Mendel, Genes and Gene Interactions

§The study of inheritance is called genetics. 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. 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.

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.

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. Mendel often learned most from offspring produced in the F 2 cross.

Recessive traits - The wrinkled seed phenotypes can only correspond to the "ss" genotype. What did Mendel figure out without knowing about genes and chromosomes? Dominant Traits - The spherical seed phenotype corresponds to offspring with one S allele "SS" or “Ss”genotypes. The spherical seed character is dominant and the character for "wrinkled seed" is termed recessive.

Recessive traits - The wrinkled seed phenotypes can only correspond to the "ss" genotype. What did Mendel figure out without knowing about genes and chromosomes? Dominant Traits - The spherical seed phenotype corresponds to offspring with one S allele "SS" or “Ss”genotypes. The spherical seed character is dominant and the character for "wrinkled seed" is termed recessive.

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.