Probability and Punnett Squares

Probability Probability is the likelihood that a specific event will occur. For example, if you flip a coin, the probability that it will land on heads is 50%, or 1 out of 2. We can use Punnett Squares to predict the probability of genetic outcomes.

Dominant and Recessive Alleles Dominant alleles are expressed in capital letters. Ex: T B A C X Dominant alleles can mask recessive alleles. This means you might have a recessive allele, but you can see the recessive trait if the dominant allele is present.

Recessive alleles are represented by lowercase letters. Ex: t b a c x Recessive alleles are masked by dominant alleles.

Homozygous and Heterozygous An organism is homozygous if both alleles are identical (both dominant or both recessive). Ex: TT, BB, aa, cc, tt

An organism is heterozygous if the alleles are different (one dominant and one recessive). Ex: Tt, Bb, Rr, Xx A heterozygous organism expresses the dominant trait, but is a carrier of the recessive trait. A heterozygous organism expresses the dominant trait, but is a carrier of the recessive trait.

TT = homozygous dominant tt = homozygous recessive Tt = heterozygous

Genotype and Phenotype Phenotype is the physical appearance of the trait. Ex: brown hair, blue eyes, tall, short, etc.

Genotype is the genetic makeup of the organism, or what the alleles actually are. Ex: BB, Tt, Rr, rr

Example A plant is tall, but has heterozygous alleles for height. What is the genotype? A dog is homozygous dominant for a straight tail. What will be the genotype if the letter that represents tail straightness is S?

Punnett Squares A Punnett square is a simple diagram that can be used to predict the outcome of a genetic cross. Punnett squares use probability to predict possible genotypes and phenotypes.

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

Example: A brown mouse (Bb) is mated with a white mouse (bb). What trait is dominant—brown or white? Which is recessive? What is the probability of producing a white mouse?

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

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

Codominance Codominance is situation in which both alleles are dominant. In a codominant individual, the phenotypes from both alleles are clearly expressed.

Multiple Alleles Many genes exist in several different forms and are said to have multiple alleles. A gene with more than two alleles is said to have multiple alleles.

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

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

Sex-Linked Traits Humans have 23 pairs of chromosomes. We have 22 pairs homologous chromosomes, called autosomes. The 23 rd pair determines gender. Females have an XX genotype. Males have an XY genotype.

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

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

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

Polygenic Inheritance Traits controlled by two or more genes are said to be polygenic. These traits often show a wide range of phenotypes. More than 4 different genes code for skin color.