WHAT DID MENDEL FIND? He discovered different laws and rules that explain factors affecting heredity.

LAW OF SEGREGATION The two alleles for a trait must separate when gametes are formed

A parent randomly passes only one allele for each trait to each offspring

LAW OF INDEPENDENT ASSORTMENT

The genes for different traits are inherited independently of each other.

RULE OF UNIT FACTORS

Each organism has two alleles for each trait

Alleles - different forms of the same gene

Genes - located on chromosomes, they control how an organism develops

Need to

Know!

RULE OF DOMINANCE The trait that is observed in the offspring is the dominant trait (uppercase-DD)

The trait that disappears in the offspring is the recessive trait (lowercase dd)

PHENOTYPE & GENOTYPE Phenotype - the way an organism looks

red hair or brown hair Genotype - the gene combination of an organism

AA or Aa or aa

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HETEROZYGOUS & HOMOZYGOUS Heterozygous - if the two alleles for a trait are different (Aa)

Homozygous - if the two alleles for a trait are the same (AA or aa)

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DETERMINING PATTERNS OF INHERITANCE A Punnett square is a chart which

shows/predicts all possible gene combinations in a cross of parents (whose genes are known).

Punnett squares are named for an English geneticist, Reginald Punnett. He discovered some basic principles of genetics, including sex linkage and sex determination.

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PUNNETT SQUARES Setting up and using a Punnett square is

quite simple once you understand how it works.  You begin by drawing a grid of perpendicular lines:

PUNNETT SQUARES Next, you put the genotype of one

parent across the top and that of the other parent down the left side.  For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:

Note that only one letter goes in each box for the parents.   It does not matter which parent is on the side or the top of the Punnett square.  

PUNNETT SQUARES Next, all you have to do is fill in the

boxes by copying the row and column-head letters across or down into the empty squares.  This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.

LET’S REVIEW! What is the phenotype? What is the genotype? What is homozygous? What is heterozygous? What do we call the trait that is

observed? What case (upper or lower) is it

written in? What about the one that disappears? What case is it written in?

LETS PRACTICE! 1. In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a cross of two plants that are heterozygous for the seed shape trait, what percentage of the offspring should have spherical seeds?

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