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Mendel, Genes
and Gene
Interactions
 The study of inheritance is called genetics.
Early theories suggested that offspring
were a blend of their parents factors (called
the “Blend Theory”). This could not explain
the appearance of recessive traits from one
generation to the next.
A monk by the name of Gregor Mendel
suspected that heredity depended on
contributions from both parents and that
specific characteristics from each parent
were passed on to their offspring.
Mendel studied pea
plants. He would
breed these and then
observed offspring
produced.
Above, a homozygous spherical seed plant is
crossed with a homozygous wrinkled seed plant.
Each parent produces gametes of only one kind,
either S or s, producing hybrid offspring with the
genotype Ss and the spherical seed phenotype.
Mendel often
learned most
from offspring
produced in
the F2 cross.
When the F1 plants self-pollinate they
produce three different genotypes and two
phenotypes "spherical seed" and
"wrinkled seed” in a classic 3:1 ratio.
What did Mendel figure out without
knowing about genes and chromosomes?
The spherical seed character is dominant
and the character for "wrinkled seed" is
termed recessive.
Dominant Traits - The spherical seed
phenotype corresponds to offspring with
one S allele "SS" or “Ss”genotypes.
Recessive traits - The wrinkled seed
phenotypes can only correspond to the "ss"
genotype.
What did Mendel figure out without
knowing about genes and chromosomes?
The spherical seed character is dominant
and the character for "wrinkled seed" is
termed recessive.
Dominant Traits - The spherical seed
phenotype corresponds to offspring with
one S allele "SS" or “Ss”genotypes.
Recessive traits - The wrinkled seed
phenotypes can only correspond to the "ss"
genotype.
Sex-Linked Inheritance: Drosophilia
In sex-linked inheritance, alleles on sex
chromosomes are inherited in predictable
patterns. For example, in Drosophilia the
locus for eye color is located on the X
chromosome. The allele for red eye color,
which is normal in wild flies, is dominant to
the mutant allele for white eyes.
Hemophilia is a sex linked trait in humans.
Males are hemizygous, receiving their only X
chromosome from their mother. Females are
heterozygous, inheriting chromosomes from
both parents.
If a female has a defective gene on one of her
two X chromosomes, she will be protected
from its effects by the normal gene on her
second X chromosome. If a male has a
mutant X and a normal Y chromosome, he
will be affected by a X-linked disease.
A son, whose mother has two normal alleles, will not be
affected by hemophilia even if the father has the disease
and the defective gene. A daughter of the same parents will
be a heterozygous carrier.
A heterozygous carrier mother and a normal father pass
the gene for hemophilia on to possibly one- half of their
children. Half the daughters will be carriers and half the
sons will be hemophilic. The rest of the siblings will be
normal. Daughters, as long as one parent is genotypically
normal, can only be carriers. The normal gene on the
second X chromosome counteracts the defect and the
daughters do not suffer from the trait. If a son receives the
defective gene from his mother, he will be hemophilic
because the Y chromosome can not counteract the
defective gene located on his X chromosome.