Probability and Probability and Punnett SquaresPunnett Squares

ProbabilityProbability

Probability is the likelihood that a Probability is the likelihood that a specific event will occur. specific event will occur.

For example, if you flip a coin, the For example, if you flip a coin, the probability that it will land on probability that it will land on heads is 50%, or 1 out of 2.heads is 50%, or 1 out of 2.

We can use Punnett Squares to We can use Punnett Squares to predict the probability of genetic predict the probability of genetic outcomes.outcomes.

Dominant and Recessive Dominant and Recessive AllelesAlleles

Dominant alleles are expressed in Dominant alleles are expressed in capital letters.capital letters.

Ex: T B A C XEx: T B A C X

Dominant alleles can mask Dominant alleles can mask recessive alleles. This means you recessive alleles. This means you might have a recessive allele, but might have a recessive allele, but you can see the recessive trait if you can see the recessive trait if the dominant allele is present.the dominant allele is present.

Recessive alleles are represented Recessive alleles are represented by lowercase letters.by lowercase letters.

Ex: t b a c xEx: t b a c x

Recessive alleles are masked by Recessive alleles are masked by dominant alleles.dominant alleles.

Homozygous and Homozygous and HeterozygousHeterozygous

An organism is homozygous if both An organism is homozygous if both alleles are identical (both dominant alleles are identical (both dominant or both recessive).or both recessive).

Ex: TT, BB, aa, cc, ttEx: TT, BB, aa, cc, tt

An organism is heterozygous if the An organism is heterozygous if the alleles are different (one dominant alleles are different (one dominant and one recessive).and one recessive).

Ex: Tt, Bb, Rr, XxEx: Tt, Bb, Rr, Xx

A heterozygous organism A heterozygous organism expresses the dominant trait, but expresses the dominant trait, but is a carrier of the recessive trait. is a carrier of the recessive trait.

TT = homozygous dominantTT = homozygous dominant

tt = homozygous recessivett = homozygous recessive

Tt = heterozygous Tt = heterozygous

Genotype and PhenotypeGenotype and Phenotype

Phenotype is the physical Phenotype is the physical appearance of the trait.appearance of the trait.

Ex: brown hair, blue eyes, tall, Ex: brown hair, blue eyes, tall, short, etc.short, etc.

Genotype is the genetic makeup of Genotype is the genetic makeup of the organism, or what the alleles the organism, or what the alleles actually are.actually are.

Ex: BB, Tt, Rr, rrEx: BB, Tt, Rr, rr

ExampleExample

A plant is tall, but has A plant is tall, but has heterozygous alleles for height. heterozygous alleles for height. What is the genotype?What is the genotype?

A dog is homozygous dominant for A dog is homozygous dominant for a straight tail. What will be the a straight tail. What will be the genotype if the letter that genotype if the letter that represents tail straightness is S?represents tail straightness is S?

Punnett SquaresPunnett Squares

A Punnett square is a simple A Punnett square is a simple diagram that can be used to diagram that can be used to predict the outcome of a genetic predict the outcome of a genetic cross.cross.

Punnett squares use probability to Punnett squares use probability to predict possible genotypes and predict possible genotypes and phenotypes.phenotypes.

In a monohybrid cross only one In a monohybrid cross only one trait is studied at a time. trait is studied at a time.

Example:Example:

A brown mouse (Bb) is mated with A brown mouse (Bb) is mated with a white mouse (bb).a white mouse (bb).

What trait is dominant—brown or What trait is dominant—brown or white? Which is recessive?white? Which is recessive?

What is the probability of What is the probability of producing a white mouse?producing a white mouse?

In beagles the trait for droopy ears In beagles the trait for droopy ears (D) is dominant to the trait for (D) is dominant to the trait for perky ears (d). What would be the perky ears (d). What would be the expected offspring if two expected offspring if two heterozygous droopy-eared dogs heterozygous droopy-eared dogs were mated?were mated?

Incomplete DominanceIncomplete Dominance

Some alleles are neither dominant Some alleles are neither dominant or recessive. In these, a blend of or recessive. In these, a blend of the two traits is expressed.the two traits is expressed.

CodominanceCodominance

Codominance is situation in which Codominance is situation in which both alleles are dominant. both alleles are dominant.

In a codominant individual, the In a codominant individual, the phenotypes from both alleles are phenotypes from both alleles are clearly expressed.clearly expressed.

Multiple AllelesMultiple Alleles

Many genes exist in several Many genes exist in several different forms and are said to different forms and are said to have multiple alleles.have multiple alleles.

A gene with more than two alleles A gene with more than two alleles is said to have multiple alleles.is said to have multiple alleles.

In human, blood type is In human, blood type is determined by multiple alleles. A determined by multiple alleles. A and B are codominant, while O is and B are codominant, while O is recessive.recessive.

A woman with AB blood has a child A woman with AB blood has a child with a man who has type O blood with a man who has type O blood (OO). What are the possible blood (OO). What are the possible blood types of the child?types of the child?

Sex-Linked TraitsSex-Linked Traits

Humans have 23 pairs of Humans have 23 pairs of chromosomes. chromosomes.

We have 22 pairs homologous We have 22 pairs homologous chromosomes, called autosomes.chromosomes, called autosomes.

The 23The 23rdrd pair determines gender. pair determines gender. Females have an XX genotype. Females have an XX genotype. Males have an XY genotype.Males have an XY genotype.

The X and the Y chromosomes are The X and the Y chromosomes are not homologous. The X not homologous. The X chromosome is much bigger than chromosome is much bigger than the Y, so there are some genes on the Y, so there are some genes on the X-chromosome that are not on the X-chromosome that are not on the Y-chromosome.the Y-chromosome.

The alleles for sex-linked traits are The alleles for sex-linked traits are written as superscripts of the X-written as superscripts of the X-chromosome.chromosome.

XXbb Y Y

X XX Xbb

Color-blindness is a sex-linked trait Color-blindness is a sex-linked trait carried on the X-chromosome.carried on the X-chromosome.Because boys only have one X-Because boys only have one X-chromosome, they are more likely chromosome, they are more likely to get color blindness. If they only to get color blindness. If they only get one bad copy of the gene, they get one bad copy of the gene, they have the disorder. Girls have to have the disorder. Girls have to get two bad copies of the gene to get two bad copies of the gene to have the disorder.have the disorder.

Polygenic InheritancePolygenic Inheritance

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

These traits often show a wide These traits often show a wide range of phenotypes.range of phenotypes.

More than 4 different genes code More than 4 different genes code for skin color.for skin color.