Gregor Mendel was an Austrian monk who studied genetics and observed how these traits were inherited over several generations.

GREGOR MENDEL

His work was REALLY important to understanding inheritance.

He studied what happened when he bred different kinds of pea plants (ex. green vs. yellow, short vs. tall, round vs. wrinkly)

From his experiments he came up with several conclusions.

GREGOR MENDEL

Biological inheritance is determined by “factors” (genes within our DNA )that are passed on from one generation to the next.

Different forms of these genes determine our physical traits.

For every gene, we inherit two alleles (one allele from each parent).

WHAT DID HE LEARN FROM HIS STUDIES?

trait: specific characteristic that varies from one individual to another

allele: one of a number of different forms of a gene

locus: the location of a gene on a chromosome

WHAT DID HE LEARN FROM HIS STUDIES?

Principle (Law) of Segregation states that the two different alleles for a gene segregate independenty from one another during meiosis

MENDEL’S THREE PRINCIPLES

Principle (Law) of Independent Assortment states that different traits sort and are inherited independently of one another when sex cells are formed

MENDEL’S THREE PRINCIPLES

For every gene, we inherit two copies (one allele from each parent)

MENDEL’S PRINCIPLES (A SUMMARY)

Blonde HairBlue Eyes

Brown HairBrown Eyes

Blonde HairBrown Eyes

If we have two alleles for every trait, how does our body pick which one to express?

IF WE HAVE TWO SETS OF MANUALS, HOW DO WE KNOW WHICH DIRECTIONS TO FOLLOW?

Blonde HairBlue Eyes

Brown HairBrown Eyes

Blonde HairBrown Eyes

When we get directions from each parent, it is a combination of the different directions that determine our traits.

IF WE HAVE TWO SETS OF MANUALS, HOW DO WE KNOW WHICH DIRECTIONS TO FOLLOW?

Principle (Law) of Dominance states that when two or more forms of a gene (alleles) exist for a single trait, some alleles are dominant while others are recessive

MENDEL’S THREE PRINCIPLES

Some alleles are dominant and others are recessive .

Organisms with a dominant allele for a particular trait WILL exhibit that trait, because the dominant allele overpowers the recessive allele.

PRINCIPLE OF DOMINANCE

A recessive trait will only show in in the organism when BOTH alleles are recessive – that is, when the dominant allele is NOT present.

PRINCIPLE OF DOMINANCE

A Punnett square is a useful tool used to determine or predict the probability that offspring will have certain traits after a genetic cross.

PUNNETT SQUARES

Probability: the likelihood that a particular event will occur Because each organism has two alleles for each gene there is an equal (50/50) chance of giving either allele to an offspring

PUNNETT SQUARES: PROBABILITY

Genotype: genetic makeup of an organism (alleles)Genotype sounds like genes

Phenotype: physical characteristics/appearance of an organism

Phenotype sounds like physical

PUNNETT SQUARES: TERMS

Homozygous: an organism with two identical alleles for a particular trait

Heterozygous: an organism with two different alleles for the same trait

PUNNETT SQUARES: TERMS

Homozygous for the tall trait – “TT” is the genotype, “tall” is the phenotype

Heterozygous for the tall trait – “Tt” is the genotype, “tall” is the phenotype

Homozygous for the short trait – “tt” is the genotype, “short” is the phenotype

LET’S USE THOSE WORDS IN AN EXAMPLE

Use letters to stand for allelesDominant alleles are noted with a

CAPITAL ( D ) letter

Recessive alleles for the same trait are noted with the lowercase ( d ) of the same letter.

PUNNETT SQUARES: RULES

Parental genotypes are shown along the top and side of the Punnett square.

Write one parent’s alleles on the top, and write the other on the side

Put one letter next to each cell, and drag alleles across or down so that there are two alleles in each square.

PUNNETT SQUARES: RULES

Each of the four squares represents a possible genotype of the offspring

These are just probabilities!

PUNNETT SQUARES: RULES

PUNNETT SQUARE PRACTICE

1. A homozygous tall (TT) pea plant is crossed with a homozygous short (tt) pea plant:

PUNNETT SQUARE PRACTICE

A homozygous red (RR) flower is crossed with a heterozygous red (Rr) flower:

PUNNETT SQUARE PRACTICE

A heterozygous funny (Ff) bird is crossed with a heterozygous funny (Ff) bird:

When you have two homozygous parents? Genotype Ratio: ________ Phenotype Ratio:________

When you have one homozygous and one heterozygous parent? Genotype Ratio: ________ Phenotype Ratio:________

When you have two heterozygous parents? Genotype Ratio: ________ Phenotype Ratio:________

OFFSPRING RATIOS

Sometimes, we need to do a Punnett square to follow more than one generation.

Generations: P F1 F2 parents first fi lial

second fi lial generation generation

MULTIPLE GENERATIONS

If you need to figure out the genotype of a parent with the dominant trait, you can do a test cross.

This is done by crossing that parent with a homozygous recessive organism.

TEST CROSSES

If any offspring show the recessive allele, you know that the dominant parent is heterozygous.

TEST CROSSES

If none of the offspring show the recessive allele, you know that the dominant parent is homozygous.

TEST CROSSES

A man with free earlobes wants to know if he is homozygous or heterozygous for that trait. He marries a woman with attached earlobes (the recessive trait). His first child has attached earlobes. The man has thus found out that he is _________________________.

EXAMPLE:

Some alleles are neither dominant nor recessive – many traits are controlled by more than one allele or by multiple genes…

GOING BEYOND DOMINANT AND RECESSIVE ALLELES

Incomplete dominance: a situation in which one allele is not completely dominant over another

Because neither trait dominates the other, sometimes the alleles are written differently (ex. RW, C rCw, DD’)

Examples are: flowers (red/pink/white),

hair (curly/wavy/straight), fur (long/short)

INCOMPLETE DOMINANCE

Notice that the offspring show a third phenotype that is different from the parents

The offspring phenotype is a MIX or BLEND of the parental traits

IDENTIFYING INCOMPLETE DOMINANCE

What happens when we cross a red (RR) flower with a white (WW) flower?

The offspring look pink

IDENTIFYING INCOMPLETE DOMINANCE

codominance: a situation in which both alleles of a gene contribute to the phenotype of the organismWhat does the prefix “co-“ mean? to share

Because neither trait dominates the other, sometimes the alleles are written differently (ex. RW, C rCw, DD’)

Examples are: cows (red/roan/white), flowers (red/spotted/white), blood type (A/AB/B)

CODOMINANCE

Notice that the offspring show a third phenotype that is different from the parents

The offspring phenotype has both parental traits appearing TOGETHER

IDENTIFYING CODOMINANCE

What happens when we cross a pure red (RR) cow with a pure white (WW) cow?

The offspring look roan -spotted with red & white

IDENTIFYING CODOMINANCE

What happens when we cross a pure red (RR) cow with a pure white (WW) cow?

The offspring look roan -spotted with red & white

IDENTIFYING CODOMINANCE

Traits controlled by two or more genes are said to be polygenic.

Polygenic traits show a wide range of phenotypes because different combinations of alleles on these genes produce many different traits.

POLYGENIC TRAITS

For example, hair color and eye color are controlled by more than one gene and therefore have more than two phenotypes.

POLYGENIC TRAITS

And, the wide range of skin color in humans is due to more than four different genes that control this trait.

POLYGENIC TRAITS