Chapter 13: Mendel and the Gene
13.1: Mendel’s Experiments with a Single Trait
Hypotheses present during Mendel’s time:
- Blending inheritance: two traits blend together for intermediate trait.(black + white=grey)
- Inheritance of acquired characteristics- giraffes acquire long necks by stretching for food,
pass trait to offspring.
First Model Organism in Genetics- Garden Peas
Model organism: small, short-lived, inexpensive to care for, able to produce large numbers of
offspring, easy to manipulate experimentally.
Genetics: branch of biology that focuses on the inheritance of traits.
Peas were a good choice because he could control which parents were involved in a mating, and
could arrange matings between individuals that differed in easily recognizable traits.
How did Mendel arrange matings?
Cut off male reproductive parts before any pollen could form (sperm cells form in pollen grains),
and brushed pollen from pollen grains of other plants on the female reproductive parts.
What traits did Mendel study?
Seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem
Phenotype: observable features of an individual ex. Eye color, hair color
Mendel began his work by mating individuals from pure lines (produce offspring identical to
themselves; result from selfing or cross-pollinating to another member of the pure-line
population). Did this so he could later compare the results to crossing with different pure-lines
Inheritance of a Single Trait
-First crossed pure lines that differed in just one trait.
-Initial experimental cross is parental generation. Their progeny are F gen1ration (first filial).
Certain traits ‘recede’: Crossed round and wrinkled. Progeny was round. Did a reciprocal
cross (mother’s phenotype in the first cross is the father’s in the second; father’s phenotype in
the first is the mother’s in the second), result was still the same. Where did determinant for
wrinkled seeds go? Dominant and Recessive Traits: F1 generation were allowed to self-pollinate. F2 generation
had approx. 3:1 ratio of smooth to wrinkled. Mendel called determinant for wrinkled recessive
and determinant for smooth dominant.
The Nature and Behavior of the Hereditary Determinants
Mendel proposed hypothesis of particulate inheritance. He maintained that the hereditary
determinants for traits do not blend together or acquire new or modified characteristics through
use. In fact, hereditary determinants maintain their integrity from generation to generation.
Instead of blending together, they act like discrete entities or particles.
Gene: Hereditary determinant for a trait.
Alleles: Different versions of same gene (dominant/recessive)
Genotype:Alleles found in a particular individual.Affects phenotype (physical traits).
Principle of segregation: Two alleles of each gene segregate/separate into different gamete cells
during the formation of eggs and sperm in the parents.
Homozygous: ex. RR or rr
Monohybrid cross: mating between parents that are both heterozygous for a trait.
-Remember: Punnett squares.
Mendel’s claims Notes
1. Peas have two alleles of each gene. True for many other organisms
2. Alleles do not blend together. Hereditary determinants maintain their
integrity from generation to generation.
3. Each gamete contains one allele of each Due to principle of segregation.Alleles of each
gene. gene segregate during formation of gametes.
4. Males and females contribute equally to When gametes fuse, offspring acquire a total of
the genotype of their offspring. two alleles for each gene, one from each parent
5. Some alleles are dominant to others. When dominant and recessive allele for same
gene found in an individual, it has dominant
13.2 Mendel’s Experiments with Two Traits
Dihybrid cross: Mating between parents that are both heterozygous for two traits. Principle of independent assortment:Alleles of different genes are transmitted independently
of one another.
Testcross: Uses a parent that contributed only recessive alleles to help determine the unknown
genotype of the second parent.
Punnett Square for Dihybrid Cross:
RY Ry rY Ry
RY RRYY RRyY rRYY rRyY
Ry RRYy RRyy rRYy rRyy
rY RrYY RryY rrYY rryY
Ry RRYy RRyy rRYy rRyy
13.3 The Chromosome Theory of Inheritance
States that chromosomes are composed of genes. The physical separation of alleles during
anaphase of meiosis I is responsible for Mendel’s principle of segregation.
Locus: Location of gene on chromosome.
Figure 13.7 Principle of Segregation: Each gamete carries only one allele for seed Figure 13.8 Principle of independent assortment: The genes for seed shape and seed color assort
independently, because they are located on different chromosomes. 13.4 Testing and Extending the Chromosome Theory
-Nettie Stevens notices that females of Tenebrio molitor diploid cells contain 20 large
chromosomes. Males contain 19 large (X) and 1 small (Y).Acted like homologs during meiosis.
X and Y contain different genes but have regions that are similar eno