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Genetics Genetics is the scientific

study of heredity of traits.

A trait is a characteristic that varies from one individual to another

Gregor Mendel is considered the father of genetics

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Genes and Dominance Mendel studied 7 different pea traits.

In crossing (mating) plants the original pair of plants is

known as the parental generation (P)

The offspring of the P generation are called F1 “first filial”

generation

The offspring of crosses between parents with different

traits are called hybrids

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All of the F1 offspring had the character of only one of the parents

From this Mendel drew 2 conclusions:

Inheritance is determined by factors that are passed from one generation to the next

The principle of dominance

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Factors that determine traits: genes

Each trait Mendel studied was controlled by one gene that occurred in contrasting forms

The contrasting forms produced the different characteristics of each trait

Ex: height trait can be short or tall

The different forms of genes are known as alleles

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Principle of Dominance States that some alleles are dominant and others are

recessive.

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Organisms with a dominant allele for a particular form of a trait will always exhibit that form of the trait.

Recessive traits will only show when the dominant allele for the trait is not present.

In Mendel’s experiment when looking at height:

Tall was dominant and short was recessive

Short plants only occurred

when the dominant allele

wasn’t present.

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Letter Representation: Upper-case letters: dominant alleles

Lower-case letters recessive alleles

Homozygous organisms have 2 identical alleles for a particular trait

TT or tt

Heterozygous organisms have two different alleles for the same trait

Tt

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Explaining the F1 Cross The dominant allele covered the recessive allele in the

F1 generation.

Traits controlled by the recessive allele showed up in some of the F2 plants- indicating that at some point the allele for shortness was separated from the allele for tallness.

How did this segregation of alleles occur?

Mendel explained it as the plant segregated from each other during the formation of the gametes

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During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces 2 types of gametes

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Genetic and Probability Probability: the likelihood an event will occur

Ex: flipping a coin

Individual probabilities are multiplied together

Past outcomes DO NOT affect future ones

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How is flipping a coin relevant to genetics?

The way alleles segregate is completely random

Probability can be used to predict the outcomes of genetic crosses.

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Punnett Squares Used to predict, determine and compare gene

combinations that can result from genetic crosses.

F1 parents are along top and side

F2 go in the 4 boxes

The letters in the squares represent alleles

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All tall plants have the same phenotype

Physical characteristics

Tall plants can have different genotypes

Genetic make-up or the actual allele combinations

Tall plants can be TT or Tt

T T

t Tt Tt

t Tt Tt

T T

T TT TT

T TT TT

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Probability & Segregation Because the allele for tallness is dominant over the allele for

shortness when a cross of F1 plants is done:

1/4 (25%) of the F2 plants have 2 alleles for tallness (TT)

1/4 boxes

½ (50%) of the F2 plants have 1 allele for tallness and 1 allele for shortness (Tt)

2/4 boxes

¼ (25%) of the F2 plants have 2 alleles for shortness (tt)

T t

T TT Tt

t Tt tt 15

Ratio of tall plants to short plants is 3:1

The means that 25% of the plants showed the trait controlled by recessive allele (short)

Probabilities predict the average outcome of a large number of events

Probability cannot predict the exact outcomes of an individual event.

The larger the number of offspring, the closer the resulting numbers will get to expected values.

T t

T TT Tt

t Tt tt

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