Traits and Inheritance

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TRAITS AND INHERITANCE

A GREAT IDEA

• Mendel figured out that when his garden pea plants were crossed, for every three pea plants that showed a dominant trait, such as purple flowers, yellow seeds, green pods, tall plants, there was one plant that exhibited recessive traits, such as white flowers, green seeds, yellow pods and short plants.• This is a 3:1 ratio• What did this tell him about how traits are passed

from parents to their offspring?

A GREAT IDEA• Mendel knew from his

experiments that there must be two sets of instructions for each characteristic .• The first-generation

plants carried these instructions for both the dominant and recessive traits.• Scientists call these

instructions for an inherited trait genes.

A GREAT IDEA

• Each parent gives one set of genes to their offspring.• The offspring then has

two forms of the same gene for each characteristic.• One from each parent.

• The different forms of a gene are called alleles.

ALLELES

• Alleles are a different form of a gene.• They occupy specific

places on a chromosome.• Dominant alleles are

shown with a capital letter• Recessive alleles are

shown with a lowercase letter

PHENOTYPES

• Genes affect the traits of an offspring.• An organism’s

appearance is know as its phenotype.

GENOTYPE

• Genotypes are a little more complicated…• Both inherited alleles together form an organism’s

genotype.• The genotype of an organism represents its exact genetic

makeup, that is, the particular set of genes it possesses.• Because the allele for purple flowers (P) is dominant, only

one P allele is needed for the plant to have purple plants. It’s genotype may be either PP or Pp

• If a plant has white flowers, we know that it’s genotype will be pp, because the allele for white flower is recessive. The organism must have two p alleles for the recessive trait to be expressed.

HOMOZYGOUS VS. HETEROZYGOUS

Homozygous• An gene with either

two dominant or two recessive alleles (PP or pp)• Root “homo” means

“same”

Heterozygous• A gene with two

different alleles (Pp)• Root “hetero” means

“different”.• The dominant trait will

be expressed.

PUNNETT SQUARES

• A Punnett Square is used to organize all the possible combinations of offspring from their parents.

WHAT ARE THE CHANCES?• Each parent has two

alleles for each gene.• When these alleles are

different, the offspring are equally likely to receive either allele.• When they are the

same, the offspring will definitely receive either the dominant or recessive allele being passed

GENOTYPE PROBABILITY

• The mathematical chance that something will happen is called probability.• Probability is most often written as a fraction or

percentage.• If you toss a coin, the probability of tossing tails is ½,

or 50%.• To find the probability of that you will toss two heads

in a row, multiply the probability of tossing the first head (1/2) , by the probability of tossing the second head (1/2).• The probability of tossing two heads in a row is ¼, or

25%

GENETIC PROBABILITY

•The same holds true when figuring out genetic probabilities!!!

GENOTYPE PROBABILITY

• For example, to have white flowers, a pea plant must receive a p allele from each parent.• Each offspring from a

Pp X Pp cross has a 50% (1/2) chance of receiving either allele from either parent.

GENOTYPE PROBABILITY

• The probability of inheriting two p alleles is ½ X ½

• This equals ¼ or 25%

MORE ABOUT TRAITS

• As you may have already figured out, things are usually more complicated than they first appear to be!• Mendel uncovered the basic principles of

how genes are passed from one generation to the next.• But as scientists learned more about

heredity, they began to find exceptions to Mendel’s principles

ONE GENE, MANY TRAITS• Sometimes one gene

influences more than one trait.

• An example of this phenomenon is shown by the white tiger.

• The while fur is determined by a single gene.

• However, this gene will also determine the tiger’s eye color, which is blue.

• The gene that controls fur color also influences eye color

INCOMPLETE DOMINANCE• Sometimes one trait is not

completely dominant over another.

• These traits do not blend together, but each allele has its own degree of influence.

• This is called incomplete dominance.

• An example is when you cross red snapdragons with while snapdragons, you will end up with pink snapdragons

MANY GENES, ONE TRAIT

• Some traits, such as the color of your skin, eyes, and hair are the result of several genes working together.• That’s why many

shades of a single color are possible.

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