Gregor Mendel, working in the mid 1800s, performed inheritance experiments using garden peas in an effort to discover how variation arose in offspring. Mendel's Experimental Procedure
Pea plants proved to be an excellent choice for Mendel's experiments because of their reproductive mechanisms and the heritable traits they exhibit. Mendel used statistical analysis on his data from garden peas leading him to formulate his particulate theory of inheritance. One Trait Inheritance
Mendel's initial experiments examined a single trait. The Punnett square is a tool which charts the possible combinations of alleles in offspring from two parents. Test crosses can be performed to determine whether dominant phenotypes are heterozygous or homozygous. Mendel formed his law of segregation from this work. Today we know that many traits are controlled by dominant and recessive alleles of genes. Genes have a specific location called the gene locus. Genotype describes the actual alleles for a gene; phenotype is the physical expression of the genotype. Two Trait Inheritance
Mendel also performed experiments looking at inheritance patterns of two traits together. From this he formulated the law of independent assortment. A test cross can also be performed to discover if individuals expressing the dominant allele are homozygous or heterozygous. Geneticists today often use Drosophila melanogaster as a test subject. The wild type fruit fly is used to discover inheritance patterns in mutant flies. Mendel's Laws and Probability
Mendel used rules of probability such as the rule of multiplication to calculate the chance of inheriting particular alleles. Mendel's Laws and Meiosis
Today's knowledge of meiosis supports Mendel's laws of inheritance. The laws revolve around the idea that the homologous chromosomes segregate, each gamete receiving one set, and that the segregation of one pair is independent of others. 10.2 Beyond Mendel's Laws
Incomplete dominance causes a heterozygote to express an intermediate phenotype. Examples are flower color in the four-o’clock plants and inheritance of wavy hair in humans. Multiple-Allele Traits
Many genes have more than one allele. In the case of ABO blood type inheritance in humans, not only is it a good example of multiple-allele traits, but it also represents codominance and complete dominance. The ABO blood group in humans is inherited by three alleles. IA and IB (conferring the A and B blood groups respectively) are codominant, and are both dominant to the recessive i allele which confers the O blood group. Polygenic Inheritance
When a trait is controlled by more than one gene, this is called polygenic inheritance. Each dominant allele in each gene has an additive affect on the phenotype. Multifactorial traits are also controlled by more than one gene and are influenced by environmental factors. Environment and the Phenotype
Some phenotypes are influenced by certain environmental factors. The study of identical twins separated at birth is important in this instance, especially in humans. Pleiotropy
In some inheritance patterns, many traits are influenced by one gene. This is often discovered as the result of human diseases such as Marfan syndrome. 10.3 Sex-Linked Inheritance
Genes carried on the sex chromosomes are called sex-linked.
Genes on the X chromosome (X-linked genes) are often studied because of the different inheritance pattern between males and females in humans and Drosophila. An X-Linked Problem
An X-linked gene in Drosophila is presented as a sample problem. In X-linked genes, females can be carriers of recessive alleles, males cannot be carriers. 10.4 Inheritance of Linked Genes
Genes found together on a chromosome form a linkage group. Gene linkage can be used to form a chromosome map. Crossing over can create recombinant gametes having a new combination of alleles. The percentage of recombinant phenotypes can be used in constructing a chromosome map.