Download Mendel and the Gene

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.
Mendel’s
experiments
Cross between varieties
Alleles of a gene at a locus
Law of
segregation
Genotype v. phenotype
Testcross
Independent assortment of genes
Segregation by chance
Overall lessons:
• Traits can be inherited as a result of possessing specific pairs of alleles
at a gene locus.
• For traits controlled by only one gene, inheritance is best explained by
Mendelian rules of inheritance.
• The law of segregation results from the randomness of meiosis I
separation of maternal and paternal chromosomes into different
gametes.
• Some (dominant) alleles do not affect phenotype when paired with
some other (recessive) alleles.
• The law of independent assortment results from the fact that some gene
loci are located on different chromosomes, so the fact that a gamete
gets a chromosome that is paternal, does not mean that the other
chromosomes it receives with be paternal.
• Even alleles on the same chromosome can be “independent” if they are
far enough apart that crossing over almost always occurs somewhere
between those two loci.