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Basic Mendelian Genetics

Basic Mendelian Genetics

Author: Andrew Graham
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How to solve a genetics problem.

Slideshow takes you step-by-step how to analyze and solve a genetics problem involving alleles that are controlled by simple dominance.

Vocabulary used with Mendelian Genetics

Trait: An inherited characteristic that’s coded for in the DNA.
Gene: A segment of DNA that codes for a specific trait.
Allele: One version of a gene – there may be different versions of a trait. Each person carries two genes for each trait. They may be the same alleles or they may be different alleles. In genetics, alleles are represented by a letter.


Chromosome: One piece of DNA that contains many genes. A human has 46 chromosomes (two sets of 23 pairs called homologous pairs). Each homologous chromosome carries the same genes as its partner, and they’re located in the same place, but the corresponding genes on homologous chromosomes may be different alleles. WHY?? - A sperm or egg cell carries 23 chromosomes – one half of each homologous pair. The sperm and egg fuse together during fertilization, each donating 23 chromosomes for a total of 46. Therefore each person inherits one set of genes from their mother and one set of genes from their father.


Dominant: These alleles are ALWAYS expressed if they are present. Dominant alleles are signified by a capital letter. (ex. B = Brown hair)
Recessive: These alleles are ONLY expressed if there is no dominant allele present. Recessive alleles are represented by a lower case letter. (ex. b = blond hair)
Homozygous: A person who carries two of the same alleles. (ex. AA or aa)
Heterozygous: A person who carries two different alleles (ex. Aa)
Genotype: Which alleles a person carries – signified by the two letters. (ex. Ff)
Phenotype: The physical expression of a trait. (ex. Brown hair or Tall)

After viewing the tutorial...

Try these three genetics problems using the instructions given in the tutorial.