCcDdEe What fraction of the progeny will be
ABCDE? (IMP : Indep Assortment, Draw?)
3/4 A x 3/4 B x 3/4 C x 3/4 D x 3/4 E = 243/1024
or AbCdE?
3/4 A x 1/4 b x 3/4 C x 1/4 d x 3/4 E = 27/1024
or have the AabbCCDdEe genotype?
½ Aa x 1/4 bb x 1/4 CC x ½ Dd x ½ Ee = 1/128
• Other crosses can also be predicted.
AaBb x aabb
Genotypes: ½ Aa, ½ aa and ½ Bb, ½ bb =
1/4 AaBb
1/4 Aabb
1/4 aaBb
1/4 aabb
Phenotypes: ½ A, ½ a and ½ B, ½ b =
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
This last cross is sometimes called a test cross because the second parent is
homozygous recessive, so whatever the genotype of the first parent is, it
will come through on the cross.
• AB x ab: All AB progeny- first parent must be AABB.
•
½ Ab, ½ AB progeny- first parent must be AABb.
•
½ AB, ½ aB progeny- first parent must be AaBB.
•
1/4 each AB, Ab, aB, ab- first parent must be AaBb.
Pedigree Analysis in Human Genetics
•
One obvious requirement for the analyses of genetic crosses as indicated here, is the need
to look at large numbers of progeny in order to obtain accurate ratios of phenotypes.
•
It is also advantageous to carry crosses out over several generations.
•
Neither of these conditions are satisfied when we apply genetic analyses to humans.
•
For example, if an individual is albino, lacking melanin, the pigment which colors the skin
and eyes, and marries a normal individual, if they have two children and both are normal
what does that mean?
It is possible that albinism is a dominant trait, and the albino parent was heterozygous
while the normal parent was homozygous recessive (Aaxaa). We would predict that ½ of
their children would be albino. But if they had only two children, it is certainly within the
realm of probability that they would both be normal.
•
•
It is also possible that albinism is a recessive trait and the normal parent was homozygous
wildtype (AAxaa). This would certainly be consistent with both children being wildtype.
Or, perhaps albinism is recessive and the normal parent was heterozygous
(Aaxaa). Then only 1/2 of the children should be albino, and it is not at all strange that
2 of 2 were normal.
Actually, the classical form of albinism is caused by an autosomal recessive
mutation (1/30000 in U.S, white skin, red eyes). Mutation affects the gene for tyrosinase,
an enzyme needed to convert tyrosine to DOPA, from which the brown pigment melanin
is formed. Melanin protects the skin from UV light by absorbing them. There are two
other kinds of albinism at least because many steps are involved from tyrosine to
melanin synthesis (thus complementation can occur, so albino parents can have
normal kids!) see fig 10.1
aaBB x AAbb→ AaBb
albino
albino
normal carrier
• The point is that we can conclude nothing from this data,
and, since we cannot carry out further crosses with the
parents or direct crosses with the progeny, for social and
time reasons, how can we deal with human genetics?
• The answer is we can't, at least not on the level that we
do genetics with peas or mice or fruit flies or a variety of
microorganisms.
• Nonetheless, there is some interest in human genetics so
we have developed some procedures.
• The first we will talk about here is pedigree analysis.
Essentially, if we can't go forward in time, we can go back
and look at family history. That is one advantage of
working with humans; we can look at past events.
• Patient walks into doctors office with rare disease. The doctor takes a family
history. Other members of the family have this disease despite its rarity. A
coincidence? An environmental problem (family members have bee drinking
water from the same well for generations)? Or is the trait inherited? The
doctor draws a family tree, perhaps questions other members of the family.
Rules common environmental factors-affected family members raised in
different localities and have different vocations. Maybe its genetic. Look at
other reports in the medical literature, contact other physicians, get more
pedigrees.
Rules for interpreting pedigrees:
Dominant alleles:
(except if trait lost)
Never skip a generation
Recessive alleles: Can skip generations, but
there must be some history in the family.