Genetics Notes

Genetics- the study of heredity, how traits or characteristics are passed from an organism to its offspring.

Gregor Mendel- Monk from Austria first studied traits or characteristics are carried by "factors" in pea plants

Gene- "factors" or units of heredity Mendel=s pea plants had 2 factors for each trait

Dominant Gene- able to mask a recessive gene so the organism shows the dominant trait, Capital letter in notation

Recessive gene- masked by the dominant gene, trait not shown, lower case letter in notation.

Purebred- having both genes of the same type (homozygous)

Hybrid- has different genes for a trait (heterozygous)

Phenotype visible characteristics or appearance

Genotype actual genetic make up

Principles of genetics

Traits are handed down from one generation to the next

Traits are controlled by genes

Genes are inherited in pairs, one from each parent

Some genes are dominant some are recessive

Dominant genes can mask recessive genes when one of each is inherited

Some genes are not dominant or recessive but blend when inherited together

Allele one gene for a trait, Mendel's' plants have 2 alleles for each trait


Incomplete dominance-(Blending) neither gene masks the other, the genes blend or show a new appearance (shows a third phenotype example red crossed with white produced pink.

Co-dominance- both alleles are expressed


Multiple alleles - when more than 2 alleles control a trait. Example human blood. A dominant, B dominant, O recessive


Phenotype (blood group)




AA or AO


BB or BO







Sex linked traits - gene for a trait is carried on one of the sex chromosomes. example color blindness, hemophilia


Scientists use probabilities to predict the results of genetic crosses

Probabilities can be shown in a Punnett Square


Chromosomes- Made of DNA, direct cells activity and pass on traits

Chromosomes control the production of proteins

Meiosis - production of gametes (sperm or egg cells) which contain half the normal number of chromosomes

Mutation - a change in genes or chromosomes which causes a new trait to be inherited

Human body cells have 46 chromosomes in 23 pairs, sex cells 23 individual chromosomes

DNA- deoxyribonucleic acid, stores information needed to make proteins

RNA- ribonucleic acid, reads DNA and carries instructions out of the nucleus

Double helix- the twisted ladder shape of DNA first described by Watson and Crick

Nitrogen Bases - Make up the “rungs of the ladder” in the DNA molecule, adenine, thymine, cytosine, guanine. Bond in Pairs Adenine to Thymine and Cytosine to Guanine