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Week 6 – Inheritance and Genetics
Chapter 12
Gregor Mendel (1822-1884)
Belief at the time was that the traits of parents were “fused” or blended, no real
idea of how that happened.
Mendel studied the garden pea (see pg 204 –206)
The pea was a good choice because there were a large number of true breeding
varieties, short generation time, easy to grow and perfect flowers
Mendel observed variations in the garden pea as to the height, flower color, seed
coat color, and seed shape. In each case there were distinct contrasting forms.
Mendel bred his plants over many generations and counted the variations in
each successive generation.
He focused on 1 or 2 traits in each experiment. After collecting all the data he
analyzed the results and derived his conclusions.
He crossed plants with different traits, and learned that the offspring showed the
dominant trait. Most often but that expressions of the recessive traits showed up
in a consistent ratio. It was Mendel’s records that showed him the way../.
Mendel found that inheritance of traits was not due to blending but instead
the passing of specific traits, or units of inheritance, what we now cal
genes
Mendel crossed a tall growing variety of pea with a short growing variety. If the
fusing theory was correct you should get an intermediate height plant…
But instead they resembled the tall parent, but it this generation were allowed to
breed there would be throwbacks to the short variety at a ratio of 3:1 (3 tall for
every one short)
No matter what trait he selected for the second generation always contained
individuals of both traits always on a 3:1 ratio.
Mendel predicted the discovery of genes (specific units of inheritance) but also
the existence of Alleles, the pairing of two alternate forms of the gene.
He deduced that during the formation of gametes (haplod reproductive cells), the
two alleles separate from one another so that each gamete has only one of the
alleles.
During sexual reproduction, the allelic pairs are restored when the sperm and
egg combine.
Today we know a gene is a portion of DNA strand…every gene contains the
information necessary to manufacture a special polypeptide or protein.
Genes are the unit of DNA which contains the information code for a specific
amino acid.
DOMINANCE
Remember that chromosonal pairs are said to be homologous, that is genes
of similar traits arranged in similar order. The gene for each trait occurs at a
particular place in the chromosome, the Locus.
This helps us identify or discuss genetics more easily…since two alternative
forms of a gene will occupy a locus, each of these forms is assigned a letter.
The dominant form is the form that always expresses itself, it is always
designated with a capital letter.
The recessive allele, the one that does not express itself in the presence of the
dominant is always expressed with a lower case letter. (see discussion on page
209)
In a homologous pair, dominant alleles mask the expression of recessive alleles
Albino …lacks melanin because they lack an enzyme that is needed to form
melanin from the amino acid tyrosine. Albanism is caused by the production of
the defective enzyme not by the gene directly.
If an individual had both a normal and a defective gene the enzyme would be
produced and albanism would not have occurred.
Genotype – the genetic makeup of an individual, may contain things not
expressed
Homozygous dominant, expressing the dominant allele
Homozygous recessive, expressing the recessive allele
Heterozygous, expressing the dominant allele
Phenotype – the actual expression of the genetic makeup
A monohybrid cross is a study of a cross between two individuals where only one
trait is studied. The probable combinations of eggs and sperm may be
represented in a Punnett square. (see discussion on pg 208 and Figure 11-6 0n
pg 209)
AA
Homozygoous – both alleles specify
the same trait
aA
Aa
Heterozygous – alleles call for
different traits
Aa
Homozygous
Principles of Inheritance
1. Inherited traits are transmitted by genes, which occur in pairs called alleles.
2. The principle of dominance is when 2 alternate forms of the same gene (2
different alleles) are present in the individual often only one…the dominant
allele…is expressed.
3. The Principle of Segregation: when gametes are formed in meiosis, the two
alleles of each gene segregate or separate and each gamete receives only
one allele.
4. The Principle of Independent Assortment: when two or more traits are
examined in a single cross, each trait is inherited without relation to the other
traits. This occurs because the alleles for each trait assort into the gamete by
chance. All possible combinations of genes will occur.
A GENOTYPE MAY BE EITHER HOMOZYGOUS OR HETEROZYGOUS
If both alleles specify the same trait then the individual is said to be homozygous.
If the alleles call for different alternate traits the individual is said to be
heterozygous
So that,

If a individual is homozygous for the dominant allele the phenotype will
reflect the dominant allele
 If an individual is homozygous for the recessive allele the phenotype
will reflect the recessive allele
 If an individual is heterozygous the dominant allele will be reflected in
the phenotype.
Read page 208 “Genotype may be either Homozygous or heterozygous”
Read “Genes Located on Different Chromosomes are inherited independently”
page 211
Dihybrid Cross mating of individuals that differ in 2 genes…
Try out the tongue roll or attached ear lobes test.
One way to discover whether the genotype of an organism that displays a
dominant phenotype is perform a test cross. The subject might be homozygous
for the dominant allele or heterozygous. By crossing with a homozygous
individual with a recessive phenotype, if the subject were heterozygous some of
the offspring should display the recessive trait but if it is homozygous all of the
offspring would reflect the dominant trait.
Rules of Probability Probability theory says that the chance of two independent
events occurring in combination is the product of their individual probabilities, so
that if two events each have a 50% chance of occurring than the chance of them
occurring together is 25%. (.5 x .5)
Incomplete Dominance – apparent blending in phenotype page 214
Codominance – page 214
Chapter 12 – human genetics page 224
GENETIC MAPPING – the human genome project (mapping all of the genetic
information carried in a human cell)
We have 23 pairs of chromosomes which contain between 50,000 and 100,000
genes. The autosomes are the first 22 chromosomes and the sex chromosomes
are the last pair.
The Y Chromosome causes maleness, therefore the genotype XX is female and
the genotype XY is male. Y chromosome is very small and carries information
only about maleness.
Men are hemizygous, that is they have an x and y sex chromsome, women are
homozygous – two X chromosomes.
Human Genetic Disorders –
Down Syndrome – 3 chromosomes of chromosome 21, metnal retardation
susceptability to disease and cancer , more common as mothers are older
Klinefelter Syndrome – XXY: phenotypically male, may be mentally retarded
Turner Syndrome : XO lack second sex chromosome, phenotypically female,
sterile, may be mentally retarded
XXY karyotype: XXY, male few other effects
Sickle Cell anemia
Cystic Fibrosis
Huntington’s Disease