NOTES FOR BIOLOGY 1001

SECTION 005

Spring 2005

DR. STEVEN POMARICO

CHAPTER 11

OBSERVABLE PATTERNS OF INHERITANCE

>>>>>>The father of genetics: Johann (Gregor) Mendel

Mendel was a monk who was trained in science at the University of Vienna.

The realization that both parents contributed to the characteristics of the offspring preceded the work of Mendel. The favorite explanation of how this occurred was the blending theory of heredity.

         -the offspring’s traits are some intermediate between the parental types.

         -Unable to explain why characteristics disappear in one generation and                    reappear in the next.

         -If this was true population would reach a uniform appearance.

Modern genetics began with Mendel, and lead to the replacement of the blending theory with the particulate theory of heredity.

Mendel developed his theory from the study of the variation of garden peas.

Why peas??

         -Genes for many different traits

---Genes are the units of heredity and are made of nucleic acid (DNA).

---Traits are the variants of an inheritable feature

For Mendel’s pea plants the traits he examined were (See fig. 11.5)

         1. Seed shape (smooth or wrinkled)

         2. Seed color (yellow or green)

         3. Pod shape (inflated or constricted)

         4. Pod color (green or yellow)

         5. Flower color (purple or white)

          6. Flower location (at branches or at the end)

         7. Plant size (tall or dwarf)

The flowers of the pea plant normally undergo self-fertilization

         -pollen and eggs come from the same flower.

Mendel started his experiments with true-breeding plant varieties which he hybridized (cross-fertilized)

---True-breeding are varieties in which self fertilization leads to offspring which are the same type as parent

Terminology for breeding (or genetics) experiments

         -P generation = parents

         -F₁ generation = hybrid offspring of the P generation

         -F₂ generation = offspring produced via self-pollination of F₁ generation

Mendel observed the transmission of the selected traits over these three generations and arrived at two principles of heredity:

                   The law of segregation

                   The law of independent assortment

>>>>>According to the law of segregation, two alleles for a character are packaged into separate gametes.

---Alleles (See fig. 11.3) are alternative forms of a gene that occupy a specific position (locus) on a specific chromosome.

In one of Mendel’s typical experiments he examined the way traits (purple flowers or white flowers) were expressed over three generations (P, F₁, and F₂) (See fig 11.2)

He found that in the F₁ generation these character traits did not blend as predicted. Instead the white color was lost.

Mendel hypothesized that the loss of the white color meant the plants had lost that allele and when he crossed the F₁ plants he would get only purple-flowered plants.

He performed the crosses and the F₂ generation had purple-flowered plants and white-flowered plants in a 3:1 ratio.

         THE HYPOTHESIS IS WRONG!!

The white allele was not lost but was masked by the presence of the purple allele.

Mendel had found that the trait for purple flowers was a dominant trait and the trait for white flowers was a recessive trait.

Mendel repeated these experiments with the six other characters and found similar 3:1 ratios in the F₂ generations. This lead him to the following four part hypothesis:

         1. Alternative forms of genes are responsible for variations in inherited              characters.

         2. For each character, an organism inherits two alleles, one from each                     parent.

         3. If the two alleles differ, one is fully expressed (dominant allele); the                    other is completely masked (recessive allele).

         4. The two alleles for each character trait segregate during gamete                    production.

---Dominant allele is an allele in which the trait is always expressed when this allele

is present.

---Recessive alleles is an allele in which the trait is masked by the presence of another allele

---Mendel’s law of segregation states that allele pairs segregate during gamete formation(meiosis), and the paired condition is restored by random fusion of gametes during fertilization

         -predicts the 3:1 ratio observed in the F₂ generation.

         -Punnett square

---Homozygous is the condition in which both the alleles are the same of a given trait.

         -all gametes carry that trait

         -homozygotes are true-breeding

---Heterozygous is the condition in which the two alleles are different for a given trait.

         -half the gametes carry one allele and half the carry the other allele.

         -heterozygotes are not true-breeding

Genotype versus Phenotype

---Phenotype is an organism’s expressed traits

         -what it looks like

---Genotype is an organism’s genetic makeup

Because the genotype is not always apparent from the phenotype, a testcross can be used to determine the genotype.

---Testcross is the breeding of an organism of unknown genotype with a homozygous recessive organism.

>>>>>>According to the law of independent assortment, each pair of alleles segregates into gametes independently

The parental crosses that Mendel did in his earlier experiments involved parental varieties which differed in a single trait (i.e., monohybrid crosses)

In later experiments Mendel crossed parental varieties that differed at two characters (i.e., dihybrid crosses) (See fig. 11.9)

---Dihyrid crosses are mating between parents that are heterozygous for two characters

By using plants which were true-breeding (homozygous) for two different characters, Mendel tested how the traits for these characters segregated.

The results lead to the law of independent assortment.

---Mendel’s law of independent assortment states that each allele pair segregates independently of other gene pairs during gamete formation.

>>>>>>The relationship between genotype and phenotype is rarely simple

INTERMEDIATE INHERITANCE or incomplete dominance (See fig. 10.10)

---Incomplete dominance is the pattern of inheritance in which the dominant phenotype is not fully expressed in the heterozygous genotype. The heterozygous genotype has a phenotype intermediate between the homozygous dominant and the homozygous recessive.

         >sounds like blending theory but it’s not

                   -recessive trait can still emerge in the F₂

                   -genotype is apparent from phenotype