Download Basic Mendelian Principles

Basic Mendelian Principles
• Mendel’s big ideas:
•
--Particulate inheritance: the
determinants of inherited traits are discrete
units that are passed between generations
unaltered, not blended together.
•
--Count large numbers of offspring.
The offspring ratios observed are
imperfect reflections of underlying simple
ratios like 3/4 : 1/4 , etc.
Experimental Organism
• Mendel worked with peas.
• Plants have sexual processes very similar to animals.
•
--The male gamete, equivalent to the sperm, is the
pollen grain.
•
--The female gamete, equivalent to the egg, is the
ovule.
• Pea plants have both sexes on the same plant, and they
can be self-pollinated (selfed): pollen from one plant is
used to fertilize ovules from the same plant. This is a
closer cross than is possible with animals.
• He found 7 lines of peas that differed from each other for
7 distinct traits.
Mendel's Traits
Monohybrid Cross
• Monohybrid cross: only one trait is examined
(although there are probably other genetic
variations present)
• We are first going to look at what happens when
plants with different traits are crossed, then go
through Mendel's explanation.
• Purple flowers vs. white flowers. The original
parental lines are true-breeding, or purebreeding. All offspring within the lines gave the
same flower color for an arbitrary number of
generations.
First Cross
• True-breeding purple x true-breeding white. All
offspring are purple. The parent lines are the P
generation; the offspring are the F1 (first filial)
generation.
• All the F1's are purple regardless of which
parent (father or mother) was purple and which
was white.
– Note: no blending occurs. The purple F1 plants look
exactly like the purple parentals.
• We say that purple is dominant because it
appears in the F1 hybrid. White is recessive
because it does not appear in the F1 hybrid.
Selfing the F1
• Self-pollinate the F1 plants to get the F2 (second
filial) generation.
– In animals with only 1 sex per individual, the
equivalent would be a brother-sister mating.
• The F2 appear in a ratio of 3/4 purple to 1/4
white.
• Note: white has re-appeared in the F2,
unchanged despite having been in a purple F1
plant. The genes are not affected by the
organism that carries them.
Selfing the F2
• Each F2 plant is selfed to produce a group of F3
offspring.
• The F3 offspring of the white F2's are all white.
The F2 white plants were all true-breeding.
• Some of the F2 purples have all purple F3
offspring. These F2 purple plants are true
breeding.
• Other F2 purples give purple and white F3's in a
ratio of 3/4 purple to 1/4 white. This is the same
behavior as the F1's had.
F2 Behavior
• The purple F2 plants come in 2 types:
some are true breeding and only give
purple offspring. Others are hybrids and
give 3/4 purple : 1/4 white offspring. They
can be distinguished by the offspring they
produce, but not by their physical
appearance.
Explanation
• Pea plants, like humans and most other
higher organisms, are diploid: there are 2
copies of each gene. One copy came
from the father and the other copy came
from the mother.
• A gene can have many different versions,
called alleles. In this case, the flower color
gene has a purple allele (symbolized P)
and a white allele (symbolized p).
More Explanation
• The true breeding lines are homozygous for the gene
being examined: both copies of the gene are the same
allele. Thus in the initial cross, one parent was PP and
the other was pp.
• Hybrid lines are heterozygous: the two copies of the
gene being examined are different. A Pp plant is a
heterozygote.
• When diploid organisms reproduce, they make gametes
(sperm and eggs) that are haploid: they have only 1 copy
of each gene. Which copy goes into the gamete is a
random process.
• The male and female gametes combine at random to
form zygotes, the first diploid cells of the next generation.
Still More
• In the case we are discussing, the PP plant produces P
gametes and the pp plant produces p gametes. These
gametes combine to form Pp F1 individuals.
• Since P (purple) is dominant to p (white), the Pp plants
are purple.
• Thus there are 2 forms of purple plant: the PP
homozygotes and the Pp heterozygotes. These plants
have the same phenotype (physical appearance) but
different genotypes (genetic constitution). Much of
genetics is an attempt to determine the relationship
between phenotypes and genotypes.
F1 -> F2 Explanation
• When the F1 (Pp) plant makes gametes, each gamete
gets either a P allele or a p allele. This happens
randomly, so 1/2 of the gametes are P and 1/2 are p.
• The gametes combine at random. So:
•
1/2 x 1/2 = 1/4 of the zygotes come from a P egg
meeting a P sperm, giving a PP F2 plant.
•
1/2 x 1/2 = 1/4 of the zygotes come from a p egg
meeting a p sperm = pp F2 plant.
•
1/2 x 1/2 = 1/4 are P egg meeting a p sperm, and 1/2
x 1/2 = 1/4 are p egg meeting a P sperm. Both of these
give a Pp F2 plant, so 1/2 of the F2 are Pp.
F2
• In summary, 1/4 of the F2 genotypes are
PP, 1/2 are Pp, and 1/4 are pp.
• Since P (purple) is dominant, both PP and
Pp plants are purple. Thus, 3/4 of the
phenotypes are purple, and 1/4 are white.
• Mendel found that this rule worked for all 7
of his traits, to within what he considered
reasonable accuracy.
Mendel Monohybrid Results
Law of Segregation
• Mendel's first law of genetics
• “The two members of a gene pair
segregate randomly and equally into the
gametes, which then combine at random
to form the next generation.”
Backcross and Test cross
• A backcross involves mating the F1 hybrid to one of the
parental types. There are 2 possible backcrosses in the
system we are examining.
– Pp x PP. Back crossing to the dominant parent. The Pp plant will
produce 1/2 P gametes and 1/2 p gametes. The PP plant will produce
only P gametes. The offspring will thus be 1/2 PP and 1/2 Pp. Both of
these types are purple, so the result of a backcross to the dominant
parent is all offspring with the dominant type.
– Pp x pp. back crossing to the recessive parent. Again, the Pp parent
produces 1/2 P gametes and 1/2 p gametes, and the pp parent
produces only p gametes. The offspring will be 1/2 Pp (purple) and 1/2
pp (white). Note this is a different ratio from the F1 x F1 (Pp x Pp)
intercross we looked at earlier.
• A test cross is a mating between a heterozygote (or
unknown) to the recessive homozygote (the tester). For
monohybrid crosses, the test cross is the same as a
backcross to the recessive parental type. The offspring
of a test cross are in the same ratio as the gametes from
the organism being tested: 1/2 P (purple) and 1/2 p
(white).
Dihybrid Cross
• A dihybrid cross examines 2 different genes.
We will use yellow vs. green pods, and round vs.
wrinkled seeds.
– Yellow (Y) is dominant to green (y).
– Round (R) is dominant to wrinkled (r).
• Initial cross: YY rr x yy RR (yellow wrinkled x
green round). The YY rr plant makes Yr
gametes ( one copy of each gene) and the yy
RR plant makes yR gametes.
• These combine to make F1 plants that are Yy
Rr, yellow round. Note that neither parent has
this phenotype.
F1 -> F2 Dihybrid
• When the Yy Rr plant is mated, it makes 4 kinds
of gamete. Each gamete gets 1 copy of each
gene, so 1/2 are Y and 1/2 are y.
Independently, 1/2 are R and 1/2 are r. Thus,
1/4 of the gametes are YR, 1/4 are Yr, 1/4 are
yR, and 1/4 are yr. This happens for both male
and female gametes.
• The gametes combine randomly. The
combinations are shown in a 16 cell Punnett
square. (draw this!)
F2 Dihybrid Results
• Examining the Punnett square, we find 1 cell (1/16 of the offspring)
that are yy rr, homozygous recessive for both traits: green wrinkled.
• Three of the cells are either yy RR or yy Rr. Both of these have the
green round phenotype, so 3/16 of the offspring are green and
round. We can symbolize this as yy R_, where the _ stands for
either R or r. As long as one copy of the gene is the dominant R
allele, it doesn’t matter what the other copy is.
• Similarly, 3 of the cells are YY rr or Yy rr, yellow wrinkled. 3/16 are
Y_ rr
• All of the other 9 cells are Y_ R_: yellow round, in various
combinations of homozygotes and heterozygotes.
• This is the fundamental ratio for the F2 from a dihybrid cross: 9/16
show both dominant traits, 3/16 show one dominant and the other
recessive, 3/16 show one recessive and the other dominant, and
1/16 show both recessive traits.
F2 dihybrid results are the
combination of 2 monohybrid
crosses
• If you just look at the individual genes
separately, you find the same 3/4 : 1/4 ratio seen
in the monohybrid cross.
• Thus, 9/16 are yellow round and 3/16 are yellow
wrinkled. This adds up to 12/16 = 3/4 yellow.
And, 3/16 are green round and 1/16 are green
wrinkled, which adds up to 4/16 = 1/4 green.
• Same is true for round vs. wrinkled.
Law of Independent Assortment
• Mendel's second law.
• “For unlinked genes, the alleles from each gene
segregate into the gametes independently of
one another.”
• Some genes are linked, which means that they
don't segregate independently of each other
and thus don't give the 9:3:3:1 ratio of F2
offspring. Linked genes are close together on
the same chromosome; we will deal with this
phenomenon later.