Mendelian Genetics

By the end of this class you should understand:

• The Mendelian model of genetics and Punnett squares

• How the structure and function of genes influences phenotypes

• The different modes of inheritance and expression of genes

• The concept of a pedigree chart and how to read one

Gregor Mendel

Austrian Monk (1822-1884)

Discovered the modern principles of genetics working in a pea garden

His work was not widely acknowledged until the 20th century

Why Pea Plants? All animals and plants use

the same DNA and chromosome structure Plants complain much less

when you force them to mate with particular individuals and take their children away for a breeding program

Many traits with different alleles at a given locus

Allele

• An allele is a particular version of a gene– How do different versions come about?

• There are many alleles for almost all genes– Many of them are functionally identical

– Sometimes the function is different for different alleles

– Some alleles are defective!

Alleles

In Mendelian genetics (simplified case) there are only two alleles, one is capitalized and the other is lowercase

In reality there are many alleles and any symbol can stand for any of them

Terminology

• The matching genes at the same locus are on homologous chromosomes

• Having two of the same allele for a gene is called homozygous

• Having two different alleles for one gene is called heterozygous

Mendel's Findings Mendel studied seven traits and started with true-breeding specimens Are true-breeding individuals homozygous or

heterozygous?

Mendel's Discoveries Mendel discovered that when

he crossed true-breeding peas for opposite traits (P generation), all the offspring had only one of the traits (F1 generation)

When these offspring were self-crossed or crossed with other F1 plants, the F2 plants had a 3:1 ratio

One trait was dominant over the other

Pattern of Inheritance All seven of the listed

traits follow the same pattern

Dihybrid Crossing What if a true-

breeding yellow smooth pea is crossed with a true-breeding green wrinkled pea?

According to Mendel's work the traits are not linked but sort randomly

Dihybrid Cross Analysis

Chromosomal Inheritance

• Half your chromosomes come from each parent (via meiosis)

• Each parent randomly passes on 1 of the 2 chromosomes s/he has

• This means each gene your parent has, you have a 50% chance of having

– This fits Mendel's model!

Probability Table

• A table of which genes get passed on is called a Punnett Square

• Mother's possible genes go on one axis, father's possible genes go on the other

Phenotype

When an organism has two different alleles, how are they expressed?

– The phenotype is what is actually expressed

What if two different organisms look the same but have different alleles?

– They have different genotypes

One of these green peas is notlike the other....

Dominant/Recessive

“Dominant” and “Recessive” are relative terms Much like “taller” and “shorter”

One allele may dominate over another Sometimes two alleles do not have a

dominant/recessive relationship If both are equally expressed they are called

codominant If the phenotype is a blend between the two

this is called incomplete dominance

Incomplete Dominance

The phenotype is a blending of the two

This means there are red proteins and white proteins here

This is very common in more complex traitsHeight, etc

“Blending theory”

Mendel's work disproved earlier ideas of “blending theory”

Blending theory states you are a literal mix of your parents

Blending theory gained support because many of your genes are codominant or incompletely dominant alleles from your parents

Developmental genes control your height, body type, facial structure, etc

Dominant/Recessive

Many genetic disorders and diseases are recessive

Only found in people who are homozygous for the allele that causes the disease

Albinism, cystic fibrosis, phenylketonuria, Tay-sachs disease, etc

These disease come from a lack of a specific enzyme or other protein

“Recessive”

Typically a “recessive” gene is a defective gene

Blue eyes are a defect in an allele for coloring the eye

This means blue eyes are recessive and parents with brown eyes can be carriers for blue eyes

Example: Cystic Fibrosis

A gene called CFTR produces a protein channel that pumps chloride ions onto the surface of mucus membranes

Through osmosis, water follows the chloride ions out

Failure to produce this protein or have it be expressed means mucus builds up in the respiratory tract and can become fatal

Cystic Fibrosis

Example: Albinism The protein melanin is the

pigment for our skin and is present in our hair and eyes as well

If one allele is defective and one is normal, what is the genotype? What is the phenotype?

If both alleles are defective, what is the genotype? What is the phenotype?

Carriers

How many copies of a working blueprint do you need to make the enzyme?

Just 1! Having 1 working

and 1 defective allele means you are healthy

Pedigree Chart A pedigree chart, or just

pedigree, shows family history for a particular condition

Can be for hair color, eye color, etc

Most commonly for a genetic disorder

Can be used to determine the nature of the inheritance

Example: Recessive Disorder Often “skips”

generations When both parents are

carriers, about 1 in 4 offspring are affected

When one parent has the condition: 1 in 2 offspring are

affected and other half are carriers

OR all are carriers

Multi-Allele Genes

There are three alleles for a marker on our red blood cells:

A, B, and O A and B markers are large and can be

detected by the immune system O marker is small and cannot be detected

As though it weren't there What allele is recessive?

ABO blood type

Six possibilities for genotype:

AA AO BB BO AB OO

Codominance

Since A and B are both fully expressed, they are codominant

O is recessive because it is only expressed when there are no other alleles present

Why does this matter?

We have an immune system! Your immune system will attack

any markers that were not in your body when you were a fetus

This includes A and B markers O type blood is the universal

donor!

See you in lab!

• Coming soon to a lab near you: more genetics!