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Chapter 6
Mendelian Genetics
Basis of Genetics
• Genetics is the study of inheritance. For
thousands of years the exact nature of it had
been debated. People thought inheritance was
blended, much like mixing two cans of paint.
Other people thought that different parts of
parents fought on the microscopic level for
dominance in the offspring.
• Gregor Mendel, an Austrian monk discovered
that inheritance was particulate. That is, each
trait is determined by a gene from either the
mother or the father, they didn’t mix.
Genes
• Genes are the segments of chromosomes which
code for traits (1 gene for 1 trait).
• Genes were discovered by Mendel, but it wasn’t
until later that chromosomes were discovered.
• Note: When Darwin published “On the Origin of
Species” he thought that heredity was mixed.
Later, Mendel’s work was the basis for the “NeoDarwinian Synthesis” which combined genetics
and natural selection.
GREGOR MENDEL
Genes
• Genes are controlled by alleles. Alleles
are either dominant or recessive.
• When two dominant alleles are in the
genotype, the organism is homozygous
dominant. When a dominant and a
recessive allele are in the genotype, the
organism is heterozygous. When two
recessive alleles are in the genotype, the
organism is homozygous recessive.
Genes
• Genes are represented by letters. The letter that
is used is the first letter of the name of the
condition that a homozygous recessive
genotype would cause: i.e. hemophilia is
represented by an H or h even though the
condition isn’t present unless the genotype is hh.
• A homozygous dominant’s genotype is HH, a
heterozygous’s genotype is Hh, and a
homozygous recessive’s is hh.
Punnett Squares
• Punnett squares are a tool we can use to determine the
possible genotypes in the offspring of two sexually
reproducing individuals.
• When one trait is observed, it is called a monohybrid
cross, for two traits, it is called a dihybrid cross, for three,
trihybrid, etc.
• To begin, take the genotypes of the organisms. There
should be two alleles for a genetic trait. Then, put the
father (or mother) above the square like columns (1 over
each column). Then put the other parent’s to the left like
rows.
• Fill out the punnett square like a multiplication table.
A punnett square in action!
Analyzing a Punnett Square
• After the square has been filled out, you can see
the possible genotypes of the offspring. What
the genotype is will determine what the
phenotype is.
• A phenotype is the outer expression of the
genotype (with environmental constraints).
• The phenotype is dominant if the genotype is
homozygous dominant or heterozygous, and
recessive if the genotype is homozygous
recessive.
• Each square represents a ¼ chance of the
genotype occurring.
Probabilities
• The main goal of mendellian genetics is to determine the
probability of a certain outcome between the mating of
two individuals.
• A probability shows which is more likely to occur.
However, unlike the fraction of ¼ which each square
represents, a probability is easier to do….just count up
the number of squares with the same genotype. This will
give you the genotypic ratio. For example: If there is
one homozygous dominant square, two heterozygote
squares, and one homozygous recessive squares, the
probability is shown by a 1:2:1 ratio.
Phenotypic Ratio
• Just as the genotypic ratio can be
determined, a phenotypic ratio can also be
determined. Just take the squares as
before: 1:2:1, then determine the
phenotype of each numbers’ square. In
this example, the phenotypic ratio is 3:1 (3
will express the dominant phenotype and 1
will show the recessive phenotype).
Test Crosses
• If the genotype of an organism is not
known, it can be bred with a homozygous
recessive individual. The offspring’s
phenotype will allow someone to
determine what the genotype was or
possibly was. This is known as a test
cross.
• In this class you will not be required to fill
one out, just know what it is.
Prediction and People
• Genetic rules (or Mendel’s Laws) apply equally to
humans.
• Many times doctors and family planners are interested to
know about the history of recessive disorders in a family.
They use a pedigree to analyze this.
• The strength of pedigrees is that they can show
recessive traits in the family, but the weakness is that
most genetic experiments are usually done with
hundreds of offspring, whereas humans might only have
one or two children.
• The end result is a probability of a certain genetic
disorder occurring, so having kids can be a gamble
genetically…..but the odds are still usually better than
Vegas.
Intermediate Inheritance
•
The phenotype doesn’t ALWAYS follow the
previously given rules. There are three ways
this happens:
1. Incomplete dominance-When two different
alleles for the same trait combine, but neither
“wins” expression over the other, the offspring
have an intermediate phenotype (this is like
blending, or mixing two cans of paint).
2. Codominance-Both alleles in the heterozygote
express themselves FULLY…not blending, but
both at once.
Intermediate Inheritance
3. Polygenic traits-Traits that are controlled
by more than one gene…hence
polygenic (poly means many).
Multiple Alleles
• Many traits have more than two alleles in the
population. An example of this is human blood
type.
• The phenotype of blood type is familiar to most
people (A, B, AB, O), but the genotypes aren’t.
• The genotypes of blood are IA, IB, and i.
• When two IA are present, the blood type is A,
similarly, blood type B occurs when two IB are
together. O happens when two i are together.
AB occurs when both IA and IB are together,
making blood type an example of codominance
as well.
Pleiotropy and Environment
• When a single gene affects more than one trait,
this is an example of pleiotropy.
• Some genes are inactive in different
environments. A pigment might be shut off
during winter that allows an animal to be
camoflauged in the Springtime.
• In chapter 8 you will learn more about the
molecular reason why different genes are turned
on or off at different times.
• Remember!: Phenotype is the combination of
the expressed genotype and the environmental
constraints or input.