Genetics: The Study of Heredity

Heredity – the passing of traits from parents to offspring Genetics – the scientific study of heredity

What factors control the inheritance of traits in organisms? Gregor Mendel (1851) was the “father of modern genetics”. Traits – physical characteristics of organisms

Mendel’s Peas Why did Mendel choose to study peas rather than other plants in the monastery garden? Pea plants have traits that exist in only two forms:  height - tall or short, have either green or yellow pods, have either wrinkled or smooth seeds.  Also, peas produce a large number of offspring quickly.

Mendel’s Experiments Purebred – an organism with the same form of a trait as the parent Genes – factors that control traits Alleles – different forms of a gene For example, in pea plants, the gene for height has one allele for tall stems and one allele for short stems.  Each plant inherits two alleles for height, one from each parent.

Mendel’s Experiments Individual alleles control the inheritance of traits.  Some alleles are dominant,while other alleles are recessive.  Dominant allele – an allele that is always expressed if it is present in the organism Recessive allele – an allele that is masked (covered up) when a dominant allele is present.  The only way for a recessive allele to be expressed is if the organism does not have a dominant allele for that trait.

Examples of Dominant and Recessive Alleles In pea plants, tall stems is dominant over short stems. If a pea plant has one allele for tall and one allele for short, then it will be tall. What combination of alleles will result in a short pea plant? Two alleles for short

Genetic Crosses and Symbols Hybrid – an organism with different alleles for a trait Letters are used to represent alleles in genetic crosses. For example, if “T” is used to represent tall stems, then “t” is used to represent short stems.  A tall pea plant could have the following combination of alleles:  TT or Tt.  A short pea plant would be written like this:  tt.

Punnett Squares:

Punnett Squares:

Phenotype: how the trait appears Genotype: the two alleles that control phenotype

Pedigree A pedigree shows phenotypes of genetically related family members. 

Incomplete Dominance When the offspring's phenotype is a combination of the parents phenotypes. 

Codominance When both alleles can be observed in a phenotype. 

Polygenic Inheritance Occurs when multiple genes determine  the phenotype of a trait. Ex. Skin color, eye color