Classical Genetics!

Subtopics are highlighted, terms are bolded, and definitions are underlined :)

Vocab:

Genotype - genetic makeup (genes); ex. TT or Tt or tt

Phenotype - observable trait (gene product); ex. Brown or white fur color

Allele - different versions of a gene; ex. yellow and green pea seeds or T or t

Homozygous - both alleles for a trait are the same; ex. TT or tt

Heterozygous - both alleles are different; ex. Tt

True-Breeding - if allowed to self-pollinate, they would produce offspring identical to themselves; ex. homozygous dominant or recessive (TT or tt)

Hybrid - the offspring of two different true-breeding varieties; ex. heterozygous (Tt)

Dominant - when only one of the two different alleles in a heterozygous individual appears to affect the trait; represented by an upper-case letter

Recessive - the allele that seems to be suppressed by the dominant allele in the heterozygous state; represented by a lower-case letter

Mendel’s Law of Dominance - in the heterozygous state, the dominant trait will be expressed in the phenotype

Mendel’s Law of Segregation - the two alleles for a character separate during the formation of gametes

Monohybrid Cross - a mating event between two individuals involving one gene

It determines the genotypes and phenotypes of the offspring based upon the genotypes of the parents.

Solving Monohybrid Problems:

Brown eyes is dominant to blue eyes. Jack is heterozygous for eye color and Jill has blue eyes. If Jack and Jill have children, what is the chance that their children will have blue eyes? List the genotypic & phenotypic ratios.

Step 1 - Develop a Key

B - Brown, b - blue

Step 2 - Determine the genotypes of the parents

 P - Bb X bb

Step 3 - Determine the various alleles of the trait that each parent may possess in their gametes

Jack - B or b, Jill - just b

Step 4 - Draw a four box Punnett square. Place the possible alleles of a trait from one parent’s gametes across the top and place the possible alleles of the other parent’s gametes along the side of the box

Step 5 - Figure out the genotypes of the potential offspring by filling in the boxes

Step 6 - Determine the genotypic ratio

1Bb : 1bb

Step 7 - Determine the phenotypic ratio

1 Brown eyed child : 1 blue eye child

Step 8 - Solve the problem

50% of the children will have blue eyes 

Test Cross - breeds an individual of unknown genotype but dominant phenotype, with a homozygous recessive individual in order to figure out whether the parent is homozygous dominant or heterozygous for a given trait

P - parental generation

F1 - Offspring of the P generation

F2 - The offspring of a cross between two individuals from the F1 generation

Mendel’s Law of Independent Assortment - genes located on different chromosomes will be inherited independently of each other

In other words, if eye color and hair color are located on different chromosomes, the eye color gene found in the gamete will not affect the type of hair color found in the gamete. Both genes are inherited independently of each other.

Linked Genes - genes located on the same chromosome that tend to be inherited together in genetic crosses

Since these genes are commonly inherited together, they don’t follow Mendel’s law of independent assortment. The closer two genes are on the same chromosome, the higher chance they will be inherited together. The farther apart the genes are on the same chromosome, the more likely it is that a crossover event will separate them.

Dihybrid Cross - cross between two individuals that includes two completely different traits; ex. Eye color & hair color

Each type of gene should have a unique letter symbol; ex. B - brown eyes, b - blue eyes; T - Brown hair, t - blonde hair

When answering these problems we will assume that the two traits are genetically unlinked (found on separate chromosomes) and therefore will assort independently of each other.

Solving a Dihybrid Cross Problem:

Step 1: Develop a key (each type of gene receives a unique letter)

B- brown eyes, b - blue eyes ; T - brown hair, t - blonde hair

Step 2: Determine the genotypes of the parents

P - BbTt X BbTt

Step 3 - Determine the various gene combinations that each parent may possess in their gametes

Egg - BT, Bt, bT, bt ; Sperm - BT, Bt, bT, bt

Step 4 - Draw a 16 box Punnett square. Place the gamete possibilities of one parent across the top of the box and place the gamete possibilities of the other parent along the side of the box

Step 5 - Figure out the genotypes of the potential offspring by filling in the box

Step 6 - Determine the phenotypic ratio

Arrange the genotypes in groups with similar phenotypes

Genotype Phenotypic Ratio:

B_T_ - Brown eyes, Brown Hair

B_tt - Brown eyes, blonde Hair

bbT_ - blue eyes, Brown Hair

bbtt - blue eyes, blonde Hair

Step 7 - Solve the problem

What are the chances that a given child will be:

Brown eyed and brown haired? Brown eyed and blonde?  Blue eyed and brown haired?  Blue eyed and blonde?

Incomplete Dominance:

Incomplete Dominance - both alleles are dominant and the heterozygote shows an intermediate phenotype

Examples include: Pink coloration in certain flowers and the blue Andalusian chicken

Co-dominance - when heterozygous for two dominant traits, the individual expresses both traits separately

The traits do not blend as they do in incomplete dominance.

Here is another chart to help remember the difference between complete dominance (which is what the monohybrid and dihybrid crosses were), incomplete dominance, and co-dominance. 

Multiple Alleles - there are more than two alleles available for a gene; ex. blood types

Antigen - a substance that can trigger an immune response causing the production of antibodies as part of the body's defense against infection and disease

Rh-factor - an antigen that is found on red blood cells

If the Rh factor antigen is present on the cells, a person is Rh-positive. If there is no Rh factor antigen, the person is Rh-negative.

Rh-positive individuals can receive + or - blood

Rh-negative individuals can only receive - blood

Blood Transfusions:

Polygenic inheritance - traits controlled by multiple genes

Genes have an additive effect upon the phenotype. Do not follow ratios predicted by Mendel’s laws. Examples - skin color, height, and eye color

Pedigrees:

Dominant Phenotype - only one dominant allele is necessary for the person to show the dominant phenotype

Dominant Phenotype - TT or Tt

Recessive Phenotype - tt

Every child with the dominant phenotype must have at least one parent with the dominant phenotype.

Recessive Phenotype - both copies of the recessive allele are needed to show the recessive phenotype

Dominant Phenotype - TT

Carrier (Dominant Phenotype) - Tt (carries the recessive allele, but does not show the recessive phenotype)

Recessive Phenotype - tt

An individual who has the recessive phenotype may or may not have parents who have the recessive phenotype.

If both parents have the recessive phenotype, then all their children will have the recessive phenotype.

Pedigree - a diagram that shows the pattern of inheritance of a gene in a family

An individual who exhibits the trait in question is represented by a filled in symbol.

Step 1: How to Identify the Inheritance Pattern within a Pedigree

First determine whether the trait of interest is dominant or recessive. If the trait is dominant, then every child who has the trait will have a parent who has the trait. (The trait doesn’t skip generations) If the trait is recessive, then the children may or may not have a parent with the trait. If there is just one situation where a child has the trait, but neither parent has the trait then you know it has to be recessive. (The trait does skip generations).

Step 2: How to Identify the Inheritance Pattern within a Pedigree

Second determine whether the trait of interest is autosomal or X-linked (sex-linked). If the trait is autosomal, then the ratio of males to females with the trait is about a 1:1 ratio. If the trait is X-linked (sex-linked), then the ratio of males to females with the trait is not equal. One gender is affected more often than the other.