Introduction to Genetics Basic Vocabulary Genetics The scientific

Introduction to Genetics Basic Vocabulary

Genetics The scientific study of heredity. Began with Austrian monk Gregor Mendel and his pea plants in the mid-1800’s. Mendel known as “The Father of Genetics” Heredity: The passing of traits from parents to offspring.

Traits are physical characteristics inherited from your parents. The genes that you inherit from your parents determines what traits you will show (express).

Alleles An allele is simply a form of a gene and its trait being expressed (shown). Example: height in pea plants Pea plants can be tall or short. Each is a form of the allele “Height”. In basic genetics, the dominant form of the trait will always show if there is at least one dominant allele inherited.

Forms of Alleles There are two basic forms of alleles – dominant and recessive. Alleles that can mask or hide another form of an allele are said to be dominant. You only need one dominant gene for it to be expressed. Capital letters are used to show a dominant allele (A). Alleles that are only expressed if two copies are inherited are said to be recessive. Lowercase letters are used to show a recessive allele (a).

Patterns of Inheritance Mendel referred to some of his plants are being “purebred” or “pure”. A plant that is purebred has inherited two copies of the same allele. The term homozygous is used when there are two copies of the SAME allele (two dominant or two recessive). AA = homozygous dominant (two dominant) aa = homozygous recessive (two recessive)

Patterns of Inheritance cont. Organisms can inherit one of each allele from their parents. If they do this, they are said to be “hybrid” or “heterozygous. ” The term heterozygous means that one of each form of an allele is inherited. Since there is a dominant allele, this is the form that is expressed. Aa = heterozygous (one of each)

Genotypes vs Phenotypes Genotypes A genotype refers to the actual forms of the alleles inherited. A genotype is written as a sequence of capital or lowercase letters. AA = homozygous dominant Aa = heterozygous aa = homozygous recessive Phenotypes A phenotype is the physical appearance of an organism due to its inherited genotype. Phenotypes are descriptions. AA = tall plant Aa = tall plant aa = short plant

Demonstrating Inheritance Patterns To show the chance of inheriting a trait, geneticists use Punnett Squares. A Punnett Square is a chart that shows all possible combinations of the alleles that can result from a genetic cross of two parents. From a Punnett Square, a geneticist can determine the probability of a trait being inherited. Probability is a number that describes how likely it is that an event will occur and can be a percentage or ratio (25% or 1 in 4 chance).

How to create a Punnett Square

Punnett Squares cont. Here is a simple cross between two parents who are BOTH heterozygous for trait “R”. The mother’s genotypes is usually written on the left and the father’s on the top.

Punnett Squares cont. Move a copy of the mother’s genes from the left into each of the boxes to the right.

Punnett Squares cont. Now move a copy of the father’s genes from the top down into the boxes below. Note that the capital letter (dominant allele) is always written first.

Punnett Squares cont. The resulting Punnett Square shows the probability of the offspring inheriting any possible traits. Since there are 4 squares, each square is ¼ or 25%.

Probability of Inheritance For a cross between two parents both heterozygous for a trait, there is: 25% chance the offspring will inherit RR (1/4) 50% chance the offspring will inherit Rr (2/4) 25% chance the offspring will inherit rr (1/4)